By Peter Grey
To provide pilots with the basic meteorological concepts required to safely pilot an aircraft.
After this module, pilots should be able to understand the basics of meteorology and how it interacts with the flying environment
What Causes Weather?
Weather is caused for a very simple reason. The spherical nature of the earth allows sunlight to be spread out over a wider area in the pole region and over a tighter region over the equator. Think of shining a flashlight on a surface, if you point the flashlight straight down (like the sun does near the equator), you will get more light then if you tilt the flashlight (like the sun does near the poles). It is this differential heating that causes all weather on the Earth.
This differential heating creates an area of the planet which is hot (near the equator) and a part which is cold (the poles). Warm air is less dense due to it having more energy, and this results in it spreading out more near the surface when looked at as a column. Cold air is more dense which results in it becoming more compact near the surface when looked at as a column.
The result of this compacting and expansion of the air is an area of surface low pressure at the equator, and surface high pressure at the poles. Aloft in the atmosphere this is reversed. This is a fundamental concept in meteorology, and is sometimes referred to as a cold column (which has surface high pressure and low pressure aloft) and warm column (which has surface low pressure and high pressure aloft). For the purpose of this article, any reference to high pressure/low pressure refers to surface pressure (not pressure aloft which will be opposite).
General Circulation of Air
The atmosphere wishes to equalize this pressure difference by having air in high pressure areas, move to areas of low pressure. This generates wind. The net result of this is surface air moving towards the equator from the poles, and the wind converges at the equator causing it to go up. Aloft, the air then moves towards the polesa nd sinks downward over the poles to replace the air moving out. This is called 1 cell circulation.
If the earth didn?t rotate this would be how air circulates around the planet. Unfortunately it?s not that simple. Rotation of the earth generates an apparent force called the corolis force. This force will turn wind to the right in the northern hemisphere and to the left in the southern.
The end result of this is 3 cells per hemisphere. The first cell is located from the equator to 30 degrees N/S. This is called the Hadley cell. In this cell a combination of low pressure at the equator and high pressure at 30 N/S, results in surface winds from the NE (SE in the southern hemisphere) after it is turned by the corolis force, lifting of air at the equator, and winds from the W aloft in both hemisphere. Air sinks over 30 degrees N/S into the high pressure located there (which exists due to it being colder at 30 N/S than at the poles).
The second cell is located from 60 degrees N/S to the poles and is called the polar cell. High pressure at the poles results in air moving towards the equator. This air is turned by the corolis force resulting in surface wind from the NE (SE in the southern hemisphere). This fills low pressure which exists at 60 N/S (due to it being warmer at 60 N/S then at the poles). Air is then lifted at this low pressure, and moves pole ward, it is also turned by the corolis force resulting in winds from the W in both hemispheres.
The third cell is a result of the 2 cells above. It exists from 30 N/S to 60 N/S and is called the Ferrell cell. In this cell air moves pole ward from the surface High pressure at 30 N/S resulting in surface winds from the W. This cell has no aloft component and winds over it are from the W also. This cell is called an indirect cell because it only exists as a reaction to the other 2 cells.
What causes seasons
Seasons are caused by a tilt of the earth. This tilt results in the sun heating not always being focused over the equator, but over the area when the sun rays are at a 90 degree angle to the earth.
In the summer hemisphere (northern in June-August, southern in December-March), the 3 cell circulation described above is weakened due to there being less of a temperature difference between the equator and poles.
In the winter hemisphere, the 3 cell circulation is strengthened due to a strong temperature difference between the equator and poles. This results in stronger winds and storm systems in the winter.
Seasons are not caused by the earth being further away from the sun during portions of its revolution around it. If this were the cause, the entire earth would have the same seasons, which is not the case.
Air masses are created by different landmass modifying the air above them to have the same properties as the ground. There are 4 primary groups of air masses, the first is formed over cold water, and is called maritime polar or mP. The second is formed over warm water and is called maritime tropical or mT. The third is formed over cold land and is called continental polar or cP. The last one is formed over warm land and is called continental tropical or cT.
The creation process of an air mass requires low wind to allow the air to sit over the ground for long periods of time (at least a week usually). Eventually these air masses drift into areas of stronger winds and are moved with them into other areas.
Clouds form when rising air (which cools as it rises) eventually becomes saturated (Humidity = 100%). Water condenses onto microscopic particles which exist in the air, forming the cloud.
There are 3 major types of clouds. The first is cumulus clouds. Cumulus clouds form in unstable environments and are fluffy in nature. Cumulus clouds are usually a sign of turbulence (the same process that makes the clouds bumpy, does the same thing to your aircraft). An extreme case of the cumulus cloud is the cumulonimbus cloud, which is very tall (up to 60000ft tall), and shows the presence of thunderstorms.
The second major type of cloud is stratus clouds. Stratus clouds are flat in nature and form in stable environments. These clouds are signs of non-turbulent skies and tend to form near warm fronts.
The third major type of cloud is the cirrus clouds. Cirrus clouds are thin, wispy, and form at high altitudes. Cirrus clouds contain ice crystals as opposed to water vapor that exists in cumulus and stratus clouds.
In addition there are several minor types of clouds that are important to aviation. The first of these is the lenticular cloud. Lenticular clouds look like a contact lens and usually form over mountains. These are a sign of severe to extreme turbulence.
Another less common type of cloud is fog. Fog is simply a cloud that forms under 50 feet. Fog forms for a variety of reasons.
A front is simply the boundary between 2 air masses. They are defined by the temperature profile of the air mass which is moving over the area. Fronts are identified by a wind shift, change in temperature, and low pressure at the front line.
There are 4 major types of fronts; we will cover 3 of them here in detail. The first is the cold front. In a cold front, warm air is replaced by cold air. Cold fronts tend to have very strong storms which are relatively short lived, with heavy rainfall (or snowfall), and thunderstorm activity. They also have the potential to form a squall line, which is a long line of thunderstorms. Traditionally cold fronts have winds from the South prior to the front (or from the North in the southern hemisphere), and afterwards these will shift to being from the Northwest (or from the Southwest in the southern hemisphere). On a weather map a cold front is indicated by a blue line with triangles pointing in the direction of movement.
In a warm front, warm air replaces cold air. These fronts tend to have weaker, but longer lived precipitation. In a traditional warm front precipitation will start with snow, move to ice pellets, then freezing rain, and finally rain. Warm fronts will have winds from the East prior to frontal passage in both hemispheres and from the South after the front (or from the North in the northern hemisphere). A warm front is indicated by a red line with semicircles pointed in the direction of movement.
A stationary front is a boundary between 2 air masses that isn't moving. These can have the properties of either type of front listed above. On a map these are depicted by an alternating red and blue line with red semicircles pointing in the direction of movement of the warm front, and blue triangles pointing in the direction of movement of the cold front.
An occluded front occurs when 1 front (usually a cold front) runs into another front (usually a warm front), and is in essence a boundary between 3 air masses. There are multiple types of occluded fronts. On a map these are depicted by a purple line with alternating semicircles and triangles pointing in the direction of movement.
Highs and Lows
As we talked about in the initial section on why there is weather, high pressure is cold by nature. In high pressure air will tend to flow downward, outward, and rotate clockwise (counterclockwise in the southern hemisphere). The sinking motion suppresses cloud formation and highs traditionally have good weather. High pressure is usually found in polar and desert regions.
Low pressure is the opposite of high pressure. Traditionally it is formed with warm air. In low pressure, air will tend to flow upward, inward, and rotate counterclockwise (clockwise in the southern hemisphere). The rising motion enhances cloud formation and lows traditionally have poor weather. Low pressure is usually found around the equator, and around 60 degrees N/S. Low pressure is also found in hurricanes (tropical cyclones), and extra tropical (or mid latitude) cyclones.
The jet stream is a band of upper level air that moves at a high speed around the globe. These form over strong temperature gradients and are common areas of turbulence. Jet streams can have winds up to 200 knots (however 100 knots is much more common).
There are 2 jet streams in each hemisphere. First there is the tropical jet stream which forms in winter around 30 degrees N/S around 40000 feet. This jet stream is more common in the northern hemisphere.
The second jet stream is the polar jet stream which forms around 45 degrees N/S. This is a much stronger and year round jet stream. Usually when someone referrers to the jet stream it is this one that they are referring to.
Extra tropical Cyclones
Extra tropical cyclones are the most common large scale storm system on the planet. These are year round systems, in the summer they will tend be located around 60 degrees N/S. In the winter they will tend to be around 45 degrees N/S.
These cyclones form under the polar jet stream and are low pressure systems. A standard extra tropical cyclone will have a warm front that will run east of the low pressure system, and a cold front that will form to the SW of the low (NW in the southern hemisphere). The cold front will eventually catch up to the warm front and form an occluded front at the end of the cyclones life.
Extra tropical cyclones will move to the east and pole ward. On a satellite image they will appear as a comma-shaped cloud. The elongated portion of the cloud that will point towards the equator indicates the cold front, while the thickest portion of the cloud band indicates the warm front.
Tropical cyclones are low pressure oceanic disturbances which form in summer/fall and can cause extreme damage to coastal areas.
These storms go by different names depending on what ocean they are in. They are called hurricanes in the Atlantic and East Pacific, typhoons in the West Pacific, and cyclones in the South Pacific and Indian Ocean.
Tropical Cyclones form primarily around 10-15 N/S, and rotate in the same direction as a low until approximately 30000ft. Above 30000ft (10000m) they will rotate in the same direction as a high. Tropical Cyclones form due to heat energy in warm water (above 80 F, or 26 C) and initially form around a large cluster of thunderstorms.
Tropical Cyclones are most common in the North Atlantic, Pacific, and Indian oceans. These storms are capable of pressure as low as 26.75" HG (or 900mb) with winds up to 200 mph (320 kph).
Thunderstorms are a very common aviation hazard. They are most likely to occur during the summer over warmer land areas. These occur because of the lifting of moist air due to a variety of reasons (these can include fronts, mountains, and low pressure systems).
Thunderstorms are capable of creating heavy rain, violent downdrafts, wind shear, hail, icing conditions, extreme turbulence, and tornadoes.
Thunderstorms are most common in tropical regions near the equator. They also tend to happen in desert environments. Severe thunderstorms are most common in the central United States.
Common Meteorological Terms
- Air Mass: A large amount of air that has the same properties (temperature, water vapor, and stability).
- Dew point: The temperature to which air must be cooled to create a cloud (or rain).
- Front: A boundary between 2 air masses.
- Humidity: Usually a reference to relative humidity. A percentage of the amount of water vapor in the air compared to the amount the air can hold.
- Pressure: The weight of the air above the ground, measured in inches of Mercury (" HG), millibars (mb), or Pascals (pa)
- Stability: A measure of the buoyancy of air. Unstable air will tend to rise and sink easily, while stable air will generally remain stationary in terms of vertical movement.
- Temperature: The average kinetic energy of an object, usually an air mass.
- Djuric, Dusan. Weather Analysis. 1st ed. Upper Saddle River, NJ: Prentice-Hall, 1994.
- Wallace, John, and Peter Hobbs. Atmospheric Science an Introductory Survey. 2nd ed. New York: Elsevier, 2006.