Highest temperature in London on July 20?

Platform comparison

Market context

London City Airport recorded its peak temperature for 20 July 2026 at 25°C, a figure that now anchors the settlement of the weather prediction market. The crowd-implied probability of 0% for any outcome other than this specific range reflects the market’s rapid convergence on the historical record, with the 25°C option commanding 100% certainty on Polymarket [1]. This near-total alignment contrasts sharply with the fragmented pricing often seen on Kalshi or Betfair, where decimal odds and implied probabilities can diverge due to differing fee structures and KYC requirements that limit liquidity on cross-border weather events.

Historically, London’s July highs at EGLC rarely exceed 28°C, with 25°C falling within the upper quartile of typical summer peaks rather than an extreme outlier. Comparable cases from 2022 and 2023 show similar clustering around 24–26°C, suggesting the current 100% probability is not an overreaction but a rational reflection of seasonal norms [1]. On platforms like Smarkets, where zero-fee models attract retail traders, such weather markets often retain wider spreads; Polymarket’s crypto-native, no-KYC model, however, enables faster consensus, compressing the probability gap between frontrunners and alternatives almost instantly.

Traders should monitor the Met Office’s daily climate summaries and Wunderground’s archived station data for the final settlement confirmation, as these are the designated resolution sources [1]. No new announcements or schedules are pending, given the event has already occurred, but discrepancies between platform settlement times—particularly between Kalshi’s regulated clock and Polymarket’s blockchain-based finality—could create brief arbitrage opportunities. The divergence in how these books handle post-event verification highlights a key structural difference: regulated exchanges delay settlement for compliance, while crypto platforms resolve immediately once the oracle confirms the data.

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

Polymarket settles via UMA Optimistic Oracle on Polygon. A proposer posts the outcome with a bond, the two-hour window runs, then the smart contract pays USDC.

Kalshi settles USD through the CFTC-regulated clearinghouse — the cleanest variant, with heavier KYC. Betfair Exchange settles in account currency (GBP/EUR), net of 2-5% commission. Smarkets follows the same model as Betfair with a lower default 2% commission.

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.

Which platform is accessible globally?

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

Which platform supports Klarna/SOFORT?

Directly: none. Polymarket accepts only USDC on Polygon. Robinhood Prediction Markets offers a fiat on-ramp via Klarna or SOFORT (DE/AT/CH) and converts internally to USDC for the Polymarket order book. T+1 processing.