Slashdot Mirror
There's No Wind Chill On Mars
sciencehabit writes: Even though daytime temperatures in the tropics of Mars can be about –20C, a summer afternoon there might feel about the same as an average winter day in southern England or Minneapolis. That's because there's virtually no wind chill on the Red Planet, according to a new study — the first to give an accurate sense of what it might feel like to spend a day walking about on our celestial neighbor. "I hadn't really thought about this before, but I'm not surprised," says Maurice Bluestein, a biomedical engineer and wind chill expert recently retired from Indiana University–Purdue University Indianapolis. The new findings, he says, "will be useful, as people planning to colonize Mars need to know what they're getting themselves into."
Meteorologists will lobby against colonization. The weather reports on Mars simply wouldn't be exciting enough without wind chill.
Wind chill works because of evaporation on the skin, right? I don't think anyone is going to be walking around on Mars outside a biosphere, in a T-shirt. If you're wearing a space suit, wind chill is totally irrelevant or am I missing something?
Less molecules to hit you, right?
Not too relevant for people, but if you're designing a solar collector to warm an underground settlement this is pretty important. You would still need some large mirrors to get enough energy to be useful. But the low atmospheric pressure would dramatically reduce the insulation requirements. Maybe just a couple of layers of reflective foil around the pipework and behind the collector to reduce radiation losses.
Similarly, if you're planning a high pressure (from a mars perspective) greenhouse this has a real bearing on heat losses.
We could get a shitload of fossil fuels and take it there (perhaps with a really -long- petrol pump pipe) and burn it off in the atmosphere, perhaps by driving lots of 4x4 SUV's. That should help ensure that Martian average temperatures rise nicely by 1 or maybe even 2 degrees Celsius in only a century.
"Is the Chief Priest an Offlian? Do dragons explode in the wood?"
Mars' atmosphere is about 1.5 % of Earth's atmospheric density. It's around 20 mBar, or eight times too thin for sustaining life even if it was pure dioxygen. For all practical purposes it's near-vacuum. And vacuum makes for a very good thermal insulator.
Maybe we deserve this world ?
"People planning on colonizing Mars ..." - that's a joke, right?
If Mars had a stock-market waiting to be rigged, or an alien military technology, or a trillion-dollar super PAC in a treasure chest ... yeah, then it might happen.
Mmmm no. Southern England may be at the same latitude as Minneapolis, but because of the North Atlantic Drift (branch of the Gulf Stream) the climate is very different. -20C is generally not encountered in Southern England even on a very cold night in a very harsh winter.
The real use for this info is understanding and designing for thermodynamic exchanges with the Martian atmosphere. The useful takeaway from this article is that heat dissipation from a source will be dominated by infrared emission rather than contact exchange with the atmosphere. Useful knowledge for the design of electronics, pressure suits, and habitats.
Somebody already beat me with the post about the surface of Mars being beyond the Armstrong limit.
I'll just reinforce that by pointing out that the atmosphere at the surface of Mars is the same density as Earth's atmosphere at 34,600 m of altitude. Feeling a bit chilly is about the LAST thing you would have to worry about on Mars. Saliva vaporizing from the surface of your tongue, tears vaporizing in your eyes, and fluids evaporating from the alveoli in your lungs will be a bit bothersome if you open your mouth and eyes before you pass out from anoxia. Ever see the space-suit-looking contraption with full helmet that you have to wear in an SR-71? Well, the ceiling of the SR-71 is a good 8700 m below 34,600. Then there's the itsy bitsy detail that Mars' atmosphere is 96% CO2.
An oxygen mask alone just won't do any good.
There's no wind chill on Mars because nobody sweats there.
You are welcome on my lawn.
Mars atmosphere, pressure/density wise, is about like what Felix Baumgartner jumped out into from balloon. Yes, pressure suit is needed if only to insure you have enough oxygen. Pure oxygen at about 1/5th atmosphere works (about 3psia). 36000 ft is about 1/4 atmosphere, so above about 40,000 ft, you need more pressure in your lungs than there is outside, so you need to wear something to keep you from overinflating or otherwise suffering barotrauma.
These places aren't comparable.
Average January temperature in Minneapolis: -9C
Average January temperature in London: 4C
There is almost no air pressure on Mars. You might enjoy the temperature for the minute or so until you body fluids start to boil. Then the experience would change.
Mars ain't the kind of place to raise your kids. In fact it's cold as hell.
Wind chill is the increased rate of cooling due to air flow.
It does not require persperation. Evaporation will add to the effect. High humidity will decrease or eliminate the effect of evaporation.
High humidity is the impetus for the Heat Index which is the slowing of evaporation cooling on the skin.
Wind chill is used as a gage to how quickly exposed skin will freeze, or get frost bite. If the ambient temperature is above freezing no velocity of wind will freeze the skin. Hypothermia maybe, or dried "wind burned" skin.
That is why you don't hear wind chill numbers like 52 degrees with a wind chill of 45 degrees.
So, what is the wind chill on Venus? Would a very dense, hot atmosphere have a wind chill?
Wind chill is the apparent temperature on exposed skin. On mars you would not have any exposed skin, therefore no wind chill. On mars exposed skin would require that you not be wearing an EVA suit, which would mean death in less than a minute for sure.
There is still an atmosphere, and if you increase the velocity of the wind, the convection heat transfer coefficient will increase, increasing the rate that heat is removed from the hot body into the cold atmosphere.
Just because there is less wind chill because of the thinner atmosphere does not mean there is NO wind chill at all.
Try going outside on a calm day in the upper reaches of the Rocky Mountains. Even with the temperature being -20F, it doesn't feel nearly as cold as 32F does with a brisk wind.
And, yes, I've done this. On one transcontinental trip, while crossing over the Sierra Nevada into California, in late December, I experienced the sudden urge to take a whiz. Despite my fears of various parts of my anatomy freezing off while at said temperature at 12,000 feet, it actually wasn't all that cold. And, yes, I did make quite a bit of "yellow snow". ;-)
There are stories of people snow-skiing in such mountain ranges while wearing bikinis.
Isn't wind chill a lower apparent temperature due to increased evaporative cooling caused by moving air?
When the atmosphere is of such low pressure than water can't maintain a liquid state, as soon as any skin is exposed, all surface water will evaporate and suck the heat away with it.
Assuming your head is still sealed so you've got air to breathe, it also wouldn't be nice having 1 bar of pressure trying to suck your blood through your skin.
Minneapolis ain't the kind of place to raise your kids. In fact it's cold as hell.
FIFY
Geez, we just got over winter here in the Great Frozen Northland and now you have to go and remind us just how much living here blows chunks. On the plus side though today is absolutely drop dead gorgeous outside. The mosquitoes and tics aren't too voracious yet and the summer humidity hasn't kicked in. Also our reputation as a frozen wasteland helps keep the riffraff away.
True story: a friend went on a band trip to Washington DC while we were in high school. While there she regaled members of other bands about how bad it is in Minnesota; it's frozen year round and we have to shoo away the polar bears. That summer some of her new friends came to Minnesota to visit and actually asked "where's the snow and polar bears?"
I guess geography wasn't her strong suit.