Summer storms are nothing new. A tense spell of hot and sultry weather broken by a sudden storm has long been the metaphorical stuff of poetry and storytelling from Basho to Shakespeare and Richard Curtis’s Four Weddings and a Funeral. But with extreme weather predicted to become a more frequent phenomenon, what exactly is the science behind summer storms?
1. Air nearer the earth is warmer
Let’s start with differences in air temperatures. Counter-intuitively perhaps, the air near the earth’s surface heats faster than the air higher up in atmosphere. Why?
The molecules that make up the earth react differently to the sun’s light particles (photons) than the molecules in the atmosphere, getting hot as they react. According to the laws of thermodynamics, heat transfers away from hot towards cool things. As a warmer body then, the earth moves its heat to a cooler place, in this case to the air particles nearby.
So, weird though it may be to imagine, the air near the earth is heating from the earth below, as well as from the sun above. It therefore gets hotter than air further away from the earth.
2. Long summer days heat the earth and its air more
In the summer, when there are longer daylight hours, there is more energy from the sun heating up the earth’s surface and thus, as we’ve understood just now, also heating up the air nearby. Meanwhile the air up in the higher layers of atmosphere remains relatively cool.
For an electrical storm to happen, you need exactly these summer conditions: a layer of warm air (nearer the earth) situated below a layer of cooler air (up in the atmosphere) – and you need the air to be moving.
3. Warm air moves upwards and is wet
Storms are caused by movement in the atmosphere, when the warm air near the earth starts to rise up towards colder layers of air. It starts to rise because hot molecules move faster and move apart. Moving apart makes the particles less densely packed, or, in other words, lighter. Warm, less dense air rises while cooler, denser air sinks.
As it moves up and away from the heat of the earth, the air begins cooling and starts falling again. This rising and overturning of air is called convection.
The other thing to note at this stage is that warm summer air contains more water droplets than cold winter air.
So we have warm air full of water droplets that start to cool and condense as they rise up and then fall down. This cooling, condensing and overturning creates clouds.
4. Cumulonimbus, ice and electricity
The warmer the weather, the more vigorous the convection, or rising and overturning of the air, and this is what causes deep layers of clouds known as cumulonimbus to form.
Inside cumulonimbus clouds there are updrafts and downdrafts. The updrafts take the moisture droplets up so high (7,620 metres or 25,000 ft) that they become supercooled. By the way, these moving droplets and ice particles have a really excellent name: hydrometeors.
When these moving icy particles happen to collide with each other, depending on the surrounding temperature and the concentration of water droplets, they transfer electrical charges.
Some are left with a negative charge, and these are heavier, like hail, and gather at the bottom of the cloud. Others have a positive charge and they are lighter crystals that are forced to the top of the cloud.
5. Negative charge attraction
Negative charge is attracted to positive charge. So the negatively-charged particles want to move towards positively-charged ones, whether within that particular cloud, another nearby cloud – or anything else that has a positive charge.
Eventually the attraction will become strong enough that the particles move as a flowing current of electricity, which we perceive as a large electrical spark – or lightning.
Lightning heats the surrounding air to approximately 10,000°C – double the temperature of the sun. We’ve seen that heating air molecules moves them. This sudden super-heating violently vibrates the air molecules – a vibration we hear as thunder. Thunder and lightning happen pretty much simultaneously, but because light travels faster than sound, we see the lightning first.
So, warmer, wetter summer air causes more vigorous convection that creates big cumulonimbus clouds, and cumulonimbus clouds are where electrical storms are born. With hotter summer temperatures predicted, bigger and more frequent storms can also be expected.