Highest temperature in Hong Kong on May 21?

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Market context

Hong Kong’s midday heat on 21 May is governed by the Observatory’s final daily extract, which is usually the cleanest way to settle a temperature range market once the day’s data are locked in. Seasonal context points to a hot late-spring regime rather than anything exceptional: May normals are already around 27.2C for the month, with recent years often running above average. In May 2025, the Observatory recorded a monthly mean maximum of 30.0C and a highest reading of 33.0C, illustrating that low-30s are plausible even before June arrives.

Against that backdrop, the crowd-implied 0% yes reading suggests the market is effectively waiting for hard data rather than pricing in a strong weather outcome. Historical comparables do not point to a single dominant temperature band, but they do show Hong Kong can move quickly from warm to very hot when the subtropical ridge strengthens. AccuWeather’s May outlook has daily highs mostly in the mid-80s to low-90s Fahrenheit, which is broadly consistent with upper-20s to low-30s Celsius and leaves the exact settlement bin highly dependent on the Observatory’s final maximum.

The main catalyst is therefore not a political calendar event but the release timing and finalisation of the Hong Kong Observatory “Daily Extract” for 21 May. Traders should watch for any late-afternoon update, but the market can only settle once the official absolute daily maximum is published. Recent Observatory reporting has emphasised warmer-than-normal conditions in Hong Kong, so any final reading near or above 30C would fit that recent pattern, while a cooler outcome would likely come from cloud, rain or a marine breeze limiting the daytime peak.

Sources: 1 · 2 · 3 · 4 · 5

Methodology

This page tracks Highest temperature in Hong Kong on May 21? across four political prediction venues. Live odds come from the Polymarket order book (the deepest political prediction-market book). Kalshi is the CFTC-regulated US alternative, Betfair the established UK sports-exchange with politics markets, Manifold the open play-money variant. PolyGram routes every trade through to Polymarket — at 0% fees.

Resolution & payout

For political markets the resolution source is decisive. Polymarket defines a concrete source per contract (e.g. AP, Reuters, official electoral commission) and uses the UMA Optimistic Oracle as the on-chain dispute mechanism. With a clearly defined outcome the USDC payout lands within minutes of the final confirmation.

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.

Is this market available outside the US?

PolyGram is available in most jurisdictions where Polymarket isn't directly accessible. Polymarket itself is geo-blocked in the US/UK/EU. Always check local regulations.

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.

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.