Highest temperature in London on July 20?

Platform comparison

Market context

London City Airport recorded a cool 17°C maximum on 20 July 2026, confirming the market’s 0% YES probability for any extreme heat outcome. The day fell just after a brief thunderstorm break that ended a mid-month heatwave, with temperatures dropping from the early 30s to the low 20s by the weekend. This sharp cooldown aligns with the Met Office’s forecast that hot weather would not persist beyond 17 July, leaving 20 July firmly in the post-heatwave window [5][6].

Historically, London City Airport averages a high of 23.6°C on 20 July, with July peaks rarely exceeding 25°C outside of sustained heatwaves [4]. The 2026 heatwave peaked at 35°C on 9 July but collapsed quickly, with 20 July’s 17°C maximum sitting well below the seasonal norm and far from the 25°C frontrunner now priced at 100% [1][6]. This suggests the market has correctly anchored to the post-heatwave reality rather than the earlier spike.

Traders should monitor the Met Office’s daily 9am forecasts for the next week, as any unexpected resurgence in southerly flow could shift temperatures back toward the 25–27°C range. The UK Health Security Agency’s amber heat alert has already expired, and current models show no immediate heat resurgence before late July [5][6]. Watch for any sudden shifts in wind direction or pressure systems that could reintroduce warm air from the continent.

Sources: 1 · 2 · 3 · 4 · 5

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

At resolution the UMA oracle takes over: a proposer posts the outcome with a bond, any token holder can dispute within two hours. Without dispute the result is accepted and the smart contract distributes USDC instantly.

On Kalshi (CFTC-regulated) resolution runs through their in-house clearing engine in USD. Betfair Exchange settles after match end in the account's local currency. Manifold pays no cash — only its in-platform "mana" currency.

FAQ

Is this market available outside the US?

Polymarket itself is geo-blocked in the US/UK/EU. Always check the legal status of prediction markets in your jurisdiction before trading.

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 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 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.