Not all the air subsiding around 30° latitude spirals out from the surface highs to become the Trades. The winds on the poleward side of each high in the southern hemisphere tend to emerge as midlatitude northwesterlies (Figure 12.1), carrying relatively warm air towards the pole. These winds encounter cold southwesterlies at around 45°S, and the convergence brings together air masses of different temperatures. The highly irregular interface is called a 'front', in this case the polar front (Figure 12.12). It is like the fronts between different water masses (Section 11.5). Fronts are discussed in the next chapter; here it is enough to know that the cold air at a front slides under the warmer air, wedging the latter upwards. William Ferrel suggested in 1856 that such ascent combines with the previous subsidence in the subtropical highs, and the polewards surface winds from them, to form part of a midlatitude circulation. This would be something like a weak
Hadley cell, but rotating in the opposite direction (Figure 12.11) and therefore intermeshing with the lower-latitude Hadley cells.
For a more sophisticated account of midlatitude winds it is necessary to consider differences between temperatures of the lower troposphere at various latitudes. There is little variation of temperature with latitude near the equator for any particular level of pressure, but there is an abrupt cooling in midlatitudes. In the first case, we say the atmosphere is barotropic. In the second case, the atmosphere is called baroclinic, i.e. imaginary surfaces of constant temperature in the atmosphere are inclined to isobaric surfaces, those of equal pressure (Note 12.H). Baroclinic conditions exist at the polar front mentioned earlier, where warm and cold air masses are adjacent. The front is usually well defined, but its location and the temperature gradient across it fluctuate daily, and it readily breaks up into frontal disturbances, consisting of tongues of cold air towards the equator behind others of warm air penetrating towards the pole. These disturbances cause the variability of winds and weather at midlatitudes (Chapter 13).
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