I forget the name they used but it has something to do with the ground temperature at this time of year which is cold so its not locking that 20deg, etc heat in at the moment so the air cools down fast, particularly with clear skies.indeed . although warm you can still feel the bite in the air even at 3 in the afternoonThe weather forecast had a go at explaining it. Some rural villages are -2deg at night and +20deg in the dayI forget the name they used but it has something to do with the ground temperature at this time of year which is cold so its not locking that 20deg, etc heat in at the moment so the air cools down fast, particularly with clear skies.
The pattern recently is going out in the jacket and returning in a t-shirt but its been very cold the last few days so would have to be generating serious heat to remove it 