Highest temperature in London on July 31?

Platform comparison

Market context

London’s payout depends on the highest reading posted at London City Airport Station before the settlement cut-off, so the key variable is not the afternoon peak in central London but the station’s official daily maximum on Wunderground. July extremes in the capital are usually well below the 2022 and 2020 outliers, yet London has produced several 37–40°C readings in recent Julys, including Heathrow at 37.8°C on 31 July 2020 and St James’s Park/Heathrow at 40.2°C on 19 July 2022.[1][3][11] Against that history, a crowd price of 0% YES implies the market is treating any qualifying heat as effectively absent, which is notably different from venues such as Polymarket that quote outcomes in discrete temperature bands as implied probability rather than a binary yes/no contract.[14]

For traders comparing venues, the practical gap is in pricing and access: Polymarket typically shows an implied probability for each range, while Kalshi, Betfair and Smarkets generally express the market through contract prices or decimal odds, with fees and spread affecting the all-in entry point. KYC also matters: exchange-style books usually have broader geoblocking and identity checks than a crypto-native venue, so the tradable audience can differ even on the same weather event. The live catalyst is the hourly temperature path at London City Airport and any late-day convective shower or sea-breeze effect that caps the maximum before the day closes; in London, the peak often arrives mid-afternoon rather than near midnight, so a settled score can be locked in well before the market window ends.[2][13][17]

Sources: 1 · 2 · 3 · 4 · 5

Methodology

We read Highest temperature in London on July 31? 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

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.

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 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.