Planes can cause rain and snow

An article in the July 1 edition of the journal "Science" suggests that aircraft movements can cause extra rain and snow around busy airports.

The article, by Andrew Heymsfield, suggests that when aircraft fly through clouds they create ice crystals that can serve as seeds around which water droplets can form. In certain circumstances, these droplets can cause precipitation on a localised basis. The area around major airports can thus sometimes get an extra amount of rain or snow. This article has been widely reported in the world press.

We were told that the Chinese Government had tried to use cloud seeding just prior to the opening ceremony of the Beijing Olympics, though with the intention of avoiding precipitation on the day of the opening ceremony. This type of seeding uses chemicals such as silver iodide to encourage precipitation, so they surely would have done it well before the ceremony, if indeed they did it.

The effects of the passage of aircraft can act in a similar way, although not needing any specific chemical substance. Of course, even if it does happen, the water droplets formed may not be enough to actually cause, or increase, precipitation.

It is also said that the effect can be to make "holes" in clouds. We have all probably seen such holes, which are invariably so placed that we stay under the cloud but other places get the sunshine.

I would suppose that the vapour trails which can be created when aircraft fly across what looks (looked!) like a clear blue sky, are a similar effect. This just has to be taken as one of the effects of a high volume of air traffic, which most people would probably classify as undesirable most of the time.

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