Download Almost all weather occurs in the lowest layer of air, or atmosphere

Almost all weather occurs in the lowest layer of air, or atmosphere, which is called the
troposphere. The troposphere is where most water is found and where most clouds form.
The layer above the troposphere is called the stratosphere. There is a little water in the
stratosphere so a few clouds form here. The stratosphere is where the ozone layer is
found (about 14 miles above the Earth’s surface). The ozone layer is important because it
protects life on Earth by absorbing some of the sun’s harmful rays.
The weather changes because the atmosphere is constantly changing. Sometimes the air
is cold and sometimes the air is warm. When the air is warm, its weight, or air pressure,
lessens. Warm air holds more water (has more humidity) than cold air. There are weather
instruments used to measure atmospheric conditions. A thermometer measures air
temperature. A barometer measures air pressure. A rain gauge measures the amount of
precipitation (rain or snow). A hydrometer measures the humidity (the amount of water in
the air). An anemometer measures wind speed. A wind vane shows which direction the
wind is blowing. One reason people measure atmospheric conditions is to predict what
the weather will be.
Though you can not feel the atmosphere weighing you down, air does have weight. The
atmosphere is pushing on us all the time and this weight is called air pressure. A
barometer is used to measure air pressure. Warm air weighs less than cold air. A mass of
warm air is called a low-pressure area. Low pressure, which is recognized by a falling
barometer (see page C66 in text), means that warm, wet, stormy weather is probably
coming. A mass of cold air is called a high-pressure area. High pressure, which is
recognized by a rising barometer, means that the weather will probably become fair.