We know this is the cause of decreasing air pressure with elevation (vertical gradient): Air pressure is a measure of the overlying air mass. (mass = vol/density) However, what causes air pressure changes in the horizontal directions? Why does air pressure change at the surface? REVIEW SLIDES:

The heating and cooling of air columns causes horizontal pressure variations aloft and at the surface. These pressure variations force the air to move from areas of higher pressure toward areas of lower pressure. In conjunction with these horizontal air motions, the air slowly sinks above the surface high and rises above the surface low.

The area shaded gray in the diagram represents a surface of constant pressure. Because of the changes in air density, a surface of constant pressure rises in warm, less- dense air and lowers in cold, more-dense air. These changes in height of a constant pressure (500-mb) surface show up as contour lines on a constant pressure (isobaric) 500-mb map.

(a) The effect of surface friction is to slow down the wind so that, near the ground, the wind crosses the isobars and blows toward lower pressure. (b) This phenomenon at the surface produces an inflow of air around a low and an outflow of air around a high. Aloft, the winds blow parallel to the lines, usually in a wavy west-to-east pattern. Both diagram (a) and (b) are in the Northern Hemisphere.

Winds and air motions associated with surface highs and lows in the Northern Hemisphere.

Atmospheric circulations Chapter 7 Scales of Circulation (micro-, meso-, macro-) Thermal Circulation Seasonal Changes (monsoon) Global Wind and Pressure Systems single-cell and three-cell circulation Inter Tropical Convergence Zone (ITCZ) Polar Front Ocean Currents Regional Climate El Niño/La Niña

Scales of atmospheric motion. The tiny microscale motions constitute a part of the larger mesoscale motions, which, in turn, are part of the much larger macroscale. Notice that as the scale becomes larger, motions observed at the smaller scale are no longer visible.

The scales of atmospheric motion with the phenomenon ’ s average size and life span. (Because the actual size of certain features may vary, some of the features fall into more than one category.)

A thermal circulation produced by the heating and cooling of the atmosphere near the ground. The H ’ s and L ’ s refer to atmospheric pressure. The lines represent surfaces of constant pressure (isobaric surfaces), where 1000 is 1000 millibars.

Development of a sea breeze and a land breeze. (a) At the surface, a sea breeze blows from the water onto the land, whereas (b) the land breeze blows from the land out over the water. Notice that the pressure at the surface changes more rapidly with the sea breeze. This situation indicates a stronger pressure gradient force and higher winds with a sea breeze.

Diagram (a) shows the general circulation of air on the side of the earth facing the sun on a nonrotating earth uniformly covered with water and with the sun directly above the equator. (Vertical air motions are highly exaggerated in the vertical.) Diagram (b) shows the names that apply to the different regions of the world and their approximate latitudes.

The idealized wind and surface- pressure distribution over a uniformly water- covered rotating earth.

Average position of the polar jet stream and the subtropical jet stream, with respect to a model of the general circulation in winter. Both jet streams are flowing from west to east.

Average sea-level pressure distribution and surface wind-flow patterns for January. The solid red line represents the position of the ITCZ.

Average sea-level pressure distribution and surface wind-flow patterns for July. The solid red line represents the position of the ITCZ.

Average position and extent of the major surface ocean currents. Cold currents are shown in blue; warm currents are shown in red.

Coastal Upwelling Ekman transport moves surface seawater offshore. Cool, nutrient-rich deep water comes up to replace displaced surface waters. Example: U.S. West Coast

Coastal Downwelling Ekman transport moves surface seawater toward shore. Water piles up, moves downward in water column Lack of marine life

In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower pressure near Indonesia produce easterly trade winds along the equator. These winds promote upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western Pacific. The trades are part of a circulation (called the Walker circulation) that typically finds rising air and heavy rain over the western Pacific and sinking air and generally dry weather over the eastern Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific becomes quite cool. This cool event is called La Ni ñ a. BONUS SLIDES: Regional Weather

In diagram (b) — atmospheric pressure decreases over the eastern Pacific and rises over the western Pacific. This change in pressure causes the trades to weaken or reverse direction. This situation enhances the countercurrent that carries warm water from the west over a vast region of the eastern tropical Pacific. The thermocline, which separates the warm water of the upper ocean from the cold water below, changes as the ocean conditions change from non-El Ni ñ o to El Ni ñ o.

Air Masses, Fronts, and Mid- latitude cyclones Chapter 8 Air Masses Fronts look at example from 1976 Polar Front Cyclone Development Clouds, Weather, Vertical Motion, & Upper Level Support

Here, a large, extremely cold winter air mass is dominating the weather over much of the United States. At almost all cities, the air is cold and dry. Upper number is air temperature (°F); bottom number is dew point (°F).

A weather map showing surface pressure systems, air masses, fronts, and isobars (in millibars) as solid gray lines. Large arrows in color show air flow. (Green-shaded area represents rain; pink-shaded area represents freezing rain and sleet; white-shaded area represents snow.)

A closer look at the surface weather associated with the cold front situated in the southern United States. (Gray lines are isobars. Green-shaded area represents rain; white-shaded area represents snow.)

A Doppler radar image showing precipitation patterns along a cold front similar to the cold front in the previous slide. Green represents light-to-moderate precipitation; yellow represents heavier precipitation; and red the most likely areas for thunderstorms. (The cold front is superimposed on the radar image.)

A vertical view of the weather across the cold front in the previous slides along the line X–X´. (active figure!)

Surface weather associated with a typical warm front. A vertical view along the dashed line P-P′ is shown in the next slide. (Green-shaded area represents rain; pink-shaded area represents freezing rain and sleet; white- shaded area represents snow.)

Vertical view of clouds, precipitation, and winds across the warm front in the previous slide along the line P–P′.

The idealized life cycle of a mid-latitude cyclonic storm (a through f) in the Northern Hemisphere based on the polar front theory. As the life cycle progresses, the system moves northeastward in a dynamic fashion. The small arrow next to each L shows the direction of storm movement.

A series of wave cyclones (a “family” of cyclones) forming along the polar front.

If lows and highs aloft were always directly above lows and highs at the surface, the surface systems would quickly dissipate. BONUS SLIDES: Convergence, Divergence, and Weather

Convergence, divergence, and vertical motions associated with surface pressure systems. Notice that for the surface storm to intensify, the upper trough of low pressure must be located to the left (or west) of the surface low.

The formation of convergence (CON) and divergence (DIV) of air with a constant wind speed (indicated by flags) in the upper troposphere. Circles represent air parcels that are moving parallel to the contour lines on a constant pressure chart. Below the area of convergence the air is sinking, and we find the surface high (H). Below the area of divergence the air is rising, and we find the surface low (L).

(a) As the polar jet stream and its area of maximum winds (the jet streak, or core) swings over a developing mid-latitude cyclone, an area of divergence (D) draws warm surface air upward, and an area of convergence (C) allows cold air to sink. The jet stream removes air above the surface storm, which causes surface pressures to drop and the storm to intensify. (b) When the surface storm moves northeastward and occludes, it no longer has the upper-level support of diverging air, and the surface storm gradually dies out.

Summary of clouds, weather, vertical motions, and upper-air support associated with a developing mid-latitude cyclone.