Understanding Global Weather

If your interest is purely in getting stuck into surfing more waves you can probably skip this section, but spare just a moment and you’ll better understand not only how and why you see waves on the coast but, if you have any interest in travel, why different coasts and countries experience such different wave climates.

Just about everything that happens on the rock we call Earth is sustained by energy that, ultimately, came from the Sun. With climate this is the absolute key driver in everything we experience. Starting at it’s very most basic the sun hits the earth and heats it’s surface, but it doesn’t do this equally everywhere. At the equator the sun sits directly overhead and the maximum possible day time energy hits land (or sea), whereas at the poles the angle of the sun is considerably lower and considerably less local energy is generated from the heat of the sun.

This might sound obvious, we know the poles are cold and the tropics are hot, but the perhaps less obvious story is how this then drives the global climate. Hot air rises, cold air sinks and this can happen on a large scale. Hot air at the equator rises up at the same time cold air at the poles sink. This convection creates a movement of air on a truly global scale, with hot air rising at the equator and heading towards the poles, before cooling and sinking and relatively warm air rising at mid latitudes and sinking back at the poles. The result, simplistically, is the division of the atmosphere into areas of rising warm and sinking cool air creating three fairly predictable ‘cells’ in both northern and southern hemisphere.

Credit: sealevel.jpl.nasa.gov

In addition to rising and sinking this movement of air is affected by the Coriolis force. The Coriolis force is the name given to a relatively simple phenomena that has a large impact on our global climate. Imagine a spinning Earth, rotating once every day. Now imagine you’re standing just a mile from the North Pole. Over the course of a day, viewed from above, you’d slowly spin around the pole drawing a small circle a few miles in diameter. A bit of school math and you can figure out that moving a few miles in a day and you’re going pretty slowly. Now imagine you’re standing somewhere hot on the equator. As the Earth spins throughout the day you’ll travel the distance of the circumference of the whole planet – about 25,000 miles. A bit more math and you can work out that your speed here must be over 1000 miles an hour. For us walking around on the surface this is pretty academic, we move, the ground moves, the air moves and everything appears (thankfully) to be standing nice and still. But for a parcel of air moving from the Equator to the Pole (as our convection explanation insists) this is something a little like jumping from a moving car and hitting stationary ground. As it heads north in the northern hemisphere it’ll be travelling faster than the air it moves into, causing it to veer to the right, likewise a parcel of air heading south will be moving slower than the air it encounters and veer left. Things are reversed in the southern hemisphere.

This Coriolis effect means that air travelling from Equator to Pole is, over hundreds of miles diverted off course. The airflow rather than steady is fractured and the good news, especially for anyone struggling to keep up so far, is that because the forces in effect are fairly constant this happens in a fairly fixed and predictable way, creating bands of alternately rising and sinking air covering about 30 degrees of latitude each. Where warm air rises the pressure on the Earths surface is lowered (as the air is ‘sucked’ from the surface) and where cold air is sinking pressure increases. Everything else being equal our nice simple earth is now split into alternate bands of high and low pressure about 30 degrees apart. If you’ve travelled a bit or check out world weather forecasts this’ll start to make some sense. Low pressure systems creating our storms tend to run in a southern band we call the roaring 40’s, likewise surfers in Hawaii and Europe will know that their winter storms come from a similar band of storms in the Northern Hemisphere.

Explaining the Seasons

The Earth doesn’t spin on an even axis, the angle of the pole to the sun changes as the year progresses, with long summer days meaning more heat at latitudes nearer the poles and long winter nights considerably cooler. Towards the equator this effect is much less pronounced and at the equator days and nights are about equal length all year long. This means that the temperature difference between the warm equatorial air and the cold polar air is more pronounced in winter than summer. This temperature difference is the driving force behind atmospheric convection which is the driving force behind our weather systems. Explaining why winter storms are generally stronger than their summer counterparts.

In addition land heats up faster than sea, this means the effects of the sun aren’t felt evenly over the earths surface. In summer the land heats up more quickly than the sea and on another scale we get convection, with warm air rising over land and sinking at sea, in winter this is reversed and warm air rising over the sea sinks as it moves over the cooler land. So within our neat, regular, global pattern we have a more chaotic local variation.

This effect is stronger in the Northern Hemisphere, there’s more land, which as we explained above changes temperature more rapidly, meaning more seasonal variation. In the Southern Hemisphere this effect is regulated by a large amount of Ocean that doesn’t change so rapidly. Of course there are seasons, but the effect is less pronounced. Ultimately this leads to the deepest and most powerful weather systems globally generally generated in the Northern Hemisphere, but more consistent large systems year round in the Southern Ocean and goes some way to explaining why different areas of the world see different seasonal swell patterns.