Highest temperature in Paris on August 21?

Platform comparison

Market context

The market is pricing a 1% chance that Paris-Le Bourget Airport will record a temperature of 40°C or higher on 21 August 2026. This represents an extreme heat event for the region, well above typical summer maxima. The settlement hinges on Weather Underground's Daily Observations table rather than summary figures, a distinction that matters when stations report multiple readings throughout the day. Traders are currently pricing this outcome as a tail-risk scenario, reflected in the USDC-denominated conditional tokens trading on Polygon at odds that heavily favour cooler temperature ranges.

August heat records in the Paris region provide context for evaluating this probability. The all-time August maximum at Le Bourget stands at 39.5°C, recorded in 1949. More recently, August 2003 saw temperatures reach 38.3°C during Europe's severe heatwave, whilst August 2022 peaked at 36.0°C. A 40°C+ reading would require conditions exceeding the station's recorded history by a meaningful margin, though climate patterns have shifted measurably over recent decades. The 1% pricing reflects this historical rarity rather than meteorological impossibility.

Traders should monitor European summer weather forecasting from mid-August 2026 onwards, particularly atmospheric pressure systems and jet stream positioning that drive extreme heat events across northern Europe. Anomalous sea surface temperatures in the Mediterranean and Atlantic during July-August could signal conditions favouring a heat dome over France. Any official heat warnings issued by Météo-France in the week preceding 21 August would likely shift market pricing, as would longer-range ensemble forecasts showing sustained high-pressure systems over the region.

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.

UK Frequently Asked Questions

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.

Is Polymarket legal in the UK?

Polymarket is accessible to UK traders but is not UKGC-licensed. It operates under US CFTC jurisdiction. UK residents face no domestic legal prohibition on using it, but UKGC consumer protections do not apply. For UKGC-regulated alternatives, Betfair Exchange and Smarkets offer similar prediction-style markets.

Do I pay tax on prediction market profits in the UK?

UKGC-licensed platform profits (Betfair, Smarkets) are typically tax-free gambling winnings for UK individuals. Polymarket profits involve USDC crypto transactions, which HMRC treats as Capital Gains Tax events. Keep full transaction records and report via Self Assessment if gains exceed £3,000 per tax year.

How do I deposit on Polymarket from the UK?

UK traders typically fund Polymarket via Coinbase UK, Kraken or Revolut — buying USDC with GBP and transferring to a Polygon-compatible wallet (MetaMask, Coinbase Wallet). Polymarket's onboarding walks you through the bridging process. Typical GBP-to-USDC conversion costs 0.5–1%.