Highest temperature in London on July 21?

Platform comparison

Market context

The Polymarket contract currently prices a YES outcome at 1%, reflecting extremely low conviction that London will record a temperature above 32°C at City Airport on 21 July 2026. This implies traders expect either moderate summer conditions or that the specific measurement threshold sits well above typical July highs for the capital. The settlement mechanism relies on Wunderground's historical data feed for the City Airport station, which records daily maxima in Celsius—a straightforward data point once the date passes, though the current pricing suggests meaningful disagreement exists about what constitutes an outlier event.

London's July temperature record stands at 40.3°C, set in 2022 during an exceptional heatwave. More typical July maxima cluster between 23–26°C, with readings above 30°C occurring roughly once per decade. The 1% probability implies traders view the market's threshold as substantially above the historical median but potentially within reach of a severe heatwave scenario. The 2022 event demonstrated that extreme heat is possible, yet remains rare enough that most summer days fall well short of triggering high-temperature outcomes.

Traders monitoring this contract should track UK Met Office seasonal forecasts and any emerging Atlantic weather patterns during June and early July 2026. Persistent high-pressure systems over the British Isles or heat domes tracking northward from continental Europe would represent the primary catalysts for elevated temperatures. Real-time weather models become reliable only 10–14 days before settlement, meaning early-season indicators offer limited predictive value for pinpointing conditions on a specific date months ahead.

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.

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 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.

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.