A thermometer is a well-known weather instrument that measures the air temperature around the device. Thermometer users increase reading accuracy by positioning the device approximately 5 feet above ground on terrain typical for the area. A level, open surface ensures that air flows easily around the thermometer. Decrease the chance of artificially high readings by locating the thermometer 100 feet or more from easily heated concrete or paved surfaces.
Barometers provide valuable air pressure information, which meteorologists use to predict weather patterns and developing storms. For example, a high air pressure reading indicates good weather. Conversely, storm centers typically show low pressure compared to areas surrounding the storm. Mercury-based barometers have largely been replaced by smaller aneroid barometers, which use a metal bellows that compresses or expands with air pressure changes. These barometers record up to a week's worth of pressure changes on paper.
Meteorologists measure wind direction and speed with an unconventional-looking instrument called an anemometer. Wind data enable forecasters to identify potential wind-related hazards and indicate the movement of weather systems. A cup anemometer consists of three centrally attached cups that spin faster as the wind blows harder. Some anemometers also include a wind vane that determines the wind's direction. A propeller anemometer resembles a tailed fan and measures the wind's direction and speed with one device.
Weather balloons carry equipment to analyze Earth's upper atmospheric range, an area inaccessible to land-based weather instruments. These latex balloons drift to altitudes of 25,000 feet or higher and carry a small device called a radiosonde. Radiosondes measure humidity, local atmospheric temperature, wind and other variables. The single-use radiosondes transmit data to land-based monitoring stations. Forecasters use this information to more accurately forecast local, regional and national weather conditions. "Hurricane hunter" aircraft also use radiosondes to track the development and progress of hurricanes.