Such air masses often plunge southward across Canada and the USA during winter, but very rarely form during the summer because the sun warms the Arctic. Continental polar cP : Cold and dry, but not as cold as Arctic air masses.
These usually form farther to the south and often dominate the weather picture across the USA during winter. Continental polar masses do form during the summer, but usually influence only the northern USA. These air masses are the ones responsible for bringing clear and pleasant weather during the summer to the North. Maritime polar mP : Cool and moist. They usually bring cloudy, damp weather to the USA. Maritime polar air masses form over the northern Atlantic and the northern Pacific oceans.
Air masses that form over the ocean, called maritime air masses, are more humid than those that form over land, called continental air masses. The second part of the name describes the temperature of the air mass, which depends on the latitude where it formed. Air masses that form near the equator or in the tropics equatorial or tropical air masses are warmer than air masses that form in polar areas or uin the Arctic polar or arctic air masses.
The word that describes humidity maritime or continental is paired with the word that describes temperature equatorial, tropical, polar or arctic. For example, if an air mass forms over a tropical ocean, it is called maritime tropical. This is a mild air stream and during the winter month in particular, can raise the air temperature several degrees above the average. This air mass has its origins over the snow fields of Eastern Europe and Russia and is only considered a winter November to April phenomena.
During the summer with the land mass considerably warmer, this air mass would be classed as a tropical continental. The weather characteristics of this air mass depend on the length of the sea track during its passage from Europe to the British Isles: this air is inherently very cold and dry and if it reaches southern Britain with a short sea track over the English Channel, the weather is characterised by clear skies and severe frosts.
With a longer sea track over the North Sea, the air becomes unstable and moisture is added giving rise to showers of rain or snow, especially near the east coast of Britain. This air mass has its origins over northern Canada and Greenland and reaches the British Isles on a north-westerly air stream. Polar maritime is the most common air mass to affect the British Isles.
This air mass starts very cold and dry but during its long passage over the relatively warm waters of the North Atlantic its temperature rises rapidly and it becomes unstable to a great depth. This air mass is characterised by frequent showers at any time of the year. In the winter months when instability convection is most vigorous over the sea, hail and thunder are common across much of the western and northern side of the British Isles. However, eastern Britain may see fewer showers as here the surface heating is reduced.
Temps will begin to fall behind front; front located at leading edge of baroclinic zone. Dew point temps turn sharply colder less humid. Pressure falls in advance; sharply rises following frontal passage.
Dew point temps usually within a few degrees of temps i. Sharp wind shift from E to S component to W to N. Pressure falls steadily in advance, then rises steadily after passage. Sharp temperature contrast o n both sides of front. Sharp dew point contrast on both sides of front. Pressure remains fairly steady. Warm to hot temps on both sides of dry line.
Much lower drier dew points on west side; higher dew points on east side. Pressure falls in advance; rises behind. Pressure falls in advance; rises behind; a surface trough may be noted.
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