The day time sky offers much beauty and intrigue, particularly in Antarctica. Many of the optical phenomena can be seen all over the world by those with a little knowledge of what to look for and an interest in observing nature. But given the extreme physical conditions in Antarctica, there are a few things which are a little special and unique to places like this. As you can probably tell based on the content of my recent icy news article, for me it is a key aspect of the Antarctic experience. Now we are on the winter side of the equinox (this morning has been the coldest so far, -27.6C), I thought it would be a good time to put together my photos of the summer sky.
The oft quoted Albert Einstein talks about kindness, beauty, and truth. So with regard to the latter, during this post I’ll be sneakily diverging off on physics and weather tangents. As one of the dieso’s on base said, “I never thought I’d learn so much….” – neither did we, Aaron, neither did we. (With all his cheeky met bashing, I have to keep him honest sometimes!)
A typical pattern over summer is the Vestfolds hills convergence – as indicated by a neat line of cumulus clouds. This is the first thing we saw when approaching Davis from the ocean. Sunny rocks are relatively hot so this draws in cooler air from the ocean in a typical seabreeze pattern. The conventional seabreeze explanation is that hot air rising in thermal currents needs to be replaced from underneath. A more technical explanation is that the heated air column expands, making the pressure at the top of the air column higher, resulting in a seabreeze which starts from pressure gradient flow at high levels, as opposed to being driven from the surface. The convergence happens where the seabreeze front meets the cold descending katabatic air from the ice plateau.
The convergence cloud frequently happens on summer afternoons, when the seabreeze effects are the strongest. While it sometimes happens on more overcast days, clear sunny days are often the best for it as there is the most solar heating on the rocks. Depending on the strength of the katabatic wind and convection (which depends on atmospheric lapse rates), it tends to happen predictably – the AAD science guys used to ask the forecasters if a rogue cloud would come and mess up their LIDAR data. Now with much colder temperatures and snow cover on the rocks, I haven’t seen any convergence cloud for maybe two months (an exception being a small cumulus or two on 27 March).
Clouds – low and mid level
The Vestfold convergence produces cumulus cloud which like stratocumulus and stratus is a low level cloud. These generally result from interaction with the earth’s surface, and in Antarctica the rule of thumb is that they are below 6500 feet. Yes we measure cloud heights in feet, for weather observations purposes and also of course aviation, which is important when we do MET(eorological)A(viation)R(eports)s in the summer.
Low level clouds are composed of tiny water droplets, the same as mist or fog. Water vapour mixed in the air (ie, humidity) is invisible. Warm air can hold a lot more water vapour before becoming saturated (which is when cloud forms). My room in Antarctica at 18 degrees celcius can hold about a glass and a half of water as vapour with 100% relative humidity (where things fog up), but the same volume of air outside now at -25 celcius can only hold about a tablespoon. No wonder we get static shocks inside all the time – once the outside air has been heated for inside it is very dry.
As air rises it cools, simply due to the fact that it is expanding as it becomes thinner air. This cooling due to spreading out (rather than transferring heat to something cold beside it) is called adiabatic cooling. Eventually this cooling air results in relative humidity of 100% and we have cloud. Condensation happens, just as when warm air cools to release its humidity on the side of a cold beer stubby.
Mid level clouds are also composed of tiny water droplets, and they occur (as a rule of thumb in Antarctica) up to a maximum of 16000 feet. Altocumulus and altostratus (the two types of mid-level cloud) can form during the day but generally they blow in – the humidity doesn’t arrive from the ground locally, it is contained within the incoming air mass. An exception is thunderstorms or towering cumulus clouds which span all three levels, but these are very rare indeed in the cold and dry air of Antarctica. Typically bad weather on the Antarctic coast comes in the form of a warm front (warmer, moister air from the southern ocean) – high cloud thickens through to mid level and finally low level cloud and snow.
A late addition – mid level clouds can make for great sunset photos.
Clouds – high level
High level clouds are cirrus, cirrostratus, and cirrocumulus. The are all composed of ice crystals, which makes them quite different to low and mid level clouds. They don’t have any of the greyish shading that you see in water droplet (low and mid level) clouds. The most powerful cloud in nature is the cumulonimbus. It is defined as as a towering cumulus that has started icing in it’s upper levels – this is a distinctive change in texture, losing the well defined cauliflower shape and becoming whispy.
This was something I was a little confused about before I did the met course – the distinctive and meaningful difference between ice and water droplets in clouds. I didn’t realise it is completely normal to have water droplet clouds even when the temperature is well below freezing. It is actually possible to cool extremely pure water to temperatures of -38C. Supercooled water droplets will freeze as soon as they come into contact with any dust particles called deposition nuclei. In practice, there are more condensation nuclei than deposition nuclei around, so water droplet clouds are common in air temperatures of -20C or even colder.
Cirrostratus can be very thin and featureless, but it produces an optical phenomenon called a halo, due to reflection and refraction of light in ice crystals. It is surprising how often this occurs, and how seldom people notice it. If you look closely at a halo, you will see red in the inside. This is the opposite to a corona (see below), which is a smaller angle and has red on the outside.
In some cases a sundog (also known as parhelia) will form at the same level of the sun, at the same 22 degree angle as with a halo. This is due to similar optical effects, this time when the ice crystals are vertically aligned from falling. They can sometimes be confused with rainbows, but only if you forget the sun is always behind you for a rainbow.
Solar pillars are a vertical beam of light caused by light reflecting off ice crystals, typically for half an hour or so around sunset (or sunrise). We’ve seen a few so far.
One of the last things I expected to see in Antarctica is mammatus. The name comes from the appearance, it is latin for breasts. Basically they are like upside down cumulus, associated with strong downdraughts rather than updraughts. Normally you would look out for them on the undersides of big storm clouds.
The first one I saw was on 7 January, when we had strong winds blowing in from a passing southern ocean cyclone system. The sky was heavily overcast although looking towards the ice plateau it was a little lighter and brighter, as it often is in the drier continental air. For some reason the mammatus was spilling over this edge. It looked quite dramatic.
Three weeks later and we had another storm, this time the mammatus came from the northeast and moved across the whole sky.
The green flash
The green flash, which can happen for just a moment after sunset, is very rarely seen. It is associated with refraction, the last of the suns rays to bend around the earths surface.
My first green flash I saw on the first night on the ship out of Hobart, and no one else I talked to managed to see it. The next one I saw was seen by several others, although no one else managed to get a photograph. It doesn’t look that spectacular on the photograph but it is quite a unique thing to see – this one lasted for several seconds. When the sun just grazes the horizon later in the winter (rather than setting directly up and down), we might have a chance to see some more.
This is becoming quite common with colder surface temperatures as we move into winter. Mirages are caused by slight bending of the light due to differences in temperatures (and therefore densities) of layers of air. A superior mirage means the image is above where the actual object really is.
Icebergs floating above the horizon – another thing you probably won’t notice unless you know to look for it.
When I first saw this mirage, operating the Zodiacs during resupply, I thought it was a superior mirage. Now, looking at it a bit closer, I think it’s an inferior mirage (image appears below actual object). Even though I did a little assignment on mirages and other similar optical illusions (such as towering and looming), it still can be confusing. As they say, it gets complicated!
The reason I think it is a inferior mirage is that the water surface (at minus one point eight celcius) is quite a bit warmer than the air above. This means the air on the surface will be warmer, and the light would bend away from the surface. I think the icebergs are exactly where they appear to be, and the sky (and inverted iceberg images) appears below the real horizon. On the left the shimmering horizontal lines look similar to the mirage you see on the highway on a hot sunny day.
Iridescence is quite common everywhere, although I’ve noticed it more than ever here in Antarctica. It can happen with all levels of cloud but I notice it with low and mid level cloud in particular.
The corona can form around the sun or the moon when there are low or mid level (water droplet) clouds. Coronas are a result of diffraction. They are quite common, but the one I saw today had me running through the snow to find a shadow to take the photo as quick as I could!
If you’ve made it this far you have done very well and you should probably just admit that you’re going to become an Antarctic tragic and figure out how to get yourself down here. My next post is probably going to talk about how damn cold it is becoming. And hopefully it won’t be long until I see my first Aurora Australis – we certainly have had more than our fair share of cloud lately.