How does the wind blow?

In our previous article, we discussed how the sun powers the weather around us. By heating the Earth, the energy from the sun, radiates across space and is absorbed by the Earth. The energy of the earth is raised as therefore so is the temperature. In this article, we will discuss how this energy powers the winds we experience from the gentle sea breeze to the destructive hurricane or typhoon.

Uneven heating causes energy

The energy of the sun heats the Earth. The solar energy is absorbed by the ground, the oceans and even the atmosphere. However the sun does not heat the earth evenly. Oceans warm differently to land, ice warms differently to rainforest. The rotation of the earth causes the day and night cycle and so only one half of the earth is exposed to the sun’s energy at any one time. In addition, the sunlight does not heat the Earth evenly. At the equator, the sun is directly overhead and passes through a small amount of atmosphere and is concentrated on a small area. At the poles the sun hits the earth at an angle, spreading the energy over a wider area. In addition, the sunlight must travel further through the atmosphere. It is clear that the solar energy is less at the poles. In addition, the seasons, caused by the rotation of the earth around the sun, causes one polar to be heated more strongly the other.

When an object is heated unevenly, it will attempt to distribute the heat so that all parts of the object are the same temperature. For example, a metal bar heated at one end will warm up throughout the length the bar as the heat energy is spread throughout the object. This process is called conduction. In fluids, like water or the gases of the atmosphere, the primary way to even out a heat imbalance is via a convection current. Conduction does occur but air is a very poor conductor and so there only significant conduction very close to the Earth’s surface.

When a gas or liquid is heated, the molecules within the gas gain kinetic energy and move faster. This energy causes the gas to expand which in turn results in the air being less dense. If the gas is unevenly heated, there will be some areas of the gas that are hotter than others which in turns means some areas of the gas will be less dense than others. The areas of the gas that are less dense, are lighter, and so they rise. When the warmer area rises and area of lower pressure remains where the warm air departed so cooler air moves in to replace it. If this cooler area is then warmed, for example by the sun’s energy in that area, then the process will continue and a convection current will begin.

We see convection currents around us wherever fluids are heated unevenly. In this atmosphere we see these in many forms. The movement of the air from the cooler zones to the lower pressure area created when the warmer air rises is felt as wind.

Local winds – Sea Breezes

As we have seen, when the atmosphere is heated unevenly, convection currents develop. When we visit the beach we often experience this through the sea breeze. When solar energy shines on both the land adjacent sea, the sea warms significantly more slowly than the land. This is because water has a high heat capacity than the land – it takes more energy to warm it up. On a warm sunny day, by the afternoon the land will be warmer than the sea and so a convection current will develop. The air over the land will rise, causing a localized low pressure and so cooler air will move from the sea to the land. This causes the onshore sea breeze we feel when we visit the beach.

At night, the situation is reversed. The higher heat capacity of the sea means it cools much more slowly than the land. An opposite convection current can then occur – the air over the sea is warmer than the land and rises, cooler air from the land moves as wind to fill the lower pressure. An offshore breeze is therefore created.

Similarly, localized winds can form in mountains and valleys. Mountain tops can be heated more strongly during the day causing the air to rise and a low pressure area to develop near the mountain top. Higher pressure wind in the valley will then move into the lower pressure area causing winds upwards from the valley floor to mountain top.

Global winds

In the same way that there is localized uneven heating within the atmosphere, we see the same effect at the global scale. The equatorial region receives significantly more solar energy than the poles. Therefore the heat causes the equatorial are to rise and create a low pressure area at the equator. We might expect that we would therefore see very consistent, constant winds from the cooler, higher pressure polar regions to the warmer, lower pressure equatorial regions. This would create a constant wind moving from pole to equator.

It is clear that there is something more than this occurring as we do not see such a polar to equatorial wind. We see larger wind patterns which, the mid latitudes, form large swirling winds.

These circular winds are a result of the rotation of the earth. The Earth is continually rotating in a easterly direction. However the gas surrounding the earth does not move with the Earth because it is not attached to the earth. Therefore the earth moves underneath the atmosphere. From the point of view of a person standing on the rotating earth, it appears the wind is veering to the right.

This is the Coriolis Effect. This effect is so great that the wind generated by the movement of the air from the cold, high pressure at the poles never reaches the equator. Multiple zones of wind flows are set up.

We can separate the globe into bands of different types of wind flows based on the latitude. If we work from the equator to the poles, we see multiple types of wind:

The Doldrums

At the equator, the strong heating means the air is the warmest and the air pressure is lowest. As winds flow towards low pressure, at the equator there is nowhere for the wind to blow towards an even lower pressure and so little wind develops. The region of very light winds that extend about 5 degrees from the equator is known as The Doldrums – causing sailors many a heartache when the wind would not fill their sails.

The Trade Winds

North of the equatorial the air typically cools and descends at approximately 30 degrees latitude. This creates a high pressure known as the sub-tropical high. This cooler, more dense air therefore floods towards the equatorial low pressure. However the Coriolis effect deflects the wind to the right and a strong easterly wind pattern results. These are the trade winds – consistent winds where sailors could take advantage of strong, easterly winds.

Westerly winds

Warm air rising from the equatorial and subtropical regions meets the polar air descending from the poles at approximately 66 degrees latitude (in both the northern and southern hemisphere). This rising, lower pressure air, known as the Sub-Polar Low, creates the predominant air flow to be from the west to the east. It also causes large low and high pressure circulation features. This is the location of many of the Asian, European and North American countries and so it is the weather caused by these systems that many people experience every day.

Polar Easterlies

At the poles, the air is coldest, densest and therefore has the highest pressure. Winds move from these poles southwards and northwards towards the warmer air. The Coriolis effect causes the wind to appear to turn to the right (towards the west) and an Easterly wind flow is set up in these regions known as the Polar Easterlies.

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