A change in barometric pressure indicates that a change in weather is coming. If air pressure
rises, a high pressure cell is on the way and clear skies can be expected. If pressure falls, a
low pressure is coming and will likely bring storm clouds. Barometric pressure data over a
larger area can be used to identify pressure systems, fronts and other weather systems.
Other instruments measure different characteristics of the atmosphere. Below is a list of a
few of these instruments, along with what they measures:
- anemometers: wind speed
2. hygrometers: humidity
3. wind vane: wind direction
4. rain gauge: the amount of liquid precipitation over a period of time
5. snow gauge: the amount of solid precipitation over a period of time
These instruments are placed in various locations so that they can check the atmospheric
characteristics of that location. Weather stations are located on land, the surface of the
sea, and in orbit all around the world (Figure16.39). According to the WMO, weather
information is collected from 15 satellites, 100 stationary buoys, 600 drifting buoys, 3,000
aircraft, 7,300 ships and some 10,000 land-based stations.
Instruments are also sent into the atmosphere in weather balloons filled with helium or
hydrogen. As the balloon ascends into the upper atmosphere, the gas in the balloon expands
until the balloon bursts. The specific altitude at which the balloon bursts depends on its
diameter and thickness, but is ordinarily about 40 km (25 miles) in altitude. The length
of the flight is ordinarily about 90 minutes. Weather balloons are intended to be used only
once, and the equipment they carry is usually not recovered.
Weather balloons containradiosondesthat measure atmospheric characteristics, such as
temperature, pressure and humidity (Figure16.40). Radiosondes in flight can be tracked
to obtain wind speed and direction. Radiosondes use a radio to communicate the data they
collect to a computer.
Radiosondes are launched from around 800 sites around the globe twice daily (at 0000 and
1200 UTC; UTC is Coordinated Universal Time; it is the same as Greenwich Mean Time —
the time in the city of Greenwich, England) at the same time to provide a profile through
the atmosphere. Special launches are done when needed for special projects. Radiosondes
can be dropped from a balloon or airplane to make measurements as they fall. This is done
to monitor storms, for example, since they are dangerous places for airplanes to fly.
Weather information can also come from remote sensing, particularly radar and satellites
(Figure16.41). Radarstands forRadioDetectionandRanging. In radar, a transmitter
sends out radio waves. The radio waves bounce off the nearest object and then return to a
receiver. Weather radar can sense many characteristics of precipitation: its location, motion,
intensity, and the likelihood of future precipitation. Most weather radar is Doppler radar,