Weather Charts.

Formation of a depression.

	The origin of depression are varied enough when we follow weather charts. Examples which come to my mind are the following: - A cold front half dead appear or go above the Sargasso sea or the Mexcio Gulf. I continue example with this farther. - A depression reaches to cross USA or Canada and come back after the Quebec. I do not represent this case.
	Then the pressure loses some millibars somewhere. Personally, I suppose that a group of torrential rain appear somewhere in a instability area, a little bit like a tropical depression.
	So the masses of air put on in movement, begin turning. Then I think that it throws hot air faster against cold air. It increase instability, cumulonimbus or nimbostratus appera in highest quantity. Every this hot air which rose in altitude causes a draught at ground level, ie.e fall of pressure. The centre of depression becomes more clear.
	The mechanism continues, fronts become more ample and more clear. The centre of depression is still at intersection of hot fronts and of cold front.
	When depression are drawn enough, it projects the cold front far forward.
	Then an occluded front appear with cold character (in summer for instance)...
	... or with hot character (in winter for instance). It is the time when the depression attains its lowest pressure.
	Then once its feeding far from her, depression fills up.

Les fronts.

	A hot front, it is when hot air pushes cold air while sliding by top. Hot and humid air forms a nimbostratus in the back, which becomes Altrostratus then Cirrostratus and Cirrus in the front. In an ideal case, we first see coming cirri. Then it thickens a bit and become cirrostrati and cirrocumulus. Rather often this begins directly at this following stage that is altostratus and altocumulus. When they are there, the rain with the nimbostratus is not far, which arrive not much after. The rain is here until the next day where this frees with altocumulus and stratocumulus. So that hot front arrive, it must be cool in front of it. Therefore it appear in our regions especially in winter.
	In summer, the cold front materializes ideally by a cumulonimbus, with storm and torrential rains which are associate there, follows by rain for several hours. But this ideal case is not very frequent. More often, it is preceded of altocumulus and stratocumulus, clouds which are also present after passage of one hot fronts. And rather often, it is no cumulonimbus which follows, but rather a nimbostratus, as in autumn winter spring. The cold front is then progressive and displayed in length, what is worth us rain for a long time. After the cold front, we have cumulus ideally. But in autumn winter spring, it is frequent to see stratocumulus and altocumulus some times after, sometimes accompanied with altostratus. In spring and in summer, the sky can sometimes free all after the passage of the cold front, for instance with a thick altostratus which stops dead. In that case, cumulus appear 30 minutes aprés!
	We see less often the occluded front in our regions, because it appear most often in north of depressions. And depressions pass often more in north of France, for instance between Ireland and Greenland. In winter, it can resemble to a hot forehead. In summer, it will have tendency to give a stormy weather, sometimes by resembling a bit to a cold forehead.

Example.