Highest temperature in Hong Kong on August 21?

Platform comparison

Market context

Hong Kong opens this Friday morning with 27°C at the Observatory, high humidity and light rain, so the day has started noticeably cooler and wetter than the recent mid-August heat burst.[1] The Thunderstorm Warning issued just after midnight adds a further drag on early heating, but the key market question is whether the rain clears quickly enough for temperatures to recover later in the afternoon.[6]

The backdrop is a city that has already seen several very hot days this month, including readings above 38°C in some areas and official warnings for very hot weather, so a top-end outcome is not impossible if sunshine breaks through.[7][8][10][15] Even so, the current setup is less conducive to an extreme maximum than the heatwave period, and a 0% crowd-implied chance on the highest bracket reflects that the market is pricing a more modest ceiling than the recent record-type conditions.

Traders should watch the Hong Kong Observatory’s morning and midday updates, especially whether the thunderstorm risk is lifted and whether the very hot weather warning returns later today.[1][6] The market cannot settle until the Observatory publishes the final Daily Extract with the “Absolute Daily Max (deg. C)” for 21 August, so any late revision to the day’s official maximum remains the decisive catalyst for resolution.[3]

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

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

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 Prediction Today. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

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.

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%.