Chapter 14: Atmospheric Motion

Chinook winds

"Chinook" originated in the Pacific Northwest and can refer to several types of winds. The most common usage is for a warm, dry wind that blows down the eastern, leeward flanks of a mountain range. These winds are so warm and dry they are called "snow eaters," for the way in which they can cause a sudden melting of snow and ice on the ground. The onset of a Chinook wind can cause a sudden rise in temperatures, especially during the winter. Pacific Northwest - Cascade, Rockies, Sierra Nevada, Montana, Black Hills (ND).

How do variations in pressure cause air to move?

Air moves because there are variations in air pressures, in density of the air, or in both. Such pressure and density variations are mostly caused by differential temperatures of the air (due to differences in solar heating) or by air currents that converge or diverge

Why the Coriolis Effect occurs

Air, like the surface, is being carried around Earth by rotation. The surface has a faster velocity near the equator than at the poles because it has to travel a greater distance in 24 hours. Therefore, as air moves toward the poles, it is rotating faster toward the east than the land over which it moves. It appears from the surface to be deflected to the east. The opposite occurs as air moves toward the equator and encounters areas with a faster surface velocity. The air appears to lag behind, deflecting to the west as if it were being left behind by Earth's rotation.

Seasonal variations in the position of the Intertropical Convergence Zone

As the overhead Sun shifts north and south within the tropics from season to season, the ITCZ shifts, too. In the northern summer, it shifts to the north. The typical June position of the ITCZ is the red line on the figure below, and the December position is the blue line. The ITCZ generally extends poleward over large landmasses in the hemisphere that is experiencing summer. This larger shift over the land than over the oceans is because of the more intense heating of land surfaces. Unlike the ITCZ, the subtropical high pressure doesn't exist in a continuous belt around earth. the ocean-covered surfaces support high pressure better than land surfaces because land heats up too much at these latitudes, especially in summer. The heated air over the land rises, counteracting the tendency for sinking air in the Hadley cell. So the subtropical high pressure tends to be more vigorous over the oceans.

Temperature-volume relationship

At constant pressure, increase in temperature increases volume; directly proportional

Pressure-volume relationship

At constant temperature, volume is inversely proportional to pressure

Monsoon

COMMON MISCONCEPTION: refers to a type of rainfall actually refers to winds that reverse directions depending on the season. One of these seasonal wind directions typically brings dry conditions and the other brings wet conditions.

Santa Ana winds

Dry, hot air from NE (Mojave Desert). CA and NV get much hotter and drier than normal. This happens when high pressure conditions prevail over the desert; wind blows toward the ocean. Causes wildfires. Coastal cities commonly affected: LA, San Diego

Northern Hemisphere low pressure pattern influence

Earth does rotate, so the Coriolis effect deflects the inward-flowing air associated with a low-pressure area. This deflection causes an inward-spiraling rotation pattern called a cyclone. In the Northern Hemisphere, the Coriolis deflection is to the right of its intended path, causing a counterclockwise rotation of air around the low-pressure zone. This counterclockwise rotation is evident in Northern Hemisphere storms.

Coriolis effect

Earth is a spinning globe, with the equatorial region having a higher spring velocity than polar regions. As a result, air moving north or south has an apparent deflection sideways caused by the rotation.

Jet streams

Fast-moving relatively narrow currents of wind flow aloft along the boundaries of the mid-latitude air currents. they all blow from west to east as a result of Earth's rotation. A

flow cells

Flows of air combine into huge, tube-shaped cells of circulating winds

Southern Hemisphere high pressure pattern influence

For a high in the Southern Hemisphere, the air flowing outward from the high is deflected to the left by the Coriolis effect. This causes the air within an anticyclone in the Southern Hemisphere to rotate counterclockwise, opposite to the pattern in the Northern Hemisphere. Therefore, cyclones and anticyclones can rotate either clockwise or counterclockwise, depending on hemisphere.

Hadley Cell

Huge cell of convecting air, where high and low pressure area and rising and descending air are linked. One occurs north of the equator, and another just south of the equator. Surface winds flowing from both sides form the ITCZ.

Is there a circulation cell in the mid-latitudes?

If there were a cell of circulating air in the mid-latitudes, as there is in the tropical and polar latitudes, then the sinking from poleward edge of the Hadley cell would cause shear that would also induce sinking at the equatorward edge of the mid-latitudes.

Southwestern U.S. Monsoon

January: Desert Southwest has a less dramatic, but still important monsoon effect. In the winter months, winds blow from various directions, and winter precipitation is from brief incursions of cold, wet air (i.e., cold fronts) from the northwest. July: During the late summer months, heating of the land surface and the resulting low pressure causes a shift in the winds. Winds from the south bring moist air northward from the Gulf of Mexico and Gulf of California, and summer thunderstorms cause precipitation to peak in August, as shown by the graph below for Tempe, Arizona. First, note the different scale needed to show the relatively small amount of precipitation. Also note that nearly as much precipitation falls in the winter from the cold fronts.

West Africa Monsoon

January: near-surface winds in West Africa largely flow from the northeast, bringing in dry air from inland areas, including the Sahara Desert, and carrying it southwest to coastal areas and father offshore. Generally results in dry weather. July: A shift in wind direction in July brings moise ocean air from several directions onto the very hot land where air has risen. This change in wind direction causes enormous differences in precipitation, as shown by the graph below for Dakar, Senegal. Along with the increase in the amount of vegetation. In Dakar and much of the region, precipitation is nearly nonexistent in January and adjacent months.

Northern Australia

January: southern summer, winds over northern Australia bring moist air from the ocean onto the heated land surface. July: wind shifts by July (winter) as the land surface cools, creating higher pressure over the land. This causes a large drop in precipitation, as shown by the graph below for Katherine, Australia. The monsoon flow in January results in plentiful rain.

Indian Monsoon

January: winds define a region where flow is clockwise and outward, centered on the light-colored area of high pressure. this high-pressure area (anticyclone) forms from cold, sinking air over Siberia. This circulation brings very dry air (from the cold interior of the continent) from the north over southern Asia and from the northwest across eastern Asia. We would predict from these wind patterns that little precipitation would occur in much of Asia at this time. July: Circulation that marked the high pressure is gone, replaced by an area of inward and counterclockwise flow over Tibet (north of Koikata). In the Northern Hemisphere, this pattern of circulation is diagnostic of a low-pressure area, which in this case is caused by warm, rising air that accompanies-warming of the Asian landmass. This circulation brings very humid air from the southwest over southern and southeastern Asia.

Siberian High

Large high-pressure zone over Asia

How do variations in insolation cause global patterns of air pressure and circulation?

Locations of low P and high P - Latitudes

Rossby waves

Meandering paths, when viewed in three dimensions, that resemble curving waves. Named after the sceintist who discovered them. Jet streams do not track around the planet along perfectly circular routes, but instead typically follow more irregular curved paths, like along the thick white line in the polar projection below.

Anticyclone

Outward spiraling rotation pattern

Air pressure

Pressure is an expression of the force exerted on an area, usually from all directions. In the case of a gas, pressure is related to the frequency of molecular collisions, as freely moving gas molecules collide with other objects, such as the walls of a container holding the gas.

Air circulation in high latitudes

Surface winds generally encircle the North Pole, blowing in a clockwise direction when viewed from above the pole. This circulation is driven by the Coriolis effect, which is very strong at these latitudes. Cold, dense air sinks near the North Pole. As it nears the surface, it then flows outward, away from the poles (to the south). Cold air flowing away from the South Pole eventually heats up enough to rise, producing a belt of low pressure. The rising air aloft turns south and descends back near the pole, completing the polar cell. The polar cell involves very cold air at such high latitudes, causing the land to largely be covered year-round in nice and snow.

Northern Hemisphere high pressure pattern influence

The Coriolis effect deflects winds to the right, but friction again slows the winds. This results in an outward-spiraling rotation pattern called an anticyclone. In the Northern Hemisphere, circulation around an anticyclone is clockwise around the high-pressure zone.

What is the sense of rotation for the Atlantic hurricanes that approach Florida?

The coriolis effect is so small at these southern latitudes that it fails to impart the necessary rotation to the storms. Once formed, the paths of hurricanes are steered by the global wind patterns, which generally move Atlantic hurricanes west (guided by the trade winds) and then north and east once they enter the latitudes of the westerlies.

What general circulation occurs aloft over the westerlies?

The main direction of surface air in the mid-latitudes is as curving arcs that move poleward away from the subtropics and then bend increasingly to the east, becoming westerlies. Circulation of air is essentially restricted to the troposphere, so the geometry of the tropopause influences the flow of air aloft. The height of the tropopause is largely controlled by temperatures in the underlying troposphere and whether the underlying air rises or sinks. As a result, the tropopause is shallower (8 km or less) over the poles, the where cold, dense air sinks, than over the equator (16 km or more), where air is warmer and rises; it increases in height from the poles across the mid-latitudes. If surface winds in the mid-latitudes and the winds on both sides are moving overall from west to east (in both hemispheres), it is logical that the winds aloft over mid-latitudes will also be westerly. Friction is less aloft than at the surface, so the upper-level westerlies over the mid-latitudes are stronger than westerlies at the surface.

Ferrel cell

The rising motion at the equatorward edge of the polar cell (the subpolar lows) would induce rising next to the poleward edge of the mid-latitudes. This rising motion, coupled with the sinking next to the Hadley cell, would logically set up upper-level circulation over the mid latitudes, with air moving from the pole toward the tropics.

How does sea-level air pressure vary globally

Two belts of high pressure encircle the globe at about 30 degrees north and 30 degrees south (the subtropics). Between these two is a belt of lower pressure straddling the equator, in the tropics. The equatorial low pressure and flanking subtropical high-pressure zones are due to the large aircurrent that rises in the tropics and descends in the subtropics. A set of low-pressure areas occurs near 60 degrees north. The lows are best developed in the oceans, and are poorly developed on land, reflecting differences in how water and land heat up and cool down. A prominent air-pressure feature on this map is a belt of extremely low pressure in the ocean just off Antarctica. This belt is so well developed in the Southern Hemisphere because of the abundant ocean surface, uninterrupted by continental landmasses at this latitude (60 degrees south). An intense high-pressure belt occurs over continental Antarctica, in contrast to the Arctic, which is mostly ocean.

______________ causes lateral movement of air in the atmosphere

Vertical motion

What causes winds?

Wind is movement of air relative to Earth's surface. It forms in response to differences in air pressure from place to place and between different heights above Earth's surface.

pressure gradient

a difference in air pressure between two adjacent areas equalized by a movement of air

subpolar lows

a series of low-pressure areas at the surface caused by south-flowing air eventually begins to heat up and rise, usually somewhere between 60 and 45 degrees latitude

pressure-gradient force

air moves from higher to lower pressure, in the simplest case perpendicular to isobars. this is due to differences in air pressure

polar easterlies

blow away from the North Pole and are deflected toward the west by Earth's rotation. Polar regions receive the least solar heating and are very cold. Surface winds move away from the poles, carrying cold air with them. Flow away from the South Pole and deflect toward the west but are mostly on the back side of the globe

Northeast trade winds

blow from the northeast and were named by sailors, who took advantage of the winds to sail from the Old World to the New World

Southeast trade winds

blow from the southeast toward the equator

Surface air circulation in mid-latitudes

characterized by distinct seasons and changing weather patterns, and from forested areas to deserts. cold surface air flowing away from the poles warms and ascends along the poleward edge of the midlatitudes. along the edge of the mid-latitudes, in the subtropics, air in the descending limb of the Hadley cell causes high pressure. These regions near 30 degrees north and south. In the Southern Hemisphere, air is driven south by high pressure in the subtropics. As this air flows south, it is deflected to the east by the Coriolis effect. The result is again west-to-east flow in the middle latitudes, forming a Southern Hemisphere belt of westerlies.

subtropical jet stream

circles around the globe at about 30 degree latitude in both hemispheres, near the boundary between the Hadley cell and westerlies aloft in the mid-latitudes.

Converging air currents

compress air into a more smaller space, increasing air pressure

The Arctic Ocean is surrounded by ___________________.

continents

Wind direction

conveyed as the direction from which the wind is blowing

Air Circulation in the Tropics

driven by intense solar heating of land and seas near the equator. The heated air rises and spreads out from the equator, setting up huge, recirculating cells of flowing air. The rising air results in a belt of tropical low pressure, and where the air descends back toward the surface is a belt of subtropical high pressure.

ridge

elongated area of high pressure

trough

elongated area of low pressure

Polar front jet stream

encircles the globe near the edge of the polar cell. It shifts position north and south from time to time, but typically resides between 45 and 60 degree latitude. As it shifts farther away from the pole, it brings cold air into the mid-latitudes.

Differences in air pressure.

generated primarily by uneven solar heating, especially as a function of distance from the equator, by temperature differences between different heights in the atmosphere, and by differences in how land and water respond to changes in temperature. Air flows from areas of higher pressure, where air sinks, to areas of lower pressure, where air rises.

Land breeze

gentle, local winds blown from the land out to the sea

Sea breeze

gentle, local winds blown from the sea out to the land

Barometer

instrument used to measure air pressure. Sealed glass tube fixed in liquid mercury. Changes in air pressure cause the liquid level in the tube to rise or fall, allowing the measurement of relative pressure. Such barometers have units of inches (or centimeters) of mercury, but more modern digital instruments record pressure in millibars (mb). 1 bar (1,000 mb) is approximately equal to the average air pressure at sea level. When air pressure is higher, it pushes down harder and forces the mercury higher in the tube. The opposite happens if air pressure is lower.

Cyclone

inward spiraling rotation pattern

Diverging air currents

move air away from an area, causing low pressure

isobars

numbered lines that connect locations with equal pressure (like contours) values are corrected to sea level.

The continent of Antarctica is completely surrounded by ___________________ and there is continuous belt of low pressure around Antarctica.

open ocean

Horse latitudes

regions near 30 degrees north and south, can have weeks without wind, with posed a hazard to early sailing ships and their cargo.

Ideal Gas Law

relates temperature, pressure, and density (mass divided by volume) if we increase a variable on one side of the equation (like increasing pressure), then one or both of the variables on the other side of the equation have to change in the same direction - density or temperature have to also change, or perhaps both do. P = R p T P = pressure R = a constant p = density T = temperature

Intertropical Convergence Zone (ITCZ)

stormy boundary near the equator where northeast and southeast trade winds meet

Icelandic low

strong low-pressure area near Iceland

Aleutian low

strong low-pressure area southwest of Alaska

Southern Hemisphere low pressure pattern influence

the air flowing inward toward a low is deflected to the left by the Coriolis effect. This causes air within a cyclone in the Southern hemisphere to rotate clockwise, opposite to what is observed in the northern Hemisphere. In either hemisphere, friction causes wind patterns within cyclones to be intermediate between straight-inward winds and circular gradient winds.

low pressure pattern influence

the pressure-gradient force pushes air laterally into the low from all directions. If the earth were not rotating and therefore had no Coriolis effect, this simple inward flow pattern would remain intact. Friction would likewise not perturb this pattern because friction does not change the direction of airflow, only the speed.

high pressure pattern influence

the pressure-gradient force pushes air laterally out in all directions. As with a low pressure, this outward flow of air is influenced by the Coriolis effect and by friction near Earth's surface. As air flows outward from the high, it is replaced by air flowing down from higher in the atmosphere.

polar cell

when flow turns back to the north after air rises to its maximum height

Rossby wave trough

where the polar jet stream curves away from the pole.

Rossby wave ridges

where the polar jet stream curves toward the pole. they bend poleward.

westerlies

winds generally blowing from the west. dominate a central belt across the United States and Europe. also occur in the Southern Hemisphere and are locally very strong because this belt is mostly over the oceans and has few continents to disrupt winds.