2 Jul 2013

What’s causing the deadly heatwave in the south west US?

Since late last week, an intense and dangerous heatwave has gripped the south west of the US, with California, Arizona and Nevada seeing record-breaking temperatures.

Wildfires have broken out, with 19 fire fighters killed in Arizona as they tried to battle a blaze that suddenly altered course as the wind changed direction.

There have also been a number of suspected heat-related deaths as people with illnesses are put under greater strain as temperatures soar beyond 38C (100F).

Las Vegas’ McCarran international airport saw the temperature reach 47C (117F) on Sunday, equalling its highest ever temperature on record.

In neighbouring California, Death Valley saw the mercury rise to 54C (129F), only a few degrees short of the highest temperature ever recorded on earth, 56.7C (134F) – coincidentally also measured at Death Valley a century ago in July 1913.

What’s caused the heatwave?

The jet stream has taken a northward path along the Pacific west coast of the US, reaching into western Canada, before diving southwards across the Midwest.

This weather pattern has allowed a big area of high pressure to build over the south west of the US, forming a block to any rain and cooler air moving in off the Pacific ocean.

As is often the case in summer, high pressure gives clear skies, lots of sunshine and building heat as air gently sinks beneath it, compressing and warming as it does so.

There are a number of other factors that contribute to building heat as well.

In northern hemisphere summers, the days are longer than the nights, which means that the sun’s energy is entering the earth’s atmosphere for more hours of the day than not.

Also, generally, under an area of high pressure, the winds are light. This allows the same bubble of hot air to sit over the same place for longer, continually heating up as the sun shines on it.

Finally, there’s the dryness of the soil. In this part of the US, as well as being in drought, there’s a lot of desert landscape.

Dry air heats up more quickly than moist air because all of the sun’s energy goes into lifting the temperature, rather than having to be shared with the process of evaporating any moisture in the ground.

How long is the heatwave going to last?

Excessive heat warnings, issued by the US National Weather Service, remain in effect until Thursday across much of California, Arizona and Nevada.

From Friday onwards, there are signs that the heat will ease slightly, taking it out of the extreme category. Nevertheless, some places will still see temperatures getting above 38C (100F).

I’ll be keeping an eye on the heatwave over the coming days and tweeting updates on Twitter – @liamdutton

Tweets by @liamdutton

2 reader comments

  1. jackie says:

    I can’t imagine how horrendous this might be and sending our condolences to the lives lost during this catastrophe and to all those affected. RIP

  2. Andrew Dundas says:

    What was the outcome of the Met Office conference that discussed why our UK weather had gone wonky?
    Might the unusual weather in N America reflect the same climate change forces that are hitting us?

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