move eastward at about 10 m/s (Figure 13.7), i.e. at about the same speed as the mid latitude westerlies in the middle troposphere (Figure 12.10). These winds are weaker nearer the equator, so the northern end of a front lags
A warm front occurs where warm air advances on colder, as may be seen in Figure 12.12 in the places where warm northerly winds strike the polar front. Such a front is drawn on a weather chart as a line with half circles (Figure
13.2). A section across a warm front is similar to that through a cold front (Figure 13.3), except that the lighter warm air moves over a retreating wedge of cold air. Warm frontal zones are even less steep than in the case of cold fronts, so the front is less evident and uplift more gradual. As a result, the first cirrus clouds may be 1,000 km ahead of the front at the surface. Commonly, the clouds gradually thicken and slowly obscure the Sun, the cloud base lowers, and eventually light rain falls.
A warm front typically advances clockwise down to the south and lies at 45-55°S, where there is little land, so they are encountered infrequently. One place where they are found is Patagonia, to the east of the southern Andes, where they occur in winter after a calm period has allowed a cold air mass to accumulate in the lee. This is called cold air damming, as the Andes block the inflow of warmer air from the west. A subsequent disturbance from the west drives warmer air from the north over the dammed air, with a warm front between the two air masses. Cold air damming also occurs on the south-east side of New Zealand's southern mountain range in winter, when warmer mT air from the north-west flows over colder, heavier mP air, deposited there a few days earlier and locked behind the mountains.
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