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Frequently asked questions

Before contacting us via our Customer Centre, please check if our frequently asked questions provide a solution to your query.

1. Help using this site
2. Questions about the weather and forecasts
3. Questions about climate change
4. Questions about units of measurement
5. Miscellaneous questions

1 Help using this site

Access keys are keyboard shortcuts which allow a user to navigate a web site without using a mouse or other pointing device. This can sometimes be quicker and may assist those with motor skill difficulties.
How to use access keys on this web site

Firstly, ensure you are looking at the latest picture by pressing Ctrl and Reload/refresh. The graphic/text will be updated when there is a significant change in the forecast. The weather symbols are valid for the time shown above the map. The associated text should always be read as this will expand and amplify the graphic.

You need to ensure that JavaScript is switched on.

How to do this in Internet Explorer:

1) From the Tools menu select Internet Options.
2) From the Security tab set the security level to Medium. JavaScript will automatically be enabled.
3) If you wish to have more control over your security settings, select the Custom security level and click on Settings. Scroll down to the bottom of the settings window and ensure that Active scripting is enabled.

How to do this in Opera:
1) From the Tools menu select Quick preferences and make sure Enable JavaScript is ticked.

How to do this in Firefox:
1) From the Tools menu select Options.
2) From the Content tab make sure the Enable JavaScript checkbox is ticked.

How to do this in Safari:
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2) From the Security tab make sure the Enable JavaScript checkbox is ticked.

You will need to look at your browser's help to see how to do this on other browsers.

You need to ensure that JavaScript is switched on. See the answer to Q1.3 for how to do this.

See our Accessibility page for instructions on how to make text larger in your browser.

This is caused by some phone software pacakges (e,g, Skype) replacing numeric strings , which resemble phone numbers, with a phone number and country code flag. You will have to look in your software package or use Google to find out how to disable this.

You need to refresh your page. We have extensive checks to ensure that data is updated on time. You might find the problem is at your ISP, contact them to ask for their pages to be updated. See question 1.11 on how to clear your cache.

If you receive a datafeed from the Met Office, we will place a link to you on our links page. For information on how to receive a datafeed, please e-mail the Customer Centre.

You need to enable cookies to use the registered services. Our policy about cookies can be found on our privacy page.

To enable cookies in Internet Explorer 5
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To enable cookies in Internet Explorer 6
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In Netscape
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You will need to look at your browser's help to see how to do this on other browsers.
Note that firewalls such as ZoneAlarm may have settings that may have to be changed to allow cookies.

You need to enable cookies to use the registered services. Our policy about cookies can be found on our privacy page. See the answer to question 1.9 for instructions about enabling cookies.

To clear the browser's cache:

In Netscape:
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3) Click on Cache
4) Click on the Clear Cache button.

In Internet Explorer:
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2)Click the Delete files button in the Temporary Internet files box.
On an Apple Mac:
In Mozilla Firefox
Select preferences/Privacy/Cache click button Clear
In Netscape 7
Select preferences/Advanced/Cache click button Clear Cache
Safari Mac OSX, go to the safari menu sixth item down, select Empty Cache click OK in the confirmation box

You will need to look at your browser's help to see how to do this on other browsers.

See our Services page.

As a result of aviation customer feedback we don't intend to provide TAFs and METARs on WAP at this time, but will keep this under review.

A weather information guide is available.

A weather warnings guide is available. Also see Advice on actions to take when severe weather is forecast.

A weather gadgets guide is available.

A RSS feeds guide is available.

A Marine forecast guide is available.

The Met Office website has not been specifically designed to enable downloads to mobile devices. Various mobile devices use different browsers and have different levels of javascript support. This means that the appearance and functionality of our website experienced on your mobile device may not be the same as experienced on your home computer.

2 Questions about the weather and forecasts

Note the answer Q1.2. The text is updated at 0600, 1200 and 1800.

Get the latest forecast or observations from the Met Office. You can also get a detailed weather forecast via your mobile phone, including rainfall, for your location or postcode. The past weather pages also give a brief overview of the climate of various regions round the world.
However, if you want detailed climate values for a specific place try World Climate. You can enter a location when prompted and this then provides long-term average values for daily maximum and overnight minimum temperatures, also rainfall, on a monthly basis. Remember that these are averages and that there can be significant variations from these figures.

You can see the decode by clicking on the 'Key' link on the pages with the weather symbols.

The wind chill or wind chill factor is the apparent temperature felt by warm blooded creatures - primarily humans - during cold and windy conditions. However, many factors contribute to the degree of discomfort experienced by human beings, including cold windy conditions, insulation, humidity, the quality and amount of clothing worn, body temperature, physical fitness, metabolic rate and psychological condition of the subject.

Jet streams are ribbons of very strong winds which move weather systems around the globe. They are found 9-16 km above the surface of the Earth, just below the tropopause. The position of a jet stream varies within the natural fluctuations of the environment. They are caused by the temperature difference between tropical air masses and polar air masses (PDF, 863 kb). What happens in one part of the world depends on what is happening elsewhere - the atmosphere is a complete environment with numerous connections.

3 Questions about climate change

Although several aspects of climate are changing, temperatures provide the clearest evidence. For many decades, temperature near the surface has been carefully measured at thousands of locations on land and at sea. There are a large number of measurements of temperature close to the Earth's surface which are global in extent, from which we can form a global average, going back to 1860. These all show temperatures higher in the past few years than at any time during the instrumental period, even allowing for measurement uncertainties and gaps in the data.

Global average land and sea temperatures (see chart above) show considerable variability from year to year, but a clear underlying trend which shows rising temperatures until about 1940, a slight downward trend from about 1940-1975, and a rise of about 0.5 °C between 1975 and the present day.
Three independent types of temperature measurement - air temperature taken at land climate stations and on ships at night (when the interfering effect of solar radiation is absent) and the temperature of the sea surface - all show good agreement from 1900 until the last couple of decades, when land temperatures have been rising at a faster rate than sea temperatures (as predicted to be the case for a global warming due to increased greenhouse gases).
Temperatures have also been measured in the atmosphere; over the last 50 years or so by weather balloons, and by satellite remote sensing since 1979. In the mid-troposphere, about 5 km above the surface, there has been a global-mean warming. Although data are sparse in tropical regions, according to sensors on weather balloons, there seems to have been little change in temperature in the tropical mid-troposphere over the past 25 years, which is not what models predict. This discrepancy and its implications are the subject of ongoing research.

The below two images show how the observations of warming over the past 30 years or so cannot be replicated in the climate model if only natural factors are included, but can be replicated once man-made factors are added. This work used global-mean warming, and it is possible that the good agreement with observations could be as a result of offsetting errors in the model, for example by the model exaggerating the effects of man-made greenhouse gases and man-made cooling aerosols.


For this reason, more detailed studies have been done looking at the patterns of changes in temperature across the surface of the Earth and through the depth of the atmosphere. The results from these enabled IPCC to pronounce in the TAR that “…most of the observed warming over the pas 50 years is likely to have been due to the increase in greenhouse gas concentrations”, which in turn have been due to emissions from human activities, notably fossil fuel burning.
Since the TAR in 2001, several new studies have strengthened this conclusion. So, whilst we cannot absolutely exclude natural variability as the cause of warming over the past few decades, and it may have played some role, it is very unlikely that this will have been the sole reason. Our best estimate is that most recent warming is due to man's activities.

No. The below image shows the results of a recent analysis at the Hadley Centre of temperature trends over the last 50 years deduced separately from measurements on the most windy nights and the least windy nights, from the same stations. If urbanisation were causing the observed warming, one would expect calmer nights to have warmed more, as it is in these conditions that the heat island effect would act to warm the station compared to its rural surroundings. In fact, the results showed no difference between trends on windier and calmer nights, confirming that urbanisation is not to blame.

Yes. The below image shows an estimate of how solar irradiance has changed over the last 150 years. There appears to have been an upward trend from about 1900 to 1960, but thereafter little long term change (just the effects of the 11-year solar cycle). Simply working from the change in the amount of solar irradiance striking the Earth, we can calculate that this would have given a rise in global temperature of about 0.1 °C. Even if the Sun had a much larger influence on climate than currently thought, changes in the Sun could not explain the warming since the 1970s.

There have been theories which seek to explain recent global warming as due to changes in the Sun's magnetic activity (which is somewhat different from the behaviour of its irradiance). We know that increasing solar activity tends to reduce the number of galactic cosmic rays entering the Earth's atmosphere. Some theories argue that galactic cosmic rays are important for the formation of clouds, and we certainly know that clouds are an important influence on climate. In this way, the theory links changes in solar magnetic activity with changes in climate. However the link between cosmic rays and clouds is unproven, and even if cosmic rays did cause variation in clouds, this may not be in the right sense to explain climate change.

Yes. The warming effect of methane and other greenhouse gases per kilogram emitted is generally greater that that of CO2. IPCC defines a quantity called Global Warming Potential which compares the warming effect of a greenhouse gas over a given time period (usually taken as 100 years) with that of CO2 (which is given a value of 1). Most gases are more 'potent' than carbon dioxide, largely because the atmosphere already contains quite high concentration of CO2, and hence its absorption of infrared radiation (the mechanism for the greenhouse effect) is already quite saturated. In fact, the additional infrared it traps is proportional to the logarithm of its concentration. For other gases, such as methane, infrared absorption is still far from saturated, and this is the main reason why its GWP is higher than that of CO2. The value of the GWP for a gas reflects not only its infrared absorbing capability, but also its lifetime in the atmosphere and its density. By definition, the GWP of CO2 is unity.

Of course, the warming effect of a particular gas will be a combination of its GWP and its total emissions, and because man-made emissions of CO2 are much greater than any other gas, its warming effect will be greater, despite its low GWP. Roughly two-thirds of the man-made warming effect over the next 100 years are projected to be due to CO2 emissions.

Yes they do, although it is by no means certain that they will continue to absorb as much as they do now. We estimate for the decade of the 1980s (see below image) that fossil-fuel burning injected on average about 5.4 GtC/yr (billion tons of carbon, in the form of carbon dioxide, per year) into the atmosphere. In addition, changes to land use, mainly deforestation, added another 1.7 GtC/yr.

We observe that atmospheric concentrations of CO2 are rising at about 2 ppm per year, which equates to an increase in the burden of carbon of some 3.3 GtC/yr. Of the remaining 3.8 GtC/yr, we estimate that oceans absorb about 1.9 GtC/yr and vegetation and soils a further 1.9 GtC/yr. Thus, the ocean and land provide a free 'buffering' service by absorbing about half of the carbon we emit. However, this may not always be the case in future. As global temperatures rise, and rainfall and temperature patterns change, we believe that several changes to carbon absorption will take place. The net result of these changes is predicted to be a reduction in the strength of the carbon sink in vegetation and soils, leaving more of the carbon dioxide that we emit left in the atmosphere, and a more rapid increase in CO2 concentration and temperature rise than without this reduction in sinks. Although there is agreement amongst modellers that climate change will reduce the natural absorption of CO2 by the biosphere, different models estimate different reductions. But the potential for enhancement of global warming from this feedback has been clearly demonstrated.

The exchange of 'man-made' carbon dioxide between man-made emissions, atmosphere, ocean and land, is about 7 GtC per year, which also shows much larger natural exchanges between atmosphere and ocean (about 90 GtC/yr) and atmosphere and land (about 60 GtC/yr). However, these natural exchanges have been in balance for many thousands of years, leading to the pre-industrial concentration of CO2 remaining steady at about 280 ppm. The effect of the additional man-made emissions is to unbalance the budget and lead to the rise in concentrations seen since about 1850.

There are many different types of aerosols — small particulates — in the atmosphere which are affected by human activity. Some, such as black carbon (soot), are emitted directly from man-made processes, and some are generated from other man-made emissions, such as the sulphate aerosols which are formed in the atmosphere from sulphur dioxide emissions from power stations, transport, etc. Some, such as mineral dust from deserts, are entirely natural, but their concentration in the atmosphere (and hence their effect on climate) could be changed if man-made climate change leads to desertification.
Some aerosols, such as sulphates, predominately act to reflect back solar radiation (both directly and indirectly) and hence exert a cooling influence on climate. Others, such as black carbon, absorb solar radiation and have a warming effect. The warming and cooling estimates of these aerosols are very uncertain, compared to the relative certainty of the effect of greenhouse gases. Nevertheless, it is very unlikely that there has been sufficient aerosol cooling to have offset the warming effect of man-made greenhouse gases, and in the future, as concentrations of cooling sulphate aerosols are likely to decline, their lessening cooling effect may have the consequence of accelerating warming. Indeed, recent Hadley Centre estimates are that the exceptionally hot 2003 summer over continental Europe would happen typically more frequently than every decade in the absence of cooling from sulphate aerosols.

'Global dimming' is the term used to describe the observations from surface instruments showing a general reduction in the amount of solar radiation reaching the ground since about 1960, globally amounting to 2–3% per decade, up to about 1990. The dimming is variable from place to place, with some sites even showing a brightening over the period, but greatest in northern mid-latitudes. However, more recent research indicates that this trend reversed in about 1990 and since then there has been some 'global brightening', although being indirectly measured from satellites these more recent estimates may be less robust. It seems likely that the reductions, and perhaps the recent increases, may be due to changes in aerosols such as sulphates and soot (black carbon). The most recent version of the Hadley Centre climate model (HadGEM1), which includes both sulphate aerosols and soot, does simulate a reduction in surface solar radiation, though not as great as that actually observed. Neither the observations nor the implications for predictions of climate change are yet clear, and this is a subject of active research.

Climate models are a mathematical description of the processes in the Earth's climate system; atmosphere, ocean, land, cryosphere. The representation of climate processes in the model are based on experimental measurements in the real atmosphere, ocean etc, and these can be chosen within the constraints of these experiments to give the best possible agreement between model simulation of current climate and observations. We evaluate their reliability in a number of ways. Firstly by comparing their representation of the current climate and observations, including not just means but variability and extremes. Secondly, by driving them with the best estimates of changes to climate forcings over the last 150 years (natural, such as volcanoes and solar radiation, and man-made such as greenhouse gases and aerosols) and comparing the simulation of climate change from the model (sometimes called a 'hindcast') with observations of trends (in, for example, global mean temperature) over the same period. This is shown in the image below.

Lastly, some validation can be carried out by comparing model simulation of climates many thousands of years ago with reconstructions of climate of the period (so called palaeoclimatologies). Validation exercises such as these provide compelling evidence that, at least in terms of gross temperature response, the model is effectively reproducing what has been observed, and this gives us confidence that the models are adequate tools for the prediction of future climates.

Although they are made by the same sort of mathematical model, weather forecasts and climate predictions are really quite different. A weather forecast tells us what the weather (for example, temperature or rainfall) is going to be at a certain place and time over the next few days; it might say, for example, that there will be a band of heavy rain moving across Somerset tomorrow mid-morning.
A climate prediction tells us about changes in the average climate, its variability and extremes. For example, it might say that the average temperature of summers in Somerset in 40–60 years time will be 4 degrees higher than it is currently, it will enjoy on average 25% more rain in winter with three times the current number of heavy rainfall events, and 50% less rain in summer. It will not make a specific forecast such as: it will be raining in Somerset on the morning of 15 October 2044.

The two major ice sheets are on Greenland and in the Antarctic. The Greenland Ice Sheet contains enough water to contribute about 7 m to sea level, and the West Antarctic ice sheet (WAIS), which is the part of the Antarctic ice sheet most vulnerable to climate change, contains about 6 m.
A sustained rise in local temperatures of about 3 °C, equivalent to a global-mean warming of about 1.5 °C, which is likely to be reached by the end of the century if man-made emissions are not controlled, would melt the Greenland Ice Sheet, although it is estimated that this would take a few thousand years. A major collapse of the WAIS is thought to be very unlikely during the 21st century, although recent measurements suggest that contributions to sea-level rise from this source may be greater than previously estimated.

Although there is a clear rising trend in globally average temperature, there are large variations in trend from region to region. This is a consequence of natural variability of climate, which gets larger as we focus on smaller and smaller areas. This results in some areas warming less than the global average — or even cooling — and some areas warming more. In addition, naturally variability tends to be greater at high latitudes. These two factors lead to the Arctic and Antarctic having a wide variety of temperature changes. For example, the Antarctic Peninsula has warmed dramatically over the past 50 years, whereas at the same time some inland areas of east Antarctica have cooled. However, recent research suggests that changes to the winds over Antarctica, which may have been brought about by stratospheric ozone depletion, have played a significant role in the peninsular warming and the continental interior cooling.

The Gulf Stream (or North Atlantic Drift, to give it its proper title) brings warmer water from lower latitudes to the north-east Atlantic, and gives north-west Europe a milder climate than it would otherwise have.
The mechanism driving circulation in the North Atlantic, of which the Gulf Stream is a part, is shown in the below image.

This mechanism could be affected by man-made global warming in several ways, for example by increased rainfall over the N Atlantic, and hence there is the potential for the Gulf Stream to be reduced, or even switched-off, by man's activities.

When we use the Hadley Centre climate model to look at the response of the N Atlantic ocean circulation to future man-made emissions, shown in the above image, we see that reductions of about 20% by 2100 are predicted, rather than a complete shutdown. Other good climate models see greater or lesser reductions, but none produces a shutdown over the next 100 years.
The Hadley Centre model has also been used to investigate the impact on climate of a hypothetical shut-down of the THC. It predicts that the whole of the northern hemisphere would be cooled, especially the north Atlantic; the UK might see a cooling of 3–5 °C. Daily minimum temperatures in central England in winter could plunge by 10 or 20 °C, and this would likely have a bigger effect on UK society than global warming. However, as was pointed out above, this is a 'what-if' scenario and not a prediction.
The model predictions of only partial shut-down of the THC seem reassuring, but we do not fully understand the reasons for the stability of the ocean circulation, and there have been recent measurements in the N Atlantic which seem to be at variance with model simulations. Hence, research continues to quantify the risk of this potentially high-impact outcome of climate change.

Over the past half million years or more, the world has alternated between ice ages and interglacials (periods between ice ages), with interglacials occurring every 100,000 years or so. We have been in the present interglacial for about 10,000 years. Evidence is strong for this behaviour to be due to changes in the Earth's orbit around the Sun, and the angle of its rotational axis, usually referred to together as 'astronomical forcing of climate'. This theory was formalised by Milankovic in the 1920s, and has been well confirmed by records from ice cores, ocean sediments, etc. Thanks to our knowledge of orbital mechanics these astronomical changes can be predicted, and it appears that astronomical forcing will be of little significance over the next 40,000 years or so, so the next ice age will be a very long time hence. Thus is it on a very different timescale to man-made global warming and cannot counteract it; if no action is taken to limit fossil-fuel emissions, for instance, climate will have changed very substantially by the time the next ice age starts.

Substantial quantities of methane are emitted naturally from wetlands, and this emission is expected to change as wetlands change. Changing rainfall patterns will cause some wetland areas to increase in extent, others to decrease, and increases in temperature will act to increase emissions from wetlands. One version of the Hadley Centre climate model includes a description of wetland methane, and this predicts an increase in natural wetland emissions by the end of the century equivalent to the amount of man-made emissions projected for that time, thus leading to a more rapid rise in methane concentrations, and hence warming.
On the other hand, the chemical reactions in the atmosphere which destroy methane are expected to become more efficient in future, largely as a result of increased water vapour. This will act as a negative feedback on methane amounts.
Methane is also stored in permafrost, and it is likely that some of this will be released as surface warming extends into the permafrost and begins to melt it.
Finally, huge amounts of methane are locked up in methane hydrates methane clathrates) in the oceans. They are currently at high enough pressures and temperatures to make them very stable. However, penetration of greenhouse effect heating into the oceans may destabilise them and allow some of the methane to escape into the atmosphere. The potential for this to happen is very poorly understood. There is concern that this may be another positive feedback not yet included in models, although there is little evidence for this from the behaviour of methane during the large temperature swings between ice ages and interglacials, and in particular over the last 50,000 years.

Not really, although there are links between the two. The depletion of ozone in the stratosphere over Antarctica (the 'ozone hole') was first discovered by scientists from the British Antarctic Survey in the mid-1980s. It is caused mainly by emissions of man-made chlorofluorocarbons (CFCs), which find their way into the stratosphere where they decompose into chlorine compounds which destroy ozone each autumn. Despite the fact that emissions of CFCs have been very severely cut back by the Montreal Protocol, because they have a lifetime of order 100 years, their concentration in the atmosphere has only recently started to turn down, and the ozone hole is expected to remain as large as it is now for decades to come, before it slowly recovers.
Links with climate change are threefold. Firstly, the CFCs which deplete ozone, and also some of their ozone-friendly replacements, are greenhouse gases and so also contribute directly to global warming. Secondly, the reduction in stratospheric ozone, both over Antarctica and more generally globally, acts to cool climate slightly (see image below).

Lastly, there is concern that increasing concentrations of CO2 from fossil-fuel burning, because it is cooling the stratosphere (see image below), aids the formation of the small particles in the stratosphere on which chemical reactions take place, and may be prolonging the ozone hole.

The impacts of climate change on society and economies will be many and various, in sectors such as agriculture, water resources, ecosystems, health, coastal communities, etc. It is too broad a topic to be covered in this FAQ, but is comprehensively addressed in the report from Working Group 2 of the IPCC TAR, which also contains a shorter Technical Summary and Summary for Policymakers.
Recent UK research on the global impacts can be seen in the April 2004 special edition of the journal Global Environmental Change, edited by M. Parry.

Plant growth depends upon several factors. Plants require sufficient warmth, moisture, light and nutrients in the soil to photosynthesise, that is, to draw down CO2 from the atmosphere into the body of the plant. If these other environmental factors are adequate then higher concentrations of CO2 in the atmosphere will indeed enable plants to grow more rapidly. However increasing CO2 concentration also changes climate, and if this becomes too warm or too dry then plants will no longer be able to take advantage of the CO2 fertilisation effect. Hence there is a balance; over the past century the enhanced growth has dominated and vegetation across the globe has acted as a vital sink for man-made CO2 emissions. But, as described in the below image, our research indicates that in future the beneficial effect of higher CO2 concentrations will be reduced as the associated climate change in some areas will reduce the ability of vegetation to absorb man-made CO2.

A further concern is the sensitivity of plant growth to concentrations of ozone, which is expected to increase in the lower atmosphere due to reactions between man-made emissions such as nitrogen oxides and hydrocarbons. Ozone can have a damaging effect on plant stomata and so there is a risk of reduction in vegetation productivity as ozone increases.

Future emissions of greenhouse gases from human activities will depend upon factors such as population growth, economic development, energy use, technological change, society's attitudes and political leadership. Obviously, we cannot know how all these factors will change, and what pathways emissions will follow in the future, but we can generate possible scenarios; the Intergovernmental Panel on Climate Change did this in its Special Report on Emissions Scenarios (SRES) in 2000. It considered various 'storylines' of how the world will develop and used models to estimate emissions which would follow from these storylines. All of the emissions scenarios are 'noninterventionist'; that is, they assume no policies to reduce emissions for the purpose of mitigating climate change.
Scenarios of climate change over the UK were published in 2002 for Defra and the UK Climate Impacts Programme (UKCIP02). They were based on climate predictions from Hadley Centre models, down to a resolution of 50 km. The headline predictions were for warming throughout the year but particularly in summer; less rain in summer but more rain in winter; greater frequency of heavy rainfall events in winter; reductions in snowfall and frosts; continued rise in sea level; increase in the frequency or height of coastal high water events. There is considerable regional variation in the changes; in broad terms they are expected to be greatest in the south and east, smallest in the north and west. The scenarios report stressed the uncertainty in the scenarios and suggested some ways of handling this. More detail is available from the UK Climate Impacts Programme web site.

It may indeed be more pleasant to have warmer temperatures in autumn, winter and spring. However, summers in the UK, especially in the south-east and in cities, could be uncomfortably warm, leading to heat-related medical problems and aggravating respiratory conditions. There is a well known link between high temperatures and mortality rates. The exceptionally hot summer of 2003 resulted in 22–35,000 additional heat-related deaths across the continent of Europe, and some ¤10 billion uninsured crop losses. On the other hand, less-cold conditions in winter would lead to fewer deaths from hypothermia.
Of course, climate change means much more than simply an increase in temperatures. The summers will probably become drier as well as hotter, leading to an increased risk of drought and pressure on water resources. Winters are likely to bring heavy rainfall events more frequently, with increased risks of urban and river flooding. As sea level rises, our coastline, especially in the south and east will be increasingly at risk, and more frequent high water events would be particularly damaging if the level of protection is not raised.

Climate change will have impacts not only on the environment, but also on society and the economy. To find out more about these, please contact the UK Climate Impacts Programme, which is based at the University of Oxford.

The four climate change scenarios developed for the UK are based on four possible future pathways of man-made greenhouse gas emissions, derived from the IPCC SRES report, as shown in the image below. IPCC states that these should not be regarded as equally probable, but there is no information on the relative likelihood of each. Some organisations are attempting to develop probabilistic emissions scenarios, but these are not yet at the point where they are reliable enough to be used as the basis for climate change scenarios. In the case of the UK climate change scenarios, it is best to consider the full range, rather than trying to identify one scenario as the most probable.

4 Questions about units of measurement

The Celsius scale is the World Meteorological Organization standard for temperature measurement and is used throughout the world by the meteorological community for global exchange of information.

From Celsius to Fahrenheit - F=9/5*C+32
From Fahrenheit to Celsius - C=5/9*(F-32)

where C is the value in Celsius and F is the value in Fahrenheit.

Multiply the millibar value by 0.02953 to get the value in inches.

The SI unit for pressure is a pascal. The worldwide meteorological community uses the hectopascal, i.e. a hundred pascals, which is the metric equivalent of a millibar. However, millibars (and inches) are still used in some public forecasts in the UK and USA.

UTC stands for Universal Time Coordinated and it is equivalent to GMT. The Royal Observatory web site has more information on the history of timekeeping. It also has a description of local time.

5 Miscellaneous questions

Many questions about hurricanes are answered on our Tropical Cyclones page.

Atmospheric pressure varies over time and space and also varies with height. Since the altitude of the barometer normally stays constant (the station height) a correction is made to the reading to make it equivalent to the mean sea-level reading. This is done so that readings from different locations can be compared, with differences due to height being removed. Aneroid barometers are normally adjusted to mean sea-level values - read the barometer's instructions to see how to adjust the instrument (normally by a screw on the back). Mercury barometers cannot be adjusted (don't tamper with them because a mercury spillage is a health hazard).
To get the value for your barometer, choose a high-pressure day, pressure values are not changing very much - you can watch the TV forecasts for such a day. Go to the Met Office's observation page and choose the station nearest to your location - on a quiet weather day the distance away from you will not be significant. Adjust the barometer to the station's pressure value. You can check your barometer on other days but will have to compensate for fast-changing pressures or distance if the pressure is low or changing fast. The observation includes information about how the pressure is changing.

For many a white Christmas means a complete covering of snow, ideally falling between midnight and midday on the 25th.
However, the definition used most widely, notably by those placing and taking bets, is for a single snow flake (perhaps amongst a shower of rain and snow mixed) to be observed falling in the 24 hours of 25 December.