'refractive index' of layers of air in the atmosphere differs, causing the stars to shimmer. Seeing is always worse earlier in the evening than later.
So, how can you forecast the likely sky conditions for your next astronomy session? Television and radio weather forecasts tend to cover large regions of the country and arc aimed at the general public, so their idea of a 'clear sky' is not necessarily the same as ours. What's needed is an even more localised forecast, such as those on weather-related websites.
These sites allow you to zoom in on your local area to get information, from simple cloud forecasts with temperature, wind speed, pressure and likely rain patterns to satellite images. Satellite images are handy for showing the current atmospheric conditions in both visual and infrared light, and how they may develop over the following hours. One website even predicts the seeing for your area (www.meteoblue. com). The lower the seeing in arcseconds, the better. Forecasting on the day is the most reliable method as cloud cover changes very quickly.
Unfortunately, forecasting is not an exact science and local conditions need to be factored in. For example, the location where this is being written - at the foot of the South Downs - can have very different conditions to those to the north and south. On a typical summer's day, with the wind coming off the sea, the skies will be clear over the coast. The air then cools as it rises over the South Downs, forming clouds. As the air descends on the north side of the Downs it will warm and dry out, leading to the cloud breaking up once again.
Weather websites have several useful displays. Satellite images are useful for determining what the cloud cover is going to be like later in an evening. They can be animated to give a view of what has gone before, indicating what is likely to follow.
Infrared images are also useful. They show the infrared energy radiated from the cloud tops and the Earth's surface. Warmer objects radiate more infrared radiation than cooler objects, so these images can be used to determine temperature. Darker areas show warmer temperatures and lighter areas show cooler temperatures. Rain falls when cloud temperatures drop and reach the dew point, so lighter colours in infrared images indicate cooler cloud tops and the
The steadiness of the atmosphere can mean the difference between a blurred picture (top) and a clear one (bottom)
The expert increased likelihood of a shower. Infrared images aren't suitable for showing low-lying cloud, because low clouds are at a similar temperature to the Earth's surface and won't show up. On the other hand, at least they're available 24 hours a day.
Visual light images show light reflected from the Earth and cloud tops back towards the satellite. They're better than infrared images for showing low-level cloud, because these are more reflective than the land or sea below. They are limited to daytime availability, however.
There are also barometric pressure charts. These are useful weather indicators, with high pressure indicating clearer conditions and lower pressure warning of unsettled conditions, cloud and stronger winds. Simple home weather stations can also tell you the current pressure, wind direction and speed. There is a strong correlation between good seeing and low wind speeds, irrespective of the atmospheric pressure.
Using a combination of information from weather websites and local knowledge is definitely a good way of obtaining a 'best guess' of what the weather holds for the next few hours. But if you're still confounded and everything conspires against you, be reassured - it's surely the perfect opportunity to stay inside in comfort and enjoy your copy of Sky at Night Magazine. ©
The Met Office's Helen Chivers explains forecasts
Why does the presence of clouds (or lack of them) often differ from what's forecast?
The weather is often changeable because the UK lies at a 'crossroads'. To the south and east is the landmass of Europe, while to the west the world's second largest ocean provides a vast supply of moisture as winds blow in from the Atlantic. Our prevailing winds generally come from the west, bringing weather systems across the country. The timing of these can make the difference between a clear or cloudy night.
What's the best way to use the Met Office site?
Check out the pages that show the latest observations: www.metoffice.gov.uk/ weather/uk/uk_latest_weather.html. Satellite images show areas of cloud and how they have been moving, clearing and developing over recent hours.
Are there any obvious signs that the coming night will be clear?
Weather fronts are areas where two air masses of different temperatures and humidities meet. A cold front marks the change from warm, humid, cloudy air to cooler, dry and much clearer air. When a cold front moves across an area there will often be a fairly dramatic change to cloud-free skies, which will last for a few hours before showers develop or cloud moves in.
What are the causes of good transparency and good seeing?
They result mainly from the clarity of the air, which in part depends on the air mass affecting the country. Air masses are created when air stagnates. As a rule, for the UK the air will be clearer when the wind is blowing from the north than the south. The clearest air can be found in an arctic maritime air mass - the air has its origins in the cold, dry region of the frozen Arctic ice cap and is blown towards the UK on northerly winds. This means the air does not pick up pollutants from continental regions and the visibility will be crisp and clear.
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