What is a heat dome, and why do they happen?
The short version
A heat dome is an unusual weather event where cold high pressure air traps a layer of warm air below it. The hot air cannot move upwards or sideways because of the air pressing down and is trapped close to the ground making the shape of a dome.
Inside the dome the trapped air circulates, picking up heat from the ground and warming the air, trapping it in a cycle of heating leading to higher and higher temperatures.
The long version
A heat dome is an unusual weather event where cold high pressure air traps a layer of warm air below it. Heat domes can be huge. On the 29th of June 2022, one caused wildfires across an area from British Columbia in Canada all the way down to Washington and Oregon in the United States, across a distance of over one thousand kilometres.
Temperatures went totally off the scale. Lytton, in Canada broke the maximum temperature record on three consecutive days running and finally reached 49.6 Celsius (121Fahrenheit). The heat started a fire that destroyed most of the town. In 2021 another heat dome nicknamed Lucifer sat across the southern mediterranean for weeks finally breaking the european heat record at 48.8C and started fires causing huge amounts of damage.
Normally weather patterns are moved around by wind created by areas of high and low pressure and by the jet stream. Jet streams are special winds around 8 to 15 kilometers (about 5 to 9 miles). Although they are very high they have a considerable effect on the weather at ground level.
What makes weather?
Our day to day weather is driven by different temperatures, pressures and water content of the air around us and these differences are what makes the weather.
What is high pressure air?
High pressure air is usually cool and dry. When air is cold the molecules are less active and this pushes them closer together. Cold air is heavier than that of a similar sized volume of warm air and this extra weight makes it push down, increasing its pressure. High pressure air usually brings with it bright sunny days that may also be a little cooler. You may have noticed on weather forecasts they say “we have a high pressure system moving in and that’s going to bring sunny weather for the next few days”.
This happens because high pressure air is heavy, as it falls it pushes out warmer air. Warm air carries more water so it usually has more clouds. As the heavy high pressure air descends towards the ground, it pushes the lighter warmer air out of the way, carrying the clouds with it. This is why high pressure brings clear sunny blue skies along with a little coolness.
What is low pressure air?
Low pressure air is usually warmer wetter air. The extra heat allows the air to carry more moisture than cooler air and the extra energy of the molecules pushes them further apart making the pressure lower. Low pressure air is lighter than the same volume of high pressure air, being light means it tends to float upwards up, but as the air rises it also cools.
As the air cools, it can’t hold as much moisture. First this water condenses to form clouds and as it cools further then these clouds will form water droplets that get larger and larger and eventually form into rain. And that’s why on weather forecasts you might hear them say “we have a low pressure weather system moving in folks so don’t forget your rain coats”
As well as high and low pressure weather systems we also have something else that helps push the weather around the globe and this is called the Jet Stream.
What is the Jet Stream?
The Jet Stream is very fast moving winds very high up in the atmosphere, they are created from the interaction between moving hot and cold air at different pressures. Air at different pressure is always trying to cancel each other out with air at high pressure naturally moving towards air at low pressure.
At the equator the sun is close to being overhead and this brings lots of heat that creates light, hot moist air at low pressure. At the north and south poles the sunlight is very weak and the air is very cold and this creates heavy, dry, air at high pressures.
If the earth was not spinning, then warm air from the equator would flow up to the poles cold and air from the poles would flow to the equator.
But the earth is spinning and this generates another force called the “Coriolis effect”. This forces the direction of the movement of air away from the poles and sends it towards the east. This breaks up the flow of air from the equator to the poles into three cells, the Hadley cell is closest to the equator, then the Ferrell cell in the middle and the Polar cell nearest the poles. This structure of three cells is repeated in both the Northern and Southern hemispheres.
The air in each cell is circulating in two directions, there is the upwards and downwards rotation of high and low pressure air inside the cell, as well as also moving from the west to the east by the coriolis force. At high altitudes the rotating air inside the cells come together with air at high pressure and low pressure and this interaction creates the Jet Streams.
So Jet Streams are like rivers of air flowing around the planet at very high speeds, created from air at high and low pressure high up in the atmosphere, and the strength of the Jet Stream is dependent on this imbalance of low and high pressures.
The Jet Streams though are also affected by what happens down on the surface. The engine of the Jet Stream is the temperature difference between the equator and the poles. When the difference is small, the jet stream is weak, when the difference is large the jet stream is strong and these changes can have dramatic effects on the weather and on the development of heat domes.
What do Jet Streams have to do with heat domes?
Normally the Jet Streams have enough strength to push weather systems around the globe, but Jet Streams are complex winds and there are times when they weaken or change direction.
They can develop meanders across the globe, forming into great huge loops across vast distances, and it’s within these loops that different weather can form called blocking patterns.
Blocking pattern as the name suggests stops the Jet Stream from doing its thing. Instead it’s the Jet Stream that changes course.
Blocking patterns have names, this one is called an Omega Blocking Pattern because it resembles the Greek Ω, this is three weather systems, two low pressure systems either side of a high pressure system in the centre with the Jet Stream snaking around them.
What makes a heat dome?
In a Blocking pattern the high pressure system pushes down, keeping out the clouds and giving sunny days, but if this high pressure is locked in place inside a blocking pattern then it’s not going anywhere, it’s just going to sit there. Underneath the high pressure system the ground is warming up, this means the air above it is also warming up, as it warms it gets lighter and wants to go up.
But it’s not going anywhere, it’s stuck under a high pressure system pressing down. The air at ground level gets hotter and hotter, but there is no escape, it starts to expand upwards into the shape of a dome.
As it expands then the air inside the dome starts to circulate, round and round, picking up more heat at ground level and feeding it into the air above. Pushing the temperatures up and up. This is why the temperature record was broken three days in a row in Lytton, each day the dome got hotter and each day the record was broken.
The question now is why? Why in 2021 have so many temperature records fallen and why do there seem to be more heat domes?
Will there be more heat domes in the future?
There has been an overall trend of warming across the globe, but the north and south poles are warming faster than anywhere else.
In June of 2020 in Verkhoyansk in Siberia, which is inside the arctic circle, a new record was set of 38C. But this was not an isolated incident, the polar regions are warming faster than at the equator.
So the temperature difference between the poles and the equator is reducing and this may be leading to a weakening of the Jet Stream, this in turn might help to explain some of the strange weather that we are getting.
Climate change is often seen as a gradual warming over a long period of time, but as the globe warms it is producing unpredictable changes. It could be that instead of change being long and gradual, it may turn out to be a series of short lived but dramatic events.