Highest temperature in London on June 21?

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Market context

London City Airport is set to record the day’s peak temperature on 21 June, with the market settling on the Celsius range that contains the highest reading logged there by noon UTC. Current live weather around the station is already warm, with London City Airport reporting 13°C earlier in the day under sunny conditions and an Amber warning for extreme heat in place from the Met Office, while the day’s forecast maximum has been around 28°C. [2][5]

That makes the 0% crowd-implied price look conservative rather than implausible: the market is effectively asking whether the airport reaches a familiar late-June high rather than something extreme. On comparable Polymarket weather books, the centre of gravity has tended to sit in the high-20s for London summer maxima, with recent London temperature markets showing 28°C and 29°C as the leading outcomes and a June climatological high near 28°C. [1][3] On Betfair, Smarkets and Kalshi, the same temperature event would usually be shown differently: decimals or price ladders rather than a pure implied-probability view, with fees and accessibility varying by venue, so a 0% screenshot on one platform does not always map cleanly to another book’s quote.

The main catalyst is the remaining afternoon evolution at London City Airport before the 12:00 UTC settlement window closes, when the highest temperature will be locked in from Wunderground’s daily history page. Traders should watch whether the heat warning translates into an early spike or whether cloud, sea breeze and wind cap the airport below the hotter forecast ranges; the Met Office and hourly forecast services both point to a very warm, sunny spell, but the exact top for the station can still be sensitive to timing and local conditions. [5][8]

Sources: 1 · 2 · 3 · 4 · 5

Methodology

This page compares Highest temperature in London on June 21? specifically across Polymarket, Kalshi, Betfair Exchange and Smarkets. Live odds come from the Polymarket order book; the other venues' contract details are maintained manually because their APIs aren't directly comparable. Every CTA routes to Polymarket Alternative, which mirrors the Polymarket order book at 0% fees.

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, 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?

Polymarket Alternative 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.

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.

How fast are USDC deposits?

Polygon credits deposits after 12 confirmations — usually under 30 seconds. Withdrawals follow the same path and land back in your wallet within minutes.

Do I need to KYC for this market?

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