- The troposphere is the atmospheric layer extending from the surface to an average altitude of about 36,000 ft. Above the troposphere is the stratosphere, mesosphere, and the thermosphere.
- Because of heating inequities, heat is transported, or circulated, from one latitude to another by a process known as convection.
- In the three-cell circulation model, the Hadley, Ferrel, and Polar cells generate predictable wind patterns and distribute heat energy.
- Pressure readings on weather maps connect points of equal pressure with lines called isobars. When isobars are spread widely apart, the pressure gradient is considered to be weak, while closely spaced isobars indicate a strong gradient.
- A high is a center of high pressure surrounded on all sides by lower pressure. Conversely, a low is an area of low pressure surrounded by higher pressure.
- A ridge is an elongated area of high pressure, while a trough is an elongated area of low pressure. A col can designate either a neutral area between two highs and two lows, or the intersection of a ridge and a trough.
- Coriolis force causes all free-moving objects to trace a curved path due to the earth's rotation. In the northern hemisphere, the deviation will be to the right of its intended path while the opposite will occur in the southern hemisphere.
- Frictional force causes a wing to shift directions when near the earth's surface.
- A sea breeze blows from the cool water to the warmer land during the day. At night, a land breeze blows from the cooler land to the warmer water.
- A cold downslope wind flows downhill from snow-covered plateaus or steep mountain slopes.
- Warm, downslope winds sometimes dramatically raise the temperature at the base of the mountain.
- Stability is the atmosphere's resistance to vertical motion.
- The rate at which temperature decreases with an increase in altitude is referred to as its lapse rate. As you ascend in the atmosphere, temperature decreases at an average rate of 2 degrees Celsius (3.5 degrees Fahrenheit) per 1,000 ft.
- When temperature increases with altitude, a temperature inversion exists.
- Evaporation is the changing of liquid water to invisible water vapor. Condensation occurs when water vapor changes to a liquid. Sublimation is the changing of ice directly to water vapor, while the transformation of water vapor to ice is known as deposition. In both cases, the liquid state is bypassed.
- Relative humidity is the actual amount of moisture in the air compared to the total amount that could be present at that temperature.
- The temperature at which air reaches a state where it can hold no more water is called the dewpoint.
- Frost forms on aircraft when the temperature of the collecting surface is at or below the dewpoint of the surrounding air and the dewpoint is below freezing. If frost is not removed from the wings before flight, it may decrease lift and increase drag to a point which seriously compromises safety.
- When the temperature/dewpoint spread reaches 4 degrees Fahrenheit (2 degrees Celsius) and continues to decrease, the air is nearing the saturation point and the probability of fog and low clouds forming increases.
- Since they normally form below 6,500 ft. AGL, stratus, stratocumulus, and nimbostratus are classified as low clouds. Altostratus and altocumulus are classified as middle clouds and have base that range from about 6,500 to 20,000 ft. AGL. High clouds have bases beginning at altitudes above 20,000 ft. AGL. The three basic types of high clouds are called cirrus, cirrostratus, and cirrocumulus. Extensive vertical development is characteristic of cumulus, towering cumulus, and cumulonimbus clouds.
- Fog is a low cloud which has its base within 50 ft. of the ground. If the fog is less than 20 ft. deep, it is called ground fog.
- Although a cloud usually forms when the atmosphere is saturated, it doesn't necessarily mean that the cloud will produce precipitation. For precipitation to occur, water or ice particles must grow in size until they can no longer be supported by the atmosphere.
- As they fall, snowflakes and raindrops may change into other types of precipitation depending on the atmospheric conditions beneath the cloud. In addition to snow and rain, falling moisture also can take the form of drizzle, ice pellets, or hail
- An airmass is a large body of air with fairly uniform temperature and moisture content. As airmass moves, it is modified by the temperature and moisture of the area over which it moves.
- Stable air is generally smooth with layered or stratiform clouds. Visibility is usually restricted, with widespread areas of clouds and steady rain or drizzle. Moist unstable air causes the formation of cumuliform clouds, showers, turbulence, and good surface visibility.
- A cold front is one where cold air is moving to displace warmer air. In a warm front, warm air is replacing cold air. A stationary front has no movement. When cold and warm fronts merge, they create an occluded front.
- Frontal discontinuities refer to the comparatively rapid changes in the meteorological characteristics of an airmass. When you cross a front, you move from one airmass into another and will normally experience changes in temperature, pressure, and wind.
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