The Winds

The troposphere is heated from the bottom up as the surface of the earth absorbs incoming solar radiation and emits infrared radiation. The infrared radiation is absorbed and reemitted numerous times by carbon dioxide and water molecules as the energy works its way bock to space. The overall result of this ongoing absorption and reemission process is the observed lapse rate, the decrease of temperature upward in the troposphere. The observed lapse rate is an average value, which means that the actual condition at a given place and time is probably higher or lower than this value. The composition of the surface varies from place to place, consisting of many different types and forms of rock, soil, water, ice snow, and so forth. These various materials absorb and emit energy at various rates, which results in an uneven heating of the surface. You may have noticed that different materials vary in their abilities to absorb and emit energy if you have ever walked barefooted across some combination of grass, concrete, asphalt, and dry sand on a hot sunny day.

Uneven heating of the earth’s surface sets the stage for convection. As a local region of air becomes heated, the increased kinetic energy of the molecules expands the mass of air, reducing its density. This less dense air is buoyant and is pushed upward by nearby cooler, denser air. This result in three general motions of air: (1) the upward movement of air over a region of greater heating, (2) the sinking of air over a cooler region and (3) a horizontal air movement between the cooler and warmer regions. In general, a horizontal movement of air is called wind, and the direction of a wind is defined as the direction from which it blows.

Air in the troposphere rises, moves as the wind, and sinks. All three of these movements are related, and all occur at the same time in different places. During a day with gentle breezes on the surface, the individual, fluffy clouds you see are forming over areas where the air is moving upward. The clear air between the clouds is over areas where the air is moving downward. On a smaller scale, air can be observed moving from a field of cool grass toward an adjacent asphalt parking lot on a calm, sunlit day. Soap bubbles or smoke will often reveal the gentle air movement of this localized convection.

Depending on local surface conditions, which are discussed in the next section, the wind usually averages about 16 km/kr (about 10 mi/hr) and has an average rising and sinking velocity of about 2 km/hr (about 1 mi/hr). These normal, average values are greatly exceeded during storms and severe weather events. A hurricane has winds that exceed 120 km/hr (about 75 mi/hr), and a thunderstorm can have updrafts and downdrafts between 50 and 100 km/hr (about 30 t0 60 mi/hr). The force exerted by such winds can be very destructive to structures on the surface. An airplane unfortunate enough to be caught in a thunderstorm can be severely damaged as it is tossed about by the updrafts and downdrafts.

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