Background
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Twice a day Sarepta offers images of the same area from the Advanced Very High Resolution Radiometer (AVHRR) on board the American NOAA satellites.
The first image is sch
eduled between 06:30 and 08:10 GMT, and the second image between 12:50 and 14:30 GMT.
The images are low resolution and cover areas of the size of 3000x5000 km, and give information on objects larger than 3 km.
NOAA images from the last two weeks can be downloaded from http://www.sarepta.org. The two daily images are available with or without coast lines. Furthermore there is a choice of either Channel 2 Visible (good for cloud levels) or Channel 4 Infrared (showing heat radiation).
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Cyclone development

Global circulation.
High and low pressure areas are signified by the "H" and "L".

The global tropospheric circulation contributes to the creation of wandering depressions, cyclones, influencing the weather conditions in Northern Europe.

A cyclone is an area of low pressure around which the winds flow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. A developing cyclone is typically accompanied by a warm front and a cold front.

Figure 1. Vertical profile of a cyclone.
Cold front left and warm front right.
The figure shows a profile of a front system.
Heavy showers at the cold front (left).
The warm sector between the two fronts is almost cloudless.
Drizzle at the warm front (right).

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The Bergen School, led by Bjerknes, has devised a simple model that shows how depressions, or low pressure systems, develop in mid-latitudes when warm and cold air masses meet. Their model has the following stages:

Waves are created on the Polar Front separating the warm (red) and cold (blue) air masses.

A warm front occurs when the air behind the front is warmer than the air in front of it. The warm air is lighter than the cold air ahead of the front. As a result, the warm air rises up and over the cold air. A broad zone of precipitation is formed at the warm front.
The gradient of the front is small, which results in widespread clouds and rain. The rain often covers a zone several hundred kilometres long in front of the warm front.
A cold front occurs when the air behind the front is colder than the air in front of it.
In this case, the coldest air undercuts the warm air ahead of the front. Heavy precipitation will follow during the passage of the cold front. Because the cold air has an easy time pushing the less dense warm air out of the way, it wells up into a fairly steeply faced blob of air that pushes the warm air more or less straight up at the frontal line (the frontal line or boundary is the line where the cold and warm air masses meet at the ground). The strong vertical lifting of the warm, moist air produces tall cumulonimbus clouds along the cold front line, with showers and thundershowers underneath.

At a cold front, the pre-frontal region (in the warm air mass) is characterized by high temperatures, high dew point, high humidity, south or southwesterly winds and falling pressure. The cumuliform clouds pop up rapidly along the front as it passes. The post-frontal region (the cold air) has low temperatures, low dew point, west or northwesterly winds and rising pressure.

The cold front gradually catches up with the warm front
The lowest pressure is found at the meeting point of the two fronts.The cold front is catching up with the warm front. The fronts occlude. This "collision" or "overlapping" is where the term "occlusion" comes from. An occluded front is essentially a cold front that is overtaking a warm front and dissolving it.
Such models are, however, rather simplistic. But they are helpful starting points when analysing and forecasting the weather in the mid-latitudes.
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