BEAUTIFUL blue sky days seem to be increasingly rare.

Those wonderful crisp mornings when there is not a cloud to be seen – horizon to horizon, clear blue – are becoming few and far between because of contrails from jet planes.

I sat in my conservatory one morning last week to enjoy breakfast and watched 17 planes go over in just 30 minutes, and there were probably just as many on other flightpaths over other parts of the Felixstowe area which I couldn’t see.

Many of the aircraft were very high – around 25,000ft – and not even landing at airports in the UK, while the rest were between 8,000ft to 12,000ft up.

Indoors, with the windows closed, they were not particularly noisy, the rumbling of one or two standing out.

The point was that when I sat down the sky was cloudless and after watching the flights with fascination for half an hour there was not much blue left to enjoy.

According to some experts, the plane trails vanish in 15 minutes.

But most scientists say this is rubbish – and it can take hours for them to disperse.

In the meantime, blown by the wind the thin trails fan out across the sky, merging into one another, blotting out the blue and the sun.

It’s impossible from ground level to guess distances up there in the ether, but the Evening Star’s weather expert Ken Blowers once told me these contrails can spread to around four miles across.

The vapour trails are condensed water made by the jet engines of the passenger and cargo planes, mostly at high altitude where the temperatures are coldest.

The hot gas cools as it is jettisoned into the surrounding colder air and forms tiny crystals which we see as spectacular trails marking the aircraft’s path – a dead giveaway showing just how planes go over an area.

On the internet, it’s possible to watch sequenced satellite photos which show how Britain, and other parts of Europe, can go from cloudless to covered with contrails in just a few hours.

The photos show the flights going in and out of the airports, and those overflying, and the trails they leave spreading and joining.

Not every plane will create a contrail and sometimes those they leave will not be long lasting – much will depend on the high altitude air temperature, wind speed, and so on.

The vapour trails are harmless – though internet conspiracies can be read which suggest governments are using them to dump all sorts of chemicals on us to test out our reactions and “control” our behaviour and our weather, even population growth!

Of course, every government denies such activity but it doesn’t stop the endless speculation from believers.

For me they are just plain annoying – creators of artificial clouds which just spoil a perfect day.