The Movement of Air
Wind due to differences in pressure
Movement of air caused by temperature or pressure differences is wind. Where there are differences of pressure between two places, a pressure gradient exists, across which air moves: from the high pressure region to the low pressure region. This movement of air, however, does not follow the quickest straight line path. In fact, the air moving from high to low pressure follows a spiralling route, outwards from high pressure and inwards towards low pressure. This is due to the rotation of the Earth beneath the moving air, which causes an apparent deflection of the wind to the right in the northern hemisphere, and left in the southern hemisphere. Consequently, air blows anticlockwise around a low pressure centre (depression) and clockwise around a high pressure centre (anticyclone) in the northern hemisphere. This situation is reversed in the southern hemisphere.
Wind caused by differences in temperature is known as convection or advection. The process of convection was described in lesson 1. In the atmosphere, convection transfers heat energy from warmer regions near the Earth’s surface to regions higher up in the atmosphere away from the heating influence of the Earth’s surface. Whereas convection is the vertical movement of air, advection involves the horizontal movement of air and heat energy transference.
Temperature differences at the Earth’s surface occur wherever there are differences in surface substances. A dark tarmacked surface will heat up more quickly on a sunny day (i.e. absorb more solar radiation) than a grassy field. Similarly, along the coast, large areas of land heat up more quickly than adjacent sea water (water has a large heat capacity and is a good conductor of heat). Air near the land surface is heated by radiation and conduction, expands and begins to rise, being lighter than the surrounding air. This is convection. To replace the rising air, cooler air is drawn in from the surface of the sea. This is advection, called a sea breeze, and can offer a pleasant cooling influence on hot summer afternoons when further inland the heat may become oppressive.
Air above the sea sinks and is again pulled in over the land. The full sea breeze circulation is shown below. A very hot summer sun may cause a sea breeze of up to 15 mph along the coast, felt in decreasing strength 20 to 25 miles inland.
Since the sea breeze owes its existence to the enhanced heating of the land under the sun, it follows that at night, when the land cools faster than the sea, a land breeze may develop. In this case, it is air above the warmer surface water that is heated and rises, pulling in air from the cooler land surface.
Inland on clear nights when the surface looses considerable radiation, surface cooling serves to set up air movements wherever there are undulations of contour. As the air becomes colder, it contracts and sinks down as far as it can move, settling into hollows, drifting down slopes and blowing down mountain sides. Large scale air movements of this nature are called Katabatic winds.
On a global scale, the same principle of temperature difference operates to develop the major wind belts. Large volumes of air rise over the equator where most solar radiation is directed, creating a demand for colder air from higher latitudes. This however, is an oversimplification of the cause of global weather. The presence of large continental land masses and vast expanses of ocean introduce further complexities to the global air movements. These are looked at in lesson 9.
Because the basic mechanism for raising air temperature occurs at ground level with the heating of the surface by the Sun, temperatures are generally higher near the Earth’s surface than further away. Nevertheless, local variations exist, caused by the slow mixing of air. Sometimes, air temperature decreases rapidly with altitude, sometimes more slowly. Occasionally, air temperature may even increase with altitude for a short distance.
As discussed, when a packet of air near the earth’s surface is heated, it rises, being lighter than the surrounding air. Whether or not this air packet continues to rise will depend upon how the temperature in the surrounding air changes with altitude. As convection continues, air pressure begins to fall, and the air packet expands. Such expansion results in loss of heat and consequent fall in temperature. (Similarly, when air descends the air compresses and its temperature rises.) The rate at which air on expansion cools is called the adiabatic lapse rate, and for dry air it is equal to 9.8C per kilometre. Adiabatic means that the air exchanges no heat with its surroundings, a condition very nearly true for rising and descending packets of air.
If the rate at which the surrounding air temperature falls is less than the adiabatic lapse rate, a rising packet of heated air will cool faster, lose its buoyancy, and sink back to its original position. In this case the atmosphere is said to be stable. If the rate at which the surrounding air temperature falls is greater than the adiabatic lapse rate, the packet of heated air will continue to rise. The atmosphere in this circumstance is said to be unstable.
When the air is saturated with water vapour, the processes are similar to those described above for dry air, but the adiabatic lapse rate is different. When saturated air rises and cools, condensation of water vapour begins, releasing latent heat. Consequently the temperature in rising moist air falls less than it otherwise would. For warmer air holding considerable water vapour, the adiabatic lapse rate may be as low as 4C per kilometre. This approaches the value for the dry adiabatic lapse rate for much cooler air carrying little water vapour. More usually in the atmosphere, unsaturated air rises, cooling at the adiabatic lapse rate until it reaches its dew point. Thereafter, it behaves like saturated air. The moisture condensing out of the air becomes visible as cloud.