Atmospheric Circulation

Earth’s Atmosphere & Oceans Earth’s atmosphere and oceans are intimately related. Gases from the water influence the climate and gases from the air affect acidity of the water and where life will survive in the ocean.

Weather vs. Climate The difference between weather and climate is that weather is what the atmosphere is doing at a given place and time for instance what is happening in Blue Hill at noon today, while climate is the average of the weather over a long period.

Water Vapor in the Atmosphere Water vapor is the form in which water is found in the atmosphere. It has varied paths of getting into the atmosphere, coming from evaporation from plants, the ground and the oceans and lakes. When it falls out of the air, it returns to Earth in some form of precipitation such as snow, rain or dew.

The Water Cycle The Water Cycle involves water vapor evaporating from the oceans and lakes into the atmosphere where as it rises, it cools and condenses forming clouds. When enough droplets join together, the condensation becomes heavy and falls out of the atmosphere as precipitation. The precipitation runs off through streams to rivers to the ocean, where it then evaporates into the atmosphere. There is one more way that water vapor gets into the atmosphere and that is through evaporation from plants called transpiration.

The Water Cycle

Air’s Mass Air has mass and it is more dense the nearer to Earth it gets because of the mass of the column of air above it. Warm air will occupy more space than the same mass of cold air, making it less dense, so it rises. Also, humid air is less dense than dry air at a given temperature because the water vapor molecules weigh less than the oxygen and nitrogen molecules that they are displacing.

Latitudes and Uneven Heating The Equator is 0o latitude. As you move away from the Equator either north or south, the latitude increases until it reaches 90o latitude at the Poles. The North Pole is 90o North and the South Pole is 90o South. At higher latitudes (like the poles), sunlight has to pass through more atmosphere and spreads out over a larger area than the same amount of sunlight at the Equator.

Latitudes

The Poles vs. the Equator

Sun’s Rays Striking Earth

Sun’s Rays & Atmosphere

Seasons and Uneven Heating Seasons: Due to the 23.5 degree tilt of the rotational axis of the Earth relative to the plane of orbit around the sun, the northern and southern hemispheres are closer or farther away from the sun at different times of the year, causing the seasons.

23.5o Rotational Axis

Seasons

Vernal & Autumnal Equinox

Seasons

Convection Currents

Heat Transport on Earth Through convection currents, the warm air rises at the Equator and moves northward toward the poles where it radiates its heat into the atmosphere, cools and sinks towards Earth’s surface and flows back toward the Equator.

Overall Convection Currents

Coriolis Effect Due to the rotation of the Earth, winds are diverted to the right. Depending on your perspective, if you are looking north it appears that the item moves east and if facing south, it looks as if they are moving west. This also causes low pressure areas to move counterclockwise and high pressure areas to move in a clockwise motion.

Atmospheric Circulation Cells In actuality, the warm air does rise at the Equator but it becomes dense enough to sink back to Earth long before reaching the poles. At about 30o north and south, the air has cooled enough to fall back to the Earth’s surface. These are known as the Hadley cells.

Hadley Cell in Red

Ferrel and Polar Cells Some of the air that descends at 30o north and south moves toward the poles. The air is heated at the surface as it moves northward and is driven by colder air moving from the poles toward the Equator. These mid-latitude cells are called the Ferrel cells. There are small cells that circulate at the poles known as the Polar cells. These are all driven by uneven heating of the air.

Ferrel Cell in Black and Polar Cell in Blue

Wind Patterns Areas where the circulation cells meet, have little or no wind at times. The doldrums is area at the surface where the two Hadley cells come together. This is also known as the Intertropical Convergence Zone or (ITCZ). The horse latitudes are also areas where there is little surface wind. They are found at the intersections of the Hadley and Ferrel cells.

Wind Patterns

Areas of Dependable Surface Winds In between the areas of convergence, are the areas of reliable winds, such as the trade winds and the westerlies. These are the areas ship captains headed for to make forward progress.

Wind Patterns

Monsoons Monsoons are affected by the heating of land and ocean and the seasonal movement of the ITCZ. In the spring the land heats faster than the ocean. Warm air over the land rises while cooler air from the ocean moves in over the land to take its place. The ocean air brings with it humidity and as the humid air is warmed by the land and rises, the water vapor condenses and rain is produced.

Monsoons In the fall, the reverse happens, the land cools more quickly than the ocean. Air above the land sinks as it cools and causes dry surface winds to move toward the ocean. The location and intensity of monsoon season depend on the location of the ITCZ. Monsoon rains are a necessity for the people of Africa and Asia as they depend on them for drinking water and farming.

Seasonal ITCZ Location

Monsoon Circulation

Sea and Land Breezes The sea breeze we feel in the summer and land breeze in the winter are actually mini-monsoons. As the land heats up in the summer more quickly than the ocean, the air above the land rises and air from the ocean fills in creating a nice, cool sea breeze. The opposite forms the land breeze.

Sea Breeze

Sea Breeze

Land Breeze

Land Breeze

El Nino & La Nina The trade winds are responsible for dragging warm surface water across the Pacific Ocean to the Far East. Every 3 to 8 years, the atmospheric circulation changes and the trade winds weaken or reverse. The warm water that has piled up on the western side of the Pacific, spreads eastward along the Equator. El Nino is a warm water current that flows back toward the Central and South America. It changes the weather patterns as well as altering ocean conditions. La Nina is colder than normal events occurring both on land and in the ocean.

El Nino

La Nina

Water Temperature for El Nino & La Nina

Air Masses An air mass is large body of air with uniform humidity and temperature. Air moving over land or water takes on the characteristics of the surface below. It will be chilly and dry if it is formed over cold, dry land. The air will be dense and cause higher pressure pushing down, thus it is called “High Pressure.” Air over warm, moist water takes on its temperature and humidity. Warm air is less dense than cooler air and rises forming a “Low Pressure” area.

High and Low Pressure Areas

Fronts Air masses usually have different densities depending on whether they are a high or low pressure area. A front is the boundary between two different air masses (areas of density) in the atmosphere. When there is a large difference in density, humidity and temperature, extreme storms can be spawned. A storm is a regional atmospheric disturbance.

Storms over Joplin, MO

Weather Map of Joplin, MO

Tornados forming near Joplin, MO

Joplin, MO

Fronts

Warm and Cold Fronts

Storms A storm is a regional atmospheric disturbance caused by low pressure areas. Rain and wind are usually associated with these systems. It is the Earth’s method of trying to reaching dynamic equilibrium for the amount of water and heat on the surface of the Earth. Hurricanes bring large amounts of water from the tropics and drop it on the higher latitudes.

Extratropical Cyclones These storms form between the Polar and Ferrel cells in the winter when the density and temperature of the cells have the most difference. This is called a Polar front. Nor’easters (northeasters) are the most violent storms that move up the eastern seaboard of the United States. They get their name from the direction from which the strongest winds blow.

Tropical Cyclones Tropical cyclones usually start near the Equator and are formed in one warm, humid air mass. They start from tropical depressions that increase in intensity to a tropical storm and finally if conditions are right to a hurricane. A hurricane gains energy from the warm water over which it is moving. The parts of the storm causing damages are the wind, rain and storm surge. The storm surge being the most dangerous part.

Tropical Cyclones The names of tropical cyclones are as follows: Eastern Pacific and Atlantic Oceans – Hurricanes Western Pacific Ocean – Typhoons Indian Ocean – Tropical cyclones Australia - Williwillis

Tropical Cyclones The eye of a tropical cyclone is usually 8 to 10 miles across and is an area of calm, sunny skies. The tropical cyclone itself can be up to 1000 miles in diameter and 50,000 feet high. When a tropical cyclone moves over land or cold water, it loses its energy due to friction over land and lack of heat to drive the storm.

Redistributing Energy These tropical cyclone are Mother Nature’s way of moving solar energy from the Equator toward the Poles to equalize the amount of energy on Earth.

A Hurricane

Hurricane Sandy

Storm Surge

Sandy’s Storm Surge

Sandy’s Storm Surge

Sandy’s in NYC

Storm Surge in NYC

The End