GLOBAL CIRCULATION

Global circulation explains how air and storm systems travel over Earth’s surface and affect our weather.

The sun heats the Earth, but Earth is not heated evenly.

“Heat” is a form of energy that is measured in calories. One calorie represents the energy required to raise the temperature of 1g of water 1ºC, so the energy required to raise the temperature of 2g of water by 1ºC would be 2 calories.

The sun in the form of solar radiation provides the earth’s heat and light. The solar radiation’s wavelenghts are such that only about HALF of the radiation that hits the upper atmosphere finally reaches the earth’s surface (the rest is either reflected back into space or absorbed into the upper layers of the atmosphere).

This short-wave radiation is absorbed by the earth’s surface, causing its temperature to increase. This is called insolation. Energy is then reradiated out from the ground as long-wave radiation, and it is this radiation that heats up the lower atmosphere near the ground, with the result that any parcel of air that is warmer than the surrounding air will rise.

At the equator, the sunlight impacts Earth at a more direct angle. Hence, this is the point on Earth which gets heated the most.

The temperature at earth’s surface and the lower atmosphere where most of the weather is found depends on two factors:

1. How much heat energy in the form of short-wave energy (solar radiation) reaches the earth’s surface.

This depends on the following factors;
a) Latitude — The more directly the sun’s rays hit the earth’s surface, the greater is the quantity of heat energy transferred.
b) Season — The earth’s axis of rotation is tilted with reference to its orbit. This tilting creates the same effect as a change of latitude and is the primary cause of the different seasons and associated temperature changes throughout the year.
c) Time — The time of day determines the amount of the sun’s heating.

2. The energy absorption (and retention) capacity of the surface:

a) Absorption — The type of surface determines how much heat energy is absorbed, i.e., its reflective quality
b) — Specific heat capacity of the surface.

In comparison, out towards Earth’s poles the sunlight ‘spreads out’ over a larger area, and so this region does not reach the same high temperatures.

This imbalance causes an area of rising air at the equator, which is also known as an ‘equatorial low’, and in turn induces ‘surface flow’ of air moving from the Earth’s poles, north and south, towards the equator.

This simplified circulation pattern is really just a concept. In reality, the earth’s rotation changes everything and makes it all just a bit more complex.

Earth’s rotation induces a phemonemon called the ‘Coriolis Force’. Coriolis Force, or ‘C’, causes the circulation of air to break up into three distinct cells.

1. Polar cell
2. Ferrel cell
3. Hadley cell

In the northern hemisphere the coriolis force acts to TURN the WIND to the RIGHT. This gives rise to:

• Westerly Flow (FROM west TO east) from 30ºN to 60ºN Latitude, and
• Easterly Flow (FROM east TO west)from Equator to 30ºN Latitude.

the Coriolis Force is an apparent force that acts on a parcel of air that is moving over the rotating earth’s surface. This means that the air does not flow simply from high to low, but is deflected to the left or right according to which hemisphere you are in. This is known as the coriolis effect. This coriolis force is a product of the earth’s rotational properties.

Other factors also affect air circulation. I.e. seasonal changes, topography and friction from objects on the surface.

A balance is always maintained by air flowing from an area of high pressure to an area of low pressure. This phenomenon is known as the ‘PGF’, ‘Pressure Gradient Force’.

A ‘PGF’ is a natural force generated by a difference in pressure across a horizontal distance; i.e., gradient between two places. It acts at right angles to the isobars and is usually responsible for starting the movement of a parcel of air from an area of high pressure to an area of low pressure.

PGF causes both horizontal movement of air, called ‘wind’, and vertical movement rising and sinking air.

In the Northern Hemisphere, low pressure systems, cyclones, will have air circulating in a counter-clockwise direction, flowing to its centre. High pressure systems, ‘anticyclones’, will circulate in a clockwise direction, flowing away from its centre.

Note to self: “HIGH CLOCK”.

As previously covered, an airmass is a large parcel of air with fairly similar temperature and humidity (moisture content) properties throughout.

A pressure system is a circulating airmass that is classified as either a low or a high, which relates to the direction of pressure change toward the center of the airmass at the surface, i.e., gets lower or higher.

A low-pressure system typically will have more than one airmass, e.g., a warm airmass incorporated into a cold unstable airmass, with fronts in between. On the other hand, a high-pressure system is typically made up of a single stable airmass.

A low-pressure system, which is also known as a depression, has the following characteristics:

1. Pressure gradient. In a low-pressure system, the surface barometric air pressure rises as you move away from its centre; in other words, its pressure drops are you move toward its center.
2. Airflow pattern. The airflow pattern of a depression can be categorized as follows:
   a) Convergence (inflow) at the lower layers
   b) Rising air at the center
   c) Divergence (outflow) in the upper layers
3. Wind direction. The wind circulates counterclockwise around a low-pressure system in the northern hemisphere and clockwise inthe southern hemisphere.
4. Airmass. A low-pressure system usually will be made up of at least two different air masses, i.e., a cold and a warm air mass, with cold and warm fronts.
5. Movement. Low-pressure systems generally are more intense than highs because they are more concentrated in terms of area and have a stronger pressure gradient (pressure change with distance). Therefore, lows move faster across the surface of the earth and tend to have a shorter life span than highs.
6. Weather. The different types of depressions are developed and classified according to their trigger phenomenon. Depressions are classified as follows:
   1. Frontal depression
   2. Thermal depression
   3. Tropical storm depressions
   4. Orographic depression