What can physics tell us about the weather?

Nov 04 2013

When most of us studied weather at school it was part of the subject of Geography. So what has science got to do with weather? The answer is everything. The weather we learned at school is more properly termed climatology: the study of weather averages and extremes, and is an important part of meteorology. Forecasting, however, is all about Physics, or understanding the forces that drive the weather we all experience.

There are three main drivers to the world’s weather.

The first of these is the sun: it heats the globe primarily around the equator, and this heat energy then flows from the hot equatorial regions to the much colder poles. All weather is driven by the energy we get from the sun.

The second driver is the spinning earth, and more especially the spinning layer of gasses that surrounds the earth. The spinning gives rise to all the circulation or vortex shapes we see in weather patterns, from the tight violent cone of a tornado to the lazy swirl of a large high pressure area.

The third driver is the plentiful supply of water on the surface of our globe. One of the characteristics of water is that it can absorb (or release) a lot of energy when it is changing phase – that is from water vapour to liquid water or from liquid water to ice. When liquid water in the tropics is evaporated, the resulting water vapour represents a great store of energy. If this water vapour is carried north or south towards the polar regions, and the vapour then condenses back into liquid water, this store of energy is released. Water is a great carrier of energy from one part of the atmosphere to another.

There are many other characteristics that are important to weather. The exact make-up of the air around us, particularly the trace gasses, is one; another is the orography – the positioning of mountains, valleys and coastlines. The behaviour of the oceans is another significant influence, especially the warm and cold currents that distribute heat energy within the ocean system.

Underlying all of this is something that we don’t often think about as relevant to weather: fluid dynamics. What is this? Air is a fluid – it flows. We normally think of fluids as substances like water or oil, but both liquids and gasses can flow so they are both classed as fluids. It is the movement of the air across the face of the globe that carries, or transports, all of that heat energy from tropical to polar regions. It is the fact that the air contains water vapour that vastly increases its energy-carrying capacity.

And it is the spinning motion of the earth that causes the flow to be so complex, with high pressure regions of lazy, slow clockwise-rotation air (in the northern hemisphere – everything is reversed south of the Equator!) to the vigorous anti-clockwise rotation of the low pressure centres that brings us our notable winter storms.

While it might be difficult to appreciate at times, we have a wonderful front seat in the theatre of weather as we watch all of these forces-of-nature work themselves out over the Atlantic, and sweep across our shores. 

This guest post was written by Gerald Fleming, who studied physics at University College Dublin and now presents the Met Éireann forecast for RTÉ.

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