My name is Chris Cox. I'm from the Cooperative Institute for Research in Environmental Sciences in Boulder, Colorado. I'm standing here today on the aft deck of the research vessel Akademik Fedorov. We're at about 81.5 degrees north and we're returning to Tromso now, Tromso, Norway from the central observatory of the MOSAiC flow, which we left about two days ago. The atmosphere is composed of a number of different gases, most prevalent of which is nitrogen, that's about 78 percent, and oxygen about 21 percent. Argon is the next largest concentration of gas, it's about one percent, and you'll notice these add up to about 100 percent. There's actually a lot of other gases in the atmosphere as well, but those appear in trace amounts, things like methane, and ozone, and CO_2. You might have noticed that I missed one very important gas, which is water vapor. Scientists generally, when they report on the gas concentrations in the atmosphere, tend to do so with respect to dry air, and that's because water vapor can be variable within the atmosphere. So it could range from very near zero percent, we have very low concentrations of water vapor here in the Arctic, to upwards of five percent, think of something like the Southeast Asian monsoon. Most of the gases are pretty well-mixed through the atmosphere. These gases, like CO_2 for instance, are well-mixed because they're long lived within the atmosphere. However, other gases such as ozone are highly reactive and so they're not very well mixed within the atmosphere. Ozone is present in a significant abundance within the stratosphere, and it actually is a protective layer for the biology on the planet because it readily absorbs ultraviolet radiation from the sun. Ozone is also sometimes present at the surface and it's generally a health hazard then, it's a byproduct of chemical processes that occur because of precursors associated with pollution. Water vapor is another gas that's present in variable concentrations through the vertical column, and that's largely because water vapor is concentrated near the surface. At the temperatures that we are accustomed to on earth, water can exist in various phases, right? So if you take this water vapor and you move it up into the atmosphere, it cools and condenses. So there's a lot of water vapor close to the surface, but very little aloft because when it condenses it then produces precipitation, it comes back down to earth. Generally speaking, people often think of the atmosphere as getting cooler with height, which is often true. This is true within the troposphere in general, and at the top of the troposphere, this is the lowest layer of the atmosphere at about 10 kilometers, we have something called the temperature inversion. This is the transition from the troposphere into the stratosphere. In the stratosphere, remember we have all that ozone, that ozone is absorbing the ultraviolet radiation. It's actually getting warmer with height as you go from the troposphere into the stratosphere, that interface there we call the tropopause. What happens when you have a temperature inversion is that the colder, denser air is lying lower, and this is stably stratifying the atmosphere. Airplanes, for instance, like to cruise in the lower part of the stratosphere because this is a very stable atmospheric layer, it's a comfortable place to ride. But we can also get temperature inversions close to the surface, and this is a characteristic of the Arctic. So on a day like today, we have what we call a radiatively clear atmosphere. You can see there's some clouds around. Most of them are fairly high and fairly thin, and in this situation, with this low sun angle that we have at this time of the year, the surface cools itself very readily. It's very effective at cooling itself. When it does so, it cools the air adjacent to the surface, and so you end up with very cold air at the surface and slightly warmer air above that, and warmer air above that. This is a persistent feature of the Arctic environment. It can be eroded in a couple of different ways, the sunlight can do it in some situations, but in particular, during this time of the year when there's very little sunlight in the system, everything is dominated by longwave radiation, and so it's a blanket effect. If you put a cloud overhead, that's essentially the way that you erode this inversion, because the cloud suppresses that cooling, it acts just like a blanket for the surface, much in the same way that my jacket is acting as a blanket for my body right now. All objects that have a temperature emit radiation. The sea ice is emitting radiation, I'm emitting radiation, polar bears out here emit radiation, and it's quite interesting to take a photograph in the infrared of a polar bear because you can see their mouth, you can see their pores, but they're so well insulated that the outer edge of their fur is actually pretty much the same temperature as everything else. So they're quite difficult to see on an infrared camera. The same would be true is if you pointed an infrared camera at me right now, you'd see my face very clearly because it's warm, but my jacket is insulating my body and keeping me warm, the outer layer of my jacket is actually quite cold. So very hot things like the sun emit high-energy, high-frequency form of radiation. There's a small slice of this electromagnetic band that's emitted by the sun that our eyes are sensitive to, we call this visible light. But there's also higher frequencies of light emitted from the sun, remember the ultraviolet light that's absorbed by the ozone and also some lower frequencies which we call near-infrared. Colder objects such as earth, earth is much colder than the sun emit frequencies that are much lower, they're lower energy, we call this infrared radiation and it just so happens that the difference in temperature between the sun and the earth is such that the total energy spectrum emitted from these two objects actually separates completely. So if we know the frequency of light that we're observing, we know the source of the light. Conveniently, we divide this into two types of radiation, we call it shortwave radiation when we refer to sunlight, and longwave radiation when we refer to infrared, sometimes also called terrestrial or thermal radiation. At MOSAiC, we just finished deploying the automated surface flux system, and we call it the ASFS. This is an atmospheric observatory that's been condensed onto a small sled, it's about the size of a car. On this sled, we have a bunch of instruments; we have this long horizontal boom that sticks out over undisturbed snow, and on the end of that boom we have a broadband radiation assembly. This is a set of instruments that measure sunlight and it also measures thermal radiation, terrestrial or longwave radiation.
