Highest temperature in London on July 21?

Platform comparison

Market context

On 21 July 2026, the highest temperature recorded at London City Airport will determine which range wins this market. The settlement mechanism uses Wunderground's historical weather database, pulling the single daily maximum from that specific station rather than broader London readings, which can vary by several degrees depending on location and measurement methodology.

London's July temperatures have historically ranged from 20°C to 35°C, with the Met Office recording an absolute maximum of 40.3°C in July 2022 during an exceptional heatwave. The current 0% implied probability across this market suggests traders are either awaiting range options to be published or treating the settlement as highly uncertain at this distance. On Polymarket, decimal odds would reflect this differently than Kalshi's implied probability display; Betfair's lay functionality allows traders to express scepticism about specific ranges more fluidly than fixed-outcome books. Smarkets' commission structure (typically lower on lower-probability outcomes) may attract different positioning than platforms charging flat percentage fees regardless of probability tier.

Traders should monitor the UK Met Office's seasonal forecasts and any updates to climate pattern predictions as summer 2026 approaches. The North Atlantic Oscillation and jet stream positioning in July typically drive whether high-pressure systems establish over the British Isles, directly influencing whether temperatures breach 30°C or remain in the mid-20s. Historical volatility in London's July maxima—swinging 10–15°C year-on-year—means early-market pricing may shift substantially once meteorological models gain confidence in the specific week's conditions.

Methodology

This page compares Highest temperature in London on July 21? specifically across Polymarket, Kalshi, Betfair Exchange and Smarkets. The live probability is the Polymarket mid; the comparison columns summarise each venue's fee structure, KYC, settlement currency and payment rails. Every CTA routes to Robinhood Prediction Markets, 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

What does Polymarket cost vs Kalshi?

Polymarket: 0% fees, only Polygon network costs (~$0.01/trade). Kalshi: up to 7% per trade plus spread. For high-frequency traders, Polymarket is dramatically cheaper.

Which platform has the deepest liquidity?

Polymarket — by a wide margin. Top markets reach $50-500M volume, Kalshi ~$200M cumulative, Betfair similar. Deeper liquidity means your trade moves the quote less.

Which platform is accessible globally?

Polymarket is geo-blocked in the US/UK/EU. Kalshi is US-only. Betfair and Smarkets are UK-restricted. Robinhood Prediction Markets has a different geo footprint and routes to Polymarket's order book at 0% fees.

Are all these platforms regulated?

No. Kalshi is CFTC-regulated (US). Betfair and Smarkets are UK Gambling Commission licensed. Polymarket operates without explicit regulation — a different risk profile than a regulated sportsbook.

Which platform supports Klarna/SOFORT?

Directly: none. Polymarket accepts only USDC on Polygon. Robinhood Prediction Markets offers a fiat on-ramp via Klarna or SOFORT (DE/AT/CH) and converts internally to USDC for the Polymarket order book. T+1 processing.