Highest temperature in London on July 21?

Platform comparison

Market context

On 21 July 2026, the highest temperature recorded at London City Airport will be measured and matched against predefined ranges. The 1% implied probability suggests traders expect a notably cool day for mid-summer in London, with the crowd pricing in conditions well below historical norms for that date. Resolution will depend on Wunderground's historical weather database, which records intraday highs at the airport station—a location that typically runs 1–2°C warmer than central London due to its riverside position and urban heat effects.

London's July temperature distribution shows considerable year-to-year variance. The 30-year average high for late July sits around 23°C, though the city has recorded 28°C+ on this date in recent decades and occasionally dipped to 18°C or lower during unsettled summers. The 2022 heatwave pushed temperatures above 40°C in parts of England, but such extremes remain statistical outliers. Traders assessing this market should note that Kalshi's decimal-odds format and Betfair's fractional odds will display the same 1% probability differently—Kalshi shows 101.00, whilst Betfair would quote 100/1—but the underlying conviction is identical. Polymarket's fee structure and KYC requirements differ from both, affecting position sizing calculations for cross-platform hedging.

Meteorological forecasts become reliable only 10–14 days before settlement. Atlantic weather patterns, jet-stream positioning, and high-pressure systems over the continent will determine whether July 21 brings typical summer warmth or cooler, unsettled conditions. Traders should monitor the UK Met Office's extended outlook from mid-July onwards, as sudden pattern shifts can move probability substantially. The settlement window closes at 12:00 UTC on the day itself, leaving minimal time for late-breaking weather data to shift positions.

Methodology

This page compares Highest temperature in London on July 21? specifically across Polymarket, Kalshi, Betfair Exchange and Smarkets. The live probability is the Polymarket mid; the comparison columns summarise each venue's fee structure, KYC, settlement currency and payment rails. Every CTA routes to Kalshi Alternative, which mirrors the Polymarket order book at 0% fees.

Resolution & payout

Polymarket settles via UMA Optimistic Oracle on Polygon. A proposer posts the outcome with a bond, the two-hour window runs, then the smart contract pays USDC.

Kalshi settles USD through the CFTC-regulated clearinghouse — the cleanest variant, with heavier KYC. Betfair Exchange settles in account currency (GBP/EUR), net of 2-5% commission. Smarkets follows the same model as Betfair with a lower default 2% commission.

FAQ

Polymarket vs Kalshi — which is better?

Depends on your location. Kalshi is CFTC-regulated, US-only with full KYC. Polymarket is global, on-chain, no KYC up to $1,500. Polymarket has ~10x higher liquidity but higher regulatory risk.

Is Betfair a Polymarket alternative?

Only partially. Betfair Exchange is UK-focused with a sports-betting emphasis; they have politics markets but with thinner liquidity than Polymarket. Settlement in GBP/EUR, 2-5% commission on winnings.

Which platform is accessible globally?

Polymarket is geo-blocked in the US/UK/EU. Kalshi is US-only. Betfair and Smarkets are UK-restricted. Kalshi Alternative has a different geo footprint and routes to Polymarket's order book at 0% fees.

Are all these platforms regulated?

No. Kalshi is CFTC-regulated (US). Betfair and Smarkets are UK Gambling Commission licensed. Polymarket operates without explicit regulation — a different risk profile than a regulated sportsbook.

Which platform supports Klarna/SOFORT?

Directly: none. Polymarket accepts only USDC on Polygon. Kalshi Alternative offers a fiat on-ramp via Klarna or SOFORT (DE/AT/CH) and converts internally to USDC for the Polymarket order book. T+1 processing.