Freezing Fog = Snow?
Well the risk of a very strange phenomenon during the next 24 hours over southern parts of the UK. As it has probably not escaped your attention if you have taken the time to look out of the window this evening, its a bit foggy out there! With the temperature below freezing, what we are in fact witnessing is freezing fog.
Lets start from the beginning. There are two measures of ‘temperature’ at the surface, one is called the air temperature, and thats what we are used to seeing on television forecasts. It is the temperature of the air, very simple. The second measure is called dew point, and this takes into account both humidity and temperature. By definition, it is the temperature at which the air can condense and form water droplets, similar to how clouds form. Fog is formed when the air temperature reaches the temperature of the ‘dew point’. If the dew point is below 0c, and the temperature reaches or even surpasses the dew point, then freezing fog occurs.
Freezing fog itself forms all sorts of very interesting and rather striking formations. As it hits sub zero surfaces (such as cars and trees), the condensed water droplets that make up the fog freeze, and form an icy coating. Depending upon how rapid this freezing is, it can form a very thick frost called ‘rime frost’. However, an even more interesting formation is that of snow.
Its quite simple to visualise. Think of how snow forms higher up in the atmosphere in clouds. Water vapour from the surface rises, cools and condenses into clouds. When a certain humidity is reached – the relative humidity figure given to this point is 100% – the condensed water, formed in this case as snow, becomes too heavy to stay within the clouds, and so falls back to the surface. In the case of freezing fog, this humidity is reached closer to the surface, but the result is the same. The fog reaches a humidity point (100% relative humidity) at which it can no longer hold any more moisture, and so this moisture is released. This is similar to drizzle that sometimes occurs during foggy conditions.
With the temperature below 0c at present, this drizzle is likely to form as either freezing drizzle, or, more likely during the next 24 hours, light snow, which we call ‘snizzle’. This snizzle can give a light covering of snow in some areas, and so do not be surprised if you wake in the morning to a small covering of snow.
There is an additional risk of light snowfall as the fog starts to lift, and additional condensation takes place as this air rises and cools. This could take place through tomorrow afternoon
And finally as a completely separate risk, there looks to be a small shortwave forming ahead of the low pressure system pushing in at the end of the week. This will push an area of rain/sleet/snow along the English channel. Current modelling suggests some of this precipitation will move along southern counties of England, mostly running to the south of the M4 motorway. However, it would not be out of the question for this to move a little further north, towards Gloucestershire, Oxfordshire and the Home Counties.
After this, Friday looks wet, windy and less cold for most, but with the risk of snowfall remaining for hills across northern England and Scotland, and thats a sign of things to come for some time, with flooding concerns once again into the weekend for western areas.
Forecaster: Kris Surtees