Lecture 12In the previous class, we looked at high pressure and low pressure in the context of influencing wind andwind direction.We will now look at high and low pressure in slightly more detail, and see how differences in air pressure across the planet are responsible for global-scale atmospheric circulation.Low Pressure: Converging air at the surfaceLow pressure areas have converging air at the surface that is ascending.Air converges at the surface when a particularly warm, low pressure spot develops. Slight cooler (slightly more high pressure) air will move towards (converge upon) the lower pressure area……because air moves from high to low pressure.High Pressure: Diverging air at the surfaceThe High pressure areas have descending air that diverges at the surface as the weight of the air mass forces air outward. Air can be descending from high above, or is cold air that doesn’t rise_.There are huge areas on the globe that are characterized by high or low pressure.Wind systems have developed around these areas.These areas are:1. Equatorial Low-Pressure Trough – ITCZ2. Sub-tropical High-Pressure Cells3. Polar high-Pressure Cells4. Subpolar Low-Pressure CellsTheir corresponding winds are:1. Trades2. Westerlies3. Polar easterlies1. Intertropical Convergence Zone (ITCZ) (aka Equatorial Low pressure trough)Occurs in the tropics between _0 and 20_° north and south latitude.Continual insolation and warming of this area creates a trough of low pressure (light, less-dense, ascending air).Surface winds (the Trades) converge along the extent of the trough.The trades are northeasterly in the northern hemisphere and southeasterly in the southern hemisphereAs the northeasterlies and southeasterlies converge, they push the air up.Thus, air rises in the ITCZ due to both convergence and convection_.This converging air is extremely moist and full of latent heat energy. Rain and thunderstorms are common as air expands, cools and condenses (We will explore condensation in a later chapter).Vertical cloud columns frequently reach the tropopause, in thunderous strength and intensity.Equatorial calm winds= doldrumsThe ITCZ shifts north and south during the year.It shifts in correspondence to the shifting of the subsolar point (the point where the direct -perpendicular- rays of the sun hit the earth).In January the ITCZ is below the equator. (The direct rays of the sun are just above the Tropic of Capricorn)In July the ITCZ is above the equator. (The direct rays of the sun are just below the Tropic of Cancer)T/F The ITCZ is stable, staying within 5 degrees of the equator throughout the year.T/F Air rises in the ITCZ because it is diverging.T /F Air rises in the ITCZ because it is converging.T/F Some convection occurs in the ITCZ.T/F The ITCZ is a high pressure areaT/F Another name for the ITCZ is the Equatorial Low Pressure TroughT/F ITCZ stands for intercontinental tropical collision zoneT/F The winds that converge in the ITCZ are known as the westerliesT/F The areas of calm that sometimes occur in the ITCZ are known as the “Horse Calms” T/F The ITCZ shifts in latitude during the year from roughly 40 degrees north to 40 degrees south.T/F Huge thunderstorms that can reach as high as the tropopause occur in the ITCZ.T/F The air rises in the ITCZ partially because of convection: continual insolation warms the area.2. Subtropical High-Pressure CellsPoleward of both sides of the ITCZ, air spreads out and is descending, creating the subtropical high-pressure cells.These cells occur between 20 and 35 degrees latitude in both hemispheres (subtropics).The air is dry because moisture has been squeezed out of it by the rains of the ITCZ The air also warms as it descends because it becomes denser and the air molecules are closer together.These subtropical high cells move north and south along with the ITCZ.They expand in summer (when the ITCZ is near) and shrink in the winter (when the ITCZ is far away).T/F Subtropical High Pressure cells occur poleward of both sides of the ITCZ, between 20 and 35 degrees latitude in both hemispheres.T/F In subtropical high pressure cells, the air is dry in part because moisture has been squeezed out of it by the rains of the ITCZ T/F The air also cools as it descends because it becomes denser and the air molecules are closer together.T/F The subtropical high pressure cells shrink in summer and expand in the winter. T/F The Bermuda high is a subtropical high pressure cell in the Pacific Ocean. False, AtlanticT/F The Azores high is a subtropical high pressure cell in the eastern Atlantic. The dry, windless, descending air of these highs led them to be called the calms of Cancer and the calms of Capricorn. These calms were deadly in the era of sailing ships (and are still dreaded by those who race). They are also called the horse latitudes: perhaps because in past centuries, when ships became becalmed and stranded, sailors destroyed the horses on board so they would not have to share water withthe livestock. Thought questions:If temperatures are higher in the tropics, will more air rise because of the increased heat? yesIf more air rises in the ITCZ, would you expect more air to sink down into the subtropical highs, since more air went up in the first place? yesWhat might that do the extent of deserts, or to the areas that experience drought when the subtropical high pressure is over them? They might grow larger The surface air that is diverging from the subtropical high-pressure cells (and those converging into the ITCZ) generates Earth’s principle surface winds. One of the most important patterns is the Hadley Cell .The Hadley Cell forms as shown in the diagram below:1. Air converges and rises at the ITCZ, forming a rainbelt marked by many thunderstorms.2a,b– at the tropopause, the air in the ITCZ (which is now cool, because it is high up, and dry, because much of the moisture has been lost through rain) moves poleward.3. The cool, dry air sinks down to form the subtropical high pressure cell. As the air sinks, it molecules compress together and the air warms. Periodic dry periods and/or deserts characterize this are.4. Equator-wards, the winds that diverge from the subtropical highs form the Trade Winds, which converge towards the ITCZ to bring the cycle back to step one.T/F The air pattern in which air rises in the tropics, spreads out poleward at the tropopause in the subtropics, descends, and then converges back into the tropics is