As the sun rises higher in the sky during the spring months, the amount of solar radiation absorbed by the ground increases. This warms the ground up, in turn heating the air near to the ground too.
As air warms, it rises, and as it rises into the atmosphere it cools and the moisture trapped within the air eventually condenses to generate clouds. Specifically, the cotton-wool, fair-weather cloud we often see is called Cumulus.
This process of rising air is called convection, and the resulting Cumulus are a form of convective cloud. If this process continues through the late morning and into the afternoon, Cumulus clouds can often spread out to fill the entire sky. A fine, sunny morning can therefore transform quickly into a rather grey, overcast afternoon, a process known as convective infill. During the spring-time months, sunshine amounts play a big role in air temperatures, and thus it can feel colder when convective infill blocks out the sun.
The rest of this week will provide a good example of springtime weather contrasts. Chilly continental air that lingered through the start of this week will be driven away by milder tropical maritime air, allowing temperatures to reach into the low-to-mid teens Celsius. However, the building warmth will be accompanied by rain and showers at times. Heading into next week, the mild conditions are likely to continue, although perhaps with largely dry days and some convective infill.