Weather Patterns. 1. Planetary Wind Belts. 2. Air Masses and Fronts. Planetary Wind Belts (1) Paula Messina

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1 Weather Patterns Paula Messina 1. Planetary Wind Belts 2. Air Masses and Fronts Planetary Wind Belts (1) Maximum Equator Hot air rises Minimum Poles Cold air sinks Effect: a general circulation from Equator to Poles 1

2 Planetary Wind Belts (2) Where air rises, it expands, it cools down, it produces clouds The Equator, therefore, is a zone of LOW PRESSURE (Low pressure is associated with clouds/rain, high temperatures) Planetary Wind Belts (3) Where air sinks, it becomes compressed. Any liquid water in it evaporates, causing the air to be very dry. The Poles, therefore, are zones of HIGH PRESSURE (High pressure is associated with cold, dry air) Planetary Wind Belts (4) 90 o N Air rising at the Equator also sinks at latitudes of 30 o N and 30 o S. Therefore, in addition to the Poles, there are high pressure zones of sinking air found at 30 o N and 30 o S. 90 o S 30 o N 30 o S 2

3 Planetary Wind Belts (5) Air rises also at latitudes of of 60oN and 60oS. Therefore, in addition to the Equator, there are low pressure zones of rising air found at 60oN and 60oS. Planetary Winds (6) n 60oN 0o 60oS 90oN So, Earth is divided into zones, each 30o of latitude thick, of alternating high and low pressure 60oN 30oN 0o 30oS 60oS 90oS Planetary Winds (7) n 90oN Remember that air moves from areas of high pressure to areas of low pressure! 60oN 30oN 0o 30oS 60oS 90oS 3

4 Planetary Winds (8) Remember the Coriolis Effect! Air is deflected to the right in the Northern Hemispehere, and is deflected to the left in the Southern Hemisphere! n n 90oN 60oN 30oN 0o 30oS 60oS 90oS Planetary Wind Belts (9) Planetary Wind Belts Polar easterlies Prevailing westerlies Tradewinds Zones of Convergence (and Low Pressure) 0o, 60o N, 60o S Zones of Divergence (and High Pressure) 30o N, 90o N, 30o S, 90o S Planetary Wind Belts (10) n So, in the US, weather patterns migrate across the country generally from southwest to northeast H H H 1004 mb 1008 mb 1012 mb 1016 mb 1020 mb 4

5 Part 2: Air Masses and Fronts What is an Air Mass? An air mass is a large quantity of air that has the temperature & humidity characteristics of the area over which it formed. Cool and moist Cool and dry Warm and dry Warm and moist Air Mass Nomenclature =maritime Polar =continental Tropical =continental Polar mt=maritime Tropical mt 5

6 Air Mass Migrations Air masses are driven by planetary winds Air masses may travel at slightly different speeds, and in slightly different directions mt Fronts When air masses collide, a front forms. Cold Fronts (1) When a cold air mass bumps into a warm air mass, a cold front forms at the boundary. In cross-section, it looks like this: Cold front 6

7 Cold Fronts (2) Cold air wedges its way under warm air (because cold air is denser) This causes the warm air to rise Cold front Cold Fronts (3) When air rises, it expands When air expands, it cools down When it cools to its dewpoint, clouds form Cold front Cold Fronts (4) Cold fronts are associated with thick ( cumuloform ) clouds, forming from rapidly rising air Cold fronts produce heavy precipitation for short periods of time Lightning is possible Cold front 7

8 Warm Fronts (1) When a warm air mass bumps into a cold air mass, a warm front forms at the boundary. Warm Fronts (2) In cross section, this is what a warm front looks like: Warm front Warm Fronts (3) Since the cold air mass is denser, it stays close to the ground. The warm air mass gently slides over the cold air mass So air is rising, but not significantly, as in the case of a cold front Warm front 8

9 Warm Fronts (4) The clouds that form are relatively flat ( stratiform ) clouds Stratiform clouds are associated with light rain or drizzle, for long periods of time. Warm front Cold Fronts vs. Warm Fronts Cold fronts tend to move rapidly, so heavy precipitation or showers last for short bursts of time Warm fronts move more slowly and their boundaries span more miles so light rain may persist for several days. Occluded Fronts (1) A warm air mass may get sandwiched between two cold air masses 9

10 Occluded Fronts (2) Two cold air masses force a warm air mass totally off the ground Occluded front Occluded Fronts (3) Clouds and precipitation are similar to those of a warm front, and a cold front in succession Occluded front Stationary Front A stationary front may be any type of front (cold, warm, or occluded) where the air masses stop moving Since the front does not move, precipitation may persist for a long period of time. 10

11 Weather Map Symbology (1) Cold front (west); warm front (east) Weather Map Symbology (2) Occluded front (west); stationary front (east) 11