Highest temperature in London on May 20?

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Market context

London turned notably warmer in the past 24-48 hours, with the Met Office reporting a daily maximum of 20°C across London monitoring stations on 20 May. That is a useful marker for the same broad air mass: the market is not looking at an extreme heat setup, but at whether the city’s warm spell can extend high enough at London City Airport to clear the relevant Celsius band for the day’s maximum.

For context, mid-May in London usually sits well below 20°C at the top end, so a reading above that level would be warmer than the seasonal norm rather than routine. The key comparison is not with summer heatwave standards, but with an ordinary spring day lifting into the low 20s. Recent public guidance from the Met Office has also pointed to warmth building across the south-east, including London, which supports why the market has shifted away from a 0% reading and towards a live temperature outcome.

The main traders’ focus should be on the day’s observed maximum at London City Airport before the market’s settlement window closes at 12:00 UTC on 20 May. The crucial drivers are cloud cover, any showers or sea-breeze effects, and whether the warm air arrives early enough to register before midday. Wunderground’s daily history page for EGLC is the final source here, so later official updates or local station adjustments matter more than general city forecasts.

Sources: 1 · 2 · 3 · 4 · 5

Methodology

This page reviews Highest temperature in London on May 20? across five venues. We show live odds for Polymarket-based markets (sourced from the Polygon order book); for other venues we list platform attributes, since the comparable contracts are not exposed via a public API on every venue. Every CTA points at PolyGram — the application we operate, where you trade directly against the Polymarket order book at 0% fees.

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.

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.

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.

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Not under $1,500 of lifetime trading volume. Above that threshold, PolyGram triggers a quick verification flow that finishes in minutes.

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.