I've Been Thunderstruck

Given that tornado season appears to be in full swing now (a recent tornado killed 11 in Mississippi), I thought it would be a good time to describe how tornadoes are rated. From time to time you might hear the weather reporter talk about an “F0″ or “F3″ tornado, and you might not know what they mean. I’m hear to set you straight!

Tornadoes are measured using the Fujita Scale, which was developed by Ted Fujita of the University of Chicago back in 1971. The scale Fujita developed is used to determine the intensity of a tornado after it has passed through an area and done its damage on human-based structures and vegetation. After a tornado has gone through an area, meteorologists and engineers will do a ground and/or aerial damage survey. If possible, they will also take into account things like eyewitness accounts, ground-swirl patterns, radar tracking and media reports and imagery. After everything has been considered, a rating of F0 to F5 is assigned to the tornado. Here is the criteria that they use to determine the rating:

Keep in mind that the rating cannot be applied until after the tornado has gone through and done its damage…the rating depends entirely on a guess of wind speed, and a measure of the damage done to structures and vegetation.

All of the world uses the Fujita Scale except the United States, which moved to the Enhanced Fujita Scale in 2007. It was revised to reflect better examinations of tornado damage surveys, so as to align wind speeds more closely with associated storm damage. Better standardizing and elucidating what was previously subjective and ambiguous, it also adds more types of structures, vegetation, expands degrees of damage, and better accounts for variables such as differences in construction quality.

The Fujita/Enhanced Fujita Scales are good ways to gauge just how bad a tornado was…not that you have to tell the people who were in them!

As the temperatures in Southern Arizona heat up, and the moisture in the air decreases, we start getting a lot of Red Flag Warnings for the various parts of the state. I’ve known intuitively what these warnings are about, especially since I was a wildland firefighter for a season, but wanted to find out some more details.

Red Flag Warnings are issues by the National Weather Service to inform firefighting authorities and land management offices that the conditions are right for wildland fire ignition and propagation. For these offices, the issuance of a Red Flag Warning helps them prepare for the potential fires brought on by drought, low humidity and high winds with potential lightning. For the general public, a Red Flag Warning means there is a high fire danger with increased probability of a quickly spreading vegetation fire in the area in the next 24 hours.

The weather criteria for fire weather watches and red flag warnings varies with each Weather Service Office’s warning area based on the local vegetation type, topography, and distance from major water sources but usually includes the daily vegetation moisture content calculations, expected afternoon high temperature, afternoon minimum relative humidity and daytime wind speed.

Related to, but of less severity than, a Red Flag Warning is a Fire Weather Watch. which alerts the public and fire fighting agencies that conditions may exist for a Red Flag Warning after the initial forecast period (12 hours). It is generally issued 12 to 48 hours in advance of the conditions, but can also be issued up to 72 hours in advance. That watch then remains in effect until it expires, is canceled, or upgraded to a Red Flag Warning.

Here’s an example of a Red Flag Warning for Flagstaff, AZ.

WWUS85 KFGZ 120342
RFWFGZ

RED FLAG WARNING
NATIONAL WEATHER SERVICE FLAGSTAFF AZ
842 PM MST WED JUN 11 2008

AZZ111>117-140-120445-
/O.EXP.KFGZ.FW.W.0023.000000T0000Z-080612T0400Z/
CHUSKA MOUNTAINS AND DEFIANCE PLATEAU (FIRE WEATHER ZONE 111)-
LITTLE COLORADO RIVER VALLEY IN COCONINO COUNTY (FIRE WEATHER
ZONE 112)-
LITTLE COLORADO RIVER VALLEY IN NAVAJO COUNTY (FIRE WEATHER ZONE
113)-
LITTLE COLORADO RIVER VALLEY IN APACHE COUNTY (FIRE WEATHER ZONE
114)-WESTERN MOGOLLON RIM (FIRE WEATHER ZONE 115)-
EASTERN MOGOLLON RIM (FIRE WEATHER ZONE 116)-
WHITE MOUNTAINS (FIRE WEATHER ZONE 117)-
NORTHEAST PLATEAUS AND MESAS SOUTH OF HWY 264 (FIRE WEATHER ZONE
140)-
842 PM MST WED JUN 11 2008

...RED FLAG WARNING WILL EXPIRE AT 9 PM MST THIS EVENING...

THE RED FLAG WARNING WILL EXPIRE AT 9 PM MST THIS EVENING. 

WINDS WILL CONTINUE TO DIMINISH AND RELATIVE HUMIDITIES WILL
INCREASE THIS EVENING...THUS THE RED FLAG WARNING WILL BE ALLOWED TO
EXPIRE.

$$

A couple days ago, as a storm was moving out of the area, we got some classic cirrus clouds in the upper atmosphere. These types of clouds are very high in the atmosphere, are traditionally thin with wispy strands, and can often herald the arrival of a storm (in this case they were remnants of a previous storm). Cirrus clouds like to live at levels above 26,000 feet (8000 meters), and are formed when water vapor freezes into ice crystals. The lack of moisture at such high altitudes is one of the reasons these clouds tend to present so thin and wispy.

Cirrus Clouds in Tucson, AZ

A lot of times in Tucson we see hair like filaments of ice crystals precipitating out of the clouds in the form of what’s called virga. These streaks often indicate the difference in the motion of air between the upper part of the cirrus cloud and the lower air below it. It appears as though rain is coming from the cloud, but it’s actually ice crystals, most of which evaporates before it hits the ground (especially in Arizona). On some days, cirrus cloud development is so extensive that they become virtually indistinguishable from one another, forming what’s called cirrostratus clouds.

Another type of cirrus cloud that you’re already very familiar with is the condensation trails, or contrails,  seen in the sky coming from planes. These trails are basically artificial clouds formed by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals. On some days, with a high level of air traffic, you can see contrails crisscrossing the sky, and these contrails often hang around for some time. It’s pretty neat!

Clouds are very cool, and an obviously integral part of weather. I intend to eventually go through all the various cloud types, hopefully using images I’ve shot with my camera. If you understand why a particular cloud type is in the area, you have a good idea of what sort of weather might be on the way.

The other day I was driving home from work, and on the south side of the Catalina Mountains in Tucson, I saw these clouds:

Tucson Lenticular Clouds

Unfortunately, because I took the picture with my iPhone, the clouds are a bit hard to see. But, if you look closely at the clouds in the center of the image, you’ll notice that they’re rounded and somewhat “flying saucer” shaped. Here’s a better picture of the same type of clouds, that I took in Mt Shasta, CA:

Mt Shasta Lenticular Clouds

These types of clouds are called Lenticular clouds, and they are stationary, lens shaped clouds that form at high altitudes. Typically, these clouds form on the downwind side of a mountain where warm, moist air is flowing, creating a series of large-scale standing waves. If the temperature at the crest of the wave drops to the dew point, moisture in the air may condense to form lenticular clouds. As the moist air moves to the trough of the wave, if it’s warm enough, it may evaporate back to vapor. Under certain conditions, long strings of lenticular clouds can form near the crest of each successive wave, creating a formation known as a wave cloud. And, because these clouds have a characteristic lens appearance and smooth saucer-like shape, they are often mistaken for UFOs (or “visual cover” for UFOs).

The air in the area of lenticular clouds is great for gliders, who seek out and ride the updrafts associated with the standing waves, but not so great for pilots of powered aircraft who are concerned with the turbulence. “Wave lift” often found in areas with lenticular clouds is often so smooth and strong that gliders can sail to incredible heights and distances. In fact, the current gliding world records for both distance (over 3,000 km; 1,864 mi) and altitude (15,460 m; 50,721 ft) were set using such lift.

I’ve always loved how lenticular clouds look. In almost every trip past Mt Shasta in California, where I have been able to see the mountain, lenticular clouds are present. I’ve also noted them quite frequently on the top of Mt Rainier in Washington. They are very beautiful clouds, but you can rest assured there are no aliens hiding in them!

In my last post, I talked a bit about the National Weather Service, and how it contributes to predicting the chances of precipitation. We learned that they send up a radiosonde twice a day, from locations all over the country, to collect data about the atmosphere. Quite a bit of that data they post online for anybody to look at, and combined with other bits of data from things like satellites and ground observations, you can get an amazing picture of how things are shaping up in your area (or almost anywhere in the world!). Let’s take a look at a few of the things you can find online at the National Weather Service web site.

Current observations are an important part of determining what will happen with the weather. If you know how conditions are now, and how they have changed from the past, you can get a clue as to how they might be in the future. One tool that the NWS provides is a Surface Plot graph.

Surface Plot Chart

The official term for this is Mesonet Observations. According to wikipedia, “a mesonet is a network of automated weather stations designed to observe mesoscale meteorological phenomena. Dry lines, squall lines, and sea breezes are examples of phenomena that can be observed by mesonets. Due to the space and time scales associated with mesoscale phenomena, weather stations comprising a mesonet will be spaced closer together and report more frequently than synoptic scale observing networks, such as ASOS. The term mesonet refers to the collective group of these weather stations, and are typically owned and operated by a common entity.” So, basically, this is a series of stations that report current observations on wind speed and direction, temperature, relative humidity, and other items. If you know how to read these reports, and can spot trends, you can tell when a frontal system moves through, or when you might expect a thunderstorm. It’s good on the ground information.

One area at the NWS site that I spend a lot of time checking during stormy periods is the local weather radar image. This image gives me a great idea of where precipitation is occurring at the moment, and in what amounts and intensity. This is particularly valuable if you are going storm chasing, as you can see where the local thunderstorm cells are dumping their rain. It also provides a time-lapse ability, so you can see what direction the storm is moving. I enjoy watching the radar, especially when storms are headed my way.

Radar Image

Now, let’s say you’re interested in what the current forecast is for your region. The NWS is particularly good at providing that information, and does so in a wonderful graphical format. You can find out great things like temperature, wind speed and direction, dew point (handy in the summer for monsoon prediction), sky cover, precipitation, and more for up to the next week or so. This is very handy for planning activities, and the graphical format of the data is exceptional.

Graphical Forecast

Finally, though by no means even close to the end of the great things you can get from the NWS, is the Weather Story that they produce from time to time. In Tucson, I tend to see these when a weather event is on the way, and right after an event. Any significant change in the expected weather pattern also seems to generate a Weather Story. Essentially, these are a couple slides that show the basics of what’s going on in the area, and what to expect in the near future. I find them interesting because it’s a good synopsis of what’s going on. I don’t know if all major locations produce weather stories, but I suspect they do.

Weather Story

So, there you have it…some of the free products that you can get from the NWS. These items only touch on the rich repository of information that is the National Weather Service. If you are interested in weather, I encourage you to take some time and look around. You’ll find data on things that you didn’t even know they collected data on!

It’s raining outside in Tucson today, which leads me to think about what they mean when they say there is an X percent chance of participation. This morning, Weather Underground says there is an 80% chance of precipitation in Tucson. Does that mean 80% of the area will get rain? That it will rain 80% of the day?

I did some research, and the answer is pretty simple : out of 100 days where the weather conditions were exactly or similar to how they are today, it rained 80 times. Pretty simple!

The questions becomes, then, how do they get that data to make such a calculation? The answer lies with the National Weather Service. Each day the NWS releases several balloons into the atmosphere from locations across the country. Those balloons, called radiosondes, are released twice a day and the information they collect is radioed to the ground where it’s collected by the National Meteorological Center near Washington, D.C, where it’s processed by computer. All the information the radiosonde collected during its rise in the atmosphere – pressure, humidity, wind speed and direction, temperature – is fed into the computer and used, in conjunction with data from ground sources, to create a 3D model of the atmosphere. That model is evaluated against various laws of fluid mechanics to predict future conditions.

Chance of precipitation for 22 February 2010.

Unfortunately, given the nature of precipitation, these percentages are really just educated guesses based on previous conditions. The atmosphere is a very fluid and dynamic entity, and many things can trigger an unexpected change in conditions. A wet day can suddenly turn sunny, just like a sunny day can suddenly turn severe. Weather forecasters have a difficult job at best, and providing a chance of precipitation number is just one way that they can help you plan your day.

By the way…a 100% chance of rain does not mean that it’s raining right this moment (unless, perhaps, you live in the Pacific Northwest). Again, it simply means that out of 100 previous days where conditions have been similar to those today, it rained every time. And of course that begs the next question…what is rain? We’ll save that for a later post!