What is the difference between a heat dome and a heat wave? Experts clarify that they are different phenomena. “The heat dome is the cause and the heat wave would be the effect,” summarizes Rubén del Campo, spokesperson for the State Meteorological Agency (Aemet).
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A heat wave is a prolonged period of time with temperatures much higher than normal. In Spain, generally, we talk about heat waves when there is a simultaneous period of extremely high temperatures affecting a large surface area of territory and lasting at least three days. This definition takes into account criteria of extent, duration, and intensity.
Right now in Spain there are high temperatures but no heat wave
What Spain is experiencing now in May is not a heat wave because, although the heat situation will be quite long-lasting and is affecting a good part of the Spanish territory, it does not reach the temperature values necessary to be called a heat wave.
For it to be so, these values should be among the top 5% of the highest temperatures of July and August recorded between 1971 and 2000 in at least 10% of the meteorological stations in Spain. “And this, at least a priori, does not seem likely to be met, although it is not impossible: we will see at the end of the episode…,” says Rubén del Campo.
Hot air at altitude
On the other hand, a heat dome is one of the atmospheric situations that can give rise to a heat wave. This is a newly popularized term. It refers to a meteorological phenomenon in which a persistent anticyclonic block causes a pocket of hot air that reaches high layers of the troposphere.
It became popular recently in June 2021 with a large heat wave that occurred in the northwest of the United States, when temperatures of 50 ºC were reached at that time.
“It could be summarized by saying that the heat dome is simply warm air at altitude,” explains Javier Martín Vide, emeritus professor of Physical Geography at UB, something that can be determined from radiosondes.
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Basically, it is an area of high pressure that covers a large geographical extent. Currently, it is located in areas of southwestern Europe; and it can also reach levels of up to 5,000 to 6,000 meters in altitude.
“These situations are characterized by being very persistent; lasting many days; they are more powerful anticyclones than usual,” adds Del Campo.
All this leads to “very stable meteorological situations, in which there is hardly any wind, hardly any vertical air currents, and this causes the heat to have no escape,” says Del Campo.
The sun heats a surface day after day and it remains confined in a certain area, so temperatures each day are a little higher because there is no ventilation, no air renewal, and that heat has no escape.
”The hot air acts like a lid, preventing cloud growth. It corresponds to anticyclonic situations, like the one occurring now, with a very powerful anticyclone indicating very high pressure,” confirms Martín Vide.
When the air descends, pressure increases… and it heats even more
And, moreover, since these are very powerful anticyclones, situations occur that cause temperatures to rise even more. Why? Usually, in all anticyclones the phenomenon of subsidence occurs: air tends to descend from high levels to the surface. But in this dome the subsidence is much more marked than usual, and that causes this descent to occur with more intensity. The air, when it descends, gains atmospheric pressure, heats up and compresses (like when a bike tire heats up and you touch the valve, which burns). And this effect is added to the stability of the powerful anticyclone, causing the very high temperatures.
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