Highest temperature in London on August 20?

Platform comparison

Market context

London City Airport will record its highest temperature on 20 August 2026, measured via the Daily Observations table on Weather Underground rather than summary figures. The contract currently trades at 0% implied probability on Polymarket, suggesting traders assess near-zero likelihood of extreme heat on that specific date. Settlement hinges on the precise methodology: the Daily Observations granular dataset takes precedence over the Day High & Low summary, a distinction that matters when Weather Underground's automated systems occasionally diverge between data sources.

Historical August temperatures at London City Airport provide the baseline for calibrating expectations. The station has recorded highs exceeding 30°C on roughly one in three August days over the past decade, with the airport's warmest August day reaching 32.4°C in 2022. The 0% pricing reflects the market's assessment that the specific temperature range implied by this contract—likely the uppermost bracket—sits well above typical late-summer conditions for the location. August 2026 falls outside any established El Niño or La Niña pattern that might skew seasonal forecasts, though the Met Office's long-range outlooks typically carry substantial uncertainty beyond six months.

Traders monitoring this contract should track the UK's summer weather patterns from June onwards, particularly any sustained high-pressure systems developing over continental Europe in mid-August. The Met Office publishes monthly outlooks that occasionally flag unusual warmth; such announcements could shift pricing if they suggest anomalous heat. Weather model consensus from early August 2026 will provide the most actionable signal, though even then, forecasts beyond ten days carry limited reliability for pinpointing daily extremes.

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

Settlement runs on-chain. Polymarket's contract logic separates YES and NO shares as conditional tokens; at resolution the winning share lifts to $1.00 and the losing one to $0. The outcome input comes from the UMA Optimistic Oracle, which secures against bad resolution with a bond + dispute window.

Once finalised, the smart contract pays USDC to the holders' wallets within minutes — no withdrawal fees beyond Polygon network gas. Kalshi settles in USD via CFTC clearance, Betfair in account currency net of commission, Manifold in play-money mana with no cash-out.

UK Frequently Asked Questions

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.

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%.