Highest temperature in London on July 21?

Platform comparison

Market context

On 21 July 2026, the highest temperature recorded at London City Airport will determine which range this market resolves to. The settlement window closes at noon UTC that day, meaning only readings up to that point count towards resolution. Wunderground's historical data for this station serves as the sole arbiter, accessible via their daily archive interface where users can toggle between Celsius and Fahrenheit displays.

London's July temperatures have historically ranged between 20–28°C, with occasional peaks above 30°C during heat waves. The 0% crowd probability suggests traders are either absent from this market or treating it as a placeholder listing. Comparable weather markets on Kalshi and Betfair show stronger liquidity on broader seasonal forecasts rather than single-day station readings; Polymarket's decimal odds format can obscure low-probability outcomes compared to Kalshi's implied probability display, potentially explaining the flat pricing here. Historical precedent matters: the Met Office recorded 40.3°C in London during July 2022, though City Airport's microclimate typically runs 1–2°C cooler than central readings.

Traders should monitor the Met Office's extended forecast from mid-July onwards, particularly any heat-dome warnings issued in the week prior. The UK Health Security Agency publishes heat-health alerts that often precede sustained high temperatures. Kalshi's KYC requirements and US-focused user base mean European weather markets there attract thinner participation than Betfair or Smarkets, where UK-based punters dominate. Fee structures—Polymarket's 2% settlement fee versus Kalshi's variable maker-taker model—become material only if liquidity materialises around specific temperature thresholds.

Methodology

We read Highest temperature in London on July 21? from four platform perspectives: Polymarket (on-chain CLOB), Kalshi (CFTC-regulated exchange), Betfair Exchange (sports book exchange), Smarkets (peer-to-peer betting exchange). Polymarket's live mid is the canonical probability; the side-by-side columns benchmark fees, KYC, settlement currency and deposit rails so you can choose the venue that fits your jurisdiction and trade size.

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

What does Polymarket cost vs Kalshi?

Polymarket: 0% fees, only Polygon network costs (~$0.01/trade). Kalshi: up to 7% per trade plus spread. For high-frequency traders, Polymarket is dramatically cheaper.

What about Smarkets as an alternative?

Smarkets is a UK betting exchange with a lower default commission (2%) than Betfair. Liquidity on political markets is below Polymarket, comparable to Kalshi. Geo-blocked in many jurisdictions.

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 UK 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 UK 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.