4 North American Winter Weather Semi-permanent highs redirect North American winds, such as cold interior southerly flow from the Canadian high.The Polar front develops a wave like pattern as air flows around lows.Figure 11.4
5 Global Precipitation Patterns Global low pressure zones around the equator and 60° latitude generate convergence at the surface, rising air and cloud formation.Zones of high pressure at 30° and the Poles experience convergence aloft with sinking, drying air.Figure 11.5
6 Example of these effects: Monsoon Wind systems that exhibit a pronounced seasonal reversal in direction.Best known monsoon is found in India and southeast Asia.Winter -- Flow is predominantly off the continent keeping the continent dry.Summer -- Flow is predominantly off the oceans keeping the continent wet.
11 SummaryLarge climate zones (deserts, monsoons, etc) depend on global wind patternAs the Earth tilts with time of year exact position of maximum large scale ascent / descent of air changes
12 HurricanesHurricanes are large, tropical storm systems that form and develop over the warm waters near the equator.They are responsible for weather that can devastate entire communities:Heavy rain -- FloodingStrong WindsVery Large Waves and Storm SurgePossibly Tornadoes
13 Tropical storms In the Atlantic they are called HURRICANES In the Pacific they are TYPHOONSIn other places (Australia, Indian Ocean) they are called CYCLONES
14 HurricanesTo be a hurricane the storm must have a distinctive eye and sustained winds over 74 mph (120 km/h)Hurricanes are typically a few hundred miles across with an eye 15 to 30 miles wideEffects of the hurricane may be felt further away
15 Tropical stormsAs a tropical storm develops it passes through a number of stagesOnce it has sustained winds above 35kts (40 mph) it is classified as a tropical stormAt this point it receives a nameAlthough not all tropical storms develop into hurricanes they are capable of damage from high waves, heavy rain and flooding (and winds)
16 HurricanesTo begin our study of hurricanes we must first look at the typical weather in the tropics.The sun typically remains high in the sky all year -- little variation in temperature with seasons.The strong sun heats the water and enhances the evaporation of the water.The general flow of air near the equator is out of the east -- Trade Winds.
18 HurricanesThe trade winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.A region of convergence (Intertropical Convergence Zone -- ITCZ) creates a band of thunderstorms near the equator.
20 HurricanesSince pressure gradients are small near the equator, we look at the flow of wind to find trough regions.A trough or ripple in the easterly flow is known as a “tropical wave.”Surface convergence occurs on the east side of this wave and surface divergence occurs on the west side.Thunderstorms tend to form on the east side of the tropical wave.
22 Conditions for Tropical Wave Development The wave must be north or south of the equator.Coriolis force is zero at the equator.Conditional InstabilityWeak vertical shearThis is different from what we want in the mid-latitudes.Warm sea-surface temperatures (SST)Typically SSTs are greater than 26oC
23 Hurricane Development Convergence at the surface leads to convergence of moisture.The convection or thunderstorms that form release latent heat which intensifies the surface low.The warm core column of air will create an upper level high pressure center.The weak shear allows the storm to remain vertical and allows for the latent heat release to enhance the surface low.