Frost vs. Freeze vs. Hard Freeze: What's the Difference?

Frost is ice on surfaces, a freeze is air at 32°F or below, and a hard freeze is 28°F or below for several hours. Each does different damage, and forecasters warn for each one separately.

The short version

Frost: ice on surfaces

Frost forms when a surface (a leaf, a car windshield, a blade of grass) cools to 32°F and moisture in the air freezes onto it. On clear, calm nights, surfaces radiate heat to the sky and cool faster than the air around them. That's why you can find frost on the lawn when the official low, measured five or six feet above the ground, was 34°F or even 36°F.

Frost needs moisture. On very dry nights, temperatures can drop below freezing with no visible frost at all. Plants still freeze; you just don't see the white coating that warns you.

Freeze: the air itself hits 32°F

A freeze means the air temperature reaches 32°F or lower. Water inside the cells of tender plants freezes, the cells rupture, and by morning the leaves turn dark and limp. Tomatoes, peppers, basil, cucumbers, squash, and beans don't survive a freeze without protection.

The frost dates on FrostCal measure exactly this: the last spring day and first fall day when the air at the weather station reached 32°F or below. Strictly speaking they're freeze dates, which is how NOAA and most frost-date sources define a frost date. See how frost dates work.

Hard freeze: 28°F and below

A hard (or "killing") freeze is usually defined as 28°F or below for several hours. At that point, semi-hardy crops like lettuce, chard, and beets take real damage, and only the hardiest vegetables carry on: kale, collards, Brussels sprouts, spinach, cabbage, and garlic and onions in the ground.

In fall, the first hard freeze is the practical end of the main vegetable season in most gardens. In spring, a late hard freeze can damage fruit tree blossoms and early perennial growth.

What the weather alerts mean

The National Weather Service issues alerts for each level during the growing season. Exact thresholds vary somewhat by local office, but typically:

• Frost Advisory: lows of about 33–36°F with conditions favoring frost (clear skies, light wind).
• Freeze Watch: a freeze is possible in the next day or two.
• Freeze Warning: lows of 32°F or below are expected.
• Hard Freeze Warning: lows of about 28°F or below are expected.

Many offices stop issuing frost and freeze alerts after the first widespread killing freeze of fall, on the assumption that the growing season is over. If you're running a cold frame or fall greens into winter, keep an eye on the forecast lows yourself.

Which plants survive what

• Need protection from any frost: tomatoes, peppers, eggplant, basil, cucumbers, squash, melons, beans, corn, sweet potatoes, zinnias, dahlias.
• Tolerate light frost (about 28–32°F): lettuce, Swiss chard, beets, carrots, peas, potatoes (foliage may die back), cauliflower, cilantro, parsley.
• Tolerate hard freezes (mid-20s°F and lower): kale, collards, cabbage, broccoli, Brussels sprouts, spinach, turnips, onions, garlic.

These are guides, not guarantees. Seedlings are more fragile than mature plants, and wind, soil moisture, and how gradually the cold arrives all change the outcome. See how to protect plants from frost for what to do when one is forecast.

Why clouds and wind matter

Clouds act like a blanket, reflecting heat back toward the ground, and wind keeps mixing slightly warmer air down to plant level. That's why the worst frosts come on still, starry nights, often right after a cold front clears out. A forecast of 33°F with clouds and a breeze is far less dangerous than 35°F under clear, calm skies.