A small but intense cyclone that forms in cold polar air advected over warmer water. These vortices often form in the subpolar North Pacific and subpolar North Atlantic equatorward of the sea ice marg
in. Horizontal scales range from several tens to several hundreds of kilometers. Because of strong winds and intense precipitation, these cyclones are sometimes referred to as "arctic hurricanes."
Cirrus-like clouds seen from spacecraft over the polar regions during summer in both hemispheres. (Abbreviated PMC.) They occur near the mesopause, at heights of roughly 85 km, and are closely related
to noctilucent clouds.
Anomalously strong radar echoes received from the mesopause region during summer at high latitudes. (Abbreviated PMSE.) They are loosely associated with the occurrence of noctilucent clouds and polar
mesospheric clouds.
A core of strong westerly winds that develop during autumn and winter in the upper stratosphere and mesosphere near the boundary of the polar night. Radiative cooling in the polar night appears to mai
ntain the required baroclinicity.
Westerly jet stream of maximum intensity near the stratopause, in the middle and subpolar latitudes of the winter hemisphere, caused by prolonged radiative cooling of the air in high latitudes during
this season.
The movement of a cold air mass from its source region; almost invariably applied to a vigorous equatorward thrust of cold polar air, a rapid equatorward movement of the polar front. (Or cold-air outb
reak.)