Mackerel Sky - A description given to the sky when full of a certain type of cirrocumulus or small individual units of altocumulus clouds which give the appearance of scales on a mackerel.
Macroburst - A large, damaging thunderstorm downburst wind with a 2.5 mile or greater diameter and lasting 5 to 20 minutes.
Maestro - A wind in the Adriatic Sea which blows from the southwest and which is accompanied by fine weather. It is especially frequent during summer on the western shore.
Mamma - Pouch-like bulges on the underside of a cloud. They may sometimes be seen underneath cumulonimbus clouds, although other cloud types may have them, as well.
Mares' Tails - Long wispy cirrus clouds thicker at one end than at the other.
Marin - A southeast wind on the French Mediterranean coast and in the maritime alps. It blows from the sea, is warm and moist and is most prevalent in spring and summer.
Mercury Barometer - An instrument used to measure atmospheric pressure. It is usually about three feet tall and uses mercury in a glass tube. The tube is closed at the top, and the open end is immersed in a cistern of mercury with the surface of the mercury exposed to the air. The height of the mercury in the tube changes with changes in atmospheric pressure.
Mesocyclone - The rotating updraft in a usually supercell thunderstorm. It is most often about 1-6 miles in diameter.
Mesoscale - A scale of size referring to a weather system which is larger than single storm clouds but smaller than the large synoptic scale systems. A mesoscale system may range anywhere from approximately 50 to several hundred miles.
Mesoscale Convective Complex (MCC) as seen by satellite.
Mesoscale Convective Complex - Frequently abreviated MCC, this is a very large mesoscale convective system (thunderstorm complex) which is round or oval in shape and most often is at its most intense stage during the night. There are quite specific minimum defined limits for size, duration and shape using satellite imagery.