Rather than consulting weather forecasts on our phones and reading off the numerical statistics for a particular location (temperature, chance of rain etc), let’s try to really understand what is happening meteorologically around us. This can be both more insightful and more interesting.
Depressions (low pressure)
Let’s jump straight to the chase with this classic diagram:
At a fixed point on the ground you are first in the way of the warm front (travelling West to East in the Northern hemisphere), followed by the cold front a few hours later. This is what you can expect in order:
Arriving warm front from the West
- Streaks of cirrus These are made of ice crystals, and lower clouds are made up of suspended water droplets (water vapour is also carried in the air as humidity, but you can’t see it). If there’s no wind the cirrus clouds can have a knotted appearance, but with an oncoming depression the winds will stretch them out to appear like “mares’ tails”, or sometimes bubbles of warm air rising high can create a “mackerel sky” patchwork of cirrocumulus
All this is reflected in weather lore:
“mackerel sky and mares’ tails make tall ships carry low sails”, since cirrus and cirrocumulus are signs of a depression on the horizon
“the bigger the ring, the nearer the rain”, halos of light are seen around the sun or moon when they shine through cirrus
- A blanket of Cirrostratus
- Pressure starts to fall and the wind backs and becomes stronger. Milky altostratus (drizzle) or heavy nimbostratus (steady rain)
Warm sector
- As we come into the warm sector the wind veers, the temperature rises and the pressure drops due to the rising warm air. So it’s relatively calm, mild and clear, but maybe with some thin stratus which can bring drizzle/fog. Drizzle is caused by condensation as ice particles fall from the cloud, melting before they hit the ground. When the sun is low in the sky and you have your back to the sun you might be lucky to see a rainbow, and if the light rays reflect twice within the rain drops you have a second fainter rainbow with the colours reversed! The sun might dry out the clouds to form stratocumulus or fracto-stratus(evaporated and blown about, so just patches of stratus remain)
Cold front
- The wind backs and it becomes gusty
- A bank of cumulus/altostratus/cumulonimbus arrives bringing heavy rain, and even thunderstorms/hail
Cold front passes
- The temperature drops, pressure rises, clear sky – or dotted with fair weather cumulus clouds. These clouds might merge and there could be showers.
If the cold front catches up with the warm front, the slice of warm air is trapping high away from the ground and we get an “occluded front” bringing unsettled weather.
Anticyclones (high pressure)
Anticyclones are slow-moving masses of sinking air. When air carrying water vapour moves upwards, eg as pockets of air bubble up in hot weather or forced over mountains, it moves above the dew point and clouds are formed (this due point level can be very striking in the sky). But the reverse happens with an anticyclone, and you generally won’t see banks of cloud. You might however get some “fair weather” cumulus (what they call a “ciel moutonné” in French since the clouds look like sheep), but they tend to evaporate quickly away. Without any clouds to trap the warmth it can get cold in winter, especially at night and early morning. On clear nights the water vapour carried in the air will condense when it comes in contact with the cold ground leaving a layer of dew (or even frost if its freezing, called ‘rime’ when it encrusts trees with ice).
Summer thunderstorms
Cumulonimbus clouds are the kings of the sky, and can spawn hail and thunder storms. They can build up quickly in hot weather, billowing up in the wind and spreading at the top in an anvil shape. We mentioned that drizzle is caused by condensation. Rain can also come about by coalescence – the water drops merge as they are tossed around the cloud until they become too heavy to be carried and fall to the ground in heavy drops. I have sometimes noticed that when the air is heavy and about to rain, the first clap of thunder seems to trigger the downpour. My guess is that the sonic shock of the lightening triggers the release of the raindrops, which then take a few seconds to reach us.
When I was young watching the weather forecast was a strangely hypnotic experience, but we didn’t really understand all this talk of pressure and fronts. Most of us don’t get weather reports from the TV any more. But the weather statistics on our phone can be rather impersonal and lack any explanation or context.
It can be fun to try to read the sky. And when you see a new cloud formation, look it up to understand how it’s formed and what it might bode. It’s easy to find charts of the pressure systems in real time:
