Increased UV Levels

You may have noticed that our forecast for UV are indicating low to moderate levels, however there is a possibility that the levels may in fact be much higher than this today. This is due to an ozone anomaly affecting the UK at the moment where levels are noticeably lower than normal. This is quite normal and similar events have occurred previously around this time of the year. We are confident that the levels should be no higher than those of a sunny day in June.

The main factors affecting the strength of UV radiation reaching the Earth’s surface are:

  • the elevation angle (height) of the sun in the sky
  • the amount of cloud, dust and pollution in the atmosphere
  • the amount of ozone gas in the stratosphere

The presence of ozone in the stratosphere is important because it absorbs much of the UV radiation before it reaches ground level. Our UV model currently accounts for sun angle and forecast cloud amounts, but uses a “climatological” value (i.e. the average concentration for this time of year over the UK) to estimate the total ozone concentration.

During the dark polar winter, the concentration of ozone in the upper atmosphere over the pole typically decreases because sunlight is a critical ingredient in making ozone in the first place.  At the same time the circulation of air around the North Pole keeps all this low ozone air at higher latitudes near the pole.   In March and April, as the sun moves north, this polar circulation begins to break down and occasionally allows pockets of low ozone air to break away. These can sometimes pass over the UK. In these situations there will be less ozone in the high atmosphere available to absorb the UV compared to the average amount used in our forecasts. This means that the UV index could be higher than currently indicated in our forecasts particularly if forecast cloud amounts are low.  We do round-up our UV values but due to this low ozone event, the UV forecast can still be lower than the actual levels.  We are currently exploring ways to incorporate low ozone events such as this into our models to improve our UV forecasts.

It is important that, if you are in an area that is particularly sunny over the next few days, you take steps to ensure that you and your family are protected from these increased UV levels.

During the next few weeks, we will continue to keep an eye on these low ozone events (which can be seen in satellite data) in order to warn the public when they are happening.

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2 Responses to Increased UV Levels

  1. jauntycyclist says:

    no mention of the role of the magnetosphere? Nor that it is weakening? Modern science seems to be all about being economical with the story talking of which for anyone who missed it here is the story of the shameful role of MetO in the ‘hide the decline story’ and the disgraceful uk establishment whitewash [mainly in parts 2 and 3]

    Climategate ‘hide the decline’ in depth explanation by Stephen McIntyre

    Stephen McIntyre deserves a medal for exposing tricks pretending to be science backed by establishment used as a tin opener to get at tax payers money.

  2. nuwurld says:

    Hi Met. Nothing I’d really disagree with here. Although maybe a little to add, if only for general knowledge.

    As you have mentioned, a portion of the solar flux, that being the highly variable UVC, is responsible for the ionisation of diatomic oxygen and the subsequent attachment of oxygen free radicals to other oxygen diatomics to produce ozone.

    The after effects of ozone production are an unstable, reactive molecule that is destructed by UVB, again a solar variable. The result of solar production and deletion of ozone results entropically spontaneous downgrade of high energy photons into a kinetic (thermal) environment with no direct subsequent radiative losses ie Sun heats atmosphere, with a highly variable spectral component.

    Total ozone in a column lags solar insolation by 1/4 phase. Maximum insulation brings about maximum production of ozone. There is no equilibrium with the solar variation.

    At present average solar extreme UV is around 59% of the solar cycle 23. Therefore ozone will be depleted. Circulation patterns like you have mentioned will be made worse if coinciding with a flaring Sun during a time of netted reduction . This is inevitable.

    However, we have to answer the question of why nature chooses lighter skin with lower insolation? Why do our bodies select more light with higher latitude?

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