Ahhh. Summer. I recall last summer looking up at a cloudless sky and seeing aircraft condensation trails – ‘contrails’ – in the sky, and then watching them grow until by late afternoon the whole sky was nearly covered. I reflected that since the radiation imbalance that we believe is leading to climate change is of the order of 0.1% of solar intensity on a sunny day (rough 1 W/m^2 as compared with 1000 W/m^2) this must be a very significant effect over quite a large region of the Earth.
I have also seen the ‘contrails’ shrink and disappear quickly after the aircraft has passed, and also seen the trail sometime stay quite still in the sky, and at other times develop ‘kinks’. All these different behaviours depend on the temperature and humidity of the atmosphere at these heights (10,000 m – well above the normal ‘weather’ clouds. The jet engines emit water vapour which is hot and quickly cools and condenses to make solid ice crystals (the temperature is around -60 °C). If the atmosphere is supersaturated – i.e. contains moisture looking for a seed on which to condense, these ice crystals can initiate cloud growth. If the atmosphere is undersaturated, then the ice crystals will simply evaporate usually slowly.
The effect on climate is hard to quantify. The high clouds reflect sunlight during the day, but at night will also additionally reflect infra red radiation back to earth, with the exact balance requiring a detailed calculation. That is why I enjoyed seeing a short under documented piece on the BBC web site showing satellite photographs of extremely large areas (more than 10% of the UK land area) of cirrus cloud being formed as a result of contrails from just a couple of aircraft. If we didn’t enjoy the benefits of flying so much, such images would cause us to abandon the practice immediately.
You can read more on this effect here