Highest temperature in London on July 21?

Platform comparison

Market context

London's highest temperature on 21 July 2026 will be measured at City Airport Station and resolved against historical records via Wunderground. The settlement window closes at 12:00 UTC that day, creating a fixed observation period. For programmatic traders, this requires automated data ingestion from Wunderground's history endpoint at settlement time, with parsing logic to extract the daily maximum from the EGLC station feed. The 2% crowd probability suggests market participants expect a notably warm outcome, likely corresponding to a temperature range above typical July maxima for the location.

London's July temperatures have historically ranged between 20–28°C, with extremes reaching 32–34°C during heat events. The UK Met Office records show that temperatures exceeding 30°C occur in roughly one in three July days during warm summers, whilst readings above 32°C are considerably rarer. The current 2% probability implies the market is pricing for an exceptionally high threshold—potentially 35°C or above—which aligns with conditions seen during the 2022 heatwave when London recorded 40.3°C, though that was in July at other stations. City Airport's urban location and measurement methodology may produce readings 1–2°C lower than inland sites.

Traders should monitor UK Met Office forecasts and the European Centre for Medium-Range Weather Forecasts (ECMWF) model runs in early July 2026 for high-pressure systems and continental air mass movements. Atmospheric blocking patterns and jet stream positioning typically determine whether heat advection reaches southern England. Settlement requires direct verification against Wunderground's historical data at the specified station, making data availability and timestamp accuracy critical for conditional order execution.

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 Bot UK. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

Is this market available outside the US?

Polymarket itself is geo-blocked in the US/UK/EU. Always check the legal status of prediction markets in your jurisdiction before trading.

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.