Highest temperature in London on August 9?

Platform comparison

Market context

The relevant event is the maximum air temperature recorded at London City Airport on 9 August 2026, with settlement tied to the Weather Underground **Daily Observations** table rather than the summary “Day High & Low” figure. In practice, that means the tradable question is not whether London felt hot, but whether the station’s final observed peak lands inside a specific Celsius band once the day’s readings are fully published and any later revisions are captured before the cutoff.

For context, London’s summer highs normally sit in the low 20s Celsius, but late-summer heat spikes into the high 20s or low 30s are not unusual in strong high-pressure setups. Recent 2026 coverage pointed to a renewed heatwave across southern Britain, with regional maxima around 30.6–32°C and forecasts for London close to 32°C, while another London forecast for 9 August put the city around 29.8°C, which explains why the market has gravitated towards a precise upper-20s-to-low-30s outcome rather than an extreme outlier.[2][3][14]

A programmatic trader would watch for the live station feed, then compare the final Daily Observations row against the market’s Celsius bins as soon as the last datapoint for the date appears. The key catalyst is not a headline forecast but whether the airport station’s afternoon peak is revised after publication; Weather Underground’s own rule gives the Daily Observations table priority, and the market remains sensitive until the first reading for 10 August is posted.[6]

Sources: 1 · 2 · 3 · 4 · 5

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

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%.