Highest temperature in London on July 21?

Platform comparison

Market context

The highest temperature recorded at London City Airport on 21 July 2026 will determine which temperature band resolves as YES. The market currently shows zero implied probability across all ranges, suggesting either minimal trader interest or genuine uncertainty about settlement mechanics. Resolution will depend on historical weather data from Wunderground's London City Airport station, a standard Met Office reporting location in East London.

July temperatures in London have historically ranged from 18°C to 36°C, with the city's record high of 40.3°C set in July 2022. The 21st sits mid-summer when warm continental air masses occasionally push temperatures into the high 20s or low 30s, though cooler Atlantic systems can suppress readings into the low 20s. Recent summers have shown increased volatility; the Met Office noted July 2023 averaged 1.5°C above the 1991–2020 baseline, whilst July 2024 remained closer to seasonal norms. This historical spread suggests multiple temperature bands could plausibly resolve across different market conditions.

The UK Met Office and BBC Weather will publish seasonal forecasts in early July 2026, providing initial directional guidance on whether high-pressure systems or Atlantic lows dominate the period. Short-range forecasts become reliable only 10–14 days before the date. Traders should monitor whether any extreme heat warnings are issued in mid-July, as these typically signal temperatures exceeding 30°C. The zero current probability likely reflects the market's early stage rather than genuine consensus that no temperature band will occur; comparable weather markets on Polymarket and Kalshi typically show distributed probabilities across multiple ranges once trading volume increases.

Methodology

Methodologically we separate two layers: the live probability (Polymarket mid-price) and the platform attributes (fee, KYC, settlement currency, payment rails). That keeps the comparison honest — a single canonical probability across the row, with the venue-by-venue trade-offs spelt out in the columns next to it.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

FAQ

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is PolyGram. 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 does Polymarket cost to trade?

Polymarket itself charges 0% — the only cost is the Polygon network fee, typically under $0.01 per transaction. Off-chain venues like Kalshi or Betfair charge 2-7% commission.

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?

On Polymarket directly, no — it's wallet-based. Intermediary brokers like PolyGram trigger KYC only above $1,500 of lifetime trading volume; under that you trade pseudonymously with a single wallet address.