Highest temperature in London on July 21?

Platform comparison

Market context

On 21 July 2026, the highest temperature recorded at London City Airport will fall into one of several defined ranges. The settlement mechanism pulls historical data from Weather Underground's daily archive for that specific station, requiring traders to verify the exact reading and corresponding temperature band before the 12:00 UTC window closes. This approach differs from broader London forecasts; City Airport sits on the Thames estuary with distinct microclimatic properties that can diverge notably from central London readings.

July temperatures at London City Airport typically range between 18–24°C, though extremes have occurred. The UK's record July high of 40.3°C (set in 2022 at Coningsby, Lincolnshire) demonstrates the tail-risk potential during heat waves, though City Airport's waterside location and urban heat-island dynamics create different baseline conditions. Historical data from the station shows that days exceeding 25°C are common in mid-July, whilst readings above 30°C remain infrequent but plausible during sustained continental air masses. Traders evaluating this market should cross-reference the station's ten-year archive on Weather Underground to calibrate expectations against the specific resolution source.

The Met Office issues extended outlooks approximately two weeks ahead; their forecast for late July 2026 will be the primary catalyst for probability shifts. Atlantic weather patterns, high-pressure positioning over the UK, and any tropical air advection become material considerations from early July onwards. Traders implementing automated monitoring should flag the Weather Underground historical page directly rather than relying on third-party weather APIs, since settlement depends on that exact source's recorded value. The 0% crowd probability suggests either low perceived tail-risk or insufficient market participation at present.

Methodology

We track Highest temperature in London on July 21? across the five venues with material prediction-market liquidity. The probability shown is the live Polymarket mid; the comparison rows summarise how each venue treats the underlying contract — fees, KYC thresholds, settlement currency, deposit options. The highlighted row marks the cheapest route into Polymarket's order book.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

FAQ

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is Polymarket Review UK. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

How does resolution work?

Through the UMA Optimistic Oracle on Polygon: a proposer submits the outcome, a two-hour challenge window opens, and USDC payouts settle automatically once the result is final.

What's the difference between YES and NO shares?

A YES share pays $1.00 if the event happens, $0 otherwise. A NO share pays $1.00 if the event doesn't happen. The market price between 0¢ and 100¢ is the implied probability.

What does Polymarket cost to trade?

Polymarket itself charges 0% — the only cost is the Polygon network fee, typically under $0.01 per transaction. Off-chain venues like Kalshi or Betfair charge 2-7% commission.

Do I need to KYC for this market?

On Polymarket directly, no — it's wallet-based. Intermediary brokers like Polymarket Review UK trigger KYC only above $1,500 of lifetime trading volume; under that you trade pseudonymously with a single wallet address.