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Weather => Forecasting => Topic started by: Hugh Westacott on August 19, 2012, 06:18:09 PM

Title: Weather forecasting by observation
Post by: Hugh Westacott on August 19, 2012, 06:18:09 PM
It is helpful to be able to forecast changes in the weather and to be able to interpret what the sun, wind and clouds are telling you. Here are some examples that I have collected from various sources but I'm not sure that they will always hold good or that  the explanation given is necessarily accurate.

Some of the old country sayings are reputed to be based on close observation. Here are some examples:

'Red sky at night, shepherd's delight' is often true if the red sky is caused by the rays of the setting sun reflected on clouds very high in the sky, indicating that a cold front has passed and that there will now be a period of settled weather.

'Red sky at morning, shepherd's warning' is usually true if the sun's rays are shining on high clouds which signal a warm front approaching bringing rain with it.

'Rain before seven, fine by eleven' is based on the observation that most depressions, unless very deep, usually pass through in four hours or so (I'm not sure that this is so true of Scottish weather!). I assume that the times are irrelevant and were chosen in order to make a helpful mnemonic.

There is also supposed to be a useful way, known as the 'cross winds rule', of determining if the weather will improve or deteriorate within a few hours. Stand with your back to the true wind (i.e. ensuring that the surface wind is not distorted by natural features such as hills). If the clouds come from the left, the weather will usually deteriorate. If it comes from the right, the weather is likely to improve. If the clouds come from behind, the weather is unlikely to change.

I'd be interested in your comments. I imagine that these methods of forecasting only apply to the UK and are only likely to be helpful to our overseas members when they  visit us.

Title: Re: Weather forecasting by observation
Post by: Lost Soul on August 19, 2012, 06:59:47 PM
Just looked up the Cross Wind Rule on the internet.  This web site aimed at the yatching community has some simply stated useful information.

http://www.boatingsidekicks.com/webdangers/weather/weather2.htm (http://www.boatingsidekicks.com/webdangers/weather/weather2.htm) To quote from it.

Place your back to the lower winds, and if the upper winds are coming from the left, the weather will most likely deteriorate. Flags blowing in the wind and the rustling of tree leaves are often useful in determining the direction of the lower winds. Generally, the direction of the air in the upper atmosphere is determined by the movement of the clouds associated with the advancing front.

Title: Re: Weather forecasting by observation
Post by: adi on August 20, 2012, 12:24:50 PM
In the Northern hemisphere when you stand with the wind to your back the low pressure is to your left, this is called the Buys Ballot's Law.

Title: Re: Weather forecasting by observation
Post by: Walking Beaver on August 20, 2012, 01:17:04 PM
Thanks a lot for that information, did not know that.

Title: Re: Weather forecasting by observation
Post by: Callum on August 20, 2012, 04:06:23 PM
Us too at the weekend, heavy rain forecast on local radio, ended up needing sunscreen 8)
Title: Re: Weather forecasting by observation
Post by: adi on August 20, 2012, 04:38:42 PM
LOL to be fair the weekend was one of those situations where despite all the information available none of the models could pin down where the weather was going to be. The only sure thing come Sunday morning was the location of the potential area for thunderstorms and on this occasion the the atmosphere played ball. Believe me when I say we can forecast where the storms are likely to happen we can not 100% forecast if they will happen. We probably have around 80% or 90% bust days.