The uneven heating of the earth’s surface from the sun is a primary factor in atmospheric circulation.
The atmosphere is in constant motion. This movement is the key factor of the various weather conditions. The uneven heating from the sun causes some areas to be warmer than others. As air warms, it ascends leading to low pressure at the surface. As air cools, it descends leading to high pressure at the surface.
FAA AC 00-6B Aviation Weather pg. 4-1 FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 12-3
Altitude is the height from a given reference point or data plane.
In aviation, there are several different types of altitude. The primary references to altitude include: indicated altitude, true altitude, absolute altitude, and pressure altitude.
• Indicated altitude is the altitude shown on the altimeter. • True altitude is the height above mean sea level (MSL). • Absolute altitude is the height above ground level (AGL). • Pressure altitude is the indicated altitude when an altimeter is set to 29.92 inches of mercury (Hg).
Density altitude is a related term but is not a reference to a specific distance from a data plane. Density altitude is pressure altitude corrected for nonstandard temperature variations. Unlike other altitudes, density altitude is a reference to performance, not actual height.
The lapse rate is the change in temperature as height increases.
The standard lapse rate is 1.98 Celsius decrease (3.57 Fahrenheit) per 1,000 feet. This change is fairly constant until reaching the tropopause. At the tropopause, the lapse rate remains constant or may increase slightly.
A temperature inversion is where the temperature raises with an increase in altitude.
A temperature inversion often occurs over land on calm clear nights. The ground cools faster than the air. The cold ground then cools the air near the surface. As a result, the air temperature increases with height. A characteristic of an inversion is stable air. If the air was not stable, the air would be mixed and there would not be an inversion. An inversion often includes haze.
A standard atmosphere is when the temperature is 15 degrees Celsius (59 degrees Fahrenheit) and barometric pressure is 29.92 Hg.
The standard atmosphere is used with reference to weather products. The standard atmosphere does not exist very often in nature but allows for comparison to a set reference or standard. The standard is then adjusted to give the pilot the proper information for the current weather conditions.
Air pressure decreases with an increase in altitude.
In a standard atmosphere, air pressure decreases at a rate of one (1) inch of mercury per 1,000 feet increase in altitude. As height increases, the volume or column of air is less. As a result, the weight of the air or pressure is less. When the pressure is less, the density of the air is less. The performance of the helicopter is less when in a low-pressure environment, as density is a factor in lift production. In addition, normally aspirated piston engines perform less efficiently in lower pressure as the lower density of air molecules means there is less air being used for combustion.
Air pressure decreases with an increase in temperature.
As the air is warmed, its volume is increased, which means there is less air density in a specific area. Pressure and density are directly related. Changes in temperature are the largest factor affecting density during horizontal flight as the temperature may be different at one location than another, when the actual elevation is the same. As height increases, the temperature decreases, which increases the pressure if everything else was equal. Air temperature decreases at a rate of 1.98 degrees Celsius per 1,000-foot increase in altitude. However, the increase in altitude lowers the air pressure at a faster rate than the decrease in temperature raises the pressure. The net result is that an increase in altitude will decrease pressure even though the temperature is lower. However, at two points with the same elevation, the lower temperature will have more pressure, resulting in better performance.
A decrease in barometric pressure will cause the altimeter to indicate a higher altitude.
The altimeter uses barometric pressure to determine the height of the aircraft. The altimeter cannot tell the difference between a decrease in pressure due to local weather conditions or a rise in altitude. For this reason, pilots should adjust the altimeter to the conditions at the nearest airport, or within 100 miles when traveling cross country.
The most concerning change in barometric pressure is when traveling from a high-pressure area to a low-pressure area. As the aircraft travels into the lower pressure, the altimeter will show an increase in altitude if it not adjusted. Assuming the pilot attempts to maintain the same indicated altitude, the true altitude of the aircraft will be lower. As such, terrain clearance could be less than expected.
Pressure altitude is the pressure read on the altimeter on a standard day. To find the pressure altitude, set the altimeter to 29.92 and read the altimeter.
The primary types of clouds are Cirriform, Nimbus, Cumuliform, and Stratiform.
Cirriform clouds are usually composed of ice crystals and are above 20,000 feet. Cirriform clouds generally occur in fair weather and point in the direction of air movement at their elevation.
Nimbus is a rain cloud. These clouds typically form between 7,000 and 15,000 feet and bring steady precipitation. Cumuliform clouds look like white, fluffy cotton balls or heaps and show the vertical motion or thermal uplift of air taking place in the atmosphere. The level at which condensation and cloud formation begins is indicated by a flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base. The tops of cumulus clouds can reach over 60,000 feet. Cumulus clouds are formed when the air is unstable. The instability of cumulus clouds will often be turbulent. If there is precipitation, it is often showery. A cumulonimbus cloud is a rain cloud.
Stratiform clouds consist of a featureless low layer that can cover the entire sky, bringing generally gray and dull weather. The cloud bases are usually only a few hundred feet above the ground. Over hills and mountains, stratiform clouds can reach ground level, then referred to as fog. Also, as fog lifts off the ground due to daytime heating, the fog forms a layer of low stratus clouds. Stratiform clouds form in stable air.
Clouds can be further classified by their height. These cloud classifications include low, medium, high or those with vertical development. The levels overlap and their limits vary with latitude. Low clouds are from the surface to 6,500 feet AGL. Middle clouds are from 6,500 feet AGL to 20,000 feet AGL. High clouds are above 20,000 feet AGL. Certain clouds are generally found at specific altitudes. Clouds with vertical development can start at low levels and rise to the tropopause.
High clouds include Cirrus, Cirrostratus, and Cirrocumulus. Cirrocumulus and cirrostratus are sometimes informally referred to as “cirriform clouds” because of their frequent association with cirrus. They are given the prefix “cirro”, but this refers more to their altitude range than their physical structure. Cirrocumulus in its pure form is actually a high cumuliform genus, and cirrostratus is stratiform, like altostratus and lower based sheet clouds.
Medium clouds include Altocumulus, Altostratus, and Nimbostratus. Mid-level clouds are composed primarily of water droplets. However, they can also be composed of supercooled liquid water droplets and/or ice crystals when temperatures are below freezing. Altostratus is usually found in the middle level, but it often extends higher. Nimbostratus clouds can be defined as dark gray, middle to low level clouds. These Nimbostratus clouds, generally, form at 6,500 feet above ground level and extend to higher and lower altitudes.
Low clouds include Cumulus, Towering Cumulus, Cumulonimbus, Stratus and Stratocumulus.
Low clouds are composed of water droplets. However, they can also be composed of supercooled liquid water droplets and/or ice crystals when temperatures are below freezing. Cumulus and Cumulonimbus usually have bases in the low level, but their vertical extent is often so great that their tops may reach into the middle and high levels. As they are near the surface, low clouds impact VFR flight and are a significant factor for helicopter flight.
Clouds with vertical development include Towering Cumulus and Cumulonimbus. 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. Towering cumulus of clouds often develop into cumulonimbus clouds or thunderstorms.
FAA AC 00-6B Aviation Weather pg. 13-1 FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 12-15
The three stages of a thunderstorm are cumulus, mature, and dissipating.
The formation of a thunderstorm requires sufficient water vapor, an unstable lapse rate, and a lifting action. The cumulus stage includes an updraft. The growth rate can exceed 3,000 feet per minute. Eventually the rising air cools and creates a downdraft that coexists with the updraft. This is the start of the mature phase. Falling precipitation is an indicator of the start of the mature phase. Downdrafts characterize the dissipating phase. Once the precipitation and downdrafts stop, the dissipating phase is over.
FAA AC 00-6B Aviation Weather pg. 19-1 FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge pg. 12-22