In case the cyclone passes the observer along the line cd, he is to the south of the centre of storm. If the change of weather is followed along this line, the appearance of high cirrus clouds on the western horizon heralds the approach of a wave cyclone.
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These cirrus or cirrostratus clouds are visible in the sky from a distance of 1600 to 2400 km from the surface front. These clouds create halo around the sun and the moon.
It is to be pointed out that the conditions of the atmosphere at the warm front depend to a large extent upon the nature of the ascending air currents of the warm sector. In case there is instability in the rising air, cirrocumulus will form.
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It produces what is known as the mackerel sky’ which forewarns the impending stormy bad weather. As the warm front draws nearer, the lower and thicker clouds cover the sky. Alto-stratus and alto-cumulus are replaced by stratocumulus and nimbus.
Sometimes the vertical movement of local air columns produces cumulonimbus clouds. These clouds are producers of local thunderstorms particularly in spring and summer.
On certain occasions, precipitation falling through the dry and cold air mass from stratus clouds above the warm front saturates the lower layers of cold air, and thus low clouds and fog are formed. In front of an approaching warm front there is a broad zone having light rain and drizzle.
As the warm front approaches along the line cd, there is a gradual increase in temperature and fall in air pressure. Relative humidity also registers a gradual increase. Winds begin to change direction from east to southeast to south to southeast.
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This is called a veering wind shift. It is clear evidence that centre of the depression has passed to the north of the observer. With the passage of warm front, the observer is now in the warm sector.
Skies become clear, temperatures rise and there are southerly winds. Depending on moisture content and instability of the warm and moist air of the warm sector, fair weather cumulus or altocumulus may be seen.
But the fair weather of the warm sector passes off quickly. Now, there is southwesterly flow of air. However, the pressure remains constant.
Since the cold front slopes upward away from the direction of storm movement, unlike the warm front, the approaching cold front is not heralded much ahead of it by clouds of any from.
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However, intense squall line thunderstorms generally precede the cold front. As the cold front approaches the observer, there are rolling black clouds accompanied by thundershowers, hail and an occasional tornado.
Heavy precipitation associated with the cold front is confined to a narrow zone and is of short duration. It is also accompanied by thunder and lightning.
Since the advancing wedge of cold air is under-running the warm and moist air and pushing it aloft, heaps of cumulonimbus clouds form along the cold front.
When the cold front passes the observer along the line cd, there is a clear wind shift, southerly air currents are replaced by westerly and northwesterly winds. There is a sudden drop in temperature.
The air pressure starts rising. Now, the region is under the influence of subsiding cool and dry polar air. The weather is fine and the skies clear.