In the UK sunbathing is common and it takes a long time to get sunburn. While part of this is due to the higher latitude, a big proportion is due to the cloud cover caused by the jet contrails. In New Zealand you will burn after 15 minutes in the sun and although the lack of ozone protection is part of the story the very clear air gives the Sun direct access to the surface and projects more heat.
Evidence of this comes from the 9/11 disaster in the USA as, when the jets were grounded for three days the temperature of the whole country rose by 1.8 C and the then settled back to normal.
The contrail cloud cover has two effects where firstly it reflects incoming heat back into space during the day and secondly it also stops heat that is nearer the ground escaping and therefore raises the temperature, which is most noticeable at night.
The combined effect is a flattening of the temperature range.
The worry here is that if we eventually get to the point where we really do start to recognize that climate change is ruining the planet and cut back hard on the big polluting industries we have locked in a nearly 2 C temperature hike across the Northern hemisphere which would be instantaneous.