Water, heat, storms and lightning
D. Indumathi, The Institute of Mathematical Sciences, Chennai

Who hasn't enjoyed a proper thunderstorm? The rain pouring down in buckets, the flash of lightning and the rolling sound of thunder. Both thunder and lightning are associated with thunderstorms. But what causes them to form in the first place?

The Earth is heated up by the Sun during the day. The land gets very hot while water (in the sea and lakes) does not heat up as much. You may have noticed this if you live on the coast, like Chennai. In the day time, you will burn your feet if you try and walk on the sand, but the moment you reach the sea and put your feet in the water, it is deliciously cool. (Do you know why that is so? See the box.)

BOX Why water does not heat up as much as land
Metals get hot very fast, but water does not. This is because of a property called specific heat. This is the amount of heat to be given to unit mass of a substance to increase its temperature by 1C. For instance, if you give 450 Joules of heat energy to 1 kg of iron, its temperature will increase by 1 degree, but to achieve the same for 1 kg of water, you need to give 4200 Joules, almost 10 times the heat. Because the specific heat of water is so high, the temperature of water in the sea hardly changes by a few degrees through the day. But sand gets very hot easily. Similarly, at night, sand (land) also loses heat easily and becomes colder than water.

Apart from this property, there are also other factors. For instance, water is light and transparent, while land is not, so land absorbs more solar radiation. Also, texture matters. A tar road is hotter than a grass lawn because the rough surface of the road absorbs more heat. In fact, sometimes it gets so hot that the tar melts! Finally, there is a complex relation between temperature and the amount of moisture in the air.
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BOX Climate change and the role of oceans
At night, after the Sun has set, sand and soil lose their heat much faster. But it turns out that Earth's oceans are more important than the land as far as the weather is concerned. For one, the oceans cover more than two-thirds of the Earth's surface. They also absorb more sunlight and store more heat. Additionally the Sun's rays penetrate the oceans to a depth of many meters, but only heat up the top layer of the sand or soil. Since water has to lose more energy than the sand (dry land) in order for the temperature to decrease, the oceans retain heat longer.
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Humans and climate change
Last year, global temperatures were 0.95C warmer than the 20th century average. Human activity is responsible for around 100% of this warming. But this temperature rise is not uniform across the Earth. The increase in temperature over land areas is almost twice as much as over the oceans (1.43C over land and 0.77C over water). Hence the continents are warming more rapidly than the oceans.

Climate and weather
While global warming is a sign of climate change, the heating and cooling of the land and sea have always been there. The daily (or short-term) changes over a region of the Earth determine the weather in that region. Stormy weather, for instance, is something locals in Chennai come to expect during the North-Eastern Monsoon that sets in about October every year.

How storms and rain form
We already saw that land gains and loses heat quickly over the day compareed to water. Since the air is thinner as you go higher above Earth's surface (troposphere), the temperature also decreases with height.

Cumulus clouds
During the day, the air is warmer and water evaporates. The moist hot air rises and forms clouds called cumulus clouds. These are white fluffy clouds, generally seen low down in the sky (about 2,000 m high). If you have been in a plane and seen them from above, they will look like cotton balls.

Cumulo-nimbus clouds
If the sun is hot enough, it can drive these cumulus clouds very high until they reach the upper end of the troposphere (called the tropopause). These are the rain-bearing clouds. The air flow inside them is complex, with warm air rising and cool air descending on air currents called drafts. The moisture in the cool air has now frozen to ice. The interaction of these two different motions (so that they sort of rub against one another) causes electricity to be generated --- actually the bottom of the cloud becomes charged.

Typically, the atmosphere acts as an insulator and so the charges just stay in the clouds. But at some point the insulation breaks down (just like a spark) and the charges flow between two clouds or from the clouds to the ground in the form of lightning. The streams of charge cause the surrounding air to become super-heated to 25,000C. This causes the airr to cool by expanding so suddenly that the air further away is compressed and causes a shock wave. The crackling of air during the discharge is heard as thunder. Typically, this happens in the afternoon, when the hot noon sun drives the moist warm air upwards to form these clouds. You may have noticed how often it rains in the afternoon of a hot day.

These cumulo-nimbus clouds not only generate lightning and thunder, they cause even tornadoes and hail.

BOX Distance of storm
How far away is your storm? You must know that light travels very fast indeed (at 300,000 km/s) while sounds travel a million times more slowly, at about 330 m/s. So you will always see the lightning before you hear the thunder that accompanies it. The lightning reaches our eyes almost instantaneously.

A general rule of thumb to know how far off a bolt of lightning is when you hear thunder, if you count the number of seconds between the flash of lightning and the sound of thunder, and then divide by 3, you'll get the distance in miles to the lightning: 3 seconds = 1 km, 9 seconds = 3 km, 0 seconds = very close.Thunder can be heard from a storm as far as 15 km away.
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Sometimes the thunder lasts a really long time. This is because as the cloud discharges to the ground, more charges from the ground reach up to meet the discharge. So there are several "forks" of lightning and each of these channels create thunder.

So when you listen to thunder, you'll first hear the thunder created by that portion of the lightning channel that is closest to you. As you continue to listen, you'll hear the sound created from the portions of the channel farther and farther away. Typically, a sharp crack or click will indicate that the lightning channel was very close. If the thunder sounds more like a rumble, the lightning was far away. The loud boom that you sometimes hear is created by the main lightning channel as it reaches the ground.

Types of Lightning
Lightning is not just a big bolt from the clouds to the ground, although that is the most commonly observed one. But there are other exotic types of lightning as well. One of the most sought after is ball lightning, where the lightning looks like a ball, but this is possibly not really lightning.

Blue Jets
One of the most exciting ones is the blue jet lightning. Although it had been seen for a long time, it was not known how blue jets formed, until scientists looked for a blue jet from above the clouds using the International Space Station. They spotted one in February 2019. It appeared above a storm over the Pacific Ocean near Australia. 

A flash of bright blue light started off the jet. It started at the top of the cloud (about 16 km above the ground) and rose to 52 km, high into the stratosphere (the next layer above the troposphere). Its speed was such that it lasted only half a second.

Why was it so short and why was it blue? asked the scientists. We know that llightning forms when electricity flows between two oppositely charged parts of clouds (or from cloud to ground). When these two regions are very close, say, about a km apart, a very short but very powerful surge of current could be produced. Since the energy was very high, it produced a blue flash. As the jet travelled out into the stratosphere, it heated the air which begins to glow. Now, the stratosphere has mostly nitrogen gas and this glows blue, giving blue jet lightning.

Red Sprites
Another strange lightning that is visible only above the clouds is a burst of red light that seems to hang in the air. They are called red sprites. A sprite is a fictional elf or fairy, almost a ghost. The red sprites were so called because they were so hard to see, above the clouds, although they can be 50 km high! Also, they last just milliseconds. But they were caught at last, by scientists who went up in planes with high speed cameras to try and photograph them.

Understanding blue jets better and red sprites will help us understand the physical and chemical composition of Earth's upper atmmosphere. Would you want to be in a plane, photographing lightning at night, when there is a storm outside?! If so, perhaps this is the career for you!
Sources: Science News for Students, Smithsonian, NOAA, Wikipedia