This is an ASOS: an Automated Surveillance Observation System. The running joke at the AMS/NOAA/NWS workshop was that “NOAA”—which correctly stands for the National Oceanic and Atmospheric Administration—in fact stands for the National Organization for the Advancement of Acronyms. I despise acronyms and have a difficult time remembering them, but when in Rome….
The reason for automated systems is simple: humans are expensive. Ideally we’d have offices to cover the entire US. That becomes prohibitively costly in parts of the American West, where population densities are low. Automatic systems like the ASOS are designed to provide 24/7 coverage of weather 365/6 days a year.
It sounds great, and much of it is. It certainly gets some data in from parts of the world where it would be prohibitively expensive to post humans. Like many other automated systems, however, the computer and machines can’t quite replace the humans, for reasons we’ll explore in this post.
As we discussed earlier, the NWS has a robust and well-developed system of identifying conditions on the ground that help meteorologists predict the weather. A lot of what meteorologists do is simple: for example, look out the window and see if it’s raining, or snowing, or hailing or sleeting. It turns out that machines have a much more difficult time making such distinctions, quite possibly because it’s difficult to get a computer or other machine to perceive where an edge is on a target. Thus, the automated systems, while essential, have some fundamental limitations that must be considered in evaluating the data they produce.
Measuring precipitation. Human stations use a rain gauge.Go out and see how full it was after 24 hours. Now, it’s more complicated. The whole apparatus looks like this:
The water flows into the center of the container through a funnel. Here you can see the first problem: what if something gets into the funnel? Well, it clogs the system. With no human around, that’s a problem. I’ve heard NWS personnel report bugs and other small critters getting lodged in the funnel and slowing or preventing drainage.
Then how do you measure water? It has a rocker mechanism that measures water in (theoretically) 0.01” increments. Not so easy if it’s clogged or if you have a real downpour, which overwhelms the system. Easier if you just have a weatherperson who goes out and eyeballs the content. However, computers have a really hard time identifying edges, something that the human eye does effortlessly. Life is filled with tradeoffs.
It’s also hard to measure water when it comes down as ice or snow, particularly since frozen water is only about 90% as dense as liquid water. These are heated so they melt whatever is frozen; I’m not sure whether they do additional corrections. More on that below.
Current precipitation measurements. Technically, the rain gauge measures accumulated precipitation. How do you find out if it’s raining? Old-school meteorologists would look out the window or stick their hand outside. Modern technology has a LEDWI (Light Emitting Diode Weather Identifier, a fancy name for a device with an infrared beam about 50 mm in diameter which uses software to analyze the scintillation pattern produced by falling precipitation to determine the size and speed of the precipitation. It’s good for distinguishing between snow and rain, but lousy at intermediate sorts of freezing or frozen water, like hail, ice pellets and sleet. Here are some pictures of it in the lab, which should give you an idea of how high off the ground it is:
Here’s a clearer picture of what it looks like outside. You can also see it on the far right of the picture at the top of this post.
Temperature and Dew Point. Old-school: read the thermometer (mercury in warmish climates; alcohol if the temperatures are likely to drop below -40°F). Take another thermometer, cover it with a wet cloth, and swing it around until the temperature stabilizes. Voilà! Your humidity measurement!
Temperature-measurement on the ASOS is fairly simple. It uses a platinum wire. The higher the temperature, the more the resistance. I don’t think it’s a linear relation, but even if it isn’t, it’s an easy thing to program and get a fairly easy measurement.
Measuring relative humidity has proven to be really difficult. Originally the ASOS had a really complicated circuit that chilled a mirror until it fogged up. The problem was that it needed to keep adjusting for a constant state of “just-having fogged,” which was problematic. These units are still used as back-up units, and look like this:
The more modern units simply measure the humidity through a sensor, and then calculates the dew point from the temperature and humidity readings. The humidity sensor is extended away from the unit (below left) and the temperature assembly on the left):
Here’s what they look like in the lab:
Wind speed and direction. Historically, one used a wind vane and anemometer, a series of 4 cups that spun around in the wind, looking something like this:
The ASOS has something new and improved, that supposedly can trace both direction of wind and speed through sonic interference. It’s a three-pronged trident, in essence. According to world-wide standards, the measuring unit has to be 10 meters (just under 40 feet) off the ground. It looks like this; the unit at the top of this post has the wind measurer truncated for obvious reasons:
This system, however, has its own problems. Apparently birds of prey like hawks love to sit on it. The Training Center had the following picture of a hawk at an airport screwing up the wind detector:
As they say, sometimes the machines need “human augmentation”: here, to chase off the birds!
Icing. At airports, where many of the ASOS units are located, icing of wings is a major concern, as it can lead to crashes (think of the planes out of Washington, D.C. which have gone down in the past few decades because they iced up.) The ASOS has a nifty little device that is, in essence, a vibrating rod:
According to the specifications, the device will lose frequencies as the “ice” changes from ice, to hoarfrost, to freezing fog, to freezing drizzle, to rime, or finally, to wet snow. Maybe so.
Cloud coverage and ceilings. Old-school: look out the window and estimate how much of the sky is covered with clouds, and how high those clouds are (best estimate/guesstimate). Of course it has a fancy and wondrous name: a ceiliometer! It shoots lasers into the air and measures reflectivity off the clouds, and, in theory, averages it out and gives you the equivalent of what the old-school meteorologist did by peering out the window. Also, it’s apparently limited to 12,000 feet in altitude, which is a serious limitation, in my view.
So that’s basically the ASOS. Pluses: it gives us data where we can’t afford to sent people. Minuses: it doesn’t give us as much or as accurate data as we could get from sending people out. I’m sure things will improve as time goes on. OTOH, it’s sobering to remember that not all data is reliable—something I really wish the superintendants of education who have never been in a classroom for more than 3 years would keep in mind when evaluating the “data” they get from standardized tests.
LessThan3ley 