the warmer, less-dense air along the equator rises, creating low pressure at the surface

the colder, more dense air at the poles sinks, creating high pressure

equatorial low-pressure trough

polar high-pressure cells

subtropical high-pressure cells

subpolar low-pressure cells

• polar high-pressure cells: thermal, 90 N 90 S, cold/dry
• subp low-pressure cells: dynamic, 60 N 60 S, cool/wet
• subt high-pressure cells: dynamic, 20-35N 20-35S, hot/dry
• equatorial low-pressure trough: thermal, 10N 10S, warm/wet

• the combo of heating and convergences forces air aloft
•  

• lie between 30 & 60 degrees latitude
• circulate air between subtropical highs & subpolar lows
• Coriolis deflection causes westerly winds 

• the winds converging on the equatorial low-pressure trough
• pick up large quantities of moisture as they return through the Hadley circulation cell for another cycle of uplift and condensation

• between 20 & 30 degrees latitude in both hemispheres, a broad high-pressure zone of hot, dry air is evident across globe
• subtropical anticyclones generally form as the air above the subtropics is mechanically pushed downward & heats by compression on its descent to the surface (descending air relatively dry)
• westerlies are stronger in the winter in both hemispheres; surface air diverging from the subtropical high-pressure cells generate Earth's principal surface winds: trade winds & westerlies
• the eastern sides of these anticyclonic systems are drier and more stable and feature cooler ocean currents than do the western sides
• the drier eastern sides of these systems and dry-summer conditions influence climate along subtropical and midlatitude west coasts
• horse latitudes: zones of windless, hot, dry desert air
• areas known as the calms of Cancer and the calms of Capricorn

• occurs in January; 2 low-pressure cyclonic cells exist over the oceans around 60 N latitude, near their namesakes: the North Pacific Aleutian low and the North Atlantic Icelandic low; both cells are dominant in the winter and weaken/disappear in summer with strengthening of high-pressure systems in the subtropics
• polar front: the area of contrast between cold air from higher latitudes and warm air from lower altitudes
• low-pressure cyclonic storms migrate out of the Aleutian & Icelandic frontal areas and may produce precipitation in North America & Europe
• in the southern hemisphere, a discontinuous belt of subpolar low-pressure systems surround Antartica

• polar high pressure cells are weak
• variable winds, cold and dry, move away from the polar region in an anticyclonic direction; they descend & diverge clockwise in the Northern Hemisphere (counterclockwise in the Southern) and form weak, variable winds of the polar easterlies
• Antartic High is stronger and more persistent, forming over the Antarctic landmass  
• the less pronounced is a polar high-pressure cell over the Arctic Ocean

zonal winds: move parallel to lines of latitude

meridional winds: move parallel to lines of longitude

• single "convection cell" per hemisphere redistributes heat
• Coriolis deflection creates easterly surface winds
• assumptions include an ocean only planet & fixed sun-angle

• each hemisphere is divided into 3 convection cells
• Polar Cell (60-90) Ferrel Cell (30-60) Hadley Cell (0-30)

• dense air formed due to very cold temperatures near the poles 
• result is sinking and and polar highs
• air moves from poles to equator
• Coriolis causes polar easterly winds
•  

• no continuous pressure "belts" around Earth, but instead "cells"
• fluctuate in strength and position on a seasonal basis: oceanic lows- strongest during winter; oceanic highs: strongest during summer; continental lows: strongest during summer; continental highs: strongest during winter 

• pressure heights decrease toward poles
• stronger pressure gradients for the winter hemisphere
• heights are higher during summer 

• upper air motions toward poles, deflected eastward by Coriolis; westerly winds dominate upper troposphere (strongest during winter & increase speed with altitude)
• jet stream: "ribbon" of fast moving, high altitude air 

• polar jet streams exist in upper troposphere above polar front
• near equator, subtropical jet stream transports moisture & energy from tropics towards the poles 

Troughs & Ridges

• ridges: high heights extending equatorward
• troughs: lower heights extending poleward
•  

• strength is greatest in the winter
• very important for meridional (north-south) transport of energy 

• occur on most coastlines
• created by the different heating characteristics of land & water surfaces
• land gains heat energy & warms faster than the water offshore during the day  

• result when mountain air cools rapidly at night and valley air gains heat energy rapidly during the day
• warm air rises upslope during the day and at night cooler air subsides downslope into the valleys 

• gravity drainage winds; layers of air at the surface cool, become denser and flow downslope  

• western intensification: as these surface currents approach the western margins of the oceans, the water piles up against the eastern shores of the continents
• upwelling current: where surface water is swept away from a coast, either by surface divergence or by offshore winds
• downwelling current: there is an accumulation of water and the excess water gravitates downwatd