Big white clouds are light and fluffy right? Wrong!
Thursday, Nov 19, 2015, 10:19 PM | Source: Pursuit
By Todd Lane
All is not black and white when it comes to clouds.
Atmospheric scientist Associate Professor Todd Lane, from the University of Melbourne's School of Earth Sciences, studies clouds and says that their dynamics are one of the biggest weaknesses of all weather and climate models.
"The work we are doing should eventually help improve prediction of rain and severe weather, especially in the tropics,'' he says.
"When we see grey clouds, they have a large concentration of water droplets," says Associate Professor Lane. "These clouds block the sunlight – we see grey clouds because we are essentially seeing a shadow.
Clouds, like thunderstorm clouds, are only grey from below.
"If you look at grey thunderclouds from above, say from an aeroplane window, they are white and fluffy because the tiny ice particles and water droplets reflect light back to you in the aeroplane."
Associate Professor Lane studies the dynamic processes of these large clouds. "Although most clouds look light and fluffy, they are in fact highly turbulent with very strong air currents within them and an enormous mass of suspended water," he says.
So how do you calculate the mass of a cloud?
According to Associate Professor Lane, clouds vary enormously in size and water content, depending on their type. A cumulus cloud might only have a diameter and depth of 1 kilometre – approximately 1 billion cubic metres in volume.
"Deeper cumulonimbus clouds can easily have a volume 100 times larger than that," says Associate Professor Lane. "The concentration of water also varies significantly throughout the cloud, but an average of 1 gram of water for every cubic metre is a good enough estimate to use.
"This means that even some of the smaller clouds have about 1 million kilograms of water in them."
"I've heard the mass of cloud water being described in terms of a certain number of elephants, but I think using cricket balls is more illustrative.
The smaller clouds have a mass equal to about 6 million cricket balls. Thunderstorms have hundreds of times more.
So how do millions of cricket balls float around in the air?
"Think of your warm breath on a cold day," says Associate Professor Lane.
"Your breath contains invisible water vapour and on a cold day it condenses into tiny water droplets we see as a puff of cloud. There are two main things that keep cloud water droplets in the air – friction and buoyancy.''
According to Associate Professor Lane it's possible to estimate the speed of a falling water droplet, determining the droplet's terminal velocity. It is a battle between surface friction and gravity.
"Gravity wins the battle for large drops and friction wins for small drops," he says.
"Cloud droplets are very small, less than 0.1 mm in diameter, so small that the terminal velocity is negligible and the droplets can remain suspended in the air or circulated by wind currents.
"Cumulus clouds form in rising air. The cloudy air becomes warmer than its surrounds and continues to rise as a buoyant thermal. These thermals transport the cloud droplets upwards; they grow from more condensation or from colliding with other drops."
"The currents circling around inside the cloud organise the cloud's shape. When the cloud droplets start to grow and reach a critical size of greater than 0.1 mm they will start to fall as rain droplets unless the updrafts are more forceful."
"The updraft of a cloud can reach speeds of 20 metres a second straight up but the terminal velocity of large rain drops is only about 5 metres a second. Drops get recirculated and grow inside the cloud until they overcome the updraft.
"This is how clouds grow giant raindrops and hailstones."
Banner image: Autumn landscape via Pixabay