Weather

Weather, the conditions in the atmosphere, especially in the layer near the ground, at any one place or time. Temperature, humidity, sunshine, wind, cloud cover, and precipitation are among the conditions taken into account. The Earth’s atmosphere is constantly changing and it is the instantaneous changes in the condition of the atmosphere that are defined as weather. The climate of any place is described most easily in terms of average conditions; temperature and precipitation are most frequently quoted. These averages are made up from observations of the weather over many years; a period of 30 years is used by the United Kingdom’s Meteorological Office to define the “average” climate.

Day-to-day changes in the weather result from the changing influence of different weather systems. These systems bring different air masses across the country. Air masses originate in various parts of the globe and when they move, they bring the conditions of their source region into a different climatic zone. Continental air masses tend to be dry, and are hot in summer but cold in winter. Maritime air masses are moist, being warmer in winter and cooler in summer than those from the continent. Cyclonic systems (depressions or “lows”) are areas of disturbed weather, characterized by low barometric pressure at the surface. In mid-latitudes these systems bring cloud, rain, and wind across the country; often there are abrupt changes in the weather as a front passes by and a different air mass arrives. In the tropics, cyclones (known there as hurricanes or typhoons) can be particularly devastating. Anticyclones (or “highs”) are much more stable systems, characterized by high barometric pressure and low wind speed. These may bring warm, sunny weather in summer but very cold weather in winter. It is this changing pattern of weather systems and the associated air masses that makes the weather so variable. Of course, there are seasonal patterns that broadly tend to repeat themselves, but no two calendar days are ever exactly alike. Nor is exactly the same weather ever experienced over any two years; this is why a period of 30 years is needed to build up a picture of the average climate of any place.

People have always observed and commented upon the weather but systematic observations have only been made in the past few hundred years. The longest series of daily observations at one site has been made at the Radcliffe Observatory in Oxford, England, where the record is unbroken since 1815. Daily observations of the weather (usually at 09.00 hours local time) are taken at thousands of places across the globe. These results are used to monitor changes in the weather and climate; maximum and minimum temperature and the rainfall total in the past 24 hours are most often observed, but ambient air temperature, wind speed and direction, cloud cover, hours of bright sunshine, visibility, soil temperature, and barometric pressure are also commonly recorded. Very soon a vast array of weather data is built up; this is the basis on which the climatological averages are calculated. To conduct these weather observations a variety of instruments is used; this can include the barometer for air pressure, the thermometer for temperature, the anemometer for wind speed, gauges for rain and snow, and the hydrometer for humidity. See Meteorology: Surface Observations.

In addition to these daily observations, many activities, in particular aviation, require much more frequent information on weather conditions. Observations are taken every three hours at airports and supplemented by the use of automated equipment that can provide instantaneous information on conditions such as runway surface temperature or wind speed. Weather conditions in remote locations are also collected automatically, data being returned to the central meteorological office via telephone lines or satellite links. Until recently, there was very little information about weather conditions across the world’s oceans and manned weather ships were used to supplement the meagre information supplied by ships and from remote islands. Today, weather satellites and rainfall radar provide a wealth of information on a very frequent basis (usually every half hour), and survey very large areas of the Earth’s surface rather than the conditions at a single place.

Using all this information—satellite data and recent observations from both manned and unmanned weather stations—weather forecasts can be prepared. Computer models provide the basic forecasts but trained forecasters are still needed to interpret the computer output and translate that information into a forecast the layperson can understand. Weather forecasts today are very good for a period of 24 hours and acceptably accurate for up to 5 days ahead. Modern technology—computer modelling and satellite observation—has transformed weather forecasting, especially for regions like western Europe and the western coast of North America where weather systems usually arrive from the ocean to the west. Hazardous weather can now be spotted in advance and appropriate warnings issued to the general public. This is most important in relation in tropical regions where advance notice of the arrival of devastating hurricanes can be crucial in terms of minimizing loss of life and property damage. In extreme cases, evacuation of the local population will follow an extreme weather warning.