How to know it’s going to rain by looking at the sky

Some hate rain, others love it – but one thing’s for sure: we all know what it is. Or do we?

Did you know that there are multiple types of rain in meteorology?

Now, while the rain itself is not different – they’re droplets of water – their origins are different.

In this article, we’ll take a look at these differences. We cover rain from convective storms as well as rain that originates in weather fronts. What’s more, we’ll also give clear examples that answer the question posed above: how to know it’s going to rain by looking at the sky?

Two types of rain – it’s strange, but true

As we mentioned before, there are two types of rain. While this sounds strange, it’s true. It’s not the rain itself that is different.

Rain is a droplet of water that falls from the sky – period.

But the origins of rain are different. While some rain falls as a result of convection and hence of a convective storm (or rain storm / thunderstorm, in plainer English), other rain falls by consequence of a weather front.

Let’s take a look at both.

First of all, a convective storm.

If you want to understand in more detail what convection is and how it is related to thunderstorms, we suggest that you take a look at our article How does a thunderstorm form? as well.

But, to avoid complexity, let’s briefly summarize its contents here as well.

A convective storm emerges in the atmosphere because heat is transferred in the atmosphere – often because of heating, or because of a dynamic trigger such as convergence of winds at a particular location. Because this air then rises (hot air is less dense than cold air), it cools down – and can contain less and less water. Eventually, it will even reach a level where it can no longer contain any water!

At that point, condensation will occur, and a gentle cumulus cloud will appear. Should the rising air be very buoyant (that is, rising very intensely), the small cumulus cloud can grow into a larger one, and eventually into a relatively aggressive one. However, only in the cumulonimbus stage, rain falls (cumulonimbus is a merger of the words cumulus and nimbus, the latter of which means rain). That’s how convection can produce rain!

Besides rain, severe convection can produce lightning as well.

Let’s now take a look at the other one: rain due to a weather front. If we want to understand how rain appears on fronts, we must first understand what a front is.

Do you know already?

If not, let’s take a look. Put briefly, a front is a separation boundary between two air masses. An air mass is an area of the atmosphere with similar characteristics, such as humidity or temperature. For example, in Europe, we often have so-called ‘subtropical air’ entering from the south, while we get ‘polar air’ from the north sometimes.

Such subtropical air is often hot and humid, while polar air is cold and dry. Maritime air, on the other hand, isn’t hot or cold, while often quite humid.

Now, in meteorology, we distinguish three kinds of weather fronts:

  • A warm front (often denoted in red). At a cold front, air that is warmer than the air currently present at one’s location enters that location.
  • A cold front (often denoted in blue). The opposite of a warm front: colder air enters the location.
  • An occluded front (often denoted in purple). It’s actually two fronts in one: as a cold front, which moves faster than a warm front, has overtaken the latter, it’s a bit of a combined front.

Now, we read before that with convective storms clouds form because warmer air rises, then gets colder, and that eventually water from this air condensates – at relative micro-scale.

At macro-scale (that is, areas much larger than the area impacted by one thunderstorm), the same processes occur, albeit a bit less aggressive than in the case of a thunderstorm. When a warm front passes, the warm air that is entering pushes the cold air away, but has to rise slightly as well (after all, it’s warmer than the cold air of the cold front).

Author: Kelvinsong at Wikipedia. License: CC BY-SA 4.0.

In the image above, we can see what happens: clouds form, but at a very gradual slope. They produce the classic gray skies – also known as stratus clouds. What’s more, when the clouds get denser and denser, particles collide and larger water droplets form. As they then fall down the skies, the stratus clouds have upgraded into what is known as nimbostratus (see the resemblance with cumulonimbus?), and it has started raining. Classic autumn rain!

At cold fronts, it’s actually cold air that pushes the warm air away. As this process is more aggressive than with a slow warm-air takeover, we often see more severe weatherphenomena occurring around cold fronts. Usually, it’s a line of storms, even with lightning activity every now and then. Rather than a warm front, which passes slowly, a cold front gets by quickly – and it’s remarkably colder when you move outside again!

Occluded fronts often combine the two. While you’ll often see longer periods of rain, there will be one or two moments when rain is actually really strong – storm-like. Not unsurprising given the fact that it’s a merger of the two others!

Now that we know what weather fronts are and how they produce rain, it’s time to look at how we can spot them. In the next two sections, we’ll take a look at spotting convective storms by looking at the sky, as well as weather fronts. First things first: let’s take a look at some storms! 🙂

Spotting convective storms in the sky

In the images below, you’ll see three characteristic images of so-called cumulonimbus clouds.

As we saw before, a cumulonimbus (or Cb) cloud is capable of producing rain, occurs because of convection, and can produce lightning, hail, wind gusts and even tornados in more extreme cases.

Generally speaking, as can be seen from the pictures, they look like fortresses in the sky – aggressive blobs of cloud, sometimes cauliflower-like, and anvil-like when they’re even more mature. If you stand still and look, it’s possible to see the air rising by looking at the edges of the cloud.

The three Cb variations below are three different stages in the lifecycle of a thunderstorm. In increasing order of maturity (and time since begin), we see a:

  1. Cumulonimbus calvus. It can be recognized by the cauliflower-like structure, but it’s often too big to be a cumulus. What’s more, weather radar indicates that rain is falling in the direction of the storm. It’s often even visible as a cell-like structure on the radar image.
  2. Cumulonimbus capillatus. It can be recognized by the hair-like structure at the top of the storm (capillatus means hair-like). That structure appears because the storm is reaching the so-called Equilibrium Level, or EL, which is when the rising air is getting colder than the environment again. As we know, air then cannot keep rising, and needs to spread out horizontally – since new air is being moved bottom to top within the cloud.
  3. Cumulonimbus incus. In those cases, the hair-like structure looks more like an anvil-like structure. It suggests a thunderstorm that is at its peak. Unless we’re dealing with a line of multicell thunderstorms, storms showing an incus (and capillatus) can deteriorate and die quickly.

Spotting weather front based rain in the sky

Now that we know what storm clouds look like, we can take a look at how to detect a weather front by looking at the sky – which can be indicative of rain in the next 24 to 72 hours.

First of all, you’ll need to think back to periods when you had nimbostratus based rain – remember, those gray skies with longer periods of rain. Where does it usually come from? For example, here in the Europe, such rain fronts often arrive from the west, because that’s where low pressure areas are born before they move towards my country.

Then, it’s time to take a look at the skies into that direction and look for what is known as cirrostratus. It’s a bit of a haze – often, the blue skies are still visible, but with a white, milky layer of clouds in front of it, like this:

Cirrostratus clouds. The sky itself is still visible, but it’s a bit hazy. Photographer: The Great Cloudwatcher, at Wikipedia. License: CC BY-SA 3.0.

This is a clear sign that a warm front is entering. However, it’s still many hours away (up to 48 hours, depending on its speed), so don’t count on immediate rain.

Soon, i.e. within a few hours, the clouds get thicker, because the warm front moves closer and closer. You’ll then see altostratus clouds – the same clouds, but then in the mid levels instead of upper levels of the atmoshpere.

Altostratus clouds. Photographer: Earth100, at Wikipedia. License: CC BY-SA 3.0

Then, a few more hours pass, and the clouds get thicker – and eventually, rain starts to fall. In this case, clouds have moved from stratus (the sun is then entirely gone, but no rain is falling yet) to nimbostratus (now, rain is falling). As Wikipedia describes it correctly at this picture, “[the clouds] often have very few visual features”.

Nimbostratus clouds, from Wikipedia. License: public domain.

Optical phenomena

Interestingly, besides the clouds, it’s possible to detect the entry of a warm front using another method – by looking for optical phenomena.

Now, while this is a very extreme example, it does indicate what I mean:

Complex halo display (22° halo, sun dogs, upper tangent arc, upper and lower Sun pillar, parhelic circle, supralateral arc) observed in Les Ménuires (elevation ≈2200 metres), Rhone-Alpes, France on January 23, 2015, during sunset at 16:30. Photographer: Gabor Szilasi, at Wikipedia. License: CC-BY-SA 4.0.

Often, a halo (the circle around the sun), or sundogs (the two lighter areas left and right of the sun) are visible less intensely – but they can be seen quite regularly!

While beautiful, they’re one of the clearest indicators that bad weather lies ahead.

Summary

In this article, we looked at rain – and more specifically, how to recognize it by looking at the skies.

We first looked at two ways that rain originates – being through a convective storm (or thunderstorm) and through a weather front, most often a warm front or occluded front, as cold fronts produce those storms.

Then, we looked at how you can look at the sky and determine whether rain is on the way. We saw that thunderstorms can be recognized by looking for cumulonimbus clouds – white, aggressive clouds, often resembling cauliflowers and in more extreme cases even anvils.

Weather front based rain can be detected by slowly increasing cloudiness, in the upper atmosphere first. The clouds get thicker and thicker with passing time, and eventually rain falls. Optical phenomena like halos, sun dogs and others are also very characteristic for the entry of weather fronts.

We hope you’ve learnt something from this article! If you did, please feel free to leave a comment in the comments section below. We would appreciate it and would love to answer any questions you have. Let’s now enjoy the weather – I hope it doesn’t rain where you’re at 🙂

References

Warm front. (2004, May 31). Wikipedia, the free encyclopedia. Retrieved September 3, 2020, from https://en.wikipedia.org/wiki/Warm_front

Cold front. (2004, May 31). Wikipedia, the free encyclopedia. Retrieved September 3, 2020, from https://en.wikipedia.org/wiki/Cold_front

How does a thunderstorm form? (2020, August 19). Mr. Weather. https://mister-weather.com/2020/08/19/how-does-a-thunderstorm-form/

What are the differences between single cell, multicell and supercell storms? (2020, August 24). Mr. Weather. https://mister-weather.com/2020/08/24/what-are-the-differences-between-single-cell-multicell-and-supercell-storms/

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