What is freezing fog?

12 12 2012

Fog has affected many areas over the last couple of days, which in some places has been freezing.

Freezing fog forms in the same way as ‘normal’ fog and is typical in winter, aided by clear skies and calm conditions. The cooling of land overnight under clear skies means that any heat radiates back into space cooling the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation of water vapour into millions of tiny water droplets to occur and fog in to form.

When temperatures are well below freezing, the tiny water droplets suspended in the air are made up of supercooled water droplets – which remain liquid even though the temperature is below freezing.  This occurs because the liquid needs a surface to freeze upon, such as a dust or pollution particles. However if there are not enough of these particles about then the water can stay as a liquid.

winter-fog

However droplets from freezing fog can freeze to surfaces on contact, forming rime. Often confused with frost, rime is a rough white deposit formed of feathery ice crystals. It can often be seen on vertical surfaces exposed to the wind – like lamp posts and pylons – as the supercooled water droplets freeze on contact as they drift past.

With warnings in force for fog people are advised to take extra care when driving in affected areas with journeys taking longer than usual. By being ‘weather aware’ our warnings help you prepare, plan and protect yourself from the impacts of severe weather.

Visit our website for more information on different types of fog.





Top ten: spookiest weather conditions

30 10 2012

As it’s halloween  tomorrow, we’ve taken a look at the top ten spookiest weather conditions. From well known scary weather – like thunder and lightning and sea mist, to lesser-known phenomena such as brocken spectre and fall streak holes.

  1. Fall streak hole. Also known as a hole punch cloud, these clouds sometimes cause people to think the world is ending, especially when wispy vigra clouds are descending from the hole. The exact conditions that cause them to occur are still debated.
  2. Sea mist. This occurs when mild air moves over the sea, which is cooler. It can be particularly spooky when sea mist comes in during the day and visibility is drastically reduced.
  3. Sunsets. Although often considered beautiful, some particularly vibrant red sunsets can create a spooky effect.
  4. Dust storms. Dust and sand storms can be whipped up rapidly by strong winds in arid regions. Dust storms can look particularly ominous as they approach as they can be up to 40 metres high.
  5. Whistling wind. Windy conditions can be scary when they blow through objects causing a whistling sound.
  6. Brocken spectre. This effect is produced when an observer stands above the upper surface of a cloud – on a mountain or high ground – with the sun behind them. When they view their shadow the light is reflected back in such a way that a spooky circular ‘glory’ appears around the point directly opposite the sun.
  7. Roll clouds. These ominous looking clouds are a type of arcus cloud usually associated with a thunderstorm or a cold front. As these rare clouds often appear to be ‘rolling’ they often cause fear that severe weather is on the way.
  8. Thunder and lightning. One of the most common forms of ‘scary weather’, thousands of thunderstorms are taking place at any one time across the globe.  The lightning you see during a thunderstorm is a large electrical spark caused by electrons moving from one place to another, while the rumble of thunder is caused by the noise of intense heating and expansion of the air along the path of the lightning.
  9. Clouds over a full moon. This spooky effect occurs when clouds partially cover a full moon.
  10. Fog. Fog forms when relatively moist and mild air close to the ground cools quickly, causing the moisture in the air to condense (at which point it becomes visible to the human eye). This normally happens in autumn and winter under clear skies, which allows heat from the ground to escape quickly to cause rapid temperature drops.

What weather conditions do you find the spookiest?





Blog about fog: why has fog been so persistent this week?

15 03 2012

Foggy weather has been especially persistent this week, not clearing all day in some areas of the UK.

Fog forms when relatively moist and mild air close to the ground cools quickly, causing the moisture in the air to condense (at which point it becomes visible to the human eye). This normally happens in autumn and winter under clear skies, which allows heat from the ground to escape quickly to cause rapid temperature drops.

Over the last few days winds have been light with an area of high pressure sitting rather stubbornly over the UK. This creates the ideal conditions for fog to form. As the fog is so dense in places the temperature has not been warm enough to cause the fog to evaporate, such as in more western parts yesterday.  However in eastern parts both yesterday and today the sun has got to work quite quickly on the fog, either lifting it into low cloud or breaking holes in it and eventually clearing it completely.

The fog is expected to clear today, with more changable weather on the way over the next few days. Keep up to date with your local forecast for the latest.

If you want to find out more about fog, have a look at our fog blog, ‘what is fog?‘ or our water in the atmosphere factsheet.





The fog blog – What is fog?

21 11 2011

With fog causing transport disruption and delays across the UK this week, there is a lot of interest in what causes this weather phenomenon.

So what is fog? The definition of fog is water droplets suspended in air – much the same as a cloud, but obviously at a lower level. The water droplets must be thick enough to reduce visibility to 1,000 metres or less. If visibility is higher than that, then it’s officially mist (there’s no upper limit for visibility for mist).

Fog forms when relatively moist and mild air close to the ground cools quickly, causing the moisture in the air to condense (at which point it becomes visible to the human eye). This normally happens in autumn and winter under clear skies, which allows heat from the ground to escape quickly to cause rapid temperature drops. Winds need to be light too, otherwise the fog will be dispersed.

Fog can also be formed in other ways, such as when warm, moist air moves over a cold ocean – something that’s more common in the UK spring when air temperatures increase but the ocean is still cold. It’s exactly the same process, but the cooling is effected in a different way.

The thickness of fog (or mist) will be determined by the amount of moisture (humidity) of the air and the amount of time the air cools for – the more time, the thicker the fog. Obviously fog can get very thick, and anything less than 100 metres would be considered thick fog.

Met Office weather forecasts

Met Office Severe Weather Warnings

What to do in severe weather








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