Air has smaller density at higher temperature if pressure is held constant. The relationship between the three properties can be very closely approximated by the equation of state of an ideal gas. Thus we may expect that, if there is difference in temperature, convective circulation where warmer air rises and cooler air sinks. In particular, if we start from the distribution with respect to latitude of annual mean temperature in the troposphere of the earth's atmosphere, we may expect such meridional-vertical circulation where air rises near the equator, sinks in high latitudes, moves away from equator at the upper level and to toward equator at the lower level.
We must consider, however, that the earth is rotating, and that the motion of air (i.e. winds) that we consider is the motion relative to the rotating solid earth. The situation is similar for the motion of water of the ocean. The set of equation of motion for a body moving on the rotating earth has terms including a kind of apparent force called Coriolis force. The magnitude of the force is proportional to the speed of the body relative to the earth, and its direction is perpendicular to the velocity vector. Because the atmosphere and the ocean are thin layers with respect to the whole earth, the effect of rotation that appears to local motions within them comes from the component of rotation about the axis in the direction of zenith (i.e. the direction just above the head of a person standing on the earth). The amount of the component varies by the latitude. It is zero at the equator and its absolute value takes maxima at the poles. Coriolis force acts to change the direction of air motion. As a result, the wind in the upper troposphere that moves away from the equator becomes westerly (flow from west to east), and the wind in the lower troposphere that moves toward the equator becomes easterly (flow from east to west). Hadley, in a paper published in 1735, explained the cause of easterly winds (called trade winds) near surface which prevails in the most part of tropics by a mechanism basically the same as this.
In the annual average picture of circulation in the tropics and the subtropics of the real world (see Fig. 4.1-5), we find upward motion near the equator, downward motion around 30o latitudes of both hemispheres, and low-level easterly wind (trade wind) between them. Also we find strong westerly wind in the upper troposphere in the higher latitudes within this band (i.e. above the downward motion). This type of circulation is called Hadley circulation. Hadley circulation transports energy to higher latitudes, and water vapor to lower latitudes.
Actually, the place where the air receives energy and the place where its density decreases are not the same. The energy that the surface in the tropics provides mostly takes the form of energy contained in water vapor (in excess to that of liquid water at the same temperature). Where the water vapor condenses, the temperature of air rises, and the density decreases. The water thus condensed drops as rain. In convection of air accompanying these phase changes of water, the area of upward motion becomes relatively small and the area of downward motion becomes large. The actual center of upward motion is shifted from the equator to the 5o -- 10o latitudes of the summer hemisphere, partly because Coriolis force is favourable for maintaining cloud clusters, and partly because of the distribution of sea surface temperature (also being influenced by the earth's rotation). The situation of Fig. 4.1-5 that the center of upward motion coincides with the equator is somewhat artificial result of annual averaging. In the surface weather charts, the upward branch of Hadley circulation appears as the intertropical convergence zone (ITCZ), and the downward branch corresponds to the subtropical anticyclonic belt.
The atmosphere in middle and high latitudes also transports energy and water vapor toward higher latitudes. The principal agent of the transport is extratropical cyclones and moving troughs and ridges of pressure accompanying them. Extratropical cyclones are generated where there is north-south difference of temperature and westerly winds prevail. They move from west to east at a speed around 10 m / s. The wavelength of the trough/ridge system accompanying them is several thousand kilometers. They act to mix warm air in lower latitudes and cold air in higher latitudes, almost horizontally (roughly speaking). Extratropical cyclones also act to maintain the westerly wind in the upper troposphere.
In laboratory experiments with rotating annular water tanks, with a constant temperature difference between outer and inner walls (corresponding lower and higher latitudes), Hadley-type circulation is found if the angular velocity of rotation is small, but the circulation becomes wavy and vortices with characteristics similar to extratropical cyclones are generated when the angular velocity exceeds some threshold. In the earth's atmosphere as well, Hadley circulation prevails in the tropics where Coriolis force is weaker, and circulation dominated by extratropical cyclones in middle and high latitudes where it is stronger. The latter type are sometimes called Rossby-type circulation, but this terminology is not so popular as Hadley circulation.
The latitude ranges where Hadley and Rossby-type circulations prevail (respectively) varies somewhat according to seasonal march. Accordingly, the zone of latitudes about 30o and 45 o, including Japan, is covered by the downward branch (i.e. subtropical anticyclones) in summer, and included in the westerly zone where extratropical cyclones pass in winter. The zone of latitudes lower than this is always covered by Hadley circulation, and the zone of latitudes higher than this may be regarded as always covered by Rossby-type circulation. Fig. 4.1-6 shows seasonal variation of the region where Hadley circulation dominates.
Overlayed with moving troughs and ridges of pressure, stationary troughs and ridges with wavelength around ten thousand kilometers as well exist in the actual middle and high latitude atmosphere, especially significantly in the northern hemisphere in winter. It is considered that these are due to the effect of heating from the ocean and the dynamic effect of mountains, as will be discussed in Section 4.1.6 .
Air masses and fronts:
When we observe air with similar properties such as temperature and humidity in a large area, we say that an air mass is formed. On the other hand, we call fronts where the properties of air change remarkably within short distance. Fronts are usually also convergence zones which means the place where air is converging from both directions. Each extratropical cyclone accompany fronts. If we look at the state averaged over a time interval longer than the lifetime of individual cyclones, we recognize the whole zone where the frequency of generation or passage of cyclones is large as a frontal zone.
I referred to Matsuno (1982) and Ogura (1999) in preparing descriptions of Sections 4.1.2 - 4.1.4. For more details of energy tranport by the atmosphere, see Masuda (2000), Peixoto and Oort (1992), Hartmann (1994). For various classification of climates proposed earlier than Nakamura et al. (1986), see Yazawa (1989) and Essenwanger (2001).