9 Aug 2012

Why the hottest month ever for the US?

The US weather service NOAA announced today that July 2012 has been the hottest month ever recorded in the US – breaking a record set in the Dust Bowl in the 1930s.

This new record comes in the midst of a severe drought that is currently affecting more than half of the country, with devastating impacts on crops and agriculture.

It’s also roused further active discussions amongst climate scientists around the world as to how much blame can be placed on human activity for yet another significant temperature record being broken.

Having read lots of stories today on this very subject, I found they all offered the raw facts but none really delved into the possible causes of this record to be broken and the contributing weather factors. So I thought I’d venture along this path and offer some explanations.

Position of the jet stream

During July in the US, there was a trend for the position of the jet stream to be further north than normal – especially across central portions of the country.

As we know from our own experiences in recent months here in the UK, the position of the jet stream determines the weather that we get.

Generally, places that lie to the south of the jet stream during the summer months tend to be drier and hotter with ridges of high pressure more prevalent.

This has been the case across the central portion of the USA in July, leading to clear skies, an abundance of sunshine and building heat – contributing to the record-breaking temperatures.

Bone dry ground

When the sun’s rays arrive in our atmosphere, they don’t heat the air at the surface directly. They actually heat the ground, which in turn warms the air just above it.

The rate at which the ground warms is determined by how moist it is. If there is moisture present, then some of the energy received by the sun is consumed by evaporating this moisture and, as a result, doesn’t all go into the heating process.

Image courtesy of NOAA – showing the central portion of the US had below normal rainfall in July

However, with much of the US in severe drought with the ground bone dry, there is no moisture to be evaporated and all of the energy goes into heating the ground.

This means that the ground heats up very quickly and more intensely which subsequently makes the air above hotter, giving higher temperatures.

Heat balance

During the summer months in the northern hemisphere, the days are longer than the nights and energy from the sun is being received for more hours than not.

This combined with the perfect atmospheric conditions of high pressure, clear skies and light winds leads to a cumulative heating effect.

Effectively, as the heat wave remains, the temperature each subsequent day and night will tend to be a little higher giving a positive feedback effect and intensifying the heat – something that was observed in the recent US heat waves.

Image courtesy of NOAA – showing much of the US had above normal temperatures in July

So whilst the debate amongst climate scientists about how much human activity can be blamed for this tumbling US record will carry on, it’s also worth noting that more everyday weather patterns can have an influence.

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