Understand The Weather has been written by Taimeir Abdelrazek. Taimeir is an airline pilot and an instructor Commander on Airbus A340/A330 in the UAE. He has an extensive experience on a varieties of civilian and military fast Jets.
Understand the weather you are facing will save you from trouble. Simple aviation weather will get you there. Weather is like a love story. It has always a beginning and always an end. Never the less the story line might be turbulent from the beginning but never to it's end. Until today, no one is able to predict the weather at 100%. It is only forecasted. Even thought with the most advanced technology available in our hands, still the efficiency of forecasting is far away to be reliable. The respect of the outside conditions and the weather phenomenas are crucial to not ending up hurt or worst.
There is nothing more looking for troubles than riding in stormy weather, hurricanes, tornados or cyclonic weather. Riding in such conditions will be lethal soon or later. So be safe and respect the outside conditions, know your limit, don't try to impress others or show off when the weather is unstable or even at any time.
Effect of the Weather on power kite
1.EFFECT OF DIFFERENCES IN AIR DENSITY
Differences in air density caused by changes in temperature result in changes in pressure. This, in turn, creates motion in the atmosphere, both vertically and horizontally, in the form of currents and wind. Motion in the atmosphere, combined with moisture, produces clouds and precipitation otherwise known as weather.
Pressure and temperature changes produce two kinds of motion in the atmosphere—vertical movement of ascending and descending currents, and horizontal movement in the form of wind. Both types of motion in he atmosphere are important as they affect the takeoff, landing, and cruise flight operations. More important, however, these motions in the atmosphere, otherwise called atmospheric circulation, causes weather changes.
3.THE CAUSE OF ATMOSPHERIC CIRCULATION
Atmospheric circulation is the movement of air around the surface of the Earth. It is caused by uneven heating of the Earth’s surface and upsets the equilibrium of the atmosphere, creating changes in air movement and atmospheric pressure. Because the Earth has a curved surface that rotates on a tilted axis while orbiting the sun, the equatorial regions of the Earth receive a greater amount of heat from the sun than the polar regions. The amount of sun that heats the Earth depends upon the time of day, time of year, and the latitude of the specific region. All of these factors affect the length of time and the angle at which sunlight strikes the surface. In general circulation theory, areas of low pressure exist over the equatorial regions, and areas of high pressure exist over the polar regions due to a difference
in temperature. Solar heating causes air to become less dense and rise in equatorial areas. The resulting low pressure allows the high-pressure air at the poles to flow along the planet’s surface toward the equator. As the warm air flows toward the poles, it cools, becoming more dense, and sinks back toward the surface. [Figure 10-8]
This pattern of air circulation is correct in theory; however, the circulation of air is modified by several forces, most importantly the rotation of the Earth.
3.1 The force created by the rotation of the Earth is known as Coriolis force. This force is not perceptible to us as we walk around because we move so slowly and travel relatively short distances compared to the size and rotation rate of the Earth. However, it does significantly affect bodies that move over great distances, such as an air mass or body of water.
The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line. The amount of deflection differs depending on the latitude.
It is greatest at the poles, and diminishes to zero at the equator. The magnitude of the Coriolis force also differs with the speed of the moving body the faster the peed, the greater the deviation. In the Northern
Hemisphere, the rotation of the Earth deflects moving air to the right and changes the general circulation pattern of the air.
The speed of the Earth’s rotation causes the general flow to break up into three distinct cells in each hemisphere.
[Figure 10-9] In the Northern Hemisphere, the warm air at the equator rises upward from the surface, travels northward, and is deflected eastward y the rotation of the Earth. By the time it has traveled one-third of the distance from the equator to the North Pole, it is no longer moving northward, but eastward.
This air cools and sinks in a belt-like area at about 30° latitude, creating an area of high pressure as it sinks toward the surface. Then it flows southward along the surface back toward the equator. The Coriolis force bends the flow to the right, thus creating the northeasterly trade winds that prevail from 30° latitude to the equator. Similar forces create circulation cells that encircle the Earth between 30° and 60° latitude, and between 60° and the poles. This circulation pattern results in the prevailing westerly winds in the conterminous United States.
Circulation patterns are further complicated by seasonal changes, differences between the surfaces of continents and oceans, and other factors. Frictional forces caused by the topography of the Earth’s surface modify the movement of the air in the atmosphere. Within 2,000 feet of the ground, the friction between the surface and the atmosphere slows the moving air.
The wind is diverted from its path because the frictional force reduces the Coriolis force. This is why the wind direction at the surface varies
somewhat from the wind direction just a few thousand feet above the Earth.
Air flows from areas of high pressure deplate into those of low pressure because air always seeks out lower pressure. In the Northern Hemisphere, this flow of air from areas of high to low pressure is deflected to the right; producing a clockwise circulation around an area of high pressure. This is also known as anti-cyclonic circulation. The opposite is true of low-pressure areas; the air flows toward a low and is deflected to create a counter clockwise or cyclonic circulation. [Figure 10-10 see below]
High-pressure systems are generally areas of dry, stable, descending air. Good weather is typically associated with high-pressure systems for this reason.
Conversely, air flows into a low-pressure area to replace rising air. This air tends to be unstable, and usually brings increasing cloudiness and precipitation. Thus, bad weather is commonly associated with areas of low pressure. A good understanding of high- and low-pressure wind patterns can be of great help when planning a flight, because a pilot can take advantage of beneficial tailwinds. [Figure 10-11 see below] and it is identical for kite. Remember low pressure is equal to wind.
When planning a flight from west to east, favorable winds would be encountered along the northern side of a high-pressure system or the southern side of a low-pressure system. On the return flight, the most favorable winds would be along the southern side of the same high-pressure system or the northern side of a low-pressure system. An added advantage is a better understanding of what type of weather to expect in a given area along a route of flight based on the prevailing areas of highs and lows. For kitesurfing knowing where are the low pressure areas will give you an advantage of finding the wind.
The theory of circulation and wind patterns is accurate for large-scale atmospheric circulation; however, it does not take into account changes to the circulation on a local scale. Local conditions, geological features, and other anomalies can change the wind direction and speed close to the Earth’s surface.
Wind will always be favorable near a high pressure at high altitude (jet stream). Where is a Jetsream there is ALWAYS strong winds at the earth's surface!!!.
Different surfaces radiate heat in varying amounts. Plowed ground, rocks, sand, and barren land give off a large amount of heat; water, trees, and other areas of vegetation tend to absorb and retain heat. The resulting uneven heating of the air creates small areas of local
circulation called convective currents. Convective currents cause the bumpy, turbulent air sometimes experienced when flying at lower altitudes during warmer weather. On a low altitude flight over
varying surfaces, updrafts are likely to occur over pavement or barren places, and downdrafts often occur over water or expansive areas of vegetation like a group of trees. Typically, these turbulent conditions can be avoided by flying at higher altitudes, even above
cumulus cloud layers. [Figure 10-12 see below]
Convective currents are particularly noticeable in areas with a landmass directly adjacent to a large body of water, such as an ocean, large lake, or other appreciable area of water. During the day, land heats faster than water, so the air over the land becomes warmer and less dense. It rises and is replaced by cooler, denser air
flowing in from over the water. This causes an onshore wind, called a sea breeze. Conversely, at night land cools faster than water, as does the corresponding air.
In this case, the warmer air over the water rises and is replaced by the cooler, denser air from the land, creating an offshore wind called a land breeze. This reverses the local wind circulation pattern. Convective currents can occur anywhere there is an uneven heating
of the Earth’s surface. [Figure 10-13 see below]
Sea Breeze—A coastal breeze blowing from sea to land caused by the temperature difference when the land surface is warmer than the sea surface. The sea breeze usually occurs during the day.
Land Breeze—A coastal breeze flowing from land to sea caused by the temperature difference when the sea surface is warmer than the adjacent land. The land breeze usually occurs at night.
Convection currents figure 1(0-12) close to the ground can affect a pilot’s ability to control the aircraft. On final approach, for example, the rising air from terrain devoid of vegetation sometimes produces a ballooning effect that can cause a pilot to overshoot the intended landing spot. On the other hand, an approach over a large body of water or an area of thick vegetation tends to create a sinking effect that can cause an unwary pilot to land short of the intended landing spot.
The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable
atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe
Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed. Adiabatic heating, or adiabatic cooling, are the terms used to describe this temperature change.
The adiabatic process takes place in all upward and downward moving air. When air rises into an area of lower pressure, it expands to a larger volume. As the molecules of air expand, the temperature of the air lowers. As a result, when a parcel of air rises, pressure decreases, volume increases, and temperature decreases. When air descends, the opposite is true.
The rate at which temperature decreases with an increase in altitude is referred to as its lapse rate. As air ascends through the atmosphere, the average rate of temperature change is 2°C (3.5°F) per 1,000 feet or 327m.
Since water vapor is lighter than air, moisture decreases air density, causing it to rise. Conversely, as moisture decreases, air becomes denser and tends to sink. Since moist air cools at a slower rate, it is generally less stable than dry air since the moist air must rise higher before its temperature cools to that of the surrounding air.
The dry adiabatic lapse rate (unsaturated air) is 3°C (5.4°F) per 1,000 feet or 327m.
The moist adiabatic lapse rate varies from 1.1°C to 2.8°C (2°F to 5°F) per 1,000 feet or 327m.
The combination of moisture and temperature determine the stability of the air and the resulting weather. Cool, dry air is very stable and resists vertical movement, which leads to good and generally clear
The greatest instability occurs when the air is moist and warm, as it is in the tropical regions in the summer. Typically, thunderstorms appear on a daily basis in these regions due to the instability of the
Adiabatic heating—A process of heating dry air through compression. As air moves downward it is compressed, resulting in a temperature increase.
Adiabatic cooling—A process of cooling the air through expansion. For example, as air moves upward, it expands with the reduction of atmospheric pressure and cools as it expands.
6.EFFECT OF OBSTRUCTIONS ON WIND
Another atmospheric hazard exists that can create problems for pilots. Obstructions on the ground affect the flow of wind and can be an unseen danger.
Ground topography and large buildings can break up the flow of the wind and create wind gusts that change rapidly in direction and speed. These obstructions range from manmade structures like hangars to large natural obstructions, such as mountains, bluffs, or canyons. It is especially important to be vigilant when flying in or out of airports that have large buildings or natural obstructions located near the runway.[Figure 10-15 ]
The intensity of the turbulence associated with ground obstructions depends on the size of the obstacle and the primary velocity of the wind. This can affect the takeoff and landing performance of any aircraft and can present a very serious hazard. During the landing phase of flight, an aircraft may “drop in” due to the turbulent air and be too low to clear obstacles during the approach.
This same condition is even more noticeable when flying in mountainous regions. [Figure 10-16 see below]
While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the
leeward side does not act in a similar manner. As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater the downward pressure and turbulence become. Due to the effect terrain has on the wind in valleys or canyons, downdrafts can be severe.
Thus, a prudent pilot is well advised to seek out a mountain qualified flight instructor and get a mountain checkout before conducting a flight in or near mountainous terrain.
8.MOISTURE AND TEMPERATURE
The atmosphere, by nature, contains moisture in the form of water vapor. The amount of moisture present in the atmosphere is dependent upon the temperature of
the air. Every 20°F (7c) increase in temperature doubles the amount of moisture the air can hold. Conversely, a decrease of 20°F (7c) cuts the capacity in half. Water is present in the atmosphere in three states:
liquid, solid, and gaseous.
All three forms can readily change to another, and all are present within the temperature ranges of the atmosphere. As water changes from one state to another, an exchange of heat takes place. These changes occur through the processes of evaporation, sublimation, condensation,
deposition, melting, or freezing. However, water vapor is added into the atmosphere only by the processes of evaporation and sublimation.
Evaporation is the changing of liquid water to water vapor. As water vapor forms, it absorbs heat from the nearest available source. This heat exchange is known as the latent heat of evaporation. A good example of
this is when the body’s perspiration evaporates. The net effect is a cooling sensation as heat is extracted from the body. Similarly, sublimation is the changing of ice directly to water vapor, completely bypassing the liquid stage. Though dry ice is not made of water, but
rather carbon dioxide, it demonstrates the principle of sublimation, when a solid turns directly into vapor.
Humidity refers to the amount of water vapor present in the atmosphere at a given time. Relative humidity is the actual amount of moisture in the air compared to the total amount of moisture the air could hold at that temperature. For example, if the current relative humidity is 65 percent, the air is holding 65 percent of the total amount of moisture that it is capable of
holding at that temperature and pressure. While much of the western United States rarely sees days of high humidity, relative humidity readings of 75 to 90 percent are not uncommon in the southern United States during warmer months. [Figure 10-20]
The relationship between dewpoint and temperature defines the concept of relative humidity. The dewpoint, given in degrees, is the temperature at which the air can hold no more moisture. When the temperature of the air is reduced to the dewpoint, the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost, clouds, rain, hail, or snow.
As moist, unstable air rises, clouds often form at the altitude where temperature and dewpoint reach the same value. When lifted, unsaturated air cools at a rate of 5.4°F (15c) per 1,000 feet and the dewpoint temperature decreases at a rate of 1°F (17c) per 1,000 feet.
This results in a convergence of temperature and dewpoint at a rate of 4.4°F (15c). Apply the convergence rate to the reported
temperature and dewpoint to determine the height of the cloud base.
11.METHODS BY WHICH AIR REACHES THE SATURATION POINT
If air reaches the saturation point while temperature and dewpoint are close together, it is highly likely that fog, low clouds, and precipitation will form.
There are four methods by which air can reach the complete
First, when warm air moves over a cold surface, the air’s temperature drops and reaches the saturation point.
Second, the saturation point may be reached when cold air and warm air mix.
Third, when air cools at night through contact with the cooler
ground, air reaches its saturation point. The fourth method occurs when air is lifted or is forced upward in the atmosphere.
As air rises, it uses heat energy to expand. As a result, the rising air loses heat rapidly. Unsaturated air loses heat at a rate of 3.0°C (5.4°F) for every 1,000 feet of altitude gain. No matter what causes the air to reach its saturation point, saturated air brings clouds, rain, and other critical weather situations.
Figure 10-21 Dubai under Sea Fog
Clouds are visible indicators and are often indicative of future weather. For clouds to form, there must be adequate water vapor and condensation nuclei, as well as a method by which the air can be cooled.
When the air cools and reaches its saturation point, the invisible
water vapor changes into a visible state. Through the processes of deposition (also referred to as sublimation) and condensation, moisture condenses or sublimates onto miniscule particles of matter like dust, salt, and smoke known as condensation nuclei.
The nuclei are important because they provide a means for the moisture to change from one state to another. Cloud type is determined by its height, shape, and behavior. They are classified according to the height of their bases as low, middle, or high clouds, as well as clouds with vertical development. [Figure 10-22]
Low clouds are those that form near the Earth’s surface and extend up to 6,500 feet AGL. They are made primarily of water droplets, but can include supercooled water droplets that induce hazardous aircraft icing.
Typical low clouds are stratus, stratocumulus, and nimbostratus. Fog is also classified as a type of low cloud formation. Clouds in this family create low ceilings, hamper visibility, and can change rapidly.
Because of this, they influence flight planning and can make Visual flight impossible.
Middle clouds form around 6,500 feet AGL and extend up to 20,000 feet AGL. They are composed of water, ice crystals, and supercooled water droplets. Typical middle-level clouds include altostratus and
altocumulus. These types of clouds may be encountered on cross-country flights at higher altitudes. Altostratus clouds can produce turbulence and may contain moderate icing. Altocumulus clouds, which usually form when altostratus clouds are breaking apart, also may contain light turbulence and icing.
Figure 10-21. Sea fog.
Condensation Nuclei-Small particles of solid matter in the air on
which water vapor condenses.
Supercooled Water Droplets-Water droplets that have been cooled below the freezing point, but are still in a liquid state.
Figure 10-22 Basic Cloud Type
12.DEW AND FROST
On cool, calm nights, the temperature of the ground and objects on the surface can cause temperatures of the surrounding air to drop below the dewpoint. When this occurs, the moisture in the air condenses and deposits itself on the ground, buildings, and other
objects like cars and aircraft. This moisture is known as dew and sometimes can be seen on grass in the morning. If the temperature is below freezing, the moisture will be deposited in the form of frost.
While dew poses no threat to an aircraft, frost poses a definite flight safety hazard. Frost disrupts the flow of air over the wing and can drastically reduce the production of lift. It also increases drag, which, when combined with lowered lift production, can eliminate the ability to take off. An aircraft must be thoroughly cleaned and free of frost prior to beginning a flight.
Fog, by definition, is a cloud that begins within 50 feet of the surface. It typically occurs when the temperature of air near the ground is cooled to the air’s dewpoint.
A cubic meter of air with 17g of water vapor at 20°C is at saturation, or 100% relative humidity. Any further cooling will cause condensation (fog, clouds, dew) to form.
Thus, 20°C is the dewpoint for this situation. If the temperature is lowered to 10°C, the air can only hold 9g of water vapor, and 8g of
water will condense as water droplets. The relative humidity will still be at 100%. If the same cubic meter of air warms to 30°C, the 17g of water vapor will produce a relative humidity of 56%. (17g is 56% of the 30g the air could hold at this temperature.)
Figure 10-20 (above). The relationship between relative humidity, temperature, and dew point.
At this point, water vapor in the air condenses and becomes visible in the form of fog. Fog is classified according to the manner in which it forms and is dependent upon the current temperature and the amount of water vapor in the air.
On clear nights, with relatively little to no wind present, radiation fog may develop. [Figure 10-21]
Usually, it forms in low-lying areas like mountain valleys. This type of fog occurs when the ground cools rapidly due to terrestrial radiation, and the surrounding air temperature reaches its dewpoint. As the sun rises and the temperature increases, radiation fog will lift and eventually burn off. Any increase in wind will also speed the dissipation of radiation fog. If radiation fog isless than 20 feet thick, it is known as ground fog.
When a layer of warm, moist air moves over a cold surface, advection fog is likely to occur. Unlike radiation fog, wind is required to form advection fog. Winds of up to 15 knots allow the fog to form and intensify; above a speed of 15 knots, the fog usually lifts and forms low stratus clouds. Advection fog is common in coastal areas where sea breezes can blow the air over cooler landmasses.
In these same coastal areas, upslope fog is likely as well. Upslope fog occurs when moist, stable air is forced up sloping land features like a mountain range.
This type of fog also requires wind for formation and continued existence. Upslope and advection fog, unlike radiation fog, may not burn off with the morning sun, but instead can persist for days. They also can extend to greater heights than radiation fog.
Steam fog, or sea smoke, forms when cold, dry air moves over warm water. As the water evaporates, it rises and resembles smoke. This type of fog is common over bodies of water during the coldest times of the year. Low-level turbulence and icing are commonly associated with steam fog.
Ice fog occurs in cold weather when the temperature is much below freezing and water vapor forms directly into ice crystals. Conditions favorable for its formation are the same as for radiation fog except for cold temperature, usually –25°F or colder. It occurs mostly in the arctic regions, but is not unknown in middle latitudes during the cold season.
RIDING DURING FOGGY CONDITIONS IS NOT ALLOWED.
High clouds form above 20,000 feet AGL and usually form only in stable air. They are made up of ice crystals and pose no real threat of turbulence or aircraft icing.
Typical high-level clouds are cirrus, cirrostratus, and cirrocumulus.
Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds. The bases of these clouds form in the low to middle cloud base region but
can extend into high altitude cloud levels.
Towering cumulus clouds indicate areas of instability in the
atmosphere, and the air around and inside them is turbulent. These types of clouds often develop into cumulonimbus clouds or thunderstorms.
Cumulonimbus clouds contain large amounts of moisture and unstable air, and usually produce hazardous weather phenomena such as lightning, hail, tornadoes, gusty winds, and wind shear.
These extensive vertical clouds can be obscured by other cloud formations and are not always visible from the ground or while in flight. When this happens, these clouds are said to be embedded, hence the term, embedded thunderstorms.
Cloud classification can be further broken down into specific cloud types according to the outward appearance and cloud composition. Knowing these terms can help identify visible clouds.
The following is a list of cloud classifications:
• Cumulus—Heaped or piled clouds.
• Stratus—Formed in layers.
• Cirrus—Ringlets; fibrous clouds;
also high-level clouds above 20,000 feet.
• Castellanus—Common base with separate vertical development; castle-like.
• Lenticularus—Lens shaped; formed over mountains in strong winds.
• Nimbus—Rain bearing clouds.
• Fracto—Ragged or broken.
• Alto—Meaning high; also middle-level clouds existing at 5,000 to 20,000 feet.
The cumulonimbus cloud is perhaps the most dangerous cloud type sake for any rider below a Cumulonimbus. Over the this has a very low base. It appears individually or in groups and is known as either an air mass or orographic thunderstorm.
Heating of the air near the Earth’s surface creates an air mass thunderstorm; the upslope motion of air in the mountainous regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are non frontal bands of thunderstorms or squall lines.
Since rising air currents cause cumulonimbus clouds, they are extremely turbulent and pose a significant hazard to flight safety. For example, if an aircraft enters a thunderstorm, the aircraft could experience updrafts and downdrafts that exceed 3,000 feet per minute or 1000m or 16m/s.
In addition, thunderstorms can produce large hailstones, damaging lightning, tornadoes, and large quantities of water, all of which are potentially hazardous to riders and equipments.
A thunderstorm makes its way through three distinct stages before dissipating. It begins with the cumulus stage, in which lifting action of the air begins. If sufficient moisture and instability are present, the
clouds continue to increase in vertical height.
Continuous, strong updrafts prohibit moisture from falling. The updraft region grows larger than the individual thermals feeding the storm.
Within approximately 15 minutes, the thunderstorm reaches the mature stage, which is the most violent time period of the thunderstorm’s life cycle. At this point, drops of moisture, whether rain or ice, are too heavy for the cloud to support and begin falling in the form of rain or hail.
This creates a downward motion of the air. Warm, rising air; cool, precipitation-induced descending air; and violent turbulence all exist within and near the cloud. Below the cloud, the down-rushing air increases surface winds and decreases the temperature. Once the
vertical motion near the top of the cloud slows down, the top of the cloud spreads out and takes on an anvil-like shape. At this point, the storm enters the dissipating stage.
This is when the downdrafts spread out and replace the updrafts needed to sustain the storm. [Figure 10-23]
It is impossible to fly over thunderstorms in light airfoil. Severe thunderstorms can punch through the tropopause and reach staggering heights of 50,000 to 60,000 feet depending on latitude.
Flying under thunderstorms can subject aircraft to rain, hail, damaging lightning, and violent turbulence. A good rule of thumb is to circumnavigate thunderstorms by at least 5 nautical miles (NM) since hail may fall for miles outside of the clouds. If flying around a thunderstorm is not an option, stay on the ground until it passes.
A ceiling, for aviation purposes, is the lowest layer of clouds reported as being broken or overcast, or the vertical visibility into an obscuration like fog or haze.
Clouds are reported as broken when five-eighths to seven-eighths of the sky is covered with clouds.
Overcast means the entire sky is covered with clouds. Current ceiling information is reported by the aviation routine weather report (METAR) and automated weather stations of various types.
Ceiling—The height above the Earth’s surface of the lowest layer of
clouds reported as broken or overcast, or the vertical visibility into an
Closely related to cloud cover and reported ceilings is visibility information. Visibility refers to the greatest horizontal distance at which prominent objects can be viewed with the naked eye.
Precipitation refers to any form of water particles that form in the atmosphere and fall to the ground. It has a profound impact on flight safety. Depending on the form of precipitation, it can reduce visibility, create icing situations, and affect landing and takeoff performance of an aircraft.
Precipitation occurs because water or ice particles in clouds grow in size until the atmosphere can no longer support them. It can occur in several forms as it falls toward the Earth, including drizzle, rain, ice pellets,hail, and ice.
Drizzle is classified as very small water droplets, smaller than 0.02 inches in diameter. Drizzle usually accompanies fog or low stratus clouds. Water droplets of larger size are referred to as rain. Rain that falls through the atmosphere but evaporates prior to striking the ground is known as virga. Freezing rain and freezing drizzle occur when the temperature of the surface is below freezing; the rain freezes on contact with the cooler surface.
If rain falls through a temperature inversion, it may freeze as it passes through the underlying cold air and fall to the ground in the form of ice pellets. Ice pellets are an indication of a temperature inversion and that freezing rain exists at a higher altitude. In the case of hail, freezing water droplets are carried up and down by drafts inside clouds, growing larger in size as they come in contact with more moisture.
Once the updrafts can no longer hold the freezing water, it falls to the
Earth in the form of hail. Hail can be pea-sized, or it an grow as large as 5 inches in diameter, larger than a softball.
Snow is precipitation in the form of ice crystals that falls at a steady rate or in snow showers that begin, change in intensity, and end rapidly. Falling snow also varies in size, being very small grains or large flakes.
Snow grains are the equivalent of drizzle in size. Precipitation in any form poses a threat to safety of flight. Often, precipitation is accompanied by low ceilings and reduced visibility. Aircraft that have ice, snow, or frost on their surfaces must be carefully cleaned prior to beginning a flight because of the possible airflow disruption and loss of lift.
Rain can contribute to water in the fuel tanks. Precipitation can
create hazards on the surface itself, making takeoffs and landings difficult, if not impossible, due to snow, ice, or pooling water and very slick surfaces.
In conclusion be very cautious with gusty wind and thunderstorms. They are unpredictables. Do not try to show off on the beach by doing a series of jumps. This is useless and very dangerous. And remember to know your limit and if you are in doubt just don't try it you have nothing to prove to any one.
Air masses are large bodies of air that take on the characteristics of the surrounding area, or source region. Asource region is typically an area in which the air remains relatively stagnant for a period of days or longer.
During this time of stagnation, the air mass takes on the temperature and moisture characteristics of the source region. Areas of stagnation can be found in polar regions, tropical oceans, and dry deserts. Air
masses are classified based on their region of origination:
• Polar or Tropical
• Maritime or Continental
A continental polar air mass forms over a polar region and brings cool, dry air with it. Maritime tropical air masses form over warm tropical waters like the Caribbean Sea and bring warm, moist air. As the air mass moves from its source region and passes over land or water, the air mass is subjected to the varying conditions of the land or water, and these modify the nature of the air mass. [Figure 10-24]
An air mass passing over a warmer surface will be warmed from below, and convective currents form, causing the air to rise. This creates an unstable air mass with good surface visibility. Moist, unstable air causes cumulus clouds, showers, and turbulence to form.
Conversely, an air mass passing over a colder surface does not form convective currents, but instead creates a stable air mass with poor surface visibility. The poor surface visibility is due to the fact that smoke, dust, and other particles cannot rise out of the air mass and are instead trapped near the surface. A stable air mass can produce low stratus clouds and fog.
A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air.
Cold fronts move more rapidly than warm fronts, progressing at a rate of 25 to 30 m.p.h. However, extreme cold fronts have been recorded moving at speeds of up to 60 m.p.h. A typical cold front moves in
a manner opposite that of a warm front; because it is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft. The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds.
The type of clouds that form depends on the stability of the warmer air mass. A cold front in the Northern Hemisphere is normally orientedin a northeast to southwest manner and can be several hundred miles long, encompassing a large area of land.
Prior to the passage of a typical cold front, cir-riform or towering cumulus clouds are present, and cumulonimbus clouds are possible. Rain showers and haze are possible due to the rapid development of
The wind from the south-southwest helps to replace the warm temperatures with the relative colder air. A high dewpoint and falling barometric pressure are indicative of imminent cold front passage.
As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky.
Depending on the intensity of the cold front, heavy rain showers form and might be accompanied by lightning, thunder, and/or hail. More severe cold fronts can also produce tornadoes. During cold front passage, the visibility will be poor, with winds variable and gusty, and the temperature and dewpoint drop rapidly. A quickly falling barometric pressure bottoms out during frontal passage, then begins a gradual increase.
After frontal passage, the towering cumulus and cumulonimbus clouds begin to dissipate to cumulus
clouds with a corresponding decrease in the precipitation. Good visibility eventually prevails with the winds from the west-northwest. Temperatures remain cooler and the barometric pressure continues to
19.4.FAST-MOVING COLD FRONT
Fast-moving cold fronts are pushed by intense pressure systems far behind the actual front. The friction between the ground and the cold front retards the movement of the front and creates a steeper frontal
This results in a very narrow band of weather, concentrated along the leading edge of the front. If the warm air being overtaken by the cold front is relatively stable, overcast skies and rain may occur for some
distance ahead of the front. If the warm air is unstable, scattered thunderstorms and rain showers may form. A continuous line of thunderstorms, or a squall line, may form along or ahead of the front. Squall lines present a serious hazard to pilots as squall type thunderstorms are intense and move quickly. Behind a fast moving
cold front, the skies usually clear rapidly and the front leaves behind gusty, turbulent winds and colder temperatures.
As air masses move across bodies of water and land, they eventually come in contact with another air mass with different characteristics. The boundary layer between two types of air masses is known as a front.
An approaching front of any type always means changes to the weather are imminent.
Air Mass—An extensive body of air having fairly uniform properties of temperature and moisture.
There are four types of fronts, which are named according to the temperature of the advancing air as it relates to the temperature of the air it is replacing. [Figure 10-25]
• Warm Front
• Cold Front
• Stationary Front
• Occluded Front
Any discussion of frontal systems must be tempered with the knowledge that no two fronts are the same. However, generalized weather conditions are associated with a specific type of front that helps identify the front.
A warm front occurs when a warm mass of air advances and replaces a body of colder air. Warm fronts move slowly, typically 10 to 25 miles per hour (m.p.h.).
The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area. Warm fronts contain warm air that often has very high humidity. As the warm air is lifted, the temperature drops and condensation occurs.
Generally, prior to the passage of a warm front, cir-riform or stratiform clouds, along with fog, can be expected to form along the frontal boundary. In the summer months, cumulonimbus clouds (thunderstorms) are likely to develop. Light to moderate precipitation is probable, usually in the form of rain, sleet, snow, or drizzle, punctuated by poor visibility. The wind blows from the south southeast, and the outside temperature is cool or cold, with increasing dewpoint. Finally, as the warm front approaches, the barometric pressure continues to fall until the front passes completely.
During the passage of a warm front, stratiform clouds are visible and drizzle may be falling. The visibility is generally poor, but improves with variable winds.
The temperature rises steadily from the inflow of relatively warmer air. For the most part, the dewpoint remains steady and the pressure levels off.
After the passage of a warm front, stratocumulus clouds predominate and rain showers are possible. The visibility eventually improves, but hazy conditions may exist for a short period after passage. The wind blows from the south-southwest. With warming temperatures, the dewpoint rises and then levels off. There is generally a slight rise in barometric pressure, followed by a decrease of barometric pressure.
19.2.FLIGHT TOWARD AN APPROACHING WARM FRONT
By studying a typical warm front, much can be learned about the general patterns and atmospheric conditions that exist when a warm front is encountered in flight. Figure 10-26 depicts a warm front advancing eastward from St. Louis, Missouri, toward Pittsburgh,
At the time of departure from Pittsburgh, the weather is good VFR with a scattered layer of cirrus clouds at 15,000 feet. As the flight progresses westward to Columbus and closer to the oncoming warm front, the clouds deepen and become increasingly stratiform in
appearance with a ceiling of 6,000 feet.
The visibility decreases to 6 miles in haze with a falling barometric
pressure. Approaching Indianapolis, the weather deteriorates to broken clouds at 2,000 feet with 3 miles visibility and rain. With the temperature and dewpoint the same, fog is likely. At St. Louis, the sky is overcast with low clouds and drizzle and the visibility is 1 mile.
Beyond Indianapolis, the ceiling and visibility would be too low to continue VFR. Therefore, it would be wise to remain in Indianapolis until the warm front had passed, which might require a day or two.
19.5.FLIGHT TOWARD AN APPROACHING COLD FRONT
Like warm fronts, not all cold fronts are the same. Examining a flight toward an approaching cold front, pilots can get a better understanding of the type of conditions that can be encountered in flight. Figure 10-27 shows a flight from Pittsburgh, Pennsylvania,
toward St. Louis, Missouri.
At the time of departure from Pittsburgh, the weather is VFR with 3 miles visibility in smoke and a scattered layer of clouds at 3,500 feet. As the flight progresses westward to Columbus and closer to the oncoming cold front, the clouds show signs of vertical development
with a broken layer at 2,500 feet. The visibility is 6 miles in haze with a falling barometric pressure.
Approaching Indianapolis, the weather has deteriorated to overcast clouds at 1,000 feet, and 3 miles visibility with thunderstorms and heavy rain showers. At St. Louis, the weather gets better with scattered clouds at 1,000 feet and a 10 mile visibility.
A pilot using sound judgment based on the knowledge of frontal conditions, would most likely remain in Indianapolis until the front had passed. Trying to fly below a line of thunderstorms or a squall line is
hazardous and foolish, and flight over the top of or around the storm is not an option. Thunderstorms can extend up to well over the capability of small airplanes and can extend in a line for 300 to 500 miles.
19.6. COMPARISON OF COLD AND WARM FRONTS
Warm fronts and cold fronts are very different in nature as are the hazards associated with each front. They vary in speed, composition, weather phenomenon, and prediction. Cold fronts, which move at 20 to 35 m.p.h., move very quickly in comparison to warm fronts,
which move at only 10 to 25 m.p.h. Cold fronts also possess a steeper frontal slope.
Violent weather activity is associated with cold fronts and the weather usually occurs along the frontal boundary, not in advance. However, squall lines can form during the summer months as far as 200 miles in advance of a severe cold front. Whereas warm fronts bring low ceilings, poor visibility, and rain, cold fronts bring sudden storms, gusty winds, turbulence, and sometimes hail or tornadoes.
Cold fronts are fast approaching with little or no warning, and they make a complete weather change in just a few hours. The weather clears rapidly after passage and drier air with unlimited visibilities prevail.
Warm fronts, on the other hand, provide advance warning of their approach and can take days to pass through a region.
Wind around a high-pressure system rotates in a clockwise fashion, while low-pressure winds rotate in a counter-clockwise manner. When two high-pressure systems are adjacent, the winds are almost in direct opposition to each other at the point of contact. Fronts are the boundaries between two areas of pressure, and therefore, wind shifts are continually occurring within a front. Shifting wind direction is most pronounced in conjunction with cold fronts.
When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the local weather for days.
This front is called a stationary front. The weather associated with a stationary front is typically a mixture that can be found in both warm and cold fronts.
An occluded front occurs when a fast-moving cold front catches up with a slow-moving warm front. As the occluded front approaches, warm front weather prevails, but is immediately followed by cold front
weather. There are two types of occluded fronts that can occur, and the temperatures of the colliding frontal systems play a large part in defining the type of front and the resulting weather. Acold front occlusion occurs when a fast-moving cold front is colder than the air
ahead of the slow-moving warm front.
When this occurs, the cold air replaces the cool air and forces the
warm front aloft into the atmosphere. Typically, the cold front occlusion creates a mixture of weather found in both warm and cold fronts, providing the air is relatively stable. A warm front occlusion occurs when the air ahead of the warm front is colder than the air of
the cold front. When this is the case, the cold front rides up and over the warm front. If the air forced aloft by the warm front occlusion is unstable, the weather will be more severe than the weather found in a cold front occlusion. Embedded thunderstorms, rain, and fog are
likely to occur.
Figure 10-28 depicts a cross-section of a typical cold front occlusion. The warm front slopes over the prevailing cooler air and produces the warm front type weather. Prior to the passage of the typical occluded
front, cirriform and stratiform clouds prevail, light to heavy precipitation is falling, visibility is poor, dewpoint is steady, and barometric pressure is falling. During the passage of the front, nimbostratus and cumulonimbus clouds predominate, and towering
cumulus may also be possible. Light to heavy precipitation is falling, visibility is poor, winds are variable, and the barometric pressure is leveling off.
After the passage of the front, nimbostratus and altostratus clouds are visible, precipitation is decreasing and clearing, and visibility is improving.
Stationary Front—A boundary between two air masses that are
Occluded Front—A frontal occlusion occurs when a fast-moving cold front catches up with a slow-moving warm front. The difference in temperature within each frontal system is a major factor in determining whether a cold or warm front occlusion occurs.