The basics of reading weather maps

GCSE science lessons

When you look at a weather map they’re covered in wiggly lines, similar to contours on a normal map. These lines, known as isobars (iso = equal, bar = pressure) join together places with same mean sea level air pressure. Knowing this, you would expect wind to flow directly from the high pressure areas to the low pressure areas in order to achieve equilibrium, however this is not the case. What actually happens is (due to the effects of the earth’s rotation) wind flows almost parallel to the isobars, with surface wind ‘leaking’ 15-30% towards low pressure. In the Southern Hemisphere the air flow is clockwise around pressure lows and anti-clockwise around highs (In New Zealand, if you look into the wind the low pressure is on your left). This is reversed in the Northern Hemisphere. Therefore, when looking at a weather map, even if wind direction indicators are not been provided, you can build a good picture of the likely prevailing air currents.

Isobars also provide an indication of wind strength. In general, the closer the isobars are together, the ‘steeper’ the change in pressure, and therefore the windier it gets. However, when isobars are tightly curved around a low, the wind speed is lower than the spacing would suggest. When they are tightly curved around a high, the wind speed is higher than you might expect.

In addition to telling us about wind direction and speed, air pressure has a critical role to play in the amount of precipitation an area is likely to receive. A low pressure system is like a giant funnel of wind spiralling inwards and upwards forcing warmish air in the centre to rise. As air rises, it experiences lower surrounding air pressure causing the rising air to expand and the air’s temperature to fall. Think about our classroom of students: in the low pressure example, there are fewer students (or air particles), generating fewer collisions. Fewer collisions result in less heat being generated leading to a drop in air temperature. As the temperature falls the water vapour in the air condenses to form rain. In contrast, in areas of high pressure air sinks and is warmed, causing clouds to evaporate. Therefore, in general low pressure = bad, high pressure = good.

This effect is similar when the air is pushed up over mountains. In New Zealand, if you have a North Westerly wind coming in from the Tasman sea, as it hits the Southern Alps the ascending air cools causing rain. In contrast, as the low passes you’re likely to get a gentler Southerly wind with the rising pressure bringing in clearer dry spells.

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