Frost in the Spring is the great enemy of the fruit grower

"When the bud scales begin to move the danger starts. The bud then puts out two tiny leaves which are known by the endearing name of "early mouse ear". At this stage damage can be done at between 7 deg and 5 deg of frost, which is 25 deg and 27 deg F. "Mouse Ear" comes three days later followed by "early green bud", "green bud", "early pink bud", "pink bud" and finally "full bloom". At each stage the susceptibility of the bud to frost gets greater by half a degree Fahrenheit until at full bloom it can be killed by a four hour frost at 30 deg F. From then onwards the susceptibility of the apple decreases by approximately half a degree every three days. The periods during which loss is likely to occur is from late March to the end of May. It is a long time to live on one's nerves."

Extract from The Cuckoo in June: Tales of a Sussex Orchard by David Atkins
ISBN 0-7089-3624-5

The author tells how, in 1954, he abandoned the secure life of a chartered accountant and bought a small apple farm in the Weald of Sussex. With no knowledge of fruit farming, he found himself at a loss in a strange rural worls. He tells, with many an amusing anecdote, how the farm survived drought, frost and hail and for a time prospered.