We May Finally Understand Why Clouds Are Different Between Earth's Hemispheres
You might think that clouds are clouds all over Earth, but that's not quite so.
If you're in the Southern Hemisphere, the clouds there are different, more abundant, and more reflective than clouds found in the Northern Hemisphere, a fact scientists are well familiar with, but have not been able to fully explain.
Now, new research sheds more light on why clouds work differently in the two hemispheres, and in particular the role that updrafts play – the upward motion of warm air that leads to condensation and formation of clouds.
The study used three years of LIDAR and radar data (2018–2021) covering Leipzig in Germany, Limassol in Cyprus, and Punta Arenas in Chile ...
What makes the region so pristine – and this extends to the Southern Hemisphere as a whole – is that a high percentage of it is ocean rather than land. That means cleaner air, fewer aerosol particles for cloud droplets to freeze around, and brighter clouds.
"Clouds ice up much less in the mid-latitudes of the Southern Hemisphere and contain more liquid water at the same temperatures," says meteorologist Patric Seifert ...
"This means that they influence the incident sunlight and also the thermal radiation emitted from the Earth's surface differently than in the north."
The study found that the differences were most pronounced in the free troposphere, air massed at higher altitudes where it's less affected by local pollution. For temperatures between -24°C and -8°C (-11.2°F and 17.6°F), clouds over Punta Arenas formed ice an average of 10 to 40 percent less often than clouds over Leipzig.
This matches neatly with previous research, but the team also discovered something new. So-called gravity waves, uplifts of air created as westerly winds from the Pacific collide with the Andes, are an important factor as well as atmospheric pollution, especially when the air is even colder. ...
Further measurements will be required to figure out how much of the excess liquid water in clouds is down to updrafts and how much is down to ice crystals.
These differences are all very intriguing on their own, but there's a problem: global climate models aren't accurate enough when it comes to representing the radiation balance of the Southern Hemisphere ...
To be as useful as possible, already complex climate models need to factor in regional differences ...
