Global Weather Patterns

AZ State Standards Concept 2: Energy in the Earth System (Both Internal and External) Understand the relationships between the Earth s land masses, oceans, and atmosphere. PO 2. Explain the mechanisms of heat transfer (convection, conduction, radiation) among the atmosphere, land masses, and oceans. PO 11. Describe the origin, life cycle, and behavior of weather systems (i.e., air mass, front, high and low systems, pressure gradients). PO 15. List the factors that determine climate (e.g., altitude, latitude, water bodies, precipitation, prevailing winds, topography).

Content Objectives SWBAT to describe the mechanisms by which water condenses to form clouds and how that affects precipitation patterns worldwide. SWBAT explain the rainshadow effect and predict locations on a map of where it would occur based on prevailing winds. SWBAT explain how monsoons function and describe the locations on Earth where they occur.

Convection of Water Vapor Warm air can hold more H 2 O vapor than cold air. Warm moist air rises and then cools. Convective plume

Water droplets collide, grow larger, and eventually become too heavy to be lifted by the convective flow any longer

Convection of Water Vapor Warm air holds more water vapor than cold air Warm moist air rises and then cools. The water vapor begins to condense into clouds. Rain occurs when the drops become too heavy to be held aloft by the convective currents.

Rule of Thumb High Pressure Means Dry Weather Low Pressure Means Wet Weather Dry Wet

World Precipitation Map

The Four Ways of Lifting Convection Topographical (Orographic) Lifting Warm air mass Frontal Lifting Cold air mass Convergence All clouds and storms are caused by the rising and cooling of air and the condensation of water vapor into clouds.

Primarily a phenomenon of hot seasons and locations Convection

Occurs year-round in mountainous regions Topographical Lifting

Primarily a spring/fall event when air of different temperatures meet Warm air mass Frontal Lifting Cold air mass

Convergence At 60 N or S or in specific locations

The Rainshadow Effect; A Special Case of Orographic Lifting

Rainshadow Effect 1) When warm moist air arrives from the ocean and begins to rise over land features, it will cool and the water vapor will condense. This is topographic lifting. 2) This forms clouds and eventually rain on the windward side of the mountains. The windward sides of the mountains often have lush forests. Temperate Rainforests in the PNW average 100-200 of rain per year! Ocean

Rainshadow Effect 1) When warm moist air arrives from the ocean and begins to rise over land features, it will cool and the water vapor will condense. This is topographic lifting. 2) As air descends into the inter-mountain trough, temperatures warm and the air gets drier. Rainfall in Seattle is only 40 per year or less 3) That is actually less than any city east of the Mississippi River! Ocean Drier inter-mountain trough

Rainshadow Effect 1) As the air rises again on the next range, topographic lifting causes air to cool, condensation into clouds, and rain to fall again on the windward side. 2) At high enough elevations it falls as snow. Up to 600-800 can fall in the Cascade Mountains per winter. Ocean Drier inter-mountain trough

1) As the clouds descend down the leeward slopes, the air warms again and the water droplets evaporate into the air. 2) The air warming and drying as it sinks is called Adiabatic Heating 3) This results in less precipitation and cloud cover. 4) That creates a Rainshadow Desert. Rainshadow Effect Rainshadow Desert Ocean Drier inter-mountain trough

Rainshadow Effect

Rainshadow Effect Ocean Drier intermountain trough Rainshadow Desert

Precipitation Map of Washington

Rainshadow Effect in Hawaii 300 20

150 of rain and 15 of rain, only 15 miles apart!

Do We Have A Rainshadow Here? 5,000 feet elevation south of Flagstaff Ever notice that it is much greener driving south to Phoenix and than driving north to Page?

Do We Have A Rainshadow Here? Ever notice that it is much greener driving south to Phoenix and than driving north to Page? 5,000 feet elevation north of Flagstaff

Monsoons come up from the south

Continental Versus Marine Climates Land heats up faster and hotter than water does. That is because rock has a lower specific heat capacity than water. Large land masses are hotter in summer and colder in winter than those near the coast.

Monsoon Seasons Monsoons are typically associated with summer rains. But, technically it refers to a seasonal changing of the wind direction associated with heating of the continental land masses.

Monsoon Flow Cold air sinks and then spreads out when it hits the ground. This high pressure prevents moisture from the ocean from coming onto land. Winter is typically dry for continental climates

Monsoon Flow Warm air rises. This creates low pressure which sucks moisture in from the ocean. Summers are typically wet for continental climates

Monsoon Seasons Winters on continents tend to be dry and cold Summers on continents tend to be hot and wet

Let s Examine the Normal Climate Patterns of North America First important distinction: Weather and Climate are different terms Weather refers to what is happening locally on a daily basis Climate refers to the normal patterns of temperature and precipitation over a large area on a seasonal basis over many years.

For Instance The average high is 86F in June in Flagstaff and June is the driest month of the year on average. Today it was only 74F and it rained 1. Does this disprove June is typically dry in this region?

Another Example Death Valley is the hottest and driest location on the continent. Yet, during my first two visits, I was rained on both times Your personal experience does not always jive with reality!

winter summer

World Precipitation Map

World Vegetation Pattern

What Triggers the Monsoons? When heat in Sonoran Desert is strong enough to overcome downdrafts from 30N to suck moisture in from Gulf of Mexico

World Precipitation Map

World Photosynthesis Patterns