Weather is the state of the atmosphere at a given time and place. Different measurements can be made to determine the condition of the atmosphere. The primary variables are barometric pressure, temperature, humidity, precipitation, wind speed, and wind direction. Meteorologists study these readings to help forecast the weather.
Most of the atmosphere is composed of nitrogen. In addition to 78 percent nitrogen, 21 percent is oxygen. The remaining 1 percent is composed of argon, carbon dioxide, and other gases. 99 percent of the atmosphere is found within 32 km of the surface of the Earth. Half of the atmosphere's weight is found from the surface up to 5.5 km in the atmosphere. Other particles found in the atmosphere include water vapor, ozone, and dust. Ozone (O3) is found at 10 to 50 km above the Earth's surface and is very important to life on Earth. It absorbs harmful UV radiation, but is being destroyed by CFCs (chlorofluorocarbons) in human products, such as aerosol sprays. The dust found in the atmosphere is made from rock and mineral particles, pollen, sea salt, chemicals, and bacteria. These dust particles cause hazy skies and provide a surface for condensation of water vapor.