In This Issue:

• Bryan Yeaton and the Weather Mobile Visit Davis
• So, What Is A Sling Psychrometer?
• Forecast From The Dry, Summer Side Of The Earth…Rain!
• Does It Help To Dash Through The Rain?
• The Devil’s In the Name?
• Too Much Data Helps and Hinders Forecasters
• You’re Brilliant! Answers to Quiz Questions
• Who You Gonna Call? Davis!

Bryan Yeaton and the Weather Mobile Visit Davis

Bryan Yeaton’s Weather Mobile boasts an on-board weather station. Despite 20,000 miles on the road, the Vantage Pro just keeps feeding Bryan weather data. He keeps the console on the seat next to him and sometimes uses the wind speed readings to verify his speedometer! Bryan’s on his way home to Mount Washington Observatory in New Hampshire, after taking his radio show, The Weather Notebook, on a cross-country car ride to Seattle and back.

But this year, with New Hampshire (and the rest of the East Coast) being “treated” to some particularly bone-chilling days, Bryan had to miss out on the dramatic mountain-top weather. Instead, he was checking out all sorts of winter weather on his Weather Notebook National Tour, presented by Subaru and Davis Instruments. Bryan fired up his Subaru Weather Mobile, equipped with a Davis Vantage Pro weather station, in December and set off in the direction of Seattle, bringing educational weather programs to schools, radio stations, and community groups along the way. He and the Weather Mobile made a big splash at WeatherFest and the American Meteorological Society annual meeting in Seattle, then turned south to start his return trip via Hayward, California, the home of Davis Instruments.

Last week, the Davis crew was delighted to find the Weather Mobile in our parking lot. Everybody hurried outside to see our familiar Vantage Pro, ingeniously mounted atop the Subaru on an adapted Thule kayak rack, and we all beamed with pride as Bryan told us that it had performed flawlessly despite a grueling 20,000-mile life on the road.

Bryan’s colleague, John Mitchell, created an ingenious mounting for the Vantage Pro. He adapted a Thule kayak rack, that not only keeps the sensors safe and accessible, but also allows Bryan to fold them down if ever he needs to go somewhere with low overhead clearance. (Storm chasers, tell us how you’ve mounted your weather station on your vehicle! Send us a photo to share in one of our future e-newsletters.)

“It’s been beat up pretty well,” Bryan told us, patting his VP like a proud papa, “and the only problem I’ve had was broken wind cups, caused by debris thrown up by a truck I was following.”

We all thought the Weather Mobile was the coolest vehicle we’d ever seen, and Bryan told us our reaction was pretty routine.

“I get all kinds of great looks, and people are always asking me about the weather station,” Bryan told us. “The only place I ever went in the Weather Mobile where it was one among many was at a meeting of storm spotters in Norman, Oklahoma. We didn’t get a second look there!”

Bryan treated our entire crew to a presentation about the Observatory and how weather readings are taken in a climate too extreme for any automated weather station. He told us that Mount Washington, at a modest 6,000 feet, has killed more people than any mountain on earth except for Mount Everest, which only recently passed it up. The reason is the frequently misleading difference between conditions at the bottom and those at the summit. Hikers have been known to set off for a hike on what seems to be a mild, sunny day only to find themselves in need of rescue in the freezing cold and high winds nearer the summit.

Mount Washington’s big claim to fame is its relentless high winds. The average daily wind speed is a gale force 35 mph, but there are hurricane force gusts every three days. On 24 days of the year, winds top 100 mph! The highest wind speed ever recorded was measured right here: 231 mph. Add that to routine winter temps in the -20’s F, and “wind chill” takes on meaning beyond the imagination.

The cold and wind make for difficult weather measurements. Wind speed is measured by a pitot-tube anemometer, and much of the hourly weather data is gathered the old fashioned way, such as by using a sling psychrometer.

Although Bryan might like to be a full-time resident at the observatory, so far, there is only one such resident: a cat named Nin. The human visitors come for a few days at a time, traveling, when conditions allow, by snow tractor to the summit. (Bryan gave us the inside scoop about the trip, describing the long ride packed in the enclosed back compartment, swaying this way and then that way. . . He also told us that the cold is something of a convenience when the end result of the inevitable motion sickness freezes into a neat, easily-disposed-of lump.)

As we watched Bryan and the Weather Mobile drive off in the general direction of home, wind cups spinning merrily, most of us felt pretty darned proud of ourselves. That’s one tough weather station and one cool weather guy!

Weather Check Quiz Question 1: Why do Nin and the human inhabitants of the Mount Washington Observatory spend so much time drinking water all winter?

Extra Credit: Mount Washington’s historians have kept careful records of fatalities on the mountain since 1849. The current total is 133. What is the most common cause of death for these 133? How many of them died in a carriage accident?

So, What Is A Sling Psychrometer?

We threw a meteorological term at you in the above story just to get you thinking. Just what is that sling psychrometer thingamabob the Mount Washington weather observers use? What does it tell them?

Many of you know how to get a relative humidity reading: just look at your Vantage Pro console, and there it is! Our weather stations measure relative humidity by using the fact that moisture changes electrical capacitance. In our sensors, a polymer layer absorbs water molecules from the air through a thin metal electrode, which causes a change in the capacitance proportional to relative humidity. The station’s computer chip simply converts the change in capacitance to relative humidity with a great deal of accuracy.

But you can’t put one of our Temp/Hum sensors out on the deck of the Mount Washington Observatory. As tough as our sensors are, they simply can’t endure such extremes of wind and ice. So when the modern way to do things won’t work, we’ve got to go back to the “old fashioned” way: using a psychrometer or hygrometer. This device takes advantage of the fact that evaporation causes cooling. (Lick your finger. Blow on it. Cool, huh?) And we know that relative humidity affects evaporation: when relative humidity is high, the air is approaching saturation and evaporation is slow. When relative humidity is low, the air is dry and evaporation happens quickly.

About a hundred years ago, meteorologists realized that they could measure the air’s ability to evaporate water, and therefore, the relative humidity, by taking two temperature measurements at the same time: one with a dry bulb thermometer, the other with a thermometer whose bulb is wrapped in moist muslin, or a wet wick. The “sling” part of the name comes from the fact that the two thermometers are mounted together, and the unit is placed on a chain with a handle that allows the observers to twirl the unit in the air. (There are automated sling psychrometers that don’t require a human twirler.) As the moisture in the wick evaporates, it causes a cooling effect, and the wet bulb thermometer will measure a lower temperature than the dry. When the temperatures on both thermometers stabilize, the observer notes them, and then converts their differences into relative humidity. Meteorologists have worked out charts that use the difference between the two temperatures to find the relative humidity.

For example, if the air is completely saturated, the water will not evaporate at all and the two thermometers will read the same. So a difference of 0º correlates to 100% relative humidity. However in dryer conditions, the wet bulb thermometer might cool quickly as the water evaporates and cause a difference of, say, 9º between the two, which correlates to a relative humidity of 44%.

You could even make your own sling psychrometer using two thermometers. First, wrap the bulb of one with muslin or cloth, and then attach them both to a thin piece of wood with wire or tape, allowing the bulbs to hang over the edge slightly. Attach a chain to the board to spin the unit. Remember to wet the cloth before taking a reading. (Check out this handy fill-in converter, which uses wet and dry bulb temperature along with pressure to ascertain relative humidity and dew point.)

(Or, just look at your VP console…. No twirling required)

Weather Check Quiz Question 2: Who is the meteorologist most well-known for his work on humidity equations and tables?

Forecast From The Dry, Summer Side Of The Earth…Rain!

Philip Riedel of Stirling, South Australia sent us a week’s worth of temperature and rainfall readings from his weather stations, just to remind us that weather is certainly unpredictable! His station, “in the driest state in the driest continent, currently experiencing water restrictions,” was reporting temps of 40ºC (104ºF) one day, and 30 mm (1.18”) of rain two days later!

“Unbelievable,” Philip concludes. If he was not using a Davis weather station, we’d have to agree…

Does It Help To Dash Through The Rain?
Here in the freezing 50ºF (10ºC) California winter, we don’t have to shovel the snow off our cars before we go out. But we do suffer, oh, yes we do! It rains here, and this year we’ve had some pretty impressive downpours.

Recently, we stood looking through the wet stuff (not a drizzle, but real, fat raindrops) at our car, sitting about 200 feet (60 meters) away in the parking lot. Having lost yet another umbrella, and having forgotten to bring a hat, we decided that the best thing to do, in order to stay as dry as possible, was to sprint through the shower, becoming a fast moving, hard-to-hit target.

But then we realized that the westerly winds would be angling the rain right into our face, and it seems that running toward an oncoming rush of water would do little to keep us dry; maybe we should walk. Dashing through the rain might also lead to wetter feet, because, as you may have guessed, we forgot to wear boots. What to do, we wondered, peering up at the grey sky in the hopes that a break in the storm would allow us to just step carefully across the parking lot and keep our ‘do looking perky.

It seems we were not the first to ponder this question. Taking the question to the great Oz, AKA the Internet, we found that several years ago a smart physicist named Doug Craigen had posted a handy fill-in calculator that would help us decide whether to dash, or walk. He made it clear that there is plenty to consider here, from the speed of rain (2 m/sec. is a drizzle, 9 m/s is a downpour) to the size of the target (our personal dimensions). We also needed to consider the wind speed and direction, and how fast we can run.

We tried several combinations of factors, and found that for our use, we would stay drier dashing rather than walking. (However, we can really move when motivated.) The strong westerly winds made it worse, but it seems that if our car was east of us and we could manage to run at exactly the same speed as the wind, we’d end up at the car perfectly dry! (Or perhaps the key is to run backwards at the exactly the same speed as the wind -- we’d not only remain dry, but could actually cause a fire…)

The website was so much fun, the rain storm had cleared by the time we finally turned off the computer and wandered out to the parking lot, where we arrived at our car perfectly dry. Proves something, huh?

Weather Check Quiz Question 3: We don’t really mind a little rain around here, as long as it is not acid rain! Is unpolluted rain neutral, basic, or acidic? Where does the acidity in “acid rain” come from?

The Devil’s In the Name?
Our story last month about Southern California’s Santa Ana winds got a few Los Angelinos thinking about how those dastardly winds got their name. Lee Craner of Agoura Hills, told us he thought the name “Santa Ana” was a “bastardization of the Spanish Santana winds, interestingly, the same origin as your Diablo winds.”

(Lee also said that the outdoor humidity reading on his Weather Monitor II, which is located about eight miles south of the Simi Valley fire, dropped to a deadly dry of 6% during the firestorms. At the same time, he was recording maximum wind speeds of 26 mph – typical, Lee says, for Santa Anas in his neighborhood.)

Michael Fox, who is a second generation native of Southern California, agrees with Lee’s ideas about the original name being Santana, not Santa Ana. “I never heard them called Santa Ana's till the late 60's,” he wrote, “and I recall my mother getting quite irritated when she heard them called by that name. Most Mexican natives I know agree with me that the original name is Santana. I also recall old weather service reports using Santana. Do you have any info on this possible controversy?”

We put the question to weather guru Jan Null, who answered by directing to us the website of the Los Angeles Almanac, which did little to lay the controversy to rest:

“The original spelling of the name of the winds is unclear, not to mention the origin. The name Santana Winds is said to be traced to Spanish California when the winds were called Devil Winds due to their heat. The reference book Los Angeles A to Z (by Leonard & Dale Pitt), credits the Santa Ana Canyon in Orange County as the origin of the name Santa Ana Winds, thereby arguing for the term Santa Anas. This might be supported by early accounts which attributed the Santa Ana riverbed running through the canyon as the source of the winds. Another account placed the origin of Santa Ana Winds with an Associated Press correspondent stationed in Santa Ana who mistakenly began using Santa Ana Winds instead of Santana Winds in a 1901 dispatch.”

So it looks like this is a mystery that won’t be easily resolved. We must admit, the idea of calling them Devil Winds has an appeal – especially in light of the terrible inferno they fueled last summer.

Jim Hathaway wrote to say that he has a good name for “the icy WNW gale we experience on the steppes at 7,280 feet along the Colorado/Wyoming border from early fall through mid-spring: Unrelenting! There's a reason so few people choose to live in this area, and I'm sure it's the wind. We also get more ‘leftover’ Pacific-origin winter moisture from the west than those to our south. I assume all of this is because we have no mountains to our west.”

But Jim, we bet those who do choose to live in your area are treated to plenty of dramatic weather. The sudden wall of mountains creates the perfect breeding place of the kind of thunderstorms and tornados that weather buffs just adore!

Weather Check Quiz Question 4: Oh no! You’re far from your VP console, and you left your new WindScribe on your yacht. There’s a breeze here at the football stadium, and you NEED to know the wind speed. What can you do? (No you can’t call someone. You left your cell phone in your Rolls.)

Too Much Data Helps and Hinders Forecasters
Admit it; we’ve all cursed the weatherman! Remember the time he swore the skies would remain sunny and you bought tickets to the big game, which turned into a cold, wet, mud fest when a surprise rainstorm dumped many inches of rain on your bare head? Tell me you didn’t think an unkind thought or two about Joe Forecaster!

On the weatherman’s word, we’ve taken the tire chains out of the trunk before heading up to the slopes only to have to buy new ones at the Truckee Ace Hardware. We’ve left the dog out in the rain all day, barbecued in the rain, rolled up long pants in the unexpected heat, shivered in a light jacket on a San Francisco spring evening, hiked with a heavy jacket tied around our waist all the way up a trail and back down, sweating. We remember each of these events clearly, but we’re famed for our selective memory. The vast majority of days, the weatherman is pretty close to right on, and the closer in time to the actual day, the better he is. Before we continue, we want to apologize for holding the poor guy to a standard nobody can reach. Even with our modern technology, computer models, state-of-the art equipment, the weather is still hard to pin down.

With the wealth of weather data now available to meteorologists, forecasting is getting more and more accurate. In fact, forecasts that are short-term, for the next 24 hours to three days, are almost always accurate. But when you are planning a ski trip or beach party, or maybe next summer’s vacation, it’s the long range forecast that matters, and that’s where accuracy starts to slip. The problem is that there are just so many variables, each one growing in complexity as we look further into the future.

As old veterans of the computer age, we know how to deal with details: feed them into a computer and see what happens. A few months ago, reader Frank Camp sent us an article from the Montgomery (MD) Journal about the NOAA’s new Weather and Climate supercomputer, which was put into service in May of 2003. Actually, it’s two computers, named Frost and Snow. Frost handles forecasting tasks, while Snow develops models for weather, climate and ocean applications. Housed at the NOAA in Gaithersburg, MD, the duo’s role is to improve both local and national forecasting. Over the next few years, the article states, “the supercomputer will be able to provide 48 times the computing power of the current weather computer.”

This must be little consolation to meteorologists who irked Sierra resort owners this winter with a prediction of a snowstorm that never happened. Science writer Carl T. Hall, of the San Francisco Chronicle, wrote a story back in November about the “snowstorm that wasn’t.” Hall was referring to a forecast for a pair of nice snowstorms scheduled to hit the Sierras back in March. We were told that we could expect three to five feet of new snow! Snowboarders and skiers started packing, while resort owners stocked up on hot cocoa and lift tickets!

But -- nothing much happened. Hall wrote that “just before the storm got to the Sierra, the jet stream split and a big chunk of the snowstorm moved to the south, causing rain in Las Vegas and glum faces in mountain ski areas.” Skiers who high-tailed it up to Tahoe could barely make a snowball with the “trace to 3 inches” of snow that actually fell.

Mark Deutschendorf, a meteorologist with the National Weather Service, wrote the forecast. He explained during a conference of Sierra weather experts just what went wrong. At the time of the forecast, there was a storm with strong winds pushing moisture inland from the Pacific. Using temperature and moisture content readings, snow was predicted. But the sophisticated models used just failed to take into account the warmer temperatures at some critical elevations. The models also failed to consider a persistent, sunny-day-making, high pressure area.

According to Steven Valiloff, a weather-radar project manager for the National Severe Storms Laboratory in Norman, Oklahoma, the problem is not really a lack of enough data, but too much data. “Even the most elaborate weather models don’t take into account enough ‘whatever,’ and there are a lot of ‘whatevers.’”

While we’d love to see improvement in forecasting and warning times for extreme weather such as tracking hurricanes, floods, and tornadoes, we kind of like the “whatevers” that make weather interesting. Frost and Snow, give it your best, but we think nature’s frost and snow will still have some surprises in store for you!

Weather Check Quiz Question 5: When the Weatherman forecasts “heavy snow” in your area, what does he or she mean?

You're Brilliant! Answers to Quiz Questions

Question 1: It’s easy to become dehydrated when you spend long hours inside a heated building, because warming cold air lowers its relative humidity. Cool air “can hold” less water vapor than warm air; its saturation point is lower than warm air. The relative humidity of air that has reached its saturation point is 100%. The “dryness” of the air is relative to that saturation point. Let’s imagine a parcel of cold air outside containing enough water vapor to reach 70% relative humidity. If we bring that parcel in and heat it up, its saturation point rises. So the same water vapor that was sufficient to be 70% relative humidity in the cold parcel, is now enough to be only 25% in the warm parcel. That warm air is dry! Pass the water please! (For more on relative humidity, see our story in the September, 2002 Weather Club E-News.)

Extra Credit: The Mount Washington Observatory website lists the name, age, and cause of death of each of these poor folks – and most often, the cause of death is falls. Hypothermia was right up there too. But the site includes all deaths, including drowning, avalanches, heart attacks, airplane accidents, “slideboard” accidents, carriage accidents (one in1880), and two “unexplained” deaths where the body was never found.

Question 2: Dr. C. F. Marvin, born October 7, 1858. According to the NOAA’s History web site, “Dr. Marvin's principal scientific contributions were in the designing, construction and standardizing of meteorological instruments of many kinds. For nearly every weather element, he developed one or more measuring or automatically recording devices, either original or modified, designed to improve the accuracy and completeness of meteorological observations and records. One of his most important contributions in this connection, and also one of his earliest, was the experimental evaluation of the constants in humidity equations and the construction of humidity tables.”

Question 3: “Clean” rain is actually slightly acidic. Distilled water is neutral (ph 7), but rainwater has carbon dioxide in it. Carbon dioxide reacts with the water to form carbonic acid, which makes the “clean” rain have a pH of about 5.6. But “acid rain” usually refers to much more acidic conditions caused by air pollutants like sulfur oxides and nitrogen oxides. In Washington D.C., the average rain pH is between 4.2 and 4.4. Source: USGS.

Question 4 : Look at Old Glory! If she is just stirring, the wind speed is about 4 to 10 mph. When she starts to beat, the wind has hit about 25 mph. She’ll stand out straight (and proud!) at about 32 mph. Over 40 and you’ll have a hard time walking, so never mind the flag. With sustained winds over 50 mph, trees can uproot… hopefully, you’re no longer at the football stadium. Here’s a very fun web page for the easily amused: choose a wind speed and watch what happens to the poor little animated guy in Beaufort Park (keep your eye on the duck…).

Question 5 : It doesn’t refer to the weight of the snowflakes! “Heavy snow” usually means four inches or more in a 12-hour period or that visibility is less than ¼ mile.

Who You Gonna Call?
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Well, that’s it for this edition. You’ll be hearing from us again next month!


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