Wednesday, September 10, 2008

Red Sky at Night and Dew on the Grass

With the daylight hours getting noticeably shorter over the last week or two it has let me try out another element of weather lore.
The item in question is the saying that "if there's dew on the grass no rain will come to pass". The idea of this is the presence of dew indicates a temperature difference which is key to good weather. Essentially, more cloud cover means higher overnight temperatures and less dew. As far as the rule goes it seems to be mostly accurate. I've seen some light rain in the day after a dewy morning but nothing serious. I'd say this rule is accurate in general for dry weather, sadly though, you have to be up pretty early for it to be any use in high summer!
The second rule I've been observing is the old concept of a red sky at sunset presaging good weather the following day. The phrase being "red sky at night shepherd's delight" where I grew up. It is important when looking at sunsets to discount the sun simply being red but the whole sky. I think this rule, in its myriad versions, is clearly accurate but you must be honest with yourself if the sky is genuinely red - not simply orangeish or with a red sun.
A third rule which has given me some good results is the presence of "halos" around the sun or moon. These occur when the light is diffused through damp air and are a portent of wet weather. If the moon has nice, clear, sharp edges then the air pressure is high so clear weather will follow. It isn't a good idea to damage your eyes by staring at the sun.
The fourth rule I'll mention is one which has arisen from my own observations which I'm still working on. From what I have observed vapour trails from planes are most prominent on days of good weather. There are no doubt some reasons for this (concerning cloud altitude, visibility and air humidity/temperature) but for the time being I'd be interested if anyone else has ever noticed a correlation.