Highest temperature in London on July 31?

Platform comparison

Market context

London’s highest temperature is measured at London City Airport Station and settled from Wunderground’s daily history page, so the key question is the day’s maximum reading rather than the lunchtime temperature or any city-wide average. At 5pm UTC on 31 July 2026, the 0% YES implied by the crowd suggests the market is pricing in an outcome below the relevant range, but that view is unusually stark given how London summer maxima can still move several degrees in the afternoon if skies clear and winds ease.

The historical frame is that late-July London heat has produced values well into the high 30s °C in extreme years, with Heathrow reaching 37.8°C on 31 July 2020 and London sites touching 40.2°C in the 19 July 2022 heatwave, while more ordinary July days sit far lower, often in the low-to-mid 20s °C. That spread matters across platforms because Polymarket shows this market as a set of discrete temperature bands with implied probability, whereas Betfair and Smarkets typically express the same view through decimal prices and charge commission on winnings, which makes a narrow band trade less directly comparable to a straight percentage quote.

The main catalysts are the afternoon forecast updates, airport-observatory observations, and whether any convective cloud or sea-breeze effect caps the day’s high near the coast. Market participants should also note that London City Airport can run a touch differently from central-London sites, so a trader watching Kalshi, Polymarket, or a betting exchange may be looking at the same weather pattern but a different settlement lens, with KYC and market access varying by venue and the payout curve depending on whether the contract is binary or banded.

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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.

Which platform has the deepest liquidity?

Polymarket — by a wide margin. Top markets reach $50-500M volume, Kalshi ~$200M cumulative, Betfair similar. Deeper liquidity means your trade moves the quote less.

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.

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.