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I was curious when a fruit tree, say, apple or pear, is safe from late frosts? For example, in this area, most years are poor in fruit production because of late frosts. So, if blossoms haven't opened up yet, but are ready to, and it frosts heavily, are the blossoms still safe? Or, if blossoms are open and it frosts, are they killed? Or, (finally!) if fruit has set, and it frosts, will the fruit be killed, or is it safe at that point?
The first scenario happened here the other day; the blossoms were just ready to burst on my pear tree, and it frosted very heavily.

Hi Taiji
Apples and pears can suffer from spring frosts in varying amounts.
In apples from the early green cluster stage they can suffer damage from prolonged exposure to temps below -2 c. The longer the exposure the more the damage. Anything over two hours is reckoned severe.
The nearer you get to full bloom the more damage that can occur.
Some varieties are more susceptible to damage than others though. Over here Bramleys will suffer a wipe-out when others have suffered little damage.
Pears are inclined to suffer more in that they generally bloom earlier than apples ...so are more likely to be hit with the early frosts. However varieties like conference , being pathnocarpic, can still carry a crop even after frost damage.
Both apples and pears can suffer damage and still give a good garden crop but skin damage or misshapen appearance due to the frost can make them uncommercial.
Damage can happen though even at just freezing point once the flowers are at full bloom, and even after the fruit has set frost can cause a separation of the skin on young fruitlets that will either cause a fruit drop or make very rough and creased skin.
On our farm here in the UK we are in a frost pocket so have to protect our orchards from these spring frosts. We use over head water protection. Freezing water continuously through the night over the trees ...it holds the ice that forms at freezing point which the blossom can take without damage.
Trouble is it needs a lot of water. In our case over 50 acres, we use a million gallons a night.
Even so the fruit mans mantra is in general.......' Frost is never as bad as you think....hail is always worse'.
If that's any consolation

By the way. A quick check to see if your fruit has been harmed is to pick a blossom off the tree and pinch it in half with your nail. If the base of the fruitlet is black....it's damaged. More often than not there will be one or two survivors in a cluster so it's only a rough guide.
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Thank you. I guess it sounds somewhat hopeful, won't know for sure til later. I will try the blossom test. Thx.

I checked my pear blossoms today; it seems like the frost maybe made them halt their march toward blooming, and now they are swelling again and ready to burst. Checked the blossoms and don't see any blackness at the base. Looks good for now, but there's still likelihood of freezing for another 2 months maybe.

Sounds like all ok so far.....good luck for a warm spring!

Thx. It's always a gamble in these parts!