Weather An Introduction to Air Masses

1. WeatherAn Introduction to Air Masses

2. 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

3. 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

4. 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

6. 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]

7. 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)

9. 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

10. Associated Weather Warm Front • wind from SE • overcast and foggy • long drizzle (expected) • gradual drop in P

11. 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

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

14. 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