Geography 12: Weather and Air Masses Weather An Introduction to Air Masses.

Geography 12: Weather and Air Masses

WeatherAn Introduction to Air Masses


What We Measure1. Temperature• Thermometer - device for measuring heat• Mean Temp. - average temp.,• Diurnal (daily) Temp. = max + min / 2 = mean daily temp.• Mean Monthly = mean daily temp / # of days in month• Mean Annual = sum of all mean monthlies / 12• Isotherm– “equal heat”– line drawn to join places of the same temp. - Temp. Map


2. Pressure• Barometer - device for measuring the weight exerted by a column

of air• Isobar - on a weather map, lines joining regions of equal P

– generally drawn at 4 mb (0.4 kPa) intervals• Air Pressure

– Sea level [= 1 atm] = 1013 hPa (hecto Pascals) =101.3 kPa (kilo Pascals)

– 1000 mb (millibars) (10 mb = 1kPa)• Weather map convention is to use only the last 3 figures in a

measurement, e.g.,– 1004.6 mb reads as 046 mb on a weather map– 994.7 mb reads as 947 mb– 99.47 kPa reads as 947 kPa


• Low Pressure– Weather: cloudy, wet, moderate (warm/cool) e.g., by

degrees of intensity: Depressions, Cyclones, Tropical Cyclones (hurricanes/typhoons), Mid-latitude Cyclones (Tornadoes)

– with air spiraling counter-clockwise about the central low• High Pressure– Weather: clear, sunny, dry, extreme (hot/cold)• e.g., Anticyclone

– with air spirally clockwise about the central high


3. Wind (& Pressure)– Anemometer - device for measuring wind speed– air flows from high P to low P = wind• the steeper the gradient, the greater the wind speed

– Wind Speeds• Beaufort scale– a numerical wind-force scale ranging from:» 0 (calm, winds less than 1.6 km/h) to» 12 (hurricane, winds in excess of 120 km/h)» [actually modified to 17- rare and catastrophic

winds up to 220 km/h]


Airmasses• Airmass - a large volume

of air whose temp and humidity are relatively uniform, and covers an extensive area

• Types:– Continental (c) - dry

(extremes of temp.)– Maritime (m) - humid

(moderate temp.)– Arctic (A) – cold– Polar (P) - cool– Tropical (T) - warm

m = over water = humid (maritime)c = over land = dry (continental)A = arctic (cold, dry) / [P = polar (cold, moist)]T = tropical (warm)


Front• a zone of transition between two airmasses

– life cycle: embryo, maturity, and decay…


Warm Front– wind from SE– overcast and foggy– long drizzle (expected)– gradual drop in P

Associated Weather


Warm Sector / Col– air temp. rising, and humid– P continues to drop– wind shifts to SW (wind direction always described FROM)– cloud thins and sunny breaks

Cold Front– rapid change in weather– sudden drop in temp.– wind from NW (therefore = cold), and strong because of P

gradient– clouds: cumulo-nimbus common– short, heavy rainfall, possible thunderstorms (summer)– sky soon clears, but it is cold for the next few days– P begins to rise


Eventually an Occluded Front forms– a cold air mass overtakes the slower warm air mass, and

completely undercuts it


Anticyclones & Cyclones1. Anticyclone– a region of descending

(clockwise [counter-clockwise in Southern hemisphere]) air forming a high P system

– Weather: clear, sunny, dry, cold

2. Cyclone– a region of ascending

(counter-clockwise [clockwise in Southern hemisphere]) air forming a low P system

– Weather: cloudy, wet, warm

Geography 12: Weather and Air Masses Weather An Introduction to Air Masses.

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