Highest temperature in London on August 21?

Platform comparison

Market context

On 21 August 2026, the highest temperature recorded at London City Airport will fall into one of several defined ranges. The market resolves using the Daily Observations table from Weather Underground rather than the Day High & Low summary—a distinction that matters when automated feeds pull data at different intervals or when station recalibration occurs mid-day. For programmatic traders, this means building resolution logic that queries the specific endpoint rather than relying on API summaries; Weather Underground's historical data structure has occasionally shown discrepancies between real-time and archived readings, so conditional orders should account for potential settlement delays whilst verification occurs.

August temperatures at London City Airport typically range between 18–24°C, with extremes rare but documented. The 0% crowd probability suggests the market may be calibrated to exclude an outlier scenario—perhaps a specific high-temperature threshold that historical August data makes genuinely unlikely. Comparable August days from 2015–2024 show that readings above 28°C occur roughly once per decade at this station, whilst temperatures exceeding 30°C are effectively unprecedented in late August. A trader evaluating this market should cross-reference the actual range brackets against Met Office historical records for August at City Airport to determine whether the current odds reflect genuine scarcity or market mispricing.

Catalysts include any sustained heatwave forecast from the UK Met Office in the weeks preceding late August 2026, though such forecasts typically carry high uncertainty beyond ten days. Traders monitoring this market should set alerts for any official heat-health alerts issued by the UK Health Security Agency, which would signal atmospheric conditions capable of pushing temperatures toward historical extremes. The resolution window closes at noon UTC on the day itself, meaning morning temperature readings will be final—a factor that favours traders with access to real-time station data feeds over those relying on end-of-day summaries.

Methodology

This page reviews Highest temperature in London on August 21? across five venues. The live probability is the Polymarket mid-price, sourced directly from the on-chain Polygon order book; the comparison columns benchmark each venue on fee structure, KYC, settlement currency and payment rails. Every CTA routes to Polymarket Bot UK, which mirrors the Polymarket order book at 0% fees.

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 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.

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%.