Weather mapping is the process of representing weather patterns and their future development and movement on a map. The process is only possible if two conditions are met. First, current weather conditions must be available at a number of widely distributed stations. Second, reports of those conditions must be transmitted to a central collecting station quickly enough to be used before the weather changes.
weather maps first came in use in the last third of the nineteenth century, when the invention of the telegraph made the transmission of weather data from far-flung observing points possible. In 1870 the newly created National Weather Service produced weather maps for limited regions of the United States.
In Norway, Vilhelm and Jakob Bjerknes developed one of the most successful early weather mapping systems. The Bjerknes convinced the Norwegian government to set up weather stations in strategically important areas throughout their country. Data from these stations was telegraphed to Bergen, where it was assembled and used to produce some of the most complete weather maps available at the time.
Today, data on weather conditions around the world are collected by more than 10,000 individual stations, hundreds of ships at sea, and a variety of instruments traveling through Earth’s atmosphere. These data are transmitted regularly four times a day: usually at 0:00 hours, 6:00 hours, 12:00 hours, and 18:00 hours (Greenwich Mean Time). The data are used by national weather services to develop weather maps and forecasts for their own regions. Overseeing these individuals forecasts is the World Meteorological Organization (WMO), an international organization with representatives from more than 130 nations. WMO is responsible for ensuring that all stations follow standard collection and transmission procedures and for the exchange of weather information among member nations.
The data sent from an observation station to the central collecting point are transmitted by a standard code consisting of number blocks. Meteorologists understand the significance of each block of numbers and translate them into specific weather conditions such as barometric pressure, temperature, and wind speed.
Many different kinds of weather maps exist. Synoptic maps show current weather conditions, while prognostic maps show weather predictions for some time in the future. Some weather maps are complex and contain a great deal of detailed information. Others are simpler and provide only general patterns and trends. Maps can be sub-divided into those that summarize weather close to Earth’s surface (surface charts) and those that describe weather at upper altitudes.
One very detailed type of map makes use of the station model. The reporting station is indicated with a small circle and the data received from that station is arranged around the circle in a predetermined pattern. Among the kinds of data plotted in the station model are cloud cover, wind direction, visibility in miles, present weather, barometric pressure, current air temperature, cloud types, dew point, and precipitation.
Most of these variables can be represented by numbers. Visibility might be indicated as 1/2 for 0.5 mi (0.8 km) and current temperature as 22 for 22°C (71°F). Other data are represented by standard symbols. Wind direction and speed are indicated by a one-edged arrow. The number of feathers on the arrow indicate the wind speed and the orientation of the arrow indicates the wind direction. A single full feather represents a wind speed of 8-12 knots; a double feather, a speed of 18-22 knots; and so on. A variety of symbols represent current weather conditions, such as * for intermittent snow fall and+, for intermittent drizzle.
Barometric pressure —Air pressure; the force exerted by a column of air at any given point.
Cloud cover —The portion of the sky that is covered by clouds at any given time and place.
Dew point —The temperature to which air must be cooled for it to become saturated.
Isobar —A line on a weather map connecting points of equal atmospheric pressure.
Surface chart —A map that shows weather conditions at and just above Earth’s surface.
Visibility —The distance to which an observer can see at any given location.
The weather map that appears in daily newspapers, television news broadcasts, or weather-related web sites can be used to predict with some degree of accuracy conditions in the next few days. It usually does not include as much station information as the more detailed maps described above. Instead the major features of the daily weather map include isobars and high and low pressure areas.
Isobars connect locations with the same barometric pressure. The pressure described by each isobar is often indicated at one end, the other, or both, in inches, millibars, or other unit. Isobars often enclose regions of high or low pressure indicated on the map as H or L.
The outer edge of a concentric series of isobars marks a front. The nature of the front is indicated by means of solid triangles, solid half-circles, or a combination of the two. An isobar with solid triangles attached represents a cold front; one with solid half-circles, a warm front; one with triangles and half-circles on opposite sides, a stationary front; and one with triangles and half-circles on the same side, an occluded front. The daily weather map may also include simplified symbols that indicate weather at a station as a T enclosed in a circle for thunderstorms, F enclosed in a circle for fog, and Z in a circle for freezing rain.
Ahrens, Donald C. Meteorology Today. Pacific Grove, CA: Brooks Cole, 2006.
David E. Newton