Highest temperature in Hong Kong on May 21?

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Market context

Hong Kong’s hottest reading for 21 May will be taken from the Hong Kong Observatory’s final daily extract, using the absolute daily maximum in degrees Celsius. The crowd-implied 0% for a “Yes” outcome appears to assume the market is unlikely to settle in the top end of the distribution, but late-May heat in Hong Kong can still be meaningful. The observatory’s own May 2025 summary showed a maximum of 33.0C on 22 May, while monthly mean maximum temperatures that month were 30.0C, underlining that isolated spikes well above the seasonal average are possible.

For context, Hong Kong’s May climate is usually hot and humid rather than extreme: normal daily highs sit around the high 20s Celsius, with the first half of the month generally a touch cooler than the final week. Recent climatology sites put typical May highs around 29C to 30C, with lows in the low-to-mid 20s C. That means the key question is not whether it will be warm, but whether the day reaches the upper 30s bracket, which has happened in very hot spells but is not the baseline case.

The main catalyst is the observatory’s finalised daily extract, which will remove any uncertainty about the official peak once published. Traders should also watch the near-term synoptic set-up: a stronger subtropical ridge over southern China favours hotter, clearer conditions, while cloud, rain bands or an onshore flow would cap the maximum. Hong Kong’s 2025 weather review described a warmer-than-usual spring and a notably hot May, so a repeat of late-spring warmth is plausible even if the exact peak remains weather-dependent.

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). The odds column is filled only where we have clean data — that avoids the made-up numbers that get a network demoted when search engines cross-check against the source venue.

Resolution & payout

Settlement runs on-chain. Polymarket's contract logic separates YES and NO shares as conditional tokens; at resolution the winning share lifts to $1.00 and the losing one to $0. The outcome input comes from the UMA Optimistic Oracle, which secures against bad resolution with a bond + dispute window.

Once finalised, the smart contract pays USDC to the holders' wallets within minutes — no withdrawal fees beyond Polygon network gas. Kalshi settles in USD via CFTC clearance, Betfair in account currency net of commission, Manifold in play-money mana with no cash-out.

FAQ

Where can I trade this market with the lowest fees?

On PolyGram, which mirrors the Polymarket order book at 0% fees. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

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 it cost to trade on PolyGram?

Zero. PolyGram routes every order to the live Polymarket order book; the only cost is the Polygon network fee, typically under $0.01 per transaction.

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.