Air Temperature

            By degrees. You can measure the temperature of the air with a thermometer. It is marked with one or two scales. The scales that are commonly used are the Fahrenheit scale and the Celsius scale. A thermometer works on the principle that a rise in temperature causes a liquid or a metal to take up more space. A lowering of the temperature causes the material to take up less space. Most thermometers are liquid thermometers. They have mercury or alcohol in them. Mercury has a silver color. Alcohol is clear, but a red or blue dye is mixed with it if it is to be used in a thermometer.
            The source of heat. Heat from the sun keeps the earth and the earth's air warm. The layers of air above the surface of the earth act in much the same way as the glass panes in a greenhouse. The air allows some of the sun's rays to come to the earth's surface. The earth's surface absorbs most of these rays and becomes warm. The warm earth, in turn, warms the air next to it. Most of the heat in the earth and in the air is held in by the layers of air above the surface of the earth. In fact, most of the heat is held in by clouds and by tiny drops of moisture in the layers of air. Because the layers of air tend to hold in heat, the temperature of the air at night is not a great deal lower than the temperature of the air during the day. But some heat does escape into space. It escapes slowly throughout the night. The coldest part of the day is usually just at dawn before the sun starts warming up the earth's surface again.
            Convection. When air is warmed, it takes up more space and becomes lighter. As it becomes lighter, it rises. Then heavier, cooler air moves down to take its place. Of course, the cool air is then warmed, and it rises as warm air. The warm air eventually becomes cooled and then sinks. As a result of this constant warming, a current of air, or wind, is created. This is called convection.

Air pressure

            Measuring air pressure. In 1643, an Italian scientist named Torricelli invented a barometer, which is an instrument used for measuring the pressure of air.
            Air pressure and temperature. The air at any place on the earth can be thought of as being in a column above that place. It is the weight of that column of air that exerts pressure. But the temperature of a column of air has an effect on the pressure caused by that air. Warm air is lighter than cold air.
            Air pressure and height above sea level. Temperature is not the only thing that affects air pressure. Air pressure becomes less and less as you go higher and higher above sea level. This is because there is less air to "weigh" the higher up you go.
            Highs and lows. This refers to regions of high air pressure and regions of low air pressure. A high is the result of a buildup of air. The air pressure is higher there than in the regions of air around it. You could think of a high as a hill of air. In contrast, a low is like a valley of air. The air pressure is lower there than in the regions of air around it. Highs are usually regions of cooler air.

Wind

            How wind is measured. Wind can be measured in two ways. One of these ways is measuring its direction. To measure the wind's direction, a wind vane is used. It points toward the direction from which the wind is coming. A second way of measuring wind is measuring its speed. To do this, an anemometer is used. The dial on an anemometer shows the speed in miles per hour or in knots. By international agreement the speed of wind is usually given in knots. A knot is a speed of 1,852 meters (about 6,075 feet or 1.15 miles) per hour. Knots are often used to measure the speed of ships and boats. Meteorologists, or people who study the weather for a living, sometimes use a scale called the Beaufort scale to estimate the speed of the wind in knots.
            Prevailing winds. The direction of major winds on the earth is greatly determined by three things: (1) the air rising at the equator and moving toward the poles; (2) the way the earth rotates, or turns on its axis; and (3) the buildup of regions of high air pressure. All these things act together so that a certain pattern of winds – called prevailing winds – is created. Warm air rising from the equator moves toward the poles by convection, and cooler air moves in to take its place. If the earth were stationary, the air would move from north to south and from south to north. The earth, however, makes one full turn from west to east every 24 hours. As the earth rotates, different parts of the earth's surface move at different speeds. The earth's surface moves fastest at the equator. It moves slowest at the poles. The farther you go from the equator toward either pole, the slower the speed is of the moving earth. Not all the air from the equator travels to the poles, however. Regions of high pressure are built up as some of the air from the equator cools and moves closer to the surface. Scientists are not exactly sure why this happens. But as the air builds up in a high, it flows "downhill" away from the high. That is, the air moves from regions of high pressure to regions of low pressure.
            Local winds. Most people in the U.S. live in a region of prevailing westerly winds. Because of this, weather moves across the states from west to east. Besides prevailing winds, there are many other things that may affect local winds. bodies of water, mountains, or buildings may affect them. A body of water takes longer to warm up than the land does. The water also takes longer to cool off. The air above the water tends to become the temperature of the water. The air above the land tends to become the temperature of the ground. For this reason, ocean or lake breezes often move toward the land during the day. Breezes tend to move away from the land at night. Near mountains, breezes often move up the mountains during the day. Then breezes tend to move down the mountains at night.

Water Vapor

            The percentage of water vapor. The moisture in the air is often called water vapor. It may also be called humidity. You may have heard someone use the term relative humidity. Whenever that term is used, the person is talking about the amount of water vapor that is in the air compared with the amount that the air could hold at that temperature. Relative humidity is given as a percentage. For example, suppose a weather report gives the relative humidity as 50%. This means that the air is holding half the water vapor that it could hold at that temperature. Temperature affects relative humidity because warm air can hold more water vapor than cold air can. When it is raining or when there is early morning dew on the ground, the relative humidity is 100% or very nearly 100%. This is because the amount of water vapor in the air is very great. But if it is a very dry day, the relative humidity could be as low as 10%.
            Measuring relative humidity. When water vapor evaporates, it cools whatever material it is on. That's why you feel cooler when you sweat on a warm day. your body is acting to keep you cool. But in order to evaporate, water must absorb heat energy from the material it is on. As the water absorbs heat and becomes water vapor, it cools the material. Water will evaporate quickly and cool something quickly when the relative humidity is low. This is because the air can hold a great deal more water vapor than it is holding. But when the relative humidity is high, water evaporates slowly and cools something slowly. An instrument known as a psychrometer may be used to measure the relative humidity. The psychrometer has two thermometers on it. One of these thermometers has a bulb that must be kept wet. The bulb of the other thermometer is kept dry. The cooling that results from evaporating water makes the wet-bulb thermometer show a lower temperature than the dry-bulb thermometer. The difference between the readings of the two thermometers can then be used in a table to find the relative humidity of the air. (See the Relative Humidity Chart)
            Clouds. Clouds do not form unless the relative humidity of the air at the height of the clouds approaches 100%. Clouds form from water vapor and dust in the air. These things are in the air all the time. However, clouds form when water vapor is cooled and condenses, or comes together in tiny drops. The water vapor condenses on the particles of dust in the air. There are many different kinds of clouds. However, there are just 3 major kinds. All other clouds are combinations of these 3 kinds. The 3 major kinds of clouds are cirrus, stratus, and cumulus. Cirrus clouds occur at very high levels in the air. They occur so high that they are made up of ice. They are white and very thin. Some people call them mare's tails because they make curved shapes in the sky somewhat like a mare's tail. Stratus clouds, on the other hand, occur at low or middle levels. They may also contain tiny pieces of ice at higher levels or during the winter season. They are often gray and make up a sheet or layer across the sky. Cumulus clouds also occur at low or middle levels and may contain tiny pieces of ice. They are white to gray and look like piled-up cotton or rolled-up pillows.
            Forms of precipitation. Clouds have water in them. But when clouds have more water in them than they can hold, some of the water falls to the ground as rain, snow, sleet, or hail. All the forms of water that fall to the ground are given one name. They are called forms of precipitation. Rain is the most common form of precipitation worldwide. It may occur as a fine drizzle, as very heavy drops, or as something in between. Rain usually starts out as snow. Snow is formed when water vapor freezes in a part of a cloud that has a temperature below freezing. If the temperature of the air near the ground is at or below freezing, the precipitation reaches the ground as snow rather than rain. Most snowflakes are six-sided ice crystals. Sleet, on the other hand, is frozen raindrops. It can only be formed if there is a layer of warm air over a layer of cold air. As the raindrops fall through the layer of cold air, the raindrops freeze into sleet. Hail is found only in thunderstorms. Currents of air lift a raindrop high up into a part of the cloud where the temperature is far below freezing. The raindrop freezes and then falls into a lower part of the cloud where water or snow collects on it. But then it may be lifted back up high in the cloud again. Suppose a hailstone is moved up and down in the cloud many times.
            The water cycle. Certain kinds of clouds produce precipitation. These clouds are an important part of the earth's weather. But you may not know that these clouds are a part of a cycle, or series of events that occur again and again. This cycle is called the water cycle. In general, the water cycle is the moving of water from the clouds to the land, from the land to the ocean, and from the ocean back into the clouds. There are many processes that work together to keep the water cycle going. Heat from the sun warms the oceans, causing water to evaporate. Then water vapor condenses on dust particles in the air to form clouds. When the particles are heavy enough, precipitation can fall as rain, snow, sleet, or hail. After the precipitation reaches the land, water flows to the ocean by means of streams and rivers. When it reaches the ocean, some of it evaporates again, continuing the water cycle. In these ways, the water cycle is an important part of the earth's weather.