Atmospheric Circulation I. Composition of the Atmosphere a) The lower atmosphere is a nearly homogenous mixture of gases i) Nitrogen- 78% ii) Oxygen- 21 % iii) Argon- 0.934% iv) Carbon Dioxide- 0.0333% b) Air is never completely dry i) Water vapor can occupy as much as 4% of its volume. (1) Mostly it’s invisible, but can also be clouds or fog (2) The residence time, the amount of time something stays in a certain location, of water vapor in the lower atmosphere is about 10 days (a) Water vapor leaves the atmosphere by condensing c) Air Density i) The temperature and water content of air greatly influence its density (1) Temperature (a) Warmer air is less dense (b) Colder air is more dense (2) Humidity- water vapor is lighter than air, therefore (a) More humid, less dense (b) Less humid, more dense ii) Adiabatic cooling or heating (1) Air becomes cooler when it expands (2) Air heats up when it is compressed. d) Warm air can hold more water vapor than cold air i) Water vapor in rising, expanding, cooling air will often condense into clouds and if it continues long enough, it may form raindrops or snowflakes, called precipitation, liquid or sold water that falls from the air to the Earth e) Atmospheric Circulation i) Powered by sunlight (1) The heat input and outflow “account” for Earth can be thought of as a heat budget. (a) Over long periods of time, the input must equal the output and therefore Earth is in thermal equilibrium. (2) Uneven Solar Heating and Latitude (a) Heat input varies with latitude (i) Polar Regions 1. Input at high latitudes is more diffuse, and spreads out over a greater area 2. Receive no sunlight during the winter (ii) Tropics 1. In low latitudes, the high solar angle distributes the same amount of sunlight over a much smaller area and is therefore more intense (iii)Mid Latitudes 1. Receive more heat in summer than in winter(b) Tropics receive more heat than they radiate into space, Polar regions lose more than they gain (i) Water is moving huge amounts of heat between tropics and poles, thus equalizing the polar tropical heat imbalance. 1. Masses of moving air, with lots of water vapor, move about 2/3 of the poleward transfer of heat 2. Ocean currents move the other third (c) Seasons (i) At mid-latitudes The Northern Hemisphere receives about three times as much solar energy per day in June as it does in December. 1. Due to the tilt of the Earth’s axis (ii) Days are longer in summer than in winter (d) Uneven Solar Heating and Atmospheric Circulation (i) Convection currents are caused by the differences in temperature between two places 1. On the earth’s surface, air heats up at the equator, rises and then moves towards the poles where it sinks. 2. This is the ideal, and is not the reality 3. Because the earth is spinning, it complicates things. (e) The Coriolis Effect (i) Any object moving freely across the globe appears to curve slightly from it initial path. In the N. Hemisphere it curves to the right, in the S. Hemisphere it is to the left. 1. The observed deflection is caused by the observer’s moving frame of reference on the spinning earth 2. Every point on the earth’s surface moves eastward at 15°/hour, but points closer to the equator move faster than those closer toward the poles 3. this plays a big role in the movements of air and water along the earth’s surface. (f) Coriolis Effect and Atmospheric Circulation Cells (i) As air rises at the equator it loses moisture by precipitation caused by expansion and cooling 1. This drier air now grows denser and radiates heat and cools a. When it’s traveled to about 30° N or S it becomes dense enough to sink back to earth b. There it causes areas of high, dry pressure and forms deserts c. It moves across the surface of the earth as the Northeasterly Trade Winds in the N. Hemisphere and the S. Easterly Trade Winds in the S. Hemisphere. d. These large atmospheric circulation cells are known as Hadley Cells. (ii) In the mid latitudes, some of the descending air at 30° turns poleward 1. A loop of air forms between 30° and 50°-60°.2. They get deflected to the right and form the Westerlies. (iii)At the poles, cold air blows toward the equator at the surface, turning west as it does so 1. This causes the polar cells. (g) Wind Patterns (i) Calm equatorial areas are known as the doldrums because there isn’t a lot of wind where the Hadley Cells converge. (ii) 30°N and S are called the subtropical highs because air return to earth creates high pressure. 1. Also called the horse latitudes. (h) Monsoons are patterns of wind circulation that change with the season (i) Monsoons are linked to the different specific heats of land and water and to the annual north-south movement of the ITCZ, InterTropical Convergence Zone. 1. EX: In Asia, the landmass draws vast quantities of warm, moist air from the Indian Ocean. a. Southerly winds drive moisture toward Asia where it rises and condenses to produce a months-long deluge (i) Sea breezes and Land Breezes (i) Sea breeze: Cooler air over the sea moves toward land as the land heats up (ii) Land breeze: At night, cooler air on land is pushed offshore as warm air from the ocean rises. (3) Tropical Cyclones (a) Tropical cyclones are great masses of warm, humid, rotating air. (i) Large tropical cyclones are called hurricanes in the N. Atlantic and E. Pacificand typhoons in the western pacific (ii) To be a hurricane or typhoon, wind speeds must be in excess of 119 km/hr (74 mph) (b) Generated within one warm, humid air mass (c) When air containing a tropical depression is heated over tropical water with temps >26°C (79°F), circular winds begin to blow in the vicinity of the wave, and some of the warm, humid air is forced upward. Condensation begins and the storm takes shape. (d) Strength comes from the heat energy that is released when water vapor condenses as liquid. (i) The cycle of evaporation and condensation carries the energy that generates wind. (e) Tons of energy (i) Rain fall can be 1 inch/hour or 20 billion metric tons/day (ii) One day releases 2.4 trillion kWh, the energy needs for the U.S. for 1 year (f) Tons of damage (i) Wind (ii) Rain (iii)Storm