Kalshi and Polymarket both run daily high-temperature markets for the same cities, often on the same day and with the same bracket structure. It is easy to assume “highest temperature in NYC today” means the same thing on both platforms. It does not. They settle off different data, and for at least one major city, off a different physical station entirely.
We checked the actual resolution rules on both platforms, then tested how often that difference changes the outcome across 102 real resolved city-days.
What each platform actually resolves off
Kalshi's daily high and low series (KXHIGH) settles off the National Weather Service's Daily Climate Report, consistently across cities. It is a government source, curated and quality-controlled by NWS meteorologists before release the following morning.
Polymarket is not consistent. Its per-market rules name different sources depending on the city:
| City | Kalshi source | Polymarket source | Same station? |
|---|---|---|---|
| NYC | NWS Daily Climate Report, Central Park | Wunderground, LaGuardia Airport | No |
| Chicago | NWS Daily Climate Report, O'Hare | Wunderground, O'Hare International Airport | Yes |
| Miami | NWS Daily Climate Report, Miami International Airport | NOAA/weather.gov, Miami International Airport | Yes |
Wunderground is a commercial property owned by The Weather Company, not a government feed. It runs its own reporting pipeline independently of NWS, even when reading from the same airport. Miami is the one city here where both platforms pull directly from the NOAA/NWS family, though through different products: Kalshi's curated daily report and Polymarket's hourly timeseries.
There are two separate variables that can diverge: which institution reads the thermometer, and which physical thermometer it is.
Does it actually matter? We checked 102 resolved city-days
Across NYC, Chicago, and Miami, comparing Kalshi's and Polymarket's settlement values on the same real day:
| City | Agreement rate | Station/source match |
|---|---|---|
| Miami | 61.8% (best) | Same station, both NOAA/NWS family |
| Chicago | 47.1% | Same station, different institution |
| NYC | 32.4% (worst) | Different station, different institution |
The pattern lines up with the mechanism. Miami, where both platforms read essentially the same government data for the same station, agrees most often. Chicago, with the same station but commercial versus government readers, sits in the middle. NYC, with a different station and institution, agrees least—worse than a coin flip.
When the platforms disagreed, it usually was not by much: a mean difference of 0.81°F and a maximum of 5°F. There is a mild skew toward Kalshi reading warmer—31 of 54 disagreements—but it is not clean or large enough to call a reliable directional bias.
What this means if you are trading both
If a strategy treats “Kalshi NYC high temperature” and “Polymarket NYC high temperature” as the same underlying event priced by two venues, that assumption holds well under a third of the time on the actual settlement value. Most disagreements are small. Near a bracket boundary, however, a 1–2°F gap between LaGuardia and Central Park is exactly the size that can flip which side of the line a contract settles on.
Cross-venue arbitrage and calibration comparisons need to account for this directly. Check each platform's named station and source rather than assuming “same city, same day” means “same outcome.”
Limitations
This is 102 city-days across three cities: a real, checked pattern, not a large-sample statistical proof. Other cities may look different depending on how close their stations are and whether Polymarket's rules point to Wunderground or NOAA directly. Both platforms have changed resolution sources on specific markets before, so current rules should always be checked against the specific contract.
FAQ
Do Kalshi and Polymarket use the same weather data?
No, not consistently. Kalshi's daily high and low markets settle off the National Weather Service's Daily Climate Report. Polymarket's source varies by city: Wunderground for some, NOAA or weather.gov directly for others.
Why does NYC disagree more often than Miami?
NYC's two platforms read different physical stations—Central Park for Kalshi and LaGuardia Airport for Polymarket—on top of different institutions reading them. Miami's platforms read the same station from the NOAA/NWS family.
How big are the disagreements when they happen?
Small on average: a mean gap of 0.81°F and a maximum of 5°F across the 102 city-days checked, with a mild skew toward Kalshi settling warmer.
Does this create an arbitrage opportunity?
Not a clean, mechanical one. The disagreements are real but small and not strongly directional. They matter most near a bracket boundary, where a 1–2°F gap can put contracts on opposite sides of a settlement line.
Where can I check a specific market's actual resolution source?
On the market page itself. Both platforms publish exact rules per market, naming the source and station. Check the specific contract rather than assuming from the city.
We track historical trade and settlement data across Kalshi and Polymarket, alongside Predict.fun, Limitless, Deribit, and Binance options, in one schema. See the API docs for current coverage if you want to run this check across more cities or a longer window.