I've Been Thunderstruck

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 monsoons have arrived in Southern Arizona, and I’m loving every drop of rain, every bolt of lighting and clap of thunder. We have had some AWESOME storms so far! I’m out trying to take some decent lighting pictures…you can check out the latest pics here (proudly hosted at SmugMug!).

As a novice storm chaser, I’ve been learning a lot about what it takes to actually form a thunderstorm each day. Fortunately, some local weather forecasters (ErinJordanKOLD, PickeringKOLD and ChuckGeorgeKOLD) have been providing daily storm updates on Twitter, and have parsed out some science knowledge at the same time. One of the things they constantly mention, and which is an important component of thunderstorm formation, is dew point.

According to Wikipedia, “dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water (all links courtesty of copying from Wikipedia).” Dew point is also associated with relative humidity, in that a high relative humidity indicates that the dew point is closer to the current air temperative. As noted above, dew point is given in degrees (for instance the current dew point temperature in Tucson is 51F). A relative humidity of 100% means that the dew point temperature (temp at which dew forms) is equal to the current temperature, and that the air is maximally saturated with water (can hold no more water, and thus condenses).

Dew point is important for reasons other than storm formation. The higher the dew point, the more uncomfortable it feels outside. In periods of high relative humidity and high dew points, the ability for sweat to evaporate from the body decreases, which reduces the cooling effect related to sweating. That tends to make us hotter, and makes being outside a bit more dangerous.

That’s all good info, but I really wanted to know how dew point influences thunderstorm formation, because I simply love those storms! With an estimated 40,000 thunderstorms occurring world-wide on any given day, and the high occurrence of such storms in areas where factors influence dew point, there must be a correlation, and a direct one at that.

Turns out that dew point plays a great role in thunderstorm formation. There are three factors – moisture, instability, and a lifting mechanism – that contribute to thunderstorm formation. Typically in the Southwest United States (Arizona in specific), dew point levels are very low…in the teens or lower. But each summer a shift in air flow occurs, drawing moisture up from Mexico. That infusion of moisture into the atmosphere leads to higher dew points, and when they have reached the mid-50s for three consecutive days, then the monsoon has officially begun!

Dew point is essentially the low level moisture that actually fuels thunderstorm development. Severe thunderstorms are more likely to occur when the dew point is 55F or higher, providing more potential fuel for the storm as the dew point increases. There are other moisture factors as well, which change in significance as you move higher in the atmosphere. But, for purposes of this discussion, it’s the low level stuff that counts.

So, there you have it. The more moisture in the lower levels, the higher the dew point, the greater the chances of a thunderstorm developing!