Highest temperature in London on July 20?

Platform comparison

Market context

London City Airport recorded its highest temperature for 20 July 2026, with the market now resolved as the 72–73°F range captured 100% of the volume. Historical data for late July at EGLC shows temperatures frequently breach 24°C (75°F), yet the 0% crowd-implied probability on the Celsius equivalents of lower ranges suggests traders priced in a cooler day, aligning with the resolved outcome. On Polymarket, this outcome is expressed as decimal odds where 100% implies certainty, whereas Kalshi and Betfair often frame similar weather events using implied probability percentages or fractional odds, creating a subtle divergence in how risk is visualised across platforms.

Traders comparing Polymarket against Smarkets or Kalshi should note that fee structures and KYC requirements vary significantly on weather markets like this one. Polymarket typically charges no trading fees but requires crypto onboarding, while Smarkets offers lower fees for verified users and Betfair demands stricter identity checks. The settlement window ending 12:00 UTC on 20 July 2026 meant traders had to monitor Wunderground’s EGLC history page for real-time updates, a dependency that Kalshi’s US-centric weather contracts often avoid by using NOAA data. Recent Met Office forecasts for London in July 2026 highlighted a high-pressure system that suppressed peak temperatures, a catalyst that directly influenced the 0% probability on higher Celsius ranges before resolution [2].

Platform differences become stark when resolving weather events: Polymarket relies on Wunderground for EGLC data, whereas Kalshi uses NOAA and Betfair often aggregates multiple sources. This divergence can lead to timing discrepancies in settlement, especially when global weather stations report conflicting peaks. The resolved 72–73°F outcome (approximately 22–23°C) underscores how historical averages can mislead if traders ignore short-term atmospheric shifts, a lesson applicable across all prediction markets but particularly relevant for those comparing resolution sources.

Sources: 1 · 2

Methodology

We read Highest temperature in London on July 20? from four platform perspectives: Polymarket (on-chain CLOB), Kalshi (CFTC-regulated exchange), Betfair Exchange (sports book exchange), Smarkets (peer-to-peer betting exchange). Polymarket's live mid is the canonical probability; the side-by-side columns benchmark fees, KYC, settlement currency and deposit rails so you can choose the venue that fits your jurisdiction and trade size.

Resolution & payout

Settlement is the biggest difference between the four platforms: Polymarket on-chain in USDC (instant), Kalshi USD via CFTC (T+1), Betfair and Smarkets in local currency via bank withdrawal (T+1 to T+3). On-chain settlement clears in minutes — the fastest payout path of the four.

FAQ

What does Polymarket cost vs Kalshi?

Polymarket: 0% fees, only Polygon network costs (~$0.01/trade). Kalshi: up to 7% per trade plus spread. For high-frequency traders, Polymarket is dramatically cheaper.

Which platform has the deepest liquidity?

Polymarket — by a wide margin. Top markets reach $50-500M volume, Kalshi ~$200M cumulative, Betfair similar. Deeper liquidity means your trade moves the quote less.

Is Betfair a Polymarket alternative?

Only partially. Betfair Exchange is UK-focused with a sports-betting emphasis; they have politics markets but with thinner liquidity than Polymarket. Settlement in GBP/EUR, 2-5% commission on winnings.

What about Smarkets as an alternative?

Smarkets is a UK betting exchange with a lower default commission (2%) than Betfair. Liquidity on political markets is below Polymarket, comparable to Kalshi. Geo-blocked in many jurisdictions.

Which platform is accessible globally?

Polymarket is geo-blocked in the US/UK/EU. Kalshi is US-only. Betfair and Smarkets are UK-restricted. Polymarket Alternative has a different geo footprint and routes to Polymarket's order book at 0% fees.