3.B : Weather Information

The atmosphere is composed of four primary gases, comprising 99.998% of the earth’s atmosphere. These gases are Nitrogen (by far the largest component), Oxygen, Argon, and Carbon Dioxide.

The atmosphere is also stratified into five vertical layers based on the vertical profile of average air temperature changes, chemical composition, movement, and density.

There is also a concept of a standard atmosphere, which addresses the reality that the atmosphere’s properties are in continuous fluctuation. This represents an average of conditions throughout the atmosphere for all latitudes, seasons, and altitudes. The standard atmosphere is a hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, and other uses.

The wind is simply air in motion, relative to the surface of the Earth. Winds cause the formation, dissipation, and redistribution of weather. There are various forces that affect the wind.

This is the most significant force impacting the wind. Wind is driven by different pressures which create a pressure gradient force. Whenever a pressure difference develops over an area the PGF makes the wind blow in an attempt to equalize the pressure differences. This force is identified by height contour gradients on constant pressure charts and by isobar gradient on surface charts. The PGF itself is directed from higher pressure to lower pressure and is perpendicular to contours and isobars. Wind speed is directly proportional to the contour/isobar gradient where closely spaced contours/isobars indicate strong winds, an widely spaced indicates light winds. Wind would flow directly from high to low pressure if the PDF was the only force acting on the wind, however because of the Earth’s rotation the Coriolis force affects the direction of wind as well.

A moving mass travels in a straight line until acted on by some outside force. However, if one views the moving mass from a rotating platform (such as the Earth) the path of the moving mass relative to this platform appears to be deflected or curved. As an illustration consider a turntable (figure above). Draw a straight line from the center to the outer edge of the turntable. The pencil will have traveled in a straight line. However, if the turntable is stopped it is apparent that the line spirals outward from the center. To the viewer on the turntable some apparent force deflected the pencil to the right. This force deflects the wind to the right in the Northern Hemisphere and left in the Southern Hemisphere.

Finally, there is friction between the wind and the terrain surface. This slows the wind, and the rougher the terrain the greater the frictional effect. Also, the stronger the wind speed the greater the friction. This frictional drag of the ground normally decreases with height, and becomes insignificant above the lowest few thousand feet or so. In the atmosphere above the friction layer only the PGF and Coriolis Force affect the horizontal motion of the air.

One of the most basic metrics used to describe the atmosphere is temperature. Heat is the total kinetic energy of the atoms and molecules composing a substance. Temperature is the numerical value representing the average kinetic energy of the atoms and molecules within matter, and a higher temperature indicates a higher average kinetic energy (and lower the the opposite). Temperature is an indicator of the internal energy of air.

Heat transfer is energy transfer such as a consequence of a temperature difference. When an object of fluid is at a different temperature than its surroundings heat transfer occurs. This transfer continues until the object/fluid and its surroundings reach equilibrium.

The heat source for the Earth is the Sun, and energy from the Sun is transferred to the Earth’s surface. There are three ways heat is transferred.

Water is much more resistant to heat transfer than land. It warms up and cools down more slowly than land, and heps moderate nearby air temperature. This is why islands and ocean cities experience smaller seasonal temperature variations than inland locations.

Temperature in the atmosphere generally decreases at an average rate of 2 degrees per 1,000 feet of altitude gain. But, in the tropopause sometimes temperature remains constant or even increases with altitude changes. This can create an isothermal layer where the temperature remains constant with height. A temperature inversion is a layer in which the temperature increases with altitude, and it can be an inversion starting at the surface (a "surface based inversion") or not (an "inversion aloft"). One key characteristic of an inversion is that it is extremely stable so that very little turbulence can occur within it.

There are a few key ingredients needed to create visible moisture and/or precipitation in the atmosphere. They are the presence of water vapor in the air, and lifting action which condenses that vapor into clouds. To produce precipitation there needs to be a growth process that allows cold droplets to grow large and heavy enough to fall as precipitation.

All clouds contain water, but only some produce precipitation. This is because cloud droplets and/or ice crystals are too small and light to fall to the ground as precipitation. There are two processes which allow cloud droplets to grow large enough to reach the ground as precipitation.

One is collision-coalescence (warm rain process) where collisions occur between cloud droplets of varying size and different fall speeds, sticking together or coalescing to form larger drops. Finally the drops become too large to be suspended in the air and fall to the ground as rain. This is the primary mechanism in warm tropical air masses where the freezing level is high.

Another process is called the ice crystal process which occur in colder clouds when both ice crystals and water droplets are present. In this situation it is easier for water vapor to deposit directly onto the ice crystals. The crystals eventually become heavy enough to fall. If it is cold near the surface it may snow, otherwise the crystals may melt to rain. This is thought to be the primary mechanism in mid and high latitudes.

The vertical distribution of temperature will often determine the type of precipitation that occurs at the surface. There are four different types of precipitation. They are:

An air mass is a large body of air with generally uniform temperature and humidity. The area from which an air mass originates is called a source region. These source regions range from extensive snow-covered polar areas to deserts to tropical oceans. The longer an air mass stays over its source region the more likely it will acquire the properties of the surface below.

These air masses are classified according to temperature and moisture properties of the source region. The temperature properties are roughly divided into three different types. One is arctic, which is an extremely deep cold air mass which develops mostly in winter over arctic surfaces of ice and snow. Another is polar which is a mass of relatively shallow cool to cold air which develops over high latitudes. The last is tropical which is a warm to hot air mass that develops over low latitudes. The moisture categorizations are continental, which is a dry air mass that develops over land, and maritime, a moist air mass that develops over water.

These different temperature and moisture types can be combined in various ways to result in the following characteristic air masses.

As air masses move around the Earth they can acquire different attributes. For instance, in the winger an arctic air mass (cold and dry) can move over the ocean picking up warmth and moisture from the warmer ocean, and become a maritime polar air mass.

Air masses can control weather for a relatively long time period, ranging from days to months. Most weather occurs along the periphery of these air masses at boundaries called "fronts". A front is a boundary or transition zone between two air masses. These fronts are classified by which type of air mass (col or warm) is replacing the other.

Fronts are usually detectable at the surface in a number of ways. There can be significant temperature change gradients. The winds usually converge. And the pressure typically decreases as a front approaches and increases afterward.

Fronts do not only exist at the surface, but they have a vertical structure as well. Cold fronts have a steep slope, and the warm air is forced up abruptly. If the warm rising air is unstable this often leads to a narrow band of showers and thunderstorms along, or just ahead of, the front. In contrast warm fronts have a gentle slope so the warm air rising along the frontal surface is gradual. This favors the development of widespread layered or stratiform cloudiness and precipitation along, and ahead of, the front if the warm rising air is stable. Stationary frontal slopes can vary, but clouds and precipitation would still form in the warm rising air along the front.

Cold fronts typically move faster than warm fronts, so in time they catch up to warm fronts. As the two fronts merge an occluded front forms. The cold are undercuts the retreating cooler air mass associated with the warm front, further lifting the already rising warm air. Clouds and precipitation can occur in the areas of frontal lift along, ahead of, and behind the surface position of an occluded front.

The part of the atmosphere in which clouds are usually present has been divided into three levels: High, Middle, and Low. In each level the clouds may be divided by type.

Turbulence is caused by convective currents, obstructions in the wind flow, and wind shear.

The ingredients of a thunderstorm are sufficient water vapor, unstable air, and a lifting mechanism. The water content is often measured using dew point, and must be present to create unstable air. Virtually all showers and thunderstorms form in an air mass that is classified as conditionally unstable.

A lifting mechanism is required in conditionally unstable air to release the instability. These mechanisms can include converging winds around surface lows and troughs, fronts, upslope flow, drylines, outflow boundaries generated by prior storms, and local winds such as sea breeze, lake breeze, land breeze, and valley breeze circulations.

Thunderstorms progress through a lifecycle, starting as a towering cumulus, moving to a mature stage, then into a dissipating stage. The towering cumulus is formed as a strong convective updraft. The updraft is a bubble of warm rising air concentrated near the top of the cloud which leaves a cloudy trail in its wake. The cell transitions to the mature stage when precipitation reaches the surface. Precipitation descends through the cloud and drags adjacent air downward, creating a strong downdraft alongside the updraft. The downdraft spreads out along the surface as a mass of cool, gusty air. The dissipating stage is marked by a strong downdraft embedded within the area of precipitation. Subsiding air replaces the updraft throughout the cloud, effectively cutting off the supply of moisture provided by the updraft. Precipitation tapers off and ends. The convective cloud gradually vaporizes from below.

There are three broad types of thunderstorms. The single cell, the multicell, and the supercell. The single cell consists of only one cell. Easily circumnavigated, except at night or when embedded in other clouds. Single cell thunderstorms are rare; almost all are multicell.

The multicell thunderstorm consists of a cluster of cells at various stages of their life cycle. As the first cell matures, it is carried downwind, and a new cell forms upwind to take its place. New cells will continue to form as long as the ingredients exist. These can be more difficult to circumnavigate since a line of thunderstorms can extend for hundreds of miles. New cells continually reform at the leading edge and the line can persist for many hours as long as the necessary ingredients exist. These are often too high to fly over, too long to fly around and too dangerous to fly under (the storms in the line can also be supercells).

The supercell storm is a dangerous convective storm that consists of primarily a single, quasi-steady rotating updraft that persists for an extended period of time. It has a very organized internal structure that enables it to produce especially dangerous weather for pilots who encounter them (updrafts may reach 9,000 fpm). A supercell may persist for hours; new cells will continue to form as long as the necessary ingredients exist.

The hazards of a thunderstorm include lightning, adverse wind, downburst, turbulence, icing, hail, rapid altimeter changes, static electricity, and tornadoes. Just about every flight hazard that might be imagined are contained within thunderstorms.

Icing is the development and accumulation of ice on the airframe. Structural icing degrades engine performance, and can destroy the smooth flow of air over the wing increasing drag while decreasing the ability to create lift. An insidious cycle develops where as power is added to compensate for the added drag the nose is lifted to maintain altitude, and the angle of attack increases. This higher angle of attack allows the fuselage and wings to collect even more ice, thus requiring more power. It can develop as a vicious cycle.

Wind tunnel and flight tests have shown that ice accumulations no thicker than a piece of coarse sandpaper can reduce lift by 30% and increase drag by 40%. Larger accretions can reduce lift even more and increase drag by 80% or more. Due to the ice aircraft may stall at a much higher airspeed and lower angle of attack than is normally the case. The actual weight of the ice on the airframe is insignificant when compared to the airflow disruption caused.

Icing is categorized into two broad categories: clear and rime icing.

Rime icing is rough, milky, and opaque ice formed by the instantaneous freezing of small supercooled water droplets after they strike the aircraft. Rime icing formation favors colder temperatures, lower liquid water content, and small droplets. It grows when droplets rapidly freeze upon striking the aircraft. The rapid freezing traps air and forms a porous, brittle, opaque, and milky colored ice.

Clear icing is a glossy, clear, or translucent ice formed by relatively slow freezing of large supercooled water droplets. Clear icing conditions exist more often in an environment with warmer temperatures, higher liquid water contents, and larger droplets. Clear ice forms when only a small portion of the drop freezes immediately while the remaining unfrozen portion flows or smears over the aircraft surface and gradually freezes. Clear ice is more hazardous than rime ice due to the following factors.

Mixed icing is, somewhat obviously, a mixture of clear and rime ice. Due to the presence of clear ice it presents much the same hazards as clear icing.

Pilots should be alert for icing any time the temperature approaches 0 degrees Celsius, and visible moisture is present. When the moisture is carried above the freezing level water becomes supercooled. This supercooled water freezes upon impact with the airframe.

When the temperature cools to about -15 degrees C, much of the remaining water vapor sublimates as ice crystals. Above this level, the amount of supercooled water decreases. Clear icing can occur at any level above the freezing level, but at high levels, icing from smaller droplets may be rime or mixed rime and clear ice. The abundance of large, supercooled water droplets makes clear icing very rapid between 0 and -15 degrees C.

For is a visible aggregate of minute water droplets that are based at the surface and reduce visibility to less than 5/8 stature mile. Fog differs from cloud only in that it’s base must be at the surface, while clouds are above the surface. There are various types of fog, generally depending upon how the fog is formed.

This type of fog is produced over a land area when radiational cooling reduces the air temperature below its dew point. Radiation fog is generally a nighttime occurrence and often does not dissipate until after sunrise. Terrestrial radiation cools the ground, the ground cools the air and when the air reaches its dew point, fog forms. Factors favoring formation: Shallow surface layer of relatively moist air beneath a dry layer, Clear skies, and Light surface winds. Radiation fog is restricted to land because water surfaces cool little from nighttime radiation. It is shallow when wind is calm. Winds up to about 5 knots mix the air slightly and tend to deepen the fog by spreading the cooling through a deeper layer. Stronger winds disperse the fog or mix the air through a still deeper layer with stratus clouds forming at the top of the mixing layer. Ground fog usually burns off rapidly after sunrise. Other radiation fog generally clears before noon (unless clouds move in over the fog).

This type of fog forms when moist air moves over a colder surface, and the subsequent cooling of that air to below its dew point. This is most common along coastal areas, but often moves deep in continental areas. Advection fog deepens as wind speed increases up to 15 knots; winds much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus clouds. The west coast of the US is quite vulnerable to advection fog which frequently forms offshore as a result of cold water and then is carried inland by the wind. It can remain for weeks, advancing over the land during the night and retreating back over the water the next morning. A pilot will notice little difference flying over advection and radiation fog

This type of fog forms as a result of moist, stable air being adiabatically cooled to or below its dewpoint as it moves up sloping terrain. Wind speeds of 5-15 knots are most favorable; stronger winds tend to lift the fog into a low layer of stratus clouds. Common along the eastern slopes of the Rockies, and somewhat less frequent east of the Appalachians. It is often quite dense and extends to high altitudes.

When warm, moist air is lifted over a front, clouds, and precipitation may form. If the cold air below is near its dewpoint, evaporation may saturate the cold air and form fog. The result is more or less a continuous zone of condensed water droplets reaching from the ground up through the clouds. This is most commonly associated with warm fronts, but can occur with others as well. It can be quite dense and continue for an extended period of time.

This type of fog forms when very cold air moves across relatively warm water, enough moisture may evaporate from the water surface to produce saturation. As the rising water vapor meets the cold air, it immediately re-condenses and rises with the air being warmed from below. Because the air is destabilized, fog appears as rising streamers that resemble steam. Steam fog is often very shallow; as the steam rises, it re-evaporates in the unsaturated air above. Pilots should expect convective turbulence flying through it.

On cool, clear nights, the temperature of the ground and objects on the surface can cause temperatures of the surrounding air to drop below the dew point. When this occurs, the moisture in the air condenses and deposits itself on the ground, buildings, and other objects like aircraft. The moisture is dew, but if the temperature is below freezing, the moisture is deposited in the form of frost. While dew poses no threat to aircraft, frost poses a definite flight safety hazard. It disrupts the smooth airflow over the wing and can drastically reduce the production of lift. It also increases drag, which when combined with lowered lift production, can adversely affect the ability to takeoff. An aircraft must be thoroughly cleaned and free of frost prior to beginning a flight.