Aeronautical Terms beginning with M

Melting

The change of ice to liquid water.

Mammato cumulus

Obsolete. Now referred to as cumulonimbus mamma.

Mare’s tail

Irregularly curved wisps of cirriform clouds.

Maritime polar air (abbreviated mP)

Also called polar air. An air mass with characteristics developed over high latitudes, especially within the subpolar highs. Maritime polar (mP) initially possesses similar properties to those of continental polar air, but in passing over warmer water it becomes unstable with a higher moisture content.

Maritime tropical air (abbreviated mT)

Also called tropical air. An air mass with characteristics developed over low latitudes. Maritime tropical air (mT), the principal type, is produced over the tropical and subtropical seas; very warm and humid.

Maximum wind axis

On a constant pressure chart, a line denoting the axis of maximum wind speeds at that constant pressure surface.

Mean sea level

The average height of the surface of the sea for all stages of tide; used as reference for elevations throughout the U.S.

Measured ceiling

A ceiling classification applied when the ceiling value has been determined by instruments or the known heights of unobscured portions of objects, other than natural landmarks.

Mercurial barometer

A barometer in which pressure is determined by balancing air pressure against the weight of a column of mercury in an evacuated glass tube.

Meteorological visibility

In U.S. observing practice, a main category of visibility which includes the subcategories of prevailing visibility and runway visibility. Meteorological visibility is a measure of horizontal visibility near the earth’s surface, based on sighting of objects in the daytime or unfocused lights of moderate intensity at night. Compare slant visibility, runway visual range, vertical visibility. Examples are surface visibility, tower visibility, and sector visibility.

Meteorology

The science of the atmosphere.

Microbarograph

An aneroid barograph designed to record atmospheric pressure changes of very small magnitudes.

Millibar (abbreviated mb.)

An internationally used unit of pressure equal to 1,000 dynes per square centimeter. It is convenient for reporting atmospheric pressure.

Mist

A popular expression for drizzle or heavy fog.

Mixing ratio

The ratio by weight of the amount of water vapor in a volume of air to the amount of dry air; usually expressed as grams per kilogram (g/kg).

Moist-adiabatic lapse rate

Saturated-adiabatic lapse rate—The rate of decrease of temperature with height as saturated air is lifted with no gain or loss of heat from outside sources; varies with temperature, being greatest at low temperatures.

Moisture

An all-inclusive term denoting water in any or all of its three states.

Monsoon

A wind that in summer blows from sea to a continental interior, bringing copious rain, and in winter blows from the interior to the sea, resulting in sustained dry weather.