Weather & Climate- Atmosphere

The general atmospheric circulation system

Planetary surface winds

Horizontal air movement is known as wind. But it can move vertically too. Winds are caused by differences in air pressure. They move from high to low. Air pressure decreases with increasing height. When air temperature increases it becomes warmer and less dense and will rise, leaving low pressure beneath. Isobars which are close together show high pressure.

In the North Hemisphere wind blows anticlockwise into a low pressure zone and clockwise outwards away from a centre of high pressure as a result of the effects of the Coriolis force & friction.
High pressure occurs where air is descending and is associated with dry weather. This is because air warms as it descends, leading to the evaporation of most water vapour.

Low pressure occurs where air is rising. It is generally linked to precipitation and windy conditions. As it ascends, it cools adiabatically and cannot hold as much water vapour. The water condenses into droplets, which become clouds at condensation level.

Coriolis Force: an effect that causes anybody that moves freely with respect to the rotating earth to veer to the right in the N hemisphere and the left in the S hemisphere.

ITCZ- inter tropical convergence zone: result of the heating of part of the Earth’s surface caused by concentrated insolation. Hot air rises. This draws in cooler air that flows across the surface to replace rising air. Air streams are drawn in from N and S where they meet.

Position of the ITCZ:
Changes according to the seasons. The sun is located directly above the TROPIC OF CANCER on 21st June which pulls the ITCZ North of the Equator whereas on the 21st December it is just over TROPIC OF CAPRICORN and the ITCZ moves into Southern Hemisphere.

FRONT : Boundary between a warm air mass and cold air mass results in frontal rainfall. 

Hadley, Ferrel & Polar Cells

• It starts in the Doldrums, an area of intense low pressure found at the equator where the intense heating (be convection) of the earth’s surface forces air to rise through the Troposphere. 
• This area is known as the Inter Tropical Convergence zone (ITCZ). 
• As this air rises it cools and condenses forming a belt of clouds. Some of this air migrates northwards in the upper Troposphere to equalise out the temperature and insolation differences of our globe. 
•  As this air migrates north it cools relative to the air around it, becomes denser and sinks to the Earth’s surface at around 30°N and S of the Equator, creating a band of high pressure. 
• Some of this air migrates (because of Pressure gradient force) back to the low pressure area at the equator to complete the first cell of the system, the Hadley cell.
• Some of the air continues towards the poles to continue equalising the temperature differences. 
• When this air reaches 60°N and S it reaches cold polar air that is migrating south. This is our second convergence zone where 2 surface air streams meet. This causes the warmer, less dense tropical air to rise through the atmosphere again creating an area of low surface pressure. 
• It is this zone where we find the mid-latitude weather systems that blight British weather. 
• Some of this air migrates back towards the Equator where it eventually sinks at 30°N and S to form the middle cell of the model, the Ferrell cell. The rest of the air migrates to the pole, where it cools and sinks creating high pressure in the Polar Regions and completing a weak polar cell. 
•  Near the Tropopause at 30°N and S and 60 °N and S we find the high speed jet stream winds.

This model has many applications and limitations. The model fails to accommodate other major transfers of energy, such as the El Nino and La Nina models of circulation from West to east or Vice Versa across The Pacific Ocean. It also fails to acknowledge the presence and impact of Geomorphological features such as the Himalayas which complexly disrupt the movement of jet streams and surface level winds within the Hadley cell on a yearly basis. 

 However, it does offer people a starting point for understanding atmospheric circulation, ad does allow for some level of prediction of the weather that affects billions of people around the globe.

Oceanic circulation 

• The second major way that heat is redistributed around our planet is by oceanic circulation or ocean currents. 
• The globes ocean currents are interlinked into a global system, which is commonly known as the Thermohaline conveyor. 
• Warm less salty water travels at the surface of our oceans driven by surface winds that blow over the top of those oceans. This water cools as it travels north and south from the Equator and increases in salinity as the salt is left behind during evaporation of the warm water. 
•  This water, now cooler and salt laden, sinks and returns to the equator as another method of balancing out the Earth’s heat budget. This mechanism is hugely important for the people of Western Europe, as a warm ocean current called the Gulf Stream brings warm ocean waters which warm Western Europe well beyond what it should be given its latitude.