Weather observations

(meteorology) An evaluation of one or more meteorological elements that describe the state of the atmosphere either at the earth's surface or aloft.

The measuring, recording, and transmitting of data of the variable elements of weather. In the United States the National Weather Service (NWS), a division of the National Oceanic and Atmospheric Administration (NOAA), has as one of its primary responsibilities the acquisition of meteorological information. The data are sent by various communication methods to the National Meteorological Center.

At the Center, the raw data are fed into large computers that are programmed to plot, analyze, and process the data and also to make prognostic weather charts. The processed data and the forecast guidance are then distributed by special National Weather Service systems and conventional telecommunications to field offices, other government agencies, and private meteorologists. They in turn prepare forecasts and warnings based on both processed and raw data. See also Weather map.

A wide variety of meteorological data are required to satisfy the needs of meteorologists, climatologists, and users in marine activities, forestry, agriculture, aviation, and other fields. This has led to a dual surface-observation program: the Synoptic Weather Program and the Basic Observations Program. See also Aeronautical meteorology; Agricultural meteorology; Industrial meteorology.

The Synoptic Weather Program is designed to assist in the preparation of forecasts and to provide data for international exchange. Worldwide surface observations are taken at standard times [0000, 0600, 1200, and 1800 Universal Time Coordinated (UTC)] and sent in synoptic code.

The Basic Observations Program routinely provides meteorological data every hour. Special observations are taken at any intervening time to report significant weather events or changes. Observation sites are located primarily at airports; a few are in urban centers. At these sites, human observers report the weather elements.

Present weather consists of a number of hydrometers, such as liquid or frozen precipitation, fog, thunderstorms, showers, and tornadoes, and of lithometers, such as haze, dust, smog, dust devils, and blowing sand. The amount of cloudiness is also reported. See also Fog; Meteorological optics; Precipitation (meteorology); Smog; Tornado.

Pressure measurements are read from either a mercury or precision aneroid barometer located at the station. A microbarograph provides a continuous record of the pressure, from which changes in specific intervals of time are reported. Pressure changes are frequently quite helpful in short-range prediction of weather events. See also Air pressure.

Temperature and humidity are measured by a hygrothermometer, located near the center of the runway complex at many airport stations. The readings are transmitted to the observation site. The temperature dial indicator is equipped with pointers to determine maximum and minimum temperature extremes. See also Humidity; Hygrometer.

Wind speed and direction measurements are telemetered into most airport stations. The equipment, consisting of an anemometer and a wind vane, is located near the center of the runway complex at participating airports; elsewhere it is placed in an unsheltered area. See also Wind measurement.

Various types of clouds and their heights are reported. The lowest height of opaque clouds covering half or more of the sky is known as the ceiling, and is normally measured by a ceilometer at first-order stations. See also Cloud.

Upper-air observations have been made by the National Weather Service with radiosondes. The radiosonde is a small, expendable instrument package that is suspended below a 6-ft-diameter (2-m) balloon filled with hydrogen or helium. As the radiosonde is carried aloft, sensors on it measure profiles of pressure, temperature, and relative humidity. By tracking the position of the radiosonde in flight with a radio direction finder or radio navigation system, such as Loran or the Global Positioning System (GPS), information on wind speed and direction aloft is also obtained.

Understanding and accurately predicting changes in the atmosphere requires adequate observations of the upper atmosphere. Radiosonde observations, plus routine aircraft reports, radar, and satellite observations, provide meteorologists with a three-dimensional picture of the atmosphere. See also Loran; Meteorological instrumentation; Satellite navigation systems; Weather observations; Wind measurement.

Weather radars distributed throughout the United States are used to observe precipitation within a radius of about 250 nmi (460 km), and associated wind fields (utilizing the Doppler principle) within about 125 nmi (230 km). The primary component of this set of weather radars is known as NEXRAD (Next Generation Weather Radar). These radars provide information on rainfall intensity, likelihood of tornadoes or severe thunderstorms, projected paths of individual storms (both ambient and within-storm wind fields), and heights of storms for short-range (up to 3 h) forecasts and warnings. See also Doppler radar; Radar meteorology.

Geostationary weather satellites near 22,000 mi (36,000 km) above the Earth transmit pictures depicting the cloud cover over vast expanses of the hemisphere. Using still photographs and animated images, the meteorologist can determine, among other things, areas of potentially severe weather and the motion of clouds and fog. In addition, the satellite does an outstanding job of tracking hurricanes over the ocean where few other observations are taken. See also Hurricane; Meteorological satellites.

Ground-based lightning detection systems detect the electromagnetic wave that emanates from the lightning path as the lightning strikes the ground. Lightning information has proven to be operationally valuable to a wide variety of users and as a supplement to other observing systems, particularly radar and satellites. See also Lightning; Lightning and surge protection; Mesometeorology; Meteorology; Weather forecasting and prediction.