Weather that affects our day-to-day lives on earth occurs in the troposphere. This is the lowest region of the atmosphere. It extends 13kilometers above the surface of the earth. Meteorologists observe the troposphere and use various tools to help them guess weather patterns. The most basic of these tools is the thermometer. This instrument measures air temperature. Another basic tool of meteorologists is the barometer, which measures air pressure. More advanced tools measure wind speed, wind direction, and water in the air. In the twentieth century, meteorologists began sending weather balloons, called radiosondes, into the atmosphere. Such balloons can be tracked by satellites to provide precise atmospheric data for meteorologists