Weather information for pilots

from JROTC.ORG

THE ELEMENTS OF THE WEATHER

Weather is the condition of the atmosphere at a given time and place. It is a combination of temperature, winds, moisture and pressure. These are called the elements of weather because they are the factors that form weather.

Heat and temperature affect weather. The temperature of the air is important because the heat energy within the atmosphere generates all the Earth’s weather processes.

The molecules in a substance are constantly in motion. This means that molecules possess energy.

Heat is the total energy of motion of all the molecules in a substance.

Temperature is the measure of that molecular motion on a scale devised by scientists. The scale is commonly called a thermometer.

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HEAT AND TEMPERATURE

The four principal ways heat is transferred from one place to another are conduction, convection, advection and radiation.

• Conduction is the transfer of heat between objects whose surfaces are in contact with each other.

• Convection is the transfer of heat in liquids or gases by currents within the liquids or gases.

• Advection is the process of lateral heat transfer.

• Radiation is the transfer of heat by radiant waves.

• The rate at which the Earth's surface is heated by solar radiation is called Insolation.

• Radiation transfers heat energy without changing the temperature of anything between the source of energy and the object heated.

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WIND

• When heated air rises and cooler air flows in to fill the lower pressure area created by the rising air, this flow is called wind.

Coriolis effect:

• The most significant influence on the creation and flow of wind is the spinning planet. The rotation of the Earth influences any object moving over its surface.

• In the example above you will notice the earth's counterclockwise rotation (E) viewed from the North Pole (NP) causes polar air moving south to be deflected to the right--toward the west. (Air moving north from the Equator will also be deflected to the right--toward the east.) This phenomena is called the Coriolis effect.

Pressure Gradient is the difference in pressure between neighboring areas of high pressure and low pressure. Air flows from the high pressure areas into the low pressure areas producing wind. The greater the pressure gradient, the stronger the wind will be.

Gravity: tends pulls the air downward producing the greatest air density near the Earth’s surface.

Local and Surface Air Movements:

Convection currents are formed along shorelines by the differing temperatures of the land and the water. During the day, these currents cause the wind to flow from the cooler water toward the warmer land. During the night, they cause the wind to blow from the cooler land towards the warmer water. This phenomena is known as the land-sea breeze.

The Jet Stream

Jet stream chart on left courtesy of the Weather Channel

• The jet stream is a narrow current of air that moves around the Northern and Southern Hemisphere of the Earth in wavelike patterns.

• It generally flows west to east.

• It moves very rapidly.

• It migrates with the seasons.

• The strongest winds are usually encountered above 30,000 feet.

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MOISTURE

• Water can be found in the atmosphere as a solid, liquid or a gas.

• Relative humidity is the ratio of the existing water vapor in the air to its possible saturation point. Dew point is the temperature at which the water vapor in the air condenses and becomes liquid.

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CLOUDS

• Fog is a cloud because it develops within the atmosphere. However, fog is classified differently because it usually forms under different conditions than a cloud.

• There are two general categories under which clouds can be classified. They are cumulus and stratus.

Cumulus clouds are dome shaped.	Stratus clouds spread out in layers.	Cirrus clouds high wispy clouds composed of ice crystals.

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AIR MASSES AND FRONTS

• The boundaries between air masses of different characteristics are called frontal zones or fronts.

• When a cold air mass replaces a warmer air mass, the leading edge of this cold air mass is called a cold front.

• A warm air mass generally contains more moisture than a cold air mass because it is less dense and can hold more moisture.

• An occluded front is formed when a warm air mass is "sandwiched" between two cold air masses.

(See illustrations page 3-13)

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SEVERE WEATHER

Thunderstorms are formed when warm, moist air is pushed upward rapidly, accompanied by powerful downdrafts of cool air. The three stages of thunderstorm development are: building, mature, and dissipating.

Tornadoes are spawned from thunderstorms. Some researchers think that a tornado is the result of variation in electrical charge within the cumulonimbus cloud. Others believe that the cause is the reaction of cold and warm air currents within the cloud.

A hurricane is a low-pressure area which goes through a period of building up force over equatorial ocean areas. Hurricanes (called typhoons in the Pacific) can be from 50 to 900 miles in diameter, with winds of 75 to 130 or more miles per hours rotating around a calm center--the eye.