Two Airports, Two Clocks: What Decides a Chicago Temperature Contract

A Chicago temperature contract in one paragraph

A Chicago temperature contract pays on a thermometer reading taken at a named airport after the day is over, never on the forecast that came before it. Each day carries a high row and a low row, and each row is a set of temperature bands; exactly one band in a row pays $1 a contract and the rest pay nothing, so the prices along a row read as the market's spread of chances for where the reading will land. The catch for a Chicago reader is that the city has two airport stations about 15 miles apart, Midway and O'Hare, and the two exchanges quoting Chicago do not read the same one.

How Chicago temperature contracts settle step by step

1. Find the airport in the rules. Kalshi's daily high and low, the KXHIGHCHI and KXLOWTCHI series, settle on the temperature "recorded at Chicago (CLIMDW)" according to The Weather Company. CLIMDW is the daily climate report the National Weather Service's Chicago office issues as its Chicago-Midway climate summary, for the station at Chicago Midway Airport. Polymarket's daily high and low settle on routine hourly reports from Chicago O'Hare International Airport, station KORD, as the Aviation Weather Center publishes them.

2. Read how the ladder is cut. Kalshi writes a Chicago row as six cells: four two-degree bands that start on odd numbers, such as 53° to 54°, with both ends included, and an open cell at each end. Polymarket writes eleven cells, two-degree bands that start on even numbers, such as 54°F to 55°F, again with an open cell at each end. The same reading can sit in the middle of one exchange's band and on the edge of the other's.

3. Know the unit before the number. Polymarket's rules take the temperature remark in each routine O'Hare report, which is given in tenths of a degree Celsius, convert it to Fahrenheit by multiplying by nine-fifths and adding 32, and round to the nearest whole degree, with an exact half rounded up. Kalshi's terms use the full precision its source reports and warn that preliminary Weather Company figures can differ from the final value through rounding and conversion.

4. Know which reports count. On the O'Hare contracts a special observation, a report whose line begins SPECI, does not count, and corrections are accepted until the first report of the following day is published. On the Midway contracts only the first official report that is not marked preliminary is used, and the exchange may hold expiration if it decides that report contains a material error.

5. Carry one invented day through. Say the warmest routine report at O'Hare carries 12.5°C. That is 54.5°F, the half rounds up, and the reading counts as 55, which pays the band from 54 to 55 on a ladder cut on even numbers. Say Midway's climate report shows a high of 54 the same day: on a ladder cut on odd numbers, that pays the band from 53 to 54. One afternoon, two honest readings, two different winning bands.

6. Watch the clock. Kalshi's Chicago rows keep trading until 11:59 p.m. local time on the day they measure and expire at the first 7 or 8 a.m. Eastern after the data is released, or a week later at most. Polymarket's resolve at the first O'Hare report of the following day or by 11:59 p.m. Eastern that day, whichever comes first, and count any report whose time stamp, in the station's local time with daylight saving included, falls on the date. The Midway climate report prints its times in local standard time, which runs an hour behind Chicago's clocks whenever daylight saving time is in effect.

7. Know what happens when the number is missing. If no reliable data arrives by expiration, Kalshi resolves every band at the last fair price it determines. Polymarket turns to Weather Underground's daily observations, and if neither source has a usable reading by its cutoff, the market resolves to the lowest band.

Where Illinois readers stand

A weather station settles the contract; it does not settle whether an exchange may offer the contract to someone in Illinois. That depends on the state's own record, including the Illinois Gaming Board's cease-and-desist letters to prediction-market exchanges, which is laid out with its dates on the legal status page. This guide reaches no conclusion about any venue. It is not legal advice and not financial advice.

Common misreadings

The forecast high is the answer. No forecast appears in either exchange's rules. A forecast for Chicago moves prices all day and decides nothing; the bands pay on what an instrument at one of the two airports recorded.

Chicago has one official temperature. For these contracts it has two, and the exchange chooses. Before reading a price, read the station line in the rules: CLIMDW is Midway, KORD is O'Hare.

Matching numbers mean matching bets. A Kalshi band from 53 to 54 and a Polymarket band from 54 to 55 share one degree and read different airports, so their prices answer different questions and are never averaged on this site.

Final means final. Kalshi notes that a "Final" label on its data provider's page is a convenience and does not mean the data is free of error, and Polymarket can keep a market open for up to seven days when a published reading is clearly wrong and a correction is expected.

The low is the overnight low. Both contracts read the lowest temperature recorded at the station for the date, so a cold front arriving at 10 p.m. can set the day's low long after sunrise.

Related guides and markets

Every open Chicago row, with its venue named, is on the Chicago weather page, and the vertical sits under the weather hub. The companion guides walk two other Illinois contracts the same way: how the Illinois gas price contract settles and how Bears playoff odds work.

This guide is not legal advice and not financial advice. It explains what a Chicago temperature contract settles on and names no band, no side and no exchange to use.