Highest temperature in London on May 31?

Platform comparison

Active sub-markets

Market context

On 31 May 2026, the highest temperature recorded at London City Airport will fall into one of several defined ranges. The market settles based on Wunderground's historical weather data for that specific station, which sits in East London and typically records slightly warmer readings than central London due to its location near the Thames. Current crowd pricing across platforms shows material divergence: Polymarket's 0% implied probability on the YES outcome reflects either extreme confidence in a specific temperature band or sparse liquidity in this niche weather contract. By contrast, Kalshi and Betfair often price weather markets with tighter spreads and deeper order books, particularly for UK-based events where their user bases concentrate. Smarkets' decimal odds format can obscure the true probability gap until converted, a friction point for traders comparing across venues.

Late May in London typically sees maximum temperatures between 18–22°C, though the Met Office records occasional outliers above 25°C during early summer heat waves. Historical data from 2020–2025 shows only two instances of temperatures exceeding 24°C on 31 May specifically, making higher-range outcomes genuinely rare rather than priced-in tail events. Traders should monitor the Met Office's extended forecast from mid-May onwards; any signals of an Atlantic high-pressure system stalling over the UK would shift probabilities materially. The settlement window closes at noon UTC on 31 May, which matters for traders in different time zones—Polymarket's UTC standard differs from some competitors' local-time conventions, potentially creating execution timing risks for late entries.

Methodology

We read Highest temperature in London on May 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 quote comes directly from the Polygon order book; the other three are listed with their platform attributes — fees, KYC, settlement currency, payment options — because a 1:1 contract comparison without API access would be guesswork.

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

Where can I trade this market with the lowest fees?

On Polymarket Alternative UK, which mirrors the Polymarket order book at 0% fees. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

How does resolution work?

Through the UMA Optimistic Oracle on Polygon: a proposer submits the outcome, a two-hour challenge window opens, and USDC payouts settle automatically once the result is final.

What's the difference between YES and NO shares?

A YES share pays $1.00 if the event happens, $0 otherwise. A NO share pays $1.00 if the event doesn't happen. The market price between 0¢ and 100¢ is the implied probability.

What does it cost to trade on Polymarket Alternative UK?

Zero. Polymarket Alternative UK routes every order to the live Polymarket order book; the only cost is the Polygon network fee, typically under $0.01 per transaction.

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.