Insights & News

Rain Delivered by Sunshine

10th April 2020

I’m sure many of us have experienced the drama of a sudden shower that seemed to come out of nowhere. Even a light one can play havoc with events from professional tennis to picnics.

How can these soaking surprises manifest on otherwise fine days? Well, believe it or not, it’s usually the sun you can blame…

The Atmosphere Is Very Moving

As you read this, the atmosphere is constantly swirling and circulating above and around you or the building you’re inside. Every movement leads to a transfer of energy – i.e. heat – to some extent or other.

When it comes to the development of rain clouds, it’s the vertical movements that matter most.

A good way to visualise this is to picture countless parcels of air circulating within the overall atmosphere. They rise when they are warmer or moister than their surroundings and fall when cooler or drier *.

As they travel, they equalise with their surroundings.

A comparatively warm parcel will lose energy to its surroundings as it rises, cooling the parcel and warming the adjacent air. In time, the parcel ceases to be warmer than its surroundings and the rising ceases.

Similarly, parcels that are cooler than their surroundings will sink with time until they equalise via warming.

This rising and falling of parcels – known as convection – works toward making the atmosphere very boring indeed – a static entity with uniform temperature and moisture content. As it is, the sun and all manner of weather systems ensure that this is never achieved.

* The warmer air is, the more its component molecules spread out, reducing weight per unit volume. The moister it is, the more water molecules it consists of, which are lighter than the nitrogen and oxygen molecules that air is otherwise predominantly made up of. Warmer or moister = buoyant.

The Sun Stirs

The diagram below illustrates an example of the atmosphere responding to surface heating by sunlight.

Diagram illustrating an example of the atmosphere responding to surface heating by sunlight.

It’s worth noting that outside the tropics and subtropics, most places have a time of year during which the sunshine is usually not strong enough to raise the surface temperature enough to initiate the development of rain clouds. During these times, fine weather is more reliable when there aren’t weather systems approaching.

As you can see, the day begins with various relatively warm and cold layers to the atmosphere (one of a few possible configurations). Radiative cooling overnight has resulted in the near-surface air being cooler than the air above it. This setup, known as an inversion, is completely stable. There is only weak convection to be found, where slightly warmer layers underlie cooler ones higher up in the atmosphere.

After sunrise, once any fog or low cloud has been ‘burned through’ (so to speak), the surface starts to be warmed by the sun’s rays. Before long, it’s become warmer than the air above it. At this point, air parcels start to rise much higher into the atmosphere.

These parcels usually carry at least a little moisture. As countless parcels equalise with the atmosphere at a similar altitude, they replace relatively cold, dry air parcels (which sink downward). In this way, moisture content increases at that height.

Now, here’s the crucial part with respect to this blog piece: If enough moisture builds, the air become saturated, forcing some moisture to condense into droplets. That means clouds.

Convective Clouds

Specifically, cumulus clouds, which are usually seen developing in the mid-levels of the atmosphere. They’re the small, ‘fluffy’ ones commonly referred to as ‘fair weather clouds’.

Ironically, they don’t always constitute dry, fine weather – if they grow large enough, they will produce some light precipitation (e.g. rain, soft hail, snow).

It’s possible for the action of weather systems to force cumulus development too, but here, I’ll keep focus on those ‘born of sunlight’.

These cumuli are small and isolated at first, but numbers and size will quickly increase if there’s enough moisture around.

Small cumuli in fair weather.

Small cumuli in fair weather. This Photo by Unknown Author is licensed under CC BY-SA

What happens next depends on temperatures in the upper troposphere ** and how much more surface heating occurs. If the upper atmosphere is entirely colder than what’s beneath, the cumulus will be able to build upward with ease, generally resulting in clouds much taller than they are wide.

If there’s an inversion up there, then the cumulus will essentially ‘hit a wall’ and be forced to grow horizontally rather than vertically. In this situation, it’s a race against time for surface temperatures to become warm enough that rising air parcels can punch through the inversion. If it doesn’t get there before the spreading cumulus fill in the sky, you’re left with one of those dreary overcast afternoons.

Peak Physique: Cumulonimbus

I expect most readers are familiar with the product of countless moisture-laden air parcels managing to rise all the way to the upper atmosphere. Towering clouds delivering heavy rain, hail or snow. Perhaps with some thunder and lightning for good measure.

These are the cumulonimbus clouds.

This isn’t the full story, though. In a gym-full of weight-lifters, there are always some particularly extreme specimens. When it comes to clouds, they are known as cumulonimbus incus. The word ‘incus’ is Latin for ‘anvil’, which aptly describes the shape these clouds tend to take on as a result of them having reached the very top of the troposphere**.

** The lowest layer of the atmosphere. Surface air very rarely makes it into the layer above (the stratosphere) – and then only a short distance.

A cumulonimbus that has reached the top of the troposphere (lowest layer of the atmosphere).

A cumulonimbus that has reached the top of the troposphere (lowest layer of the atmosphere). This Photo by Unknown Author is licensed under CC BY-SA

You see, this top is marked by a big temperature inversion, which generally prevents air parcels from rising through it. So, the top of the cumulonimbus flattens out against it and then expands in the direction of the wind, leading to the anvil shape.

These phenomenal structures are the most likely to become severe thunderstorms, bringing dangerous weather conditions such as torrential rain, damaging winds and large hail. Whether they reach this ferocity or not mainly depends on just how much moisture is being delivered to them by convection and other air movement.

Is that the limit, then? Well, no… there’s one more level to which certain cumulonimbus may ascend, albeit not so much literally this time.

Serious Supercells

Most cumulonimbus are only able to sustain for half an hour or so before collapsing. This is because the precipitation they bring act to cool the surface and put a stop to the convective process they depend upon. However, it’s possible for strong upper-atmosphere winds to displace the precipitation away from the main area of rising motion. Then, a storm may sustain for up to several hours.

In this situation, the effect of Earth’s spin tends to cause the rising air to rotate, producing a small-scale circulation known as a mesocyclone. This is what makes a supercell – it’s an especially large, rotating thunderstorm.

A supercell struts its stuff over the Great Plains. This staggering appearance been likened to an alien ‘mothership’.

A supercell struts its stuff over the Great Plains. Yes, they really can look like this! This staggering appearance been likened to an alien ‘mothership’. This Photo by Unknown Author is licensed under CC BY-SA-NC

These can produce the most severe, destructive weather of all, including tornadoes. If I’ve got you hooked, you can read some excellent summary details here.

Due to a variety of factors combined, the strongest supercells are almost exclusively observed in the USA. I’m planning a blog entry on that subject very soon – watch this space!


James Peacock MSc

Head Meteorologist at MetSwift



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