Highest temperature in London on August 4?

Platform comparison

Market context

London’s hottest reading on 4 August will be set by the London City Airport station, not by a city-wide average, so the key question is whether the day briefly clears the low-20s Celsius forecast or stays nearer the mid-teens to low-20s shown by current models. AccuWeather’s August outlook puts London’s daily highs broadly between 67°F and 78°F, while other August guides cluster nearer 21–22°C on average, which leaves room for a warm spike but not a strong heat-event profile.[1][3][4][9]

That backdrop helps explain why the crowd-implied 0% YES on a specific temperature band may be less about certainty and more about market structure. On Polymarket, traders usually see an implied probability directly; on Kalshi, prices are also quoted as probabilities, but fees and contract granularity can differ; on Betfair and Smarkets, the same weather view is expressed as decimal odds or exchange prices, with the effective probability shaped by commission rather than an all-in price. London’s August 4 climatology is usually modest — roughly low-20s Celsius with occasional rain — and recent forecast pages for early August show highs around 22–28°C depending on source, so a late-morning run-up or a sun-break could still matter if the airport station records a short-lived peak.[2][3][5][9]

For traders, the main catalysts are the intraday forecast updates rather than scheduled announcements. The contract resolves on the highest temperature recorded at London City Airport on the day, so the most relevant inputs are changes in cloud cover, rain timing, and any afternoon warming after the morning settlement window; the Wunderground history page is the official reference for the final print, but live forecast revisions can move exchange prices well before then.[5][7][10] On a platform-comparison basis, that means Polymarket and Kalshi may reprice more cleanly in probability terms, while Betfair and Smarkets can look cheaper or tighter depending on commission, depth, and whether the market is accessible to your jurisdiction.

Sources: 1 · 2 · 3 · 4 · 5

Methodology

This page compares Highest temperature in London on August 4? specifically across Polymarket, Kalshi, Betfair Exchange and Smarkets. The live probability is the Polymarket mid; the comparison columns summarise each venue's fee structure, KYC, settlement currency and payment rails. Every CTA routes to Robinhood Prediction Markets, which mirrors the Polymarket order book at 0% fees.

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

Polymarket vs Kalshi — which is better?

Depends on your location. Kalshi is CFTC-regulated, US-only with full KYC. Polymarket is global, on-chain, no KYC up to $1,500. Polymarket has ~10x higher liquidity but higher regulatory risk.

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.