The soil will quite possibly be starting to warm up.
A sure sign of this is the emergence of annual and perennial weeds.
Do not be tempted to sow / plant out too early.
Plants sown / planted later have a way of catching up with those that were sown / planted earlier, this can be generally put down to soil temperatures, rather than ambient temperatures!
Plants sown / planted in cold soils are generally held 'in check' ( stop growing) assuming the don't die off first!