There is more cloud over islands
Cloudy islands¶
The International Conference on Clouds and Precipitation (ICCP) is held every four years, most recently in Jeju, Korea. Following the precedent set in previous years [1], Jeju was chosen as it is the cloudiest place in Korea [2].
The average cloud cover over the Korean Peninsular (as detected by satellite)
Jeju isn't the only cloudy island - islands are very often cloudier than the surrounding ocean and some of the cloudiest places in the world are islands (even if they sometimes have a nearby bit of ocean with less cloud).
Why is this, and should you cancel your next holiday to Tahiti?
Cloudiness around three different island groups - South Georgia, Hawai'i and Vanuatu
Why are islands cloudy?¶
Looking at a whole year of satellite images, we see that the Jeju weather is often very similar to the surrounding ocean. This is what you might expect - if there is a big storm in the area, it is not going to be a storm only over the land (or only over the ocean). However, there are also quite a lot of days (particularly in summer and autumn - the middle and upper rows of the image), where the island is cloudy but the surrounding ocean is relatively clear.
An image a day over Jeju island
Obviously the islands are not wetter than the surrounding ocean! It turns out there are a few reasons why islands are cloudy.
Islands are tall¶
Clouds form in cooling, rising air - what easier way to make air rise than to force it over a mountain? In the same way that mountains are often cloudy over land, islands are a high point above the ocean. This is pretty common - all three of the island groups above (South Georgia, Hawai'i and Vanuatu) are mountainous islands [3]
Clouds over the Hawai'ian islands (MODIS Aqua via NASA Worldview)
These type of island clouds can even generate a rain shadow (or a 'cloud shadow' in this case) - an area of less rain (and cloud) immediately downwind of the island (if the island is high enough). In many cases, this effect is strong enough to see in individual images - such at this one of South Georgia.
A cloud shadow downwind of South Georgia (outlined with a thin black line, the wind is blowing from the bottom left/South West). The mountains here reach almost 3,000m/10,000ft, making both the island cloudy and the downwind area relatively clear. (MODIS Aqua via NASA Worldview)
Islands are dry¶
Clouds don't only form from air that is forced to rise by mountains, they can also form when air at lower altitudes is heated. This can create convective clouds (which we often see starting as little cumulus clouds).
Water is hard to heat up - one of the reasons why rivers are less cloudy than the surrounding land. Being drier than the surrounding ocean, islands are considerably easier to heat up with the sun. This means that that convection starts earlier over islands, making lots of little cumulus clouds and creating some cloudy islands (even if the island is almost flat).
Cumulus clouds over Dolok/Yos Sudarso/Kolepom Island. This island is almost all less than 30m above sea level - lower than much of London, Manhattan and the Netherlands. (MODIS Aqua via NASA Worldview)
These effects don't happen in isolation - they both contribute to increasing island cloudiness. The nearby Tiwi Islands form a regular thundercloud known as Hector, due to the island warming during the day and the height of the island helping to trigger Hector.
The Hector cloud - forming due to the combination of island cloudiness effects (Wikimedia/Djambalawa)
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Land Ho!¶
So why don't we see this from the ground? It turns out that you do!
Clouds forming over islands have been used by sailors for centuries to help spot islands from further away than the island is easily visible.
Cloud forming over Nevis (Reddit/AspectPale5097).
The formation of these island cloud depends on the properties of the island - here you see a cloud forming over the higher island, but not the larger one just to the left. This is the impact of air being forced to rise over the mountain.
Cloud forming over an island in the Torres Strait (Reddit)
But a mountain/hill is not required. Where you have several island close to each other, these clouds can then be related to the size of the island.
A collection of small clouds over islands viewed from an aircraft (Reddit/DonBigote)
These island clouds are surprisingly common, although not something you see easily from the land (or at the very least, not something you take lots of photos of if you want to get people to visit your tropical paradise).
What a lovely picture, neatly avoiding the clouds above the island (Wikimedia/Argenberg)
Sea survival¶
This means that clouds can help you find land if you are lost at sea! As mentioned in Dougal Robertson's sea survival guide under 'landfall phenomena':
Lenticular cumulus often develops over a land mass or fixed cumulus cloud, which appears in a cloudless sky or amidst moving clouds, may result from vertical wind currents above an island.
Cumulus clouds are not unique to land, but over ocean they move with the wind. If you see a cumulus cloud that is fixed in position, that can indicate land.
This might not be a common situation - but if it happens to you, knowing a bit about clouds might just help!
I like this one as it even has an island with a cloud over it! (Survival Systems Inc.).
This article was published from under the sea, thanks to the future.
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Notes¶
| [1] | The prevous two conferences (held every four years) were in Pune, India (although online due to covid) and Manchester, UK - also two cloudy places. Seems reasonable for a conference about clouds! |
| [2] | It might also have something to do with the very large conference centre on Jeju (ICC Jeju), but who am I to say? |
| [3] | In the case of Hawai'i, the mountains are high enough to make it a valuable place for all manner of scientific research - from astronomical telescopes to atmospheric gas monitoring. |