Warm dry air that originates over a continental land mass. Major semi-permanent source regions in the northern hemisphere are the subtropical anticyclones over North Africa and Arabia, together with parts of Central Asia, where there are light winds and upper tropospheric subsidence. An additional source region occurs over the southwestern United States and Mexico. In summer, the Eurasian source region expands to become a wide belt running from the western Mediterranean almost to the Sea of Japan. In the southern hemisphere, only central Australia is a permanent source. In winter, the only other significant source lies in southeastern Africa, but this shrinks in summer, when maritime tropical air invades the continent. A small summertime source region exists over Argentina. The cT air that originates in the subtropical anticyclones in summer is extremely hot and dry, but often unstable at low levels.
Subjects: Meteorology and Climatology.