Highest temperature in London on August 21?

Platform comparison

Market context

On 21 August 2026, the highest temperature recorded at London City Airport will determine which range this market settles into. The resolution hinges on the Daily Observations table from Weather Underground rather than the Day High & Low summary—a distinction that matters when automated systems pull data. Traders building conditional orders or API-driven monitoring should flag this specification early; mismatched data sources have historically caused settlement disputes on weather markets, particularly when intraday peaks occur outside standard reporting windows.

August temperatures in London typically range from 20–24 °C, with extremes occasionally reaching 28–30 °C during heat waves. Historical precedent shows that summer 2022 saw London exceed 40 °C, though such outliers remain rare. The 0% crowd probability suggests the market may be pricing in either a narrow range definition or baseline expectations for mild conditions. Traders evaluating this market should cross-reference the exact temperature bands offered; a 1–2 °C difference in range boundaries can shift expected value significantly when historical volatility is factored in.

The UK Met Office publishes seasonal outlooks and heat-health alerts that typically activate in July, signalling whether August will trend warmer or cooler than normal. Monitoring their monthly forecasts from June 2026 onwards will provide early signal on whether atmospheric patterns favour heat accumulation. Additionally, any El Niño or La Niña phase active in mid-2026 could influence European summer conditions. Programmatic traders should set alerts for Met Office updates and cross-correlate with European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble data as August approaches.

Methodology

We track Highest temperature in London on August 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.

UK Frequently Asked Questions

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.

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.

Is Polymarket legal in the UK?

Polymarket is accessible to UK traders but is not UKGC-licensed. It operates under US CFTC jurisdiction. UK residents face no domestic legal prohibition on using it, but UKGC consumer protections do not apply. For UKGC-regulated alternatives, Betfair Exchange and Smarkets offer similar prediction-style markets.

Do I pay tax on prediction market profits in the UK?

UKGC-licensed platform profits (Betfair, Smarkets) are typically tax-free gambling winnings for UK individuals. Polymarket profits involve USDC crypto transactions, which HMRC treats as Capital Gains Tax events. Keep full transaction records and report via Self Assessment if gains exceed £3,000 per tax year.

How do I deposit on Polymarket from the UK?

UK traders typically fund Polymarket via Coinbase UK, Kraken or Revolut — buying USDC with GBP and transferring to a Polygon-compatible wallet (MetaMask, Coinbase Wallet). Polymarket's onboarding walks you through the bridging process. Typical GBP-to-USDC conversion costs 0.5–1%.