Weather Fundamentals

Weather Almanac, (2004)

THE EARTH'S ATMOSPHERE

Composition

Air is a mixture of several gases. When completely dry, it is about 78% nitrogen and 21% oxygen. The remaining 1% is other gases such as argon, carbon dioxide, neon, helium, and others. However, in nature, air is never completely dry. It always contains some water vapor in amounts varying from almost none to 5% by volume. As water vapor content increases, the other gases decrease proportionately.

Vertical structure

The atmosphere is classified into layers, or spheres, by characteristics exhibited in these layers.

The troposphere is the layer from the surface to an average altitude of about 7 mi. It is characterized by an overall decrease of temperature with increasing altitude. The height of the troposphere varies with latitude and seasons. It slopes from about 20,000 ft over the poles to about 65,000 ft over the equator; and it is higher in summer than in winter.

At the top of the troposphere is the tropopause , a very thin layer marking the boundary between the troposphere and the layer above. A relationship between the height of the tropopause and certain weather phenomena has been documented.

Above the tropopause is the stratosphere . This layer is typified by relatively small changes in temperature with height except for a warming trend near the top.

Density

Air is matter and has weight. Since it is gaseous, it is compressible. Pressure the atmosphere exerts on the surface is the result of the weight of the air above. Thus, air near the surface is much more dense than air at high altitudes.

TEMPERATURE

Temperature scales

Two commonly used temperature scales are Celsius (°C), or centigrade, and Fahrenheit (°F). The Celsius scale is used exclusively for upper air temperatures and is rapidly becoming the world standard for surface temperatures also.

Traditionally, two common temperature references are the melting point of pure ice and the boiling point of pure water at sea level. The melting point of ice is 32°F (0°C); the boiling point of water is 212°F (100°C). Thus, the difference between melting and boiling is 100°C, or 180°F; the ratio between degrees Celsius and Fahrenheit is 100/180 or 5/9. Since 0°F is 32°F colder than 0°C, you must apply this difference when comparing temperatures on the two scales. You can convert from one scale to the other using one of the following formulae: C = 5/9 (F − 32) or F = 9/5 C 32 where C is degrees Celsius and F is degrees Fahrenheit.

Heat and temperature

Heat is a form of energy. When a substance contains heat, it exhibits the property that is measured as temperature—the degree of "hotness" or "coldness." A specific amount of heat absorbed by or removed from a substance raises or lowers its temperature a definite amount. However, the amount of temperature change depends on characteristics of the substance. Each substance has its unique temperature change for the specific change in heat. For example, if a land surface and a water surface have the same temperature and an equal amount of heat is added, the land surface becomes hotter than the water surface. Conversely, with equal heat loss, the land becomes colder than the water.

The Earth receives energy from the Sun in the form of solar radiation. Earth and its atmosphere reflect about 55% of the radiation and absorb the remaining 45%, converting it to heat. The Earth, in turn, radiates energy, and this outgoing radiation is terrestrial radiation. It is evident that the average heat gained from incoming solar radiation must equal heat lost through terrestrial radiation in order to keep the Earth from getting progressively hotter or colder. However, this balance is worldwide; regional and local imbalances that create temperature variations should also be considered.

Temperature variations

The amount of solar energy received by any region varies with time of day, with seasons, and with latitude. These differences in solar energy create temperature variations. Temperatures

(Courtesy of U.S. Air Force.)

also vary with differences in topographical surface and with altitude. These temperature variations create forces that drive the atmosphere in its endless motions.

Day-to-night (diurnal) variation of temperature

Diurnal variation is the change in temperature from day to night brought about by the daily rotation of the Earth. Earth receives heat during the day from solar radiation but continually loses heat by terrestrial radiation. Warming and cooling depend on an imbalance of solar and terrestrial radiation. During the day, solar radiation exceeds terrestrial radiation and the surface becomes warmer. At night, solar radiation ceases, but terrestrial radiation continues and cools the surface. Cooling continues after sunrise until solar radiation again exceeds terrestrial radiation. Minimum temperature usually occurs after sunrise, sometimes as much as one hour after. The continued cooling after sunrise is one reason that fog sometimes forms shortly after the Sun is above the horizon.