The UK experiences some of its largest diurnal temperature ranges in early spring due to a combination of factors.
By March and April the sun is much higher in the sky and the days are lengthening, allowing more energy to be delivered and more efficiently. After the long winter, however, the ground and lower atmosphere remain chilly allowing rapid overnight cooling.
Clear skies underneath a spring high pressure systems can increase this contrast further.
At night heat is lost quickly and until the spring equilux, the nights are still longer than the days, all leading to cold mornings.
During the day as the sun rises, those same now sunny skies allow rapid warming as the sun is more directly overhead, leading to warm afternoons.
Dry air heats and cools more quickly than humid air and this can further widen the temperature gap.
These factors combine to increase our diurnal temperature range during the early stages of spring, especially during settled, sunny spells with light winds.