Insights & News

Deceiving Clouds: What Falls Above Doesn’t Fall Below

15th February 2021

13th Feb 2021 snapshot from the Weather Radar. It shows snowfall in pink, mixed rain/snow in green and rain in blue.

Last Saturday (13th Feb 2021), a band of snow slowly crossed Wales & much of England from the west.

Yet if not for forecast and radar weather apps, it could easily have gone without notice.

You see, the conditions beneath the clouds that day were highly abnormal for this part of the world.

There was very dry (and cold) air blowing in from the southeast – much drier than usually experienced in the UK with its maritime (ocean driven, generally humid) climate.

Let’s take a look at how we can detect such dry airmasses.

Don’t worry, it doesn’t require subjecting yourself to dry, chapped hands!

Wet Bulbs, Dew Points & Relative Humidity

I expect most of you are familiar with air temperature measurement using a typical thermometer. This is known as the ‘dry bulb’ temperature and is what you see in weather forecasts for overnight lows and daytime highs.

But did you know that there is another widely taken measure known as the wet bulb temperature? The name is literal – it’s taken using a thermometer wrapped in damp cloth! What it tells us is how low the temperature can drop because of water evaporating.

Yes, evaporating water lowers temperature – the water goes from a lower energy state (liquid) to a higher one (gas) by taking energy – i.e. heat – out of the air around it. That’s why sweating works so well – as it evaporates off you, it cools your skin.

Anyway, using the dry and wet bulb temperatures, we can figure out something known as the dew point. This handy measure tells us at what temperature the air becomes saturated with moisture. At that point, you have clouds – be that way up in the sky or down by the ground as fog.

Not only that, but by comparing the dew point and dry bulb temperature, we can tell how far away the air is from being saturated at any given time and place. That’s known as the relative humidity (RH), expressed as %, with saturation occurring at 100%.

Dew Points Way Down

Last Saturday afternoon, dry bulb temperatures across England & Wales were generally 0 to 3°C. Meanwhile, dew points were in the range of -8 to -5°C. That’s exceptionally low by UK standards.

This huge difference meant the relative humidity was only about halfway toward saturation; 50-60% in most parts. This essentially means the air had a lot of ‘room’ for storing more moisture.

Enter scene: Snow from that band of cloud crossing Wales & England.

The snowflakes leave the nice moist cloud layer to find themselves in a relatively parched environment. The low RH air chips away at them and before long, most, or all of them have dwindled to nothing.

Only the largest snowflakes were able to make it down to the surface.

Bemusing Apps

This led to a confusing situation: Many weather apps indicated that snow was falling when it clearly wasn’t. Meanwhile, radar apps showed precipitation when none could be seen first-hand.

The trouble is, the forecast models that feed weather apps predict what will fall out of the cloud, not what will reach the surface. Meanwhile, radars scan a swathe of altitudes up to the cloud base, but their measurements are blended for use in apps, so we can’t see that the precipitation stops above the surface.

Snow’s Most Vulnerable

When it comes to surviving low RH air to reach the surface, size isn’t everything. In fact, it can be detrimental to be larger, depending on density.

To have the best chance of making it, precipitation must have the smallest surface area to weight ratio.

Raindrops score highly and are also very streamlined for a faster fall. The less time spent in the dry air, the better!

Then you have hail and frozen rain, which have a lower weight to surface area ratio but still much more than snow.

Yes, delicate, vulnerable snow. Composed of elaborate ice crystals, the have a lot of airy spaces, which gives them low weight and high surface area. It also means they’re not very streamlined. So, they fall slowly – sometimes really slowly, taking hours to reach the surface! That’s a long time to do battle with dry air.

Long story short, when dry air’s about, snow is least likely to make it to the surface, rain most likely. So, low RH air tends to have a big impact during freezing cold winter weather. On Saturday, many spots saw little or no snow, but some graupel – which is a sort of low-density hail. This sits between snow and hail on the survival scale, so was more able to reach the surface.

Evaporating Rain Takes More Heat

Loss of precipitation – especially raindrops – to dry air is also often seen during warm or hot weather. In such conditions, the relative humidity can become very low, due to daytime heating by the sun.

You see, the dew point doesn’t tend to change much in response to sunshine – but the dry bulb temperature does. As the sun heats the air, the dry bulb temperature rises increasingly far above the dew point, reducing the RH.

It’s not uncommon for every single raindrop to evaporate well above the surface. When this happens, you see a formation known as ‘virga’. It resembles ghostly jellyfish drifting across the sky!

Something to look out for in the longer days ahead.

James Peacock
Head Meteorologist at MetSwift

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