Understanding of clouds, fog, and dew

Most of the names given to clouds (cirrus, cumulus, stratus, nimbus, and their combinations) were coined in 1803 by the English meteorologist Luke Howard. Howard’s effort was not simply taxonomic; he recognized that clouds reflect in their shapes and changing forms “the general causes which effect all the variations of the atmosphere.”

After Guericke’s experiments it was widely believed that water vapour condenses into cloud as soon as the air containing it cools to the dew point. That this is not necessarily so was proved by Paul-Jean Coulier of France from experiments reported in 1875. Coulier found that the sudden expansion of air in glass flasks failed to produce an artificial cloud if the air in the system was filtered through cotton wool. He concluded that dust in the air was essential to the formation of cloud in the flask.

From about the mid-1820s, efforts were made to classify precipitation in terms of the causes behind the lowering of temperature. In 1841 the American astronomer-meteorologist Elias Loomis recognized the following causes: warm air coming into contact with cold earth or water, responsible for fog; mixing of warm and cold currents, which commonly results in light rains; and sudden transport of air into high regions, as by flow up a mountain slope or by warm currents riding over an opposing current of cold air, which may produce heavy rains.

Observation and study of storms

Storms, particularly tropical revolving storms, were subjects of much interest. As early as 1697 some of the more spectacular features of revolving storms were recorded in William Dampier’s New Voyage Round the World. On July 4, 1687, Dampier’s ship survived the passage of what he called a “tuffoon” off the coast of China. The captain’s vivid account of this experience clearly describes the calm central eye of the storm and the passage of winds from opposite directions as the storm moved past. In 1828 Heinrich Wilhelm Dove, a Prussian meteorologist, recognized that tropical revolving storms are traveling systems with strong winds moving counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The whirlwind character of these storms was independently established by the American meteorologist William C. Redfield in the case of the September hurricane that struck New England in 1821. He noted that in central Connecticut the trees had been toppled toward the northwest, whereas some 80 kilometres westward they had fallen in the opposite direction. Redfield identified the belt between the Equator and the tropics as the region in which hurricanes are generated, and he recognized how the tracks of these storms tend to veer eastward when they enter the belt of westerly winds at about latitude 30° N. In 1849 Sir William Reid, a British meteorologist and military engineer, studied the revolving storms that occur south of the Equator in the Indian Ocean and confirmed that they have reversed rotations and curvatures of path compared with those of the Northern Hemisphere. Capt. Henry Piddington subsequently investigated revolving storms affecting the Bay of Bengal and Arabian Sea, and in 1855 he named these cyclones in his Sailor’s Horn-book for the Laws of Storms in all Parts of the World.

Beginning in 1835, James Pollard Espy, an American meteorologist, began extensive studies of storms from which he developed a theory to explain their sources of energy. Radially convergent winds, he believed, cause the air to rise in their area of collision. Upward movement of moist air is attended by condensation and precipitation. Latent heat released through the change of vapour to cloud or water causes further expansion and rising of the air. The higher the moist air rises the more the equilibrium of the system is disturbed, and this equilibrium cannot be restored until moist air at the surface ceases to flow toward the ascending column.

That radially convergent winds are not necessary to the rising of large air masses was demonstrated by Loomis in the case of a great storm that passed across the northeastern United States in December 1836. From his studies of wind patterns, changes of temperature, and changes in barometric pressure, he concluded that a cold northwest wind had displaced a wind blowing from the southeast by flowing under it. The southeast wind made its escape by ascending from Earth’s surface. Loomis had recognized what today would be called a frontal surface.

Weather and climate

Modern meteorology began when the daily weather map was developed as a device for analysis and forecasting, and the instrument that made this kind of map possible was the electromagnetic telegraph. In the United States the first telegraph line was strung in 1844 between Washington, D.C., and Baltimore. Concurrently with the expansion of telegraphic networks, the physicist Joseph Henry arranged for telegraph companies to have meteorological instruments in exchange for current data on weather telegraphed to the Smithsonian Institution. Some 500 stations had joined this cooperative effort by 1860. The Civil War temporarily prevented further expansion, but, meanwhile, a disaster of a different order had accelerated development of synoptic meteorology in Europe. On Nov. 14, 1854, an unexpected storm wrecked British and French warships off Balaklava on the Crimean Peninsula. Had word of the approaching storm been telegraphed to this port in the Black Sea, the ships might have been saved. This mischance led in 1856 to the establishment of a national storm-warning service in France. In 1863 the Paris Observatory began publishing the first weather maps in modern format.

The first national weather service in the United States began operations in 1871 as an agency of the Department of War. The initial objective was to provide storm warnings for the Gulf and Atlantic coasts and the Great Lakes. In 1877 forecasts of temperature changes and precipitation averaged 74 percent in accuracy, as compared with 79 percent for cold-wave warnings. After 1878 daily weather maps were published.

Synoptic meteorology made possible the tracking of storm systems over wide areas. In 1868 the British meteorologist Alexander Buchan published a map showing the travels of a cyclonic depression across North America, the Atlantic, and into northern Europe. In the judgment of Sir Napier Shaw, Buchan’s study marks the entry of modern meteorology, with “the weather map as its main feature and forecasting its avowed object.”

In addition to weather maps, a variety of other kinds of maps showing regional variations in the components of weather and climate were produced. In 1817 Alexander von Humboldt published a map showing the distribution of mean annual temperatures over the greater part of the Northern Hemisphere. Humboldt was the first to use isothermal lines in mapping temperature. Buchan drew the first maps of mean monthly and annual pressure for the entire world. Published in 1869, these maps added much to knowledge of the general circulation of the atmosphere. In 1886 Léon-Philippe Teisserenc de Bort of France published maps showing mean annual cloudiness over Earth for each month and the year. The first world map of precipitation showing mean annual precipitation by isohyets was the work of Loomis in 1882. This work was further refined in 1899 by the maps of the British cartographer Andrew John Herbertson, which showed precipitation for each month of the year.

Although the 19th century was still in the age of meteorologic and climatological exploration, broad syntheses of old information thus kept pace with acquisition of the new fairly well. For example, Julius Hann’s massive Handbuch der Klimatologie (“Handbook of Climatology”), first issued in 1883, is mainly a compendium of works published in the Meteorologische Zeitschrift (“Journal of Meteorology”). The Handbuch was kept current in revised editions until 1911, and this work is still sometimes called the most skillfully written account of world climate.