The source region for cA and cP air is the same as for its counterpart in winter. It is a predominantly dry air mass and produces gen-erally fair weather over the continent and the British Isles. The visibility is usually reduced because of haze and smoke in the surface layers. As cA and cP air stream southward, the lower layers become unstable; and eventually convective clouds and showers develop in the later stages of their life cycles.

Maritime tropical air that arrives over Europe usually originates over the southern portion of the North Atlantic under the influence of the Azores anticyclone. Maritime tropical air is marked by pronounced stability in the lower layers and typical warm-mass cloud and weather conditions. Relatively high temperatures accompany the influx of mT air, and the moisture content is greater than in any other air mass observed in the middle latitudes of Europe.

Visibility is, as a rule, reduced because of the presence of fog and drizzle, which are frequently observed with an influx of mT air. Maritime tropical air in winter exists only in western Europe. By the time it reaches Russia, it is generally found aloft and greatly modified.

In general, mT air has the same properties as its counterpart in winter with the exception that it is less stable over land because of surface heating. Additionally, this air mass loses its maritime characteristics soon after passing inland. Over water, mT air is still a typical warm air mass. Sea fog frequently occurs in the approaches to the English Channel during the spring and early summer. Visibility in mT air is generally better in summer than in winter, particularly over land where convection currents usually develop. Maritime tropical air flowing over the Mediterranean in summer usually changes to a cold mass, since the water temperature of the Mediterranean is then slightly higher than that of the air. Weak convection currents prevail, usually sufficiently strong to form cumulus clouds but seldom sufficiently strong to produce showers.

The continental tropical air that arrives over Europe in winter originates over North Africa. By the time it reaches central Europe, it differs little from mT air. In general, a cT air mass is much more prevalent over southern Europe than over central or western Europe. Although the moisture content of cT air is less than that observed in mT air, the visibility is not much better, primarily because of the dust that cT air picks up while over Africa. This air mass con-stitutes the major source of heat for the develop-ment of the Mediterranean cyclonic storms, most common during the winter and spring months.

The cT air usually develops over North Africa, Asia Minor, and the southern Balkans. At its source region, the air is dry and warm as well as unstable. The North African air mass is the hot-test air mass on record in the world. In its north-ward flow over southern Europe, cT air absorbs moisture and increases its convective instability. The summer showers and thunderstorms observed over southern Europe are often produced by a modified cT air mass. This air mass is much more prevalent over southern Europe than is its winter counterpart.

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