When the temperatures drop to 30 -32 for a short time, you get a light frost.
When the dew on the grass, car top freezes as snow flakes , it is a frost. You might no even see snowy white stuff on your lawn or car but may have had frost.
Now if the temp dip more and stick around longer then you will have "Freeze".

Seysonn

Here is a good definition of frost vs freeze vs hard freeze.

Here is a link that might be useful: Frost vs hard freeze

Do I need to wait to transplant my tomatoes until there is absolutely no frost? The last frost date for my area apparently doesn't even exist... it just says "rare." But I definitely have seen that light frost on my back lawn a couple times this month.

From what I have read it is far too early to be planting them in Berkley. Normally seed starting time indoors isn't until Feb and plant out is April weather permitting.

But if you want to push it then check your soil temp. If the soil temp 3" down is at least 60 degrees you can plant them but you will have to cover and protect them on the chilly nites. If the temp is less than 60 degrees then it is a waste of time to plant them yet.

Post over on the CA Gardening forum for recommendations from local growers.

Dave

digdirts's advice is really good. When he says weather permitting, that is the part I'd take more seriously than temperature in that moderated climate. It is not until the end of April that the rains give you a break from a cool climate that is extremely moderated, and then becomes nice 'n dry. Your weather is different from most of us in that your whole year is sandwiched between January nightly lows typically around 42 F lows and August daily highs around 74 F.

With night time temperatures not breaking 50 F till the end of May, the rain will promote weakened plants and disease if you start too soon. So, taking "weather permitting" a step further, the time to put your plants in the ground there is the first 10-day forecast of minimal rain after April 15 or so, but not later than early May, and then hope ;-).

Your season runs well through the end of October, giving you about 165 days of great weather even if you starting late as May 7. That is a really long season, so you want to have plants that are very healthy ... like running a marathon in your climate. At the beginning, don't make an early sprint so you'll have the energy to make a strong finish. Summer will be a fantastic run there! If you do want an early sprint and don't have many plants, the thing to do is get larger transplant pots and bring them out on days it doesn't rain, and otherwise keep them near a window if you have such a place on rainy days would nicely give as much as a month earlier earlier if you are enthusiastic about moving pots around.

"Climatemps.com" has a convenient linkable graph that matches up with the weather service numbers

PC

Thanks for the advice. I guess what I'm doing right now will be a science experiment then haha! I followed advice from Burpee's app, and started some tomato seeds in early December. I have 5 growing right now that I've been taking outside every morning around 7 am and bringing inside at 7 pm.

I probably have a few more weeks until they start outgrowing their peet pellets. Maybe I'll put a few in pots I have lying around, and a couple directly into the raised bed I have slated for them--which is the warmest part of the yard by far. I'll probably start a few more in a few weeks just to be safe.

The rain comment is somewhat amusing though... there hasn't been any real rain for about a month. Wish the drought would end. It rained in November like I've never even seen--hope that wasn't the last of it.

I'm scientific by practice and outlook, and the question is of interest. In gardening terms, it involves the difference between leaf surface ice crystals and actual plant leaf cellular destruction.

Frost itself can destroy some plants (basil is a great example - the leaves just turn black - and I don't know why). But true freezing causes expansion of the plants cells bursting them (unscientifically, turns them to mush).

So in general, think of a frost as a leaf surface thing and a freeze as a deep down thing.

"The rain comment is somewhat amusing though... there hasn't been any real rain for about a month. Wish the drought would end. It rained in November like I've never even seen--hope that wasn't the last of it."

LOL! That's the part about hoping for the best.

November was pretty much a drought there if you look back at the weather report, which ended on November 30. From November 30 till Jan 1 you got nearly 1 foot of rain, if you don't forget the 2 inches on New Year's Day. That is tied with our second heaviest month in over the past 6 years here (in humid northern Florida).

Someone from AZ once posted here that if you have time , that gaming the weather with different start date worked best for him like you decided to try and since you happen to have Early May weather right now it could be tempting ;-)

PC

Pupilla - maybe I'm remembering the dates of our big storm wrong. For the bay area it was pretty wild. We have an avocado tree in the backyard that I was sure was going to blow over from the wind and our backyard was like a swamp for about a week. Other than that week of intense rain there hasn't been much more than a drizzle.

I think I may go ahead and start planting in a couple weeks. The sun is already noticeably higher in the sky than a couple weeks ago so the tomatoes should get a solid 6+ hours of direct sun.

I don't mean to provide a local weather report, but this winter has been all rain. A few years ago, we had accumulated a total of nearly 3 feet of snow (which is about 3" of rain). So far this year in Maryland, we have merely had 6" of rain.

Which I certainly prefer... A good drizzle every few days soaks in nicely. Snow stays mostly unavailable to the plants until it all melts suddenly.

I have a perfect way of knowing what soaks in and what runs off. I live downhill in my neighborhood, and there is a drainage easement (artificial creek running to a storm drain) on one side of the yard. With regular drizzle, the easement is nearly dry. When there is massive snowmelt, it is a torrent. All the water that flows past me is NOT soaking in. And from my POV, that is wasted water.

The coldclimategardening link above is a good one. You can get frost (as in, white crystals on a windshield or a rooftop) when the air temperature is above 32F, and the air is humid. That's because the particular surface cools especially efficiently. But that's not going to hurt most plants. A freeze is when the temp drops to 32F or below. As noted above, a "hard freeze" is something much lower. 28F or so.

This is one of many forecasting stones seen over the net.