Farmer's Almanac Frost Dates vs. NOAA Station Data: What's Different?

Farmers Almanac frost dates are one of the most common ways gardeners time their planting, and they are often treated as a different source from government weather data. They are not. Both the Old Farmer’s Almanac and the Farmers’ Almanac say on their own websites that their frost dates are calculated from NOAA’s 1991-2020 U.S. Climate Normals. The real differences come down to two choices: which probability level is shown, and which weather station a location is matched to.

The Short Answer

Where the Almanac Frost Dates Come From

There are two separate publications with similar names, and both publish frost dates.

The Old Farmer’s Almanac (almanac.com) runs a ZIP code lookup. Its results page states: “Our frost dates are based on 1991–2020 climate data from the National Oceanic and Atmospheric Administration (NOAA). We use a 30% probability threshold, meaning there’s about a 3 in 10 chance frost will occur before or after the given date.” Its main frost dates page describes the last spring frost as the average final date when temperatures may dip to 32°F, and its FAQ adds that frost dates are averages and you should always check your local forecast before planting warm-season crops.

The Farmers’ Almanac (farmersalmanac.com) publishes a list of average frost dates by city rather than a ZIP lookup. That page describes its dates as estimated light frost dates (29-32°F), says the probability of frost after the spring date or before the fall date is 30%, and gives its source as the United States Annual/Seasonal Climate Normals (1991-2020).

So the frost dates from both almanacs are climate statistics, not long-range predictions for a particular year. The Old Farmer’s Almanac also links to a separate seasonal frost map for the current year; that is a different product, and this comparison does not cover it. Both almanacs are open about their method, and both remind readers that the dates are a guide rather than a guarantee.

How the Farmer’s Almanac Frost Date Calculator Works

When you look up Farmers Almanac frost dates by ZIP code on almanac.com, the tool shows a short table: the nearest climate station, its altitude, the last spring frost, the first fall frost and the growing season length in days. For downtown Chicago (60602), for example, it names the Chicago Northerly Island station at 583 feet, with a last spring frost of April 17 and a first fall frost of November 1. The page says it uses “data from the nearest official weather station and your ZIP or Postal Code,” and notes that microclimates vary, so your yard may run warmer or cooler than nearby towns.

The page does not explain how it decides which station is nearest, or whether it ever skips a closer station with an incomplete record. Since we could not find that on their site, we won’t guess. The station name is printed with every result, which is the most useful detail on the page: it tells you exactly whose thermometer your dates come from.

What NOAA’s 1991-2020 Climate Normals Are

NOAA’s National Centers for Environmental Information (NCEI) defines a normal as the 30-year average of a weather variable, calculated over a uniform period. The current U.S. Climate Normals cover 1991 to 2020. They were first released in May 2021, with statistics for 23 sites reissued in 2023, and they include more than 7,300 stations reporting temperature. Alongside average temperatures and rainfall, the normals include agricultural statistics such as growing degree days and counts of days below freezing.

For frost, NCEI publishes freeze and frost probability dates for each station. NC State Extension’s guide to these tables explains that thresholds run from 36°F down to 16°F in 4°F steps, and that the three most useful for growers are 36°F (frost), 32°F (freeze) and 28°F (hard freeze). Each threshold comes with dates at several probability levels. NC State describes the 90% date as an aggressive, high-risk choice, the 50% date as a moderate one, and the 10% date as a conservative, low-risk choice.

Median, 30% and 1-in-10: Same Data, Different Risk

This is the biggest reason two frost date tools disagree even when they read the same station. For the last spring frost at 32°F:

• Median (50%) date: half of years have a later last frost. This is a 50/50 date.
• Almanac (30%) date: about 3 years in 10 have a later last frost. At the same station, it falls a few days to a week after the median.
• 1-in-10 (10%) date: only 1 year in 10 has a later last frost. In the five cities below, it falls 12 to 17 days after the median.

Fall works in reverse. The 30% and 10% first fall frost dates come earlier than the median, because they mark the dates by which a frost has already arrived in 30% or 10% of years. None of these is the right one in every case. They answer different questions about how much risk you want to take.

Five Cities Side by Side

Here are our station dates for five cities, from NOAA’s 1991-2020 normals at a 32°F threshold. Our tool matches each ZIP code to the nearest NOAA station, choosing an airport station within 15 miles first where there is one.

For reference, here is what the Old Farmer’s Almanac ZIP tool showed for the same five ZIP codes when we checked in October 2026. These are its 30% probability dates, with the station it names:

Boston and Dallas are the cleanest comparison because both tools use the same station. In Boston, the Almanac’s April 8 falls between our median of April 4 and our 1-in-10 date of April 17. In Dallas, its March 9 sits between March 2 and March 19. The fall dates line up the same way. That is exactly what a 30% date should do, so the numbers agree once you account for the probability level.

In Chicago, Denver and Atlanta, the two tools picked different stations, so the gap mixes both effects. Denver’s Almanac date of May 4 matches our median, for example, even though a 30% date would normally run a little later. The Denver Water Department station is simply a different site from Stapleton, with its own record.

Why the Nearest Station by ZIP Matters

Frost depends heavily on local conditions: elevation, nearby water, pavement and whether the station sits in a low spot where cold air drains on calm nights. Two stations a few miles apart can tell very different stories.

Boston shows how much. Logan Airport, on the harbor 3 miles from downtown, has a median last spring frost of April 4. Bedford’s Hanscom Field, 14 miles inland, has a median of April 28. That is more than three weeks apart for two stations in the same metro area. Atlanta is closer but still noticeable: Fulton County Airport’s median is March 24, while Hartsfield-Jackson, the station the Almanac names, has a median of March 15 in the same NOAA data.

This is why a ZIP code lookup that names its station is more useful than a single date for a whole state or zone. It gives you a real place to compare against. If you live inland from a coastal airport, in a valley, or at a higher elevation than the station, expect your own frost dates to run later in spring and earlier in fall than the station’s. A hardiness zone is a separate measure from a frost date, and our guide to USDA hardiness zones explained covers what the zone number tells you.

Which Date Should You Plan Around?

Use whichever tool you like, but know which probability you are reading. A practical way to use them together:

• Count seed-starting dates from the median. It is the middle of the range and the standard planning point, and our Seed Starting Calculator does the counting back from it.
• Hold tender transplants until the 30% or 1-in-10 date if a late frost would cost you plants you can’t replace. Tomatoes, peppers and basil are the usual examples.
• Check which station you got. If it is a warm coastal or downtown site and you are not, lean toward the later date.
• Watch the forecast near the date. Both almanacs say this too. A climate date tells you the odds, while the 7-day forecast tells you about this week.

Once you have picked a date, our beginner vegetable garden guide shows how to plan around your last frost, and the planting calendar lists what to sow by month.

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Sources & further reading

The Old Farmer’s Almanac’s method (NOAA 1991-2020 climate data, a 30% probability threshold, the nearest climate station to a ZIP code, the 32°F definition) and its five city results were read directly from its First and Last Frost Dates by ZIP Code page and its ZIP results pages in October 2026. The Farmers’ Almanac’s method comes from its average frost dates page. Details of the 1991-2020 normals come from NOAA NCEI’s U.S. Climate Normals page, and the thresholds and probability levels in NCEI’s freeze and frost tables are explained in NC State Extension’s guide to interpreting freeze/frost probabilities. Our station dates come from the same NOAA normals; see our methodology page, the Frost Date Calculator and frost dates by city and zone. For what temperatures actually damage plants, see what temperature frost forms at.

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Last updated: October 13, 2026.