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Clouds and rain

The animation above, provided by the Met Office, shows a progression of waves on the polar front (where the yellow/green tropical air meets the blue polar air) over the course of a few days. You can see wave after wave growing and breaking.

In Step 1.5 you explored why we get depressions and how they move warm air polewards. Now let’s break the wave’s lifecycle down so we can explore the weather that develops.

In the four stages in Figure 1, the grey shading shows the region of continuous cloud cover. The red warm front and blue cold front denote the boundaries of the central wave of warm air – often called the warm sector. The occluded front (purple) occurs when the wave breaks, and the cold front catches up with the warm front.

Cold fronts are marked with triangles (reminiscent of icicles) and warm fronts with semi-circles (reminiscent of the warm sun).

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Figure 1: Four stages in the lifecycle of a depression

Before we go into further detail, let’s take a step back and think about some basic physics. The first thing to remember is that warm air rises, and that warm air rising is called convection.

As warm air rises, there is less atmosphere left above it and so the pressure falls. In physics, the ideal gas laws tell us that

PV/T = constant

Where P is pressure, V is volume and T is temperature.

So, if the pressure falls, the gas must cool. As the rising air cools, the rate of evaporation becomes less than the rate of condensation until eventually cloud droplets form. As water vapour condenses into liquid water, latent heat is released providing the energy source for the developing storm. And the latent heat generated is significant. For example, there is enough heat released in a small cumulus cloud to power an average home for 17 years. As the storm continues to develop, some of the cloud droplets will become large enough to fall as rain.

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Figure 2: Cumulus clouds are indicative of convection. You can sometimes watch them billowing upwards as the air rises within them. ©RMetS

So now, let’s look at a cross section through our developing depression (stage 2 in Figure 1) and see what’s going on in 3 dimensions.

The cross section follows the brown line, going from the cold air behind the cold front, through the cold front, the warm air in the middle and the warm front and then into the cold air ahead of the depression. Remember that the depression is moving from west to east. As the cold air on the left pushes into the warm air ahead of it, the warm air has to rise, and the cold air pushes underneath it, giving a cold front which slopes backwards. At the same time, where the warm air meets the cold air ahead of it, the warm air rises up and over the cold air, giving a forwards facing warm front. In both places the air is rising, and this is where the cloud and rain are found.

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Figure 3: A cross section through a depression at stage 2 © Dr Peter Inness

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Figure 4: The weather associated with a warm front © Crown Copyright, adapted from image supplied by Met Office.

Imagine you’re standing on the ground in this scenario. What weather will you experience? Think about standing near the right hand edge of the diagram above. As the system approaches you from the west, the first thing that reaches you is the warm front, far above your head. The clouds are on the front, so you will see high clouds, typically wispy, ice clouds called cirrus.

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Figure 5: Cirrus clouds.© RMetS

As the front moves towards you, it gets lower and similarly the clouds get lower and heavier. You’re going to need your umbrella. On the warm front, you’ll typically see featureless stratus or ‘sheets’ of cloud, becoming nimbostratus or ‘rainy sheets’ with a prolonged spell of rain when the front is near the ground.

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Figure 6: Nimbostratus. © RMetS

As the front crosses you on the ground, the temperature rises as there is a shift from the cold to the warm air, typically by a few °C. Winds turn clockwise, also known as a wind ‘veer’ (eg from southwesterly to westerly). The air pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage. In the warm sector, the air may still be rising and it may still be cloudy and drizzly.

The pressure begins to fall increasingly rapidly as the cold front approaches and rain usually starts not long before it arrives, becoming heavy for a short time. The cold front tends to be steeper than the warm front, with more dramatic weather. The cloud type on the front is a thunder cloud: cumulonimbus, with associated heavy rain and maybe hail, thunder, lightning or even a tornado. This is often accompanied by an increase in wind. The passage of the front is usually marked by a sharp change from falling to rising pressure and a veer (it changes its direction in a clockwise sense, eg from westerly to northwesterly) in the wind. As the rain dies away, the cloud lifts and breaks and, although there is sunshine, the air temperature falls.

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Figure 7: The weather associated with a cold front © Crown Copyright, adapted from image supplied by Met Office.

The saying ‘red sky at night, shepherds delight; red sky in the morning, shepherds warning’ is linked to the passing of a depression. If the Sun, setting in the west, shines its light on a depression in the east giving a red sky, it is a sign than the depression has passed and that better weather can be expected for a bit. On the other hand if the Sun, rising in the east, shines its light on a depression in the west, then that depression has yet to hit the observer and is about to bring bad weather.

Now let’s consider what’s going on a bit later in the lifecycle of the depression (stage 3 in Figure 1). As with the previous scenario we’ll start by looking at a cross section, this time through the occluded front, shown in Figure 8.

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Figure 8: A cross section through a depression at stage 3 © Dr Peter Inness

You can see the cold front has caught up with the warm front and has been pushed up. On the ground, the cold air from ahead of the system is directly in contact with the cold air behind. If you imagine you’re standing on the ground again you won’t notice much of a temperature change as the occluded front passes. However, the remaining warm air, squeezed higher up in the atmosphere, is still rising, so you’re going to still need your umbrella as there will still be cloud and rain.

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This video is from the free online course:

Come Rain or Shine: Understanding the Weather

University of Reading