The Washington PostDemocracy Dies in Darkness

Multi-day heatwave topples records in London and Paris. This is what’s behind it.

An omega block is a weather pattern where high pressure is blocked by two surrounding areas of low pressure, eventually forming what looks like an omega in the jet stream. This jet stream forecast is for Wednesday over Europe. (

This post has been updated.

A dangerous pattern of long-term heat is setting up over Europe this week, and forecast models suggest it could stretch beyond the weekend. Temperatures are running up to 30 degrees above average and long-term heat records are falling in the U.K and France.

The unusual aspect of this heatwave is not only how early it’s coming — heat of this magnitude is more typical later in the summer — but how long it seems likely to last.

England experienced its hottest July day on record Wednesday as the temperature at London Heathrow soared to 98.1 degrees (36.7 Celsius) according to the U.K. Met Office, breaking the previous record set in 2006.

Several locations across France established their hottest temperature ever recorded in any month. In Paris, the mercury surged to 103.5 degrees (39.7 Celsius) on Wednesday afternoon, which is the second highest temperature the city has ever recorded, according to Meteo-France.

Wimbledon had its hottest day in recorded history on Wednesday, climbing to a stifling 96 degrees (35.7 Celsius).

A huge ridge of high pressure is building over the continent in what we call an “omega block,” where two areas of low pressure book-end the ridge of high pressure and force the flow of air farther and farther north. This pumps warm air northward, and the clear skies and calm winds under the high allow the Sun to roast the surface without periodic ventilation in the form of a cold front or a trough.

In turn, a positive feedback can set up which can be hard to break — the high temperatures strengthen the ridge, the ridge leads to more heat, and so on.

The “omega block” is the same kind of weather pattern that took hold and didn’t let go during the catastrophic heatwave of July 2003, when tens of thousands of people died from heat-related illness. 14,000 of those fatalities occurred in France alone.

The Guardian reports that French authorities have opened registries where at-risk isolated populations can sign up to be checked on. “I don’t think this heatwave will have the same consequences as the one in 2003 because we weren’t as prepared at that time,” Ségolène Royal, the French ecology minister, told the Guardian.

Forecast models suggest this pattern will continue through at least the end of the week as the high pressure inches into Eastern Europe. Long-rage models are predicting temperatures to remain above average in Europe through the end of July.

But in the meantime, the clear skies and calm winds are baking the air in Spain and France, and highs are soaring past 100 degrees in the second day of heat. Highs will be running up to 30 degrees above average through at least Friday, and warm overnight lows in the 70s and low 80s will not provide much relief from the heat of the day.

A code orange weather alert is in effect across much of these two countries. MeteoFrance is using particularly strong language to describe the heatwave, noting that even those in good health are at risk from the heat.

On Monday, Madrid set a new record for highest temperature in the month of June at a sweltering 104 degrees. It hit that high temperature again on Tuesday. Analysis from the Spanish meteorological agency AEMet shows highs surging past 110 degrees in southern Spain near Cordoba.

Forecast models suggest Wednesday, Friday and Saturday will be the hottest days in northern France this week. Paris is expected to climb to over 100 degrees on Wednesday, and seems likely to stay in at least the 90s through early next week.

Jason Samenow contributed to this story.