Highest temperature in London on July 20?

Platform comparison

Market context

London City Airport is expected to record its highest temperature for 20 July 2026 within the 25°C range, a forecast that has driven the Polymarket contract to price the “25°C” outcome at 100% probability, while the “YES” option for any temperature above that sits at 0% [1]. This pricing reflects the market’s confidence that the heatwave peaking earlier in the week will have broken by the settlement date, with forecasts indicating a sharp cooldown as a cool western breeze arrives on 19 July, pushing afternoon highs down to 25°C [4].

Historically, London July temperatures rarely sustain extreme heat beyond mid-week without a significant weather shift, and the 2026 heatwave—which saw peaks of 36°C on 15 July—follows this pattern by tapering off just before the 20th [4]. The current 0% probability for higher ranges aligns with comparable cases where late-July days in London settle near average summer highs after a preceding spike, rather than extending record-breaking conditions.

Traders should monitor the official Wunderground daily history for EGLC once the day closes, as this is the definitive resolution source, and watch for any unexpected meteorological updates from the Met Office or BBC Weather that might contradict the current cooldown narrative [2][3]. With settlement ending at 2026-07-20T12:00:00Z, the on-chain mechanics on Polygon using USDC and conditional tokens will lock in the final temperature range once the data is published, leaving no room for post-event ambiguity.

Sources: 1 · 2 · 3 · 4

Methodology

Methodologically we separate two layers: the live probability (Polymarket mid-price) and the platform attributes (fee, KYC, settlement currency, payment rails). That keeps the comparison honest — a single canonical probability across the row, with the venue-by-venue trade-offs spelt out in the columns next to it.

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 Scam?. 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.

How fast are USDC deposits?

Polygon credits deposits after 12 confirmations — usually under 30 seconds. Withdrawals follow the same path and land back in your wallet within minutes.

Do I need to KYC for this market?

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

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.