Air mass: an extremely large body of air (thousands of square kilometers) whose properties of temperature and moisture are fairly similar in any horizontal direction at any given altitude.

The properties of an air mass are determined by the type of surface over which it develops – known as the source region. The source region should be generally flat and of uniform composition, with light surface winds; such as ice- and snow-covered arctic plains in winter and subtropical oceans and desert regions in summer. Air masses are characterized by TEMPERATURE and MOISTURE Photo courtesy of NOAA To have a uniform air mass in temperature and moisture, the air mass must stay in its source region for several days or weeks.

Air Mass Temperature Classifications: Temperature of an air mass is classified by the general characteristics of its source region A = Arctic P = Polar T = Tropical

Airmass moisture characteristics: m = maritime (ocean) surfaces c = continental (land) surfaces The combination of temperature and moisture gives us five basic air mass types that affect Europe

A cP mP mTcT Air masses that affect weather in Europe

Continental tropical, cT

Continental polar, cP

Maritime tropical, mT

Maritime polar, mP

Arctic

Outside of Europe there are a couple of other types of air masses that affect regional weather. In terms of temperature, there are two other types of air masses that are not seen Europe: Equatorial (E) Antarctic (AA) We still use the same designations for humidity Maritime - forming over ocean surfaces- (m), which create relatively humid air masses Continental – forming over land surfaces - (c), which create relatively dry air masses

Arctic (A, or cA) Extremely low temperatures and very little moisture (often don’t bother with the “c” designation since these are so cold they are always “dry”) usually originate north of the Arctic Circle, where days of 24 hour darkness allow the air to cool to sometimes record-breaking low temperatures form over ice- and snow-covered regions of Siberia, the Arctic Basin, Greenland, and northern North America.. these air masses often plunge south across Europe during winter, but very rarely form during the summer because the sun warms the Arctic. Photo courtesy of NOAA

Arctic air masses move about as a shallow area of high pressure, commonly known as an "Arctic High". Clear skies, extremely low humidity, and very high atmospheric pressure prevail. Northerly surface winds transport the colder air southward. Even though the Alps provide a significant topographical barrier, arctic air masses entering Europe can still move all the way to North Africa. An arctic outbreak that spreads southward can bring record-breaking cold temperatures to the continent. Austria’s record low: -36.6C, 11 Feb 1929 at Zwettl ( source: wortrivfeb.html ) wortrivfeb.html Arctic air masses are responsible for bringing below zero Fahrenheit (below -18C) weather to every state in the U.S., including Florida.

Continental Polar (cP)  very cold, having developed over sub-polar regions (not as cold as Arctic air masses)  very dry, due to cold and developing over land  form farther south in the subpolar Siberia and eastern continental Europe  common across continent during winter  do form in summer, but mostly only in interior Siberia and Russia  typically bring clear and pleasant weather during the summer

Maritime Polar (mP)  very cool and moist  typically bring cloudy, damp weather  form over northern Pacific and northern Atlantic Oceans  most often influence the British Isles and Scandinavia.  Maritime polar air masses can form any time of the year and are usually not as cold as continental polar air masses.

Maritime Tropical (mT) very warm and very humid – develop in tropical and sub- tropical latitudes originate over warm waters of southern north Atlantic Ocean can form year around, but are most common in summer important source of moisture feeding storms all year round responsible for hot, humid days of summer across southern Europe and much of the south and eastern half of North America

Continental Tropical (cT)  very warm; develop in lower sub-tropical latitudes  very dry because of formation over land  form over the deserts of North Africa and southwest Asia  bring heat to southern Europe during summer  as air mass moves eastward, moisture is evaporated into it, making it more mT  these air masses rarely form in winter Austria’s highest temperature: 39.7C, on 27 July 1893 in Dellach im Drautal

 boundary between cT and mT is often called the dryline, and is a favored location for thunderstorm initiation

Air moving off an elevated mountain plateau is “well- mixed”: (a) its potential temperature, θ, is nearly constant with height [so its stability borders between neutral and absolutely unstable], and (b) its mixing ratio is also nearly constant with height This dry, warm, well-mixed layer moves out over a surface layer of warm and very moist air. At the interface is a temperature inversion. Moist, conditionally unstable air from this surface layer must break through the inversion and rise through the EML Typical dryline thunderstorm setup

Typical “synoptic” setup for a US, southern plains dryline Rocky mountains (4500 m elevation) stretch from central Colorado, through New Mexico, and into Mexico Gulf of Mexico provides mT air with high (> 20 g/kg) mixing ratios Surface low (cyclone) forms on lee side of mountains, and circulation around this cyclone brings dry cT air into contact with moist mT air. Interface can be violent!

Case study: 29 May 2004 DDC sounding location

Dryline over eastern India

Mean position of mesoscale features during Indian / Bangladeshi severe events

Air masses do not remain in their source region indefinitely but move from place to place changing their temperature, humidity, and/or stability. The modification in air mass occurs primarily by i)exchanging heat or moisture, or both, with the surface over which the air mass travels ii)radiative cooling, and iii)processes associated with large-scale vertical motion. Photo courtesy of NOAA