M

Mainly cloudy – more than half cloud cover but with some breaks in the cloud. It can also be described as cloudy with some sunny periods.

Mainly sunny – mainly sunny means sunny with some cloudy periods.

Mammatus (or mamma clouds) – these clouds appear as hanging, rounded protuberances or pouches on the under-surface of a cloud. With thunderstorms, mammatus are seen on the underside of the anvil. These clouds do not produce tornadoes, funnels, hail, or any other type of severe weather, although they often accompany severe thunderstorms.

Maritime air mass – an air mass that forms over water. It is usually humid, and may be cold or warm.

Maximum temperature – the highest temperature during a specified time period.

Mean temperature – the average of a series of temperatures taken over a period of time, such as a day or a month.

Medium cloud – altostratus, altocumulus and nimbostratus.

Meteorology – the study of the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth's surface, the oceans, and life in general.

Mild – in winter, mild refers to four to seven degrees Celsius above normal.

Minimum temperature – the lowest temperature during a specified time period.

Mist – consists of microscopic water droplets suspended in the air, which produces a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog. In Australia, it is described as similar to fog, but visibility remains more than a kilometer.

Moderate risk – severe thunderstorms are expected to affect between five and 10 percent of the area.

Moderate visibility – five to 10 kilometers’ visibility.

Monsoon – a persistent seasonal wind, often responsible for seasonal precipitation regime. It is most commonly used to describe meteorological changes in southern and eastern Asia.

Morning – sunrise to noon or midnight to noon depending on context.

Mountain breeze – system of winds that blow downhill during the night.

Muggy – colloquially descriptive of warm and especially humid weather.

Multivortex tornado – a tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.

Mushroom – a thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.