Composition and Structure of the Atmosphere
What is Weather?
Weather is the state of the atmosphere at a given time and place. A complete description of the weather includes the amount and type of clouds. Rain, snow, thunderstorms, lightning, and even dust storms are part of the weather. Measurements of temperature, pressure, wind speed and direction, and the amount of moisture in the air are also included in a description of the weather. Weather is studied and predicted by scientists called meteorologists. The science of meteorology is the study of the entire atmosphere, including its weather. To understand and predict the weather, meteorologists must first understand how the atmosphere heats and cools, how clouds form and produce rain, and what makes the wind blow. Meteorologists also study subjects not obviously related to weather. Such subjects include the composition of the atmosphere, the atmospheres of other planets, and the causes of past and present climates.
Observing the Weather
Much can be learned about the weather without numerous instruments. Direct observation tells whether it is cloudy or raining. Wind direction and speed can be estimated with little practice. It is easy to tell whether the air is warm or cold. Even humidity and pressure have observable effects. Certain clouds come with fair weather and other clouds foretell rain. Farmers, sailors, and others dependent on weather become quite skilled at predicting weather from watching the clouds. For example, in the verse Mackerel scales and mares’ tails
Make lofty ships carry low sails,
A cloud formation predicts a coming storm. The wind is also related to weather changes. For example, folk wisdom says that winds blowing out of the east bring rain. Wind direction is show by flags or blowing dust. The actual scale, named for Sir Francis Beaufort, relates the wind speed to its effects. This scale is shown below.
| Calm, sea like a mirror.
| Light air, ripples only.
| Light breeze, small wavelets (0.2m). Crests have a glassy appearance.
| Gentle breeze, large wavelets (0.6m), crests begin to break.
| Moderate breeze, small waves (1m), some white horses.
| Fresh breeze, moderate waves (1.8m), many white horses.
| Strong breeze, large waves (3m), probably some spray.
| Near gale, mounting sea (4m) with foam blown in streaks downwind.
| Gale, moderately high waves (5.5m), crests break into spindrift.
| Strong gale, high waves (7m), dense foam, visibility affected.
| Storm, very high waves (9m), heavy sea roll, visibility impaired. Surface generally white.
| Violent storm, exceptionally high waves (11m), visibility poor.
| Hurricane, 14m waves, air filled with foam and spray, visibility bad.
| The temperature cannot be guessed accurately, but people feel heat and cold. In the winter, people feel colder when the wind is blowing. The actual temperature can be converted into the temperature the body feels by using a chart for the windchill factor. This adjusted temperature describes the danger of frostbite. In the winter, weather broadcasts often include temperatures adjusted for windchill. Air pressure cannot be felt, but changes in the air pressure can. The most common effect of pressure change is the “popping” of the ears. Ear popping occurs ding the takeoff or landing of an airplane, a ride in a rapid elevator, or even a ride in a car over a high mountain pass. In all cases, the ears pop because the air pressure falls with height. To try to predict weather, you need to observe the clouds, wind, temperature, humidity, air pressure, and precipitation over a period of time. Look for patterns in how...
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