Air Mass Weather

Lesson 33/34

What is an Air Mass?

• An air mass is a large volume of air covering many thousands of square miles and having fairly uniform properties.

• The characteristic of the air mass depends upon:a) the source regionb) the surface over which it is travelling

What is an Air Mass?

• The main source regions are the stable high pressure belts:– Sub-tropical– Polar

Air Mass Modification

• As air masses move away from their source region, they are affected by moisture and heat exchanges from the surface.– They may be heated from below,

• this causes instability.– They may be cooled from below,

• this increases the stability.– A sea track increases the moisture at low level,– A land track will not modify the moisture content to

any great degree.

Air Masses Affecting the UK

Source Regions of Air Masses

• Worlds stable high pressure regions.– Classified chiefly on the basis of two primary factors:

• Temperature» Polar» Arctic» Tropical

• Surface of origin:» Maritime» Continental

Cold Air-masses

• Polar Maritime– Between Greenland and N. Canada

• Arctic Maritime– From above the Arctic Circle

• Polar Continental– From the Siberian High over central Asia.

Warm Air-masses

• Tropical Maritime– From the Azores High in the mid Atlantic Ocean.

• Tropical Continental– from the North African sub-tropical high pressure

belt.

Summary of Air Masses

• Summer & Winter Air Masses:– Tropical Maritime – Polar Maritime– Returning Polar Maritime

Summary of Air Masses

• Winter Only– Polar Continental

• Short sea track.• Long sea track.

– Arctic Maritime• Summer Only

– Tropical Continental

Air Mass Properties

• Characteristic properties are:– Uniform mean temperature– Moisture content– ELR– Stability– Weather– Visibility

Air Mass Characteristics

• Tropical air cooled from below

• Polar air heated from below

Tropical Continental

L

H

Tc Air Mass PropertiesWinter Summer (only)

Temperature Very warm

Humidity Relatively dry.

Change of lapserate

Little change

Stability Generally stable

Weather Little cloud/Fair WxCu/ Poss CB if seatrack.

Visibility Mod to poor. Thickhaze.

Polar Continental (Short Sea

Track)

L

H

Pc (Short Sea Track) Air Mass PropertiesWinter(Only) Summer

Temperature Very cold

Humidity Vry dry

Change of lapserate

Little change

Stability Stable

Weather Clear with frost/fogor St

Visibility Poor

Very dry

Polar Continental (Long Sea

Track)

Pc(Long Sea Track)Air Mass Properties

Winter (only) Summer

Temperature Cold

Humidity Moist in lowerlayers

Change of lapserate

Heated from below

Stability Unstable

Weather Rain or SNSH, thickSC, embd Cu.

Visibility Good but poor inSNSH.

Tropical Maritime

Tm Air Mass PropertiesWinter Summer

Temperature Near seatemperature

Warm

Humidity Very moist Very moist

Change of lapserate

Cooled from below Cooled from below

Stability Stable Stable aloft

Weather Low St and DZpenetrates wellinland

Broken Sc/DZRA oncoasts

Visibility Mod topoor/Advectionfog/Cloud coveringhills

Mod to poor/seafog/cloud coveringhiils

Polar Maritime

Pm Air Mass PropertiesWinter Summer

Temperature Rather cold Cool

Humidity Moist Moist

Change of lapserate

Heated from below Heated from below

Stability Unstable Unstable

Weather Cu/CB, showersmainly on W coast

Cu/CB, showerspenetrating wellinland.

Visibility Good Good

Arctic Maritime

Am Air Mass Properties

Winter (only) Summer

Temperature Colder than Pm.

Humidity Fairly moist aftersea track.

Change of lapserate

Heated from below.

Stability Very unstable.

Weather Heavy snow SNSHfrom freq. CB.

Visibility Very good outsideshowers.

Returning Polar Maritime

rPm Air Mass PropertiesWinter Summer

Temperature Warmer than Pm. Cool

Humidity Moist Moist

Change of lapserate

Lowest layerscooled from below.

Cooled from below

Stability Stable in lowerlayers, unstablealoft.

Stable in lowerlayers, unstable aloft.

Weather St, Sc, Ac, possiblelight rain.

Turreted Ac, prob ofshowers and Ts overland.

Visibility Moderate Moderate