Hi All
Every six hours, one of the two staff meteorologists goes outside and takes a "surface ob[servation]". The code for blowing snow in the "metar" (another name for the ob) is BLSN, and we've been seeing a lot of that lately. There is very little precipitation here, in that if you attempted to measure snowfall it would be perhaps a quarter inch or less annually. Of course, there isn't actually very much data on this, nor is there any good way to actually measure this. When the snow/ice falls it tends to form into fluffy layers, which eventually compact down into what we presume is something like a quarter of an inch. The snow is also very dry here--if you take some volume of reasonably compacted snow and melt it down, you get much less water than if you did this with similarly compacted snow elsewhere. There are several "snowstake" lines that extend out from the station, and every year the meteorologists measure snow accumulation from the station to 20 km out. Average annual accumulation is about 8 inches--nearly all of this is drift from other parts of the antarctic plateau.
We see a lot more drift when there are objects for the snow to drift around, such as buildings on the station. The dome here was built in the early 70s and was on the surface of the snow. It is now nearly buried; there is a ramp leading down into the entrance, and the side walls are about 30 feet high.
The last week the winds have been blowing steady at about 25 knots, gusting to 35 on occasion. It is astonishing how quickly drifts accumulate under these conditions. There is a door in the unheated hallway of the hypertats that we all use--early last week you opened the door and the snow was at the bottom of the doorway. The first day the wind started blowing, I went to leave and opened the door onto a 2 foot high shelf of snow that I shoveled away. About 20 hours later my friend Meghan opened the door to a 4 foot high mound of snow! By yesterday, there was a small little well cut into this: I open the door, and there is just enough room for me to stand and close the door behind me. I scramble over a 5 foot high ridge, which drops off only a few feet later. I don't even bother standing up until I'm up around the corner of the whole hypertat area, there are so many new sastrugi and weird drifts!
The entrances to the arches and the dome have similarly been blocked in all week. Everyone comes in covered in snow from their various slides and scrambles and shoveling, trying to find a way in or out of a door. The main entrance to the dome has two large doors that open inward, like some garages (well, most garage doors swing outward). They are held closed by a big 4x4 cross beam, but the weight of the snow accumulating outside had caused them to bulge inward, and the little crack between them let a lot of snow inside. On Saturday, during the weekly station clean up (we are assigned to clean various places around the station every Saturday before the all-hands meeting--the bar, the pool room, the public computer room, etc) one group was assigned to shovel all this inside drift far enough away from the doors that they could be opened. There was an 8 foot high mound about 15 feet across by 5 feet wide!
Jake, one of my cribbage buddies, is our main equipment operator; he's the one who will plow away the snow, and bring snow to the Chades snowmelter, etc. The cat[erpillar]s can be operated only if the temps are warmer than -80F. They actually can physically run at colder temps, but are much more likely to break a hose, leaking hydraulic fluid and being a maintenance nuisance. So, we wait for warm days for Jake and the cargo people to operate. When the wind blows as hard as it has been, the temps always warm up. In the winter there is an inversion layer--the surface is cooled by radiation, thus the air near the surface is the coldest. As you go up, the air is not as cold, since it is farther away from the "cold sink" of the surface. This is a stable situation, so there tends to be quite a temperature gradient as you go up from the ground. When the wind picks up, all this air is mixed up, bringing warmer air down to the surface (the official temperature readings are taken 2 m from the snow surface). However, as we saw last week, if the winds pick up too much, there is so much blowing snow that visibility drops to almost nothing. For several days, it was warm enough to operate, but no one could see anything! Besides, there isn't a lot of point in digging out a door if the winds are still high--it will drift right back in a matter of hours. The winds did calm down a bit on Sunday, and Jake was able to clear out the entrance to the dome and the hypertats.
Temps also generally warm up if it is cloudy here. If it is clear, the heat radiating from the surface is able to pass out of the atmosphere. When clouds are present, they absorb the energy radiated from the surface and then re-radiate it in all directions, including back down towards the surface. The weather at south pole is hard to predict, yet very easy to predict. Weather forecasting anywhere is difficult because the atmosphere is such a complicated system, and it is impossible to fully specify every detail, or to have a model large enough to include everything. (For more on this topic, I suggest reading the book _Chaos_ by James Gleick.) Forecasting here is also hampered by the fact that there are very few weather observing stations on the antarctic continent, and by the fact that weather systems tend to start at the pole--our weather is more interesting and useful to the coastal stations for their forecasting than theirs is to us. The met department here launches a balloon every morning; attached to the balloon is a radiosonde which measures and transmits temperature, pressure, wind speed and direction, and relative humidity. The balloon and sonde go up to about 30,000 ft before the balloon bursts. This data is very useful for forecasting weather on other parts of the continent.
At the south pole, we unofficially get our forecasts from something called the "quick and easy south pole forecast", which I've put up on my website. Since we are at the pole, all points are technically north. We work off a "grid" to indicate directions--we define grid north as the zero longitude (the Greenwich Meridian) and south as the 180 degree longitude. The weather here is pretty well determined by where the wind comes from. If you inspect the quick and easy guide while also looking at a topographical map of the continent, you will find some explanation for the three sectors. The forecast map is divided into three sections. If the wind is blowing from 020 to 120, which is where they normally come from, we can expect good visibility and weather (often abbreviated wx). This is because the wind has blown down from the higher, colder, drier part of the antarctic plateau; as the air sinks it warms and dries out, thus it does not bring much moisture with it. Winds from 120 to 260 almost never occur here; they bring very warm, cloudy weather with precipitation. These winds come off the Pacific Ocean and are quite wet. The winds from 260 to 020 come off the Atlantic Ocean and are also quite wet.
The winds today are blowing from 030 and are again at about 20 knots.
/andrea
copyright 2001 Andrea Grant
