Frost Dates: First and last frost dates for Mt Hamilton, CA
What's a safe temperature for tender plants? When the nightly temperature falls, frost can form, even above 32°, because the air is colder high above the ground and the frost can form up there and then fall down onto your garden and do some damage, even if it's 36° on the ground. So many factors come into play, including wind, concrete, houses, trees and other structures, etc etc etc. Because of all this, you might want to consider 36° as "the danger zone".
In your average springtime, you have a 90% chance that there will be no 36° nights by June 17. In other words, you can pretty much count on being safe from frost by that day. But we want to get those tomatoes in the ground as soon as possible, right? We see that there's still an 80% chance of 36° on May 17, so we don't dare plant that early. We wait a few days and by May 29 we are at the 50/50 point. At this point, we are close and we can start watching the weather forecast. If the upcoming week's forecast doesn't show below 40°, then it's probably okay to risk planting out your plants.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| Last 16° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| Last 20° | Feb 6 | Jan 21 | Jan 6 | Dec 15 | -- | -- | -- | -- | -- |
| Last 24° | Apr 14 | Mar 26 | Mar 13 | Feb 28 | Feb 15 | Feb 1 | Jan 10 | -- | -- |
| Last 28° | May 14 | May 3 | Apr 26 | Apr 19 | Apr 13 | Apr 7 | Mar 31 | Mar 23 | Mar 13 |
| Last 32° | Jun 6 | May 27 | May 20 | May 14 | May 8 | May 2 | Apr 26 | Apr 19 | Apr 9 |
| Last 36° | Jun 17 | Jun 10 | Jun 6 | Jun 2 | May 29 | May 26 | May 22 | May 17 | May 11 |
The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:
- You have a small 20% chance of getting 32° by October 28.
- There is a 50% chance of being hit by a 32° frost starting around November 15
- You have a 80% chance of seeing 32° by December 3
- Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.
| Temperature | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| First 16° | -- | -- | -- | -- | -- | -- | -- | -- | -- |
| First 20° | Dec 9 | Dec 28 | Jan 16 | -- | -- | -- | -- | -- | -- |
| First 24° | Nov 16 | Dec 5 | Dec 19 | Jan 2 | Jan 16 | Feb 2 | Mar 4 | -- | -- |
| First 28° | Nov 7 | Nov 17 | Nov 25 | Dec 1 | Dec 7 | Dec 13 | Dec 19 | Dec 26 | Jan 6 |
| First 32° | Oct 19 | Oct 28 | Nov 4 | Nov 10 | Nov 15 | Nov 20 | Nov 26 | Dec 3 | Dec 12 |
| First 36° | Oct 7 | Oct 14 | Oct 19 | Oct 23 | Oct 27 | Oct 31 | Nov 4 | Nov 9 | Nov 15 |
Data is provided by the NOAA National Centers for Environmental Information.