- The rate of pressure change in a direction
- perpendicular to the isobars (horizontal distance).
While the pressure gradient force causes air to flow from high pressure to low pressure across the isobar pattern, another force acts upon the wind to determine its direction. The Coriolis force, created by the Earth’s rotation, diverts the air to the right–with respect to its initial direction of motion–regardless of whether the air is near a high or a low pressure system. The result of these two forces flow perpendicular to the pressure gradient force. This also means that gradient winds flow parallel to the isobars, and the resulting circulation flows clockwise around highs, and counterclockwise around lows. Finally, gradient winds are found above 2000 feet AGL.
Buy's Ballot Law
This law states that if the wind is at your back, the area of lower pressure will be to your left. When standing on the Earth’s surface, the low will be slightly forward or directly left because the winds flow across the isobars.
a narrow band of strong winds of 50 knots or more that meanders vertically and horizontally around the hemisphere in wave-like patterns. The jet streams (polar and subtropical) have a profound influence on weather patterns.
During the day, the pressure over the warm land becomes lower than that over the colder water. The cool air over the water moves toward the lower pressure, replacing the warm air over the land that moved upward. The resulting onshore wind, blowing from the sea, is called a sea breeze, with speeds sometimes reaching 15 to 20 knots.
At night, the circulation is reversed so that the air movement is from land to sea, producing an offshore wind called the land breeze.
At night, the air in contact with the mountain slope is cooled by outgoing terrestrial radiation and becomes more dense than the surrounding air. As the denser air flows downhill, from the top of the mountain, it is called the mountain wind, and a circulation opposite to the daytime pattern forms.
This downward motion forces the warmer air near the ground up the mountain, and since it is then flowing from the valley.
when air contains the maximum amount of water vapor it can hold for that temperature.
Dew Point Temperature (
the temperature at which saturation occurs. The dew point is a direct indication of the amount of moisture present in the air. The higher the dew point, the greater chances for clouds, fog, or precipitation.
Relative humidity (RH)
Another measure of atmospheric moisture is the relative humidity, which is the percent of saturation of the air. The air can become saturated (RH = 100%) by one of two ways.
If the air is cooled, the falling air temperature decreases the dew point spread closer to zero, while the RH rises closer to 100%.
If evaporation occurs, this adds moisture to the atmosphere, increasing the dew point, which again lowers the dew point spread and increases the RH.
Characteristics of Precipitation
1. Showers - Characterized by a sudden beginning and ending, and abruptly changing intensity and/or sky conditions. Showers are associated with cumuliform clouds.
2. Continuous - Also known as steady (not showery). Intensity changes gradually, if at all. Continuous precipitation is associated with stratiform clouds.
3. Intermittent - Stops and restarts at least once during the hour. Intermittent precipitation may be showery or steady, and therefore may be associated with cumuliform or stratiform clouds.
Different Types of Precipitation
1. Drizzle – Very small droplets of water that appear to float in the atmosphere.
2. Freezing drizzle – Drizzle that freezes on impact with objects.
3. Rain – Precipitation in the form of water droplets that are larger than drizzle and fall to the ground.
4. Freezing rain – Rain that freezes on impact with objects.
5. Hail or graupel – A form of precipitation composed of irregular lumps of ice that develop in severe thunderstorms, consisting of alternate opaque and clear layers of ice in most cases. Water drops, which are carried upward by vertical currents, freeze into ice pellets, start falling, accumulate a coating of water, and are carried upward again, causing the water to freeze. A repetition of this process increases the size of the hailstone. It does not lead to the formation of structural ice, but it can cause structural damage to aircraft.
6. Ice pellets or sleet – Small translucent and irregularly shaped particles of ice. They form when rain falls through air with temperatures below freezing. They usually bounce when hitting hard ground and make a noise on impact. Ice pellets do not produce structural icing unless mixed with super-cooled water.
7. Snow – White or translucent ice crystals, usually of branched hexagonal or star-like form that connect to one another forming snowflakes. When condensation takes place at temperatures below freezing, water vapor changes directly into minute ice crystals. A number of these crystals unite to form a single snowflake. Partially melted, or “wet” snow, can lead to structural icing.
8. Snow grains – Very small white, opaque grains of ice. When the grains hit the ground, they do not bounce or shatter. They usually fall in small quantities from stratus-type clouds, never as showers.
Principle Cloud Groups
1. Low clouds, ranging from just above the surface to 6500 feet AGL.
2. Middle clouds with bases between 6500 and 20,000 feet AGL.
3. High clouds found above 20,000 feet AGL.
4. Special clouds with extensive vertical development
Weather Conditions Assoc. w/ Low Clouds
Low clouds frequently present serious hazards to flying. The most serious hazard is the proximity of the cloud base to the surface of the Earth. Some of the low cloud types hide hills, making a collision with the terrain a very real danger, and visibility within low clouds is very poor. Low clouds may also hide thunderstorms. If the clouds are at or below freezing temperatures, icing may result. Icing accumulates faster in low clouds since they are generally denser than middle and high clouds. Turbulence varies from none at all to moderate turbulence. Expect turbulence in and below the clouds. Precipitation from low clouds is generally light rain or drizzle. If the word nimbo or nimbus appears, beware that these clouds are producing violent, or heavy, precipitation.
Weather Conditions Assoc. w/ Middle Clouds
They are composed of ice crystals, water droplets, or a mixture of the two. A special cloud, nimbostratus, produces continuous rain, snow, or ice pellets. The cloud base will extend down to about 1000 feet AGL, and fog is often present. Expect poor visibility and low ceilings with very slow clearing.
Visibility in middle clouds varies depending on cloud density from ½ mile to a few feet. Turbulence may be encountered in middle clouds. Frequently these clouds are dark and turbulent enough to make formation flying difficult. Icing is common due to the presence of super-cooled water droplets. Rain, rain and snow mixed, or snow can be encountered in thick middle clouds. Virga, which is rain or snow that evaporates before reaching the ground, may be encountered below these clouds.
Weather Conditions Assoc. w/ High Clouds
The names of the high clouds will contain the prefix cirro- or the word cirrus. High clouds have little effect on flying except for moderate turbulence and limited visibility associated with dense jet stream cirrus. Since high clouds are composed mostly of ice crystals, they have no precipitation and do not constitute an icing hazard. Severe or extreme turbulence is often found in the anvil cirrus of thunderstorms.
Weather Conditions Assoc. w/ Special Clouds with Extensive Vertical Development
Towering cumulus are clouds nearing the thunderstorm stage. They can produce heavy rain showers and moderate turbulence in and near the cloud. Icing is common above the freezing level.
Cumulonimbus clouds are thunderstorm clouds. A cumulonimbus cloud is sometimes referred to as a “CB.” Cumulonimbus is an exceedingly dangerous cloud, with numerous hazards to flight such as severe to extreme turbulence, hail, icing, lightning, and other hazards.
4 Methods of Lifting
- 1. convergence
- 2. frontal
- 3. orographic
- 4. thermal
Flight Conditions w/ stable atmosphere
Cumuliform clouds develop with unstable conditions and stratiform clouds develop with stable conditions.
Flight Conditions w/ Unstable Atomosphere