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

Geography 12: Weather and Air Masses What We Measure 1. 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

Geography 12: Weather and Air Masses 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., – mb reads as 046 mb on a weather map – mb reads as 947 mb – kPa reads as 947 kPa

Geography 12: Weather and Air Masses 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

Geography 12: Weather and Air Masses

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]

Geography 12: Weather and Air Masses 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)

Geography 12: Weather and Air Masses

Front a zone of transition between two airmasses – life cycle: embryo, maturity, and decay… Cold Front 1. Steep gradient 2. Forces warm up along 3. Cumulo-nimbus clouds 4. Heavy rain of short duration 5. Cold Wind 6. Low Temp. Warm Sector 1. aka Col (=saddle) 2. Warmer air 3. Thin cloud or clear 4. “Good” weather Warm Front 1. Gentler gradient 2. Warm air rises 3. Nimbo-stratus (ppt) 4. Long duration of fog, mist or drizzle 5. Cirrus clouds precede

Geography 12: Weather and Air Masses Warm Front – wind from SE – overcast and foggy – long drizzle (expected) – gradual drop in P Associated Weather

Geography 12: Weather and Air Masses 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

Geography 12: Weather and Air Masses

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

Geography 12: Weather and Air Masses Anticyclones & Cyclones 1. 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