THUN, LAKE OF, in the Swiss canton of Bern, the second lake (the first being that of Brienz) into which the river Aar (q.v.) expands. These two lakes occupy an ancient terminal basin of a glacier and are separated by a lacustrine delta on which Inter laken stands. Lake Thun (with the town of Thun at its north western end) is I 1 m. long, 2 m. wide, its maximum depth is 712 ft., while its area is 181 sq.m., and its altitude 1,837 ft. THUNDER, the noise which accompanies or follows a flash of lightning (q.v.), on account of the air disturbance caused by the sudden heating and expansion of air during the electrical dis charge. (See ATMOSPHERIC ELECTRICITY and METEOROLOGY.) THUNDERSTORMS. Since the middle of the 18th cen tury it has been realized that lightning is an electric spark, and that thunder is the noise produced by the spark. The natural tendency was to assume that the meteorological phenomena of the thunderstorm, and especially the heavy rainfall, were pro duced by electricity. The modern view is that the rainstorm is the primary phenomenon and that the electricity is incidental., The common feature of thunderstorms is their great vertical extent. The base of a cloud may be between I and 2 km. above the ground. The summit may reach the level of the cirrus clouds, at a height of 9 km. or more.
To explain the motive power required for the production of such a cloud we must consider the behaviour of dry air and moist air when subject to changes of pressure. Air is heated by compression and cooled by expansion. If, however, the air con tains drops of water, the rise of temperature causes evaporation, and some of the energy used in compressing the air is converted into latent heat. The same increase of pressure produces less rise of temperature in wet air than in dry air. Conversely, the ex pansion of saturated air in which condensation is taking place will be accompanied by a smaller fall of temperature than the expansion of dry air. Further, if two masses of air have the same temperature initially, and both are cooled by the reduction of pressure, the wet air will not only be the warmer at the end of the process but will also be lighter, and occupy more space. In the atmosphere, where masses of air can move freely, those masses which contain cloud tend, on the whole, to be the more buoyant.
Thus we have a general explanation of the formation of cumulus cloud. The air in the lower part of the atmosphere is warm, and contains water in the form of vapour. If some of this air is carried upwards to a level where the pressure is lower, the tem perature falls, and at a certain height condensation begins. The air brought from below may be at the same temperature as the surrounding air at this height, but as condensation proceeds the rising air remains comparatively warm, and it may be lighter than its surroundings. A cloud has begun to form, and, as the top of the cloud rises, air from below takes its place, and the cloud tends to grow. The conditions favourable for the growth of a cumulus cloud are : The upper air into which the cloud grows is cold, preferably with a lapse rate approaching the limiting value Io° C per kilometer. (2) The lower air contains much water vapour, and is not too stable; if vigorous convection is taking place, the lapse rate in this lower air may even exceed 1o° C per kilometer. By the time the cloud has grown to a height of some kilometers, the difference between the pressure at the bottom of the cloud and at other points at the same level will be very great, and the upflow of air through the cloud very violent. In the rising air condensation proceeds rapidly. Some of the drops are carried up by the air. They are frozen to form soft hail, and then covered by more ice to become true hail. The raindrops or hail stones get thrown out of the more vigorous ascending currents and are able to fall through and out of the clouds.
Experience shows that these are the circumstances in which the electrical phenomena which characterize thunderstorms occur. According to some observers there is no true thunderstorm without the formation of false cirrus at the top of the cloud. In the early stages of growth the air surrounding the top of a cloud is com paratively dry, and the parts of the cloud carried away by eddies re-evaporate. When the cloud towers up to the cirrus level, where the temperature is so low that very little moisture can be held in the gaseous state, evaporation can no longer take place at the top of the cloud, and when the ascending air mixes with the other air of that height, numerous ice crystals build up the cap of false cirrus.