Highest temperature in London on August 21?

Platform comparison

Market context

The market concerns the highest temperature recorded at London City Airport on 21 August 2026, measured in degrees Celsius via Weather Underground's Daily Observations table. The current crowd-implied probability sits at 0%, suggesting traders expect the actual high to fall outside the resolution range specified in the market's terms. This represents an extreme consensus position, with no meaningful probability mass assigned to any outcome within the defined bracket.

London's August temperatures have historically ranged between 18°C and 28°C, with extremes occasionally reaching 30°C or above during heat waves. The Met Office records show that August 2022 saw a peak of 40.3°C in parts of southern England, though London City Airport's readings typically run 2–4°C cooler than inland locations. August 2023 peaked at 26°C in the capital, whilst August 2024 remained moderate at around 24°C. These patterns suggest the market's zero probability reflects either an unusually narrow resolution band or trader uncertainty about the specific measurement methodology at the airport station.

Traders should monitor long-range weather forecasts released in late July and early August 2026, particularly from the Met Office and European Centre for Medium-Range Weather Forecasts. Atmospheric pressure systems and jet stream positioning in the weeks preceding 21 August will determine whether high-pressure systems bring sustained heat or whether Atlantic low-pressure systems deliver cooler, wetter conditions. The distinction between Weather Underground's Daily Observations table and the Day High & Low summary section is material; traders should verify which source the platform will ultimately reference, as discrepancies between automated station data and summary figures have historically affected resolution disputes on weather markets.

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

At resolution the UMA oracle takes over: a proposer posts the outcome with a bond, any token holder can dispute within two hours. Without dispute the result is accepted and the smart contract distributes USDC instantly.

On Kalshi (CFTC-regulated) resolution runs through their in-house clearing engine in USD. Betfair Exchange settles after match end in the account's local currency. Manifold pays no cash — only its in-platform "mana" currency.

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 Best Prediction Markets 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 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.

What are the best prediction markets for UK traders in 2026?

For UKGC-regulated markets: Betfair Exchange (sports, politics) and Smarkets (sports, politics, lower commission than Betfair). For broader global event prediction: Polymarket (deepest liquidity, 0% fee, USDC settlement). For US-regulated option: Kalshi (USD, limited UK payment options). Most serious UK traders use a combination of Betfair for regulated sports and Polymarket for broader markets.