Meteorology

I

Introduction

Meteorology, scientific study of the Earth's atmosphere. It includes the study of day-to-day variations and predictions of weather conditions (synoptic meteorology); the study of atmospheric motion (dynamic meteorology); the study of electrical, radiative, hygrological, optical, and other physical properties of the atmosphere (physical meteorology); the study of climate, average and extreme weather conditions over long periods of time (climatology); the study of the variation of meteorological elements close to the ground over a small area (micrometeorology); and studies of many other phenomena. The study of the highest parts of the atmosphere (above about 50 km/30 mi) generally involves the use of special techniques and disciplines, and is termed aeronomy. The term aerology has been applied to the study of conditions in the free atmosphere anywhere away from the ground.

II

History

The scholars of ancient Greece were greatly interested in the atmosphere. In the 4th century bc Aristotle wrote a treatise called Meteorologica, dealing with the “study of things lifted up”; about one third of the treatise is devoted to atmospheric phenomena, and it is from this work that the modern term “meteorology” is derived. Throughout history much of the progress in the discovery of the laws of physics and chemistry was stimulated by curiosity about atmospheric phenomena.

Weather forecasting has challenged the human mind from the earliest times, with much ancient worldly wisdom identified with weather lore and weather almanacs. Little progress was made in scientific forecasting, however, until the 19th century, when developments in the fields of thermodynamics and hydrodynamics provided the theoretical basis for meteorology. Exact measurements of the atmosphere are also of the greatest importance in meteorology, and the advance of the science has been furthered by the invention of suitable instruments for observation and by the organization of networks of observation stations to gather weather data. Weather records for individual localities were made as early as the 14th century, but not until the 17th century were any systematic observations made over extended areas. Slow communications also hampered the development of weather forecasting, and it was not until the invention of the telegraph in the mid-19th century that weather data from an entire country could be transmitted to a central point and correlated for the making of a forecast.

One of the most significant milestones in the development of the modern science of meteorology occurred in the World War I period, when a number of Norwegian meteorologists, led by Vilhelm Bjerknes, made intensive studies of the nature of fronts and discovered that interactions between warm and cold air masses at their boundaries generate the cyclones (often called depressions or “lows”) found in temperate latitudes. Later meteorological work was aided by the invention of apparatus such as the rawinsonde, described below, which made possible the investigation of atmospheric conditions at high altitudes. Immediately following the World War I period, a British mathematician, Lewis Fry Richardson, made the first significant attempt to obtain numerical solutions of the atmospheric equations for the prediction of meteorological elements. Although his efforts were not successful at the time, they contributed to the explosive progress in numerical weather prediction of today.

III

Weather Observation

Improved observations of high-level winds during and following World War II provided the basis for new theories of weather forecasting and revealed the necessity for changes in older concepts of the general atmospheric circulations. During this period major contributions to meteorological science were made by the Swedish-born meteorologist Carl-Gustav Rossby and his colleagues in the United States. The so-called jet stream, a fast-moving river of air circling the globe high in the atmosphere, was discovered. In 1950, through the use of computers, it became possible to apply the fundamental theories of hydrodynamics and thermodynamics to the problem of weather forecasting, and today such computers are employed regularly to provide weather predictions for government, commerce, industry, shipping, aviation, agriculture, and the general public.

A

Surface Observations

Observations made at ground level are more numerous than those made at upper levels. They include the measurement of air pressure, temperature, humidity, wind direction and speed, the amount and height of clouds, visibility, and precipitation (the amount of rain or snow that has fallen).

For the measurement of air pressure, the mercury barometer is the accepted standard. Aneroid barometers are also useful, particularly on ships and when used in the recording form called a barograph to show the trend of pressure change over a period of time. All barometric readings used in meteorological work are corrected for variations resulting from temperature and the height of the station, so that pressures from different stations may be directly compared.

For the observation of temperature many different types of thermometers are employed. For most purposes an ordinary thermometer covering an appropriate range is satisfactory. It is important to place the thermometer in such a way as to minimize the effects of sunshine during the day and of heat loss by radiation at night, thus yielding values of the air temperature representative of the general area.

The instrument most often used at weather observatories is the hygrometer. A special type of this instrument, known as the psychrometer, consists of two thermometers to measure the dry-bulb and wet-bulb temperatures. A more recent device to measure humidity is based on the fact that certain substances undergo changes in electrical resistance with changes in humidity. Instruments making use of this principle are commonly used in rawinsondes, high-level atmospheric sounding devices.

The most common instrument for measuring the direction of the wind is the weather vane, which keeps pointing into the wind and which is connected to an indicating dial or to a series of electronic switches that provide electrical output related to the wind direction. Wind speed is measured by means of a cup anemometer, an instrument consisting of three or four cups mounted about a vertical axis. The anemometer spins faster as the speed of the wind increases, and some form of device for counting its revolutions is used to gauge the wind speed.

Precipitation is measured by a rain gauge or a snow gauge. A rain gauge consists of an upright cylinder, open at the top to catch the rain and calibrated either in millimetres or in inches, so that the total depth of the precipitated water may be measured. The snow gauge also is a cylinder, which is thrust into the snow to collect a core of snow. This core is melted and measured in terms of the equivalent depth of water, thus making the measurement compatible with rainfall observations. Snow-depth measurements are also made with a staff gauge, which is similar to an ordinary ruler.

Recent advances in the field of electronics have been accompanied by concurrent developments in the use of electronic weather instruments. One such device is weather radar, which makes possible the detection of hurricanes, tornadoes, and other severe storms over distances of several hundred kilometres. For such purposes, the radar echoes from precipitation associated with the disturbance are used to track its course; the most modern weather radars can also detect local wind fields within areas of precipitation. Other electronic weather instruments include the ceilometer, used for measuring cloud (or ceiling) heights, and the transmissometer, which measures the total effect of smoke, fog, and other restrictions to vision in the atmosphere. The ceilometer and transmissometer together provide instrumental measurements that are extremely important for the take-off and landing of aircraft.