Highest temperature in London on June 25?

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Market context

London is currently experiencing an extreme heatwave, with the Met Office issuing an amber alert for dangerous temperatures that could reach 40C in the capital over the coming days. On 25 June 2026, the London City Airport recorded a high of 25C at 08:00 BST, though forecasts suggest the mercury will soar significantly higher later in the day, potentially breaking previous records for the hottest June day in the UK. This real-world volatility directly challenges the current crowd-implied probability of 0% YES, which appears to ignore the immediate atmospheric conditions.

Historical precedents frame this probability as highly questionable; just yesterday, 24 June, saw records broken for the hottest June day in the UK with temperatures hitting 34.8C, and the 23rd reached 33.9C at Hampstead. With the heatwave forecast to spike to 36C on Tuesday and 40C on Wednesday, the likelihood of a 25C maximum at London City Airport being the highest temperature seems implausible, suggesting the market is mispricing the event. Traders comparing platforms should note that Polymarket’s decimal odds format may offer clearer entry points than Kalshi’s implied probability structure, while fee structures and KYC requirements at Betfair or Smarkets could further diverge on liquidity for this specific weather event.

The primary catalyst to watch is the Met Office’s extended amber alert, which remains in force until the end of Thursday, indicating sustained extreme heat conditions. Recent reporting from The Standard confirms the heat could spiral to 40C, making the 25C threshold a likely floor rather than a ceiling for the day’s maximum. Platforms diverge here: Kalshi’s KYC reach may limit participation compared to the more accessible Polymarket, while fee differences between Smarkets and Betfair could alter the final settlement price. Traders must monitor the hourly Wunderground updates for London City Airport, as the settlement window closes at 12:00 UTC on 25 June 2026, and any delay in data could skew the resolution.

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Methodology

We read Highest temperature in London on June 25? 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

Settlement is the biggest difference between the four platforms: Polymarket on-chain in USDC (instant), Kalshi USD via CFTC (T+1), Betfair and Smarkets in local currency via bank withdrawal (T+1 to T+3). PolyGram routes every trade directly into Polymarket's on-chain settlement, which is why payouts land fastest.

FAQ

Where can I trade this market with the lowest fees?

On PolyGram, 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.

Is this market available outside the US?

PolyGram is available in most jurisdictions where Polymarket isn't directly accessible. Polymarket itself is geo-blocked in the US/UK/EU. Always check local regulations.

What does it cost to trade on PolyGram?

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

Do I need to KYC for this market?

Not under $1,500 of lifetime trading volume. Above that threshold, PolyGram triggers a quick verification flow that finishes in minutes.

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.