We have a lot of sunshine on the horizon as a ridge of high pressure has positioned itself just to the east of the Reno area, bringing in a dry and stable flow. High temperatures on Saturday and Sunday will range from the low to mid-90s with generally light winds, before a dry cold front Sunday night helps to drop the highs into the upper-80s on Monday and Tuesday. We may get a few clouds out of the front Monday, but those should clear out by Tuesday.

Yesterday I mentioned that while we have been hot, at least it’s been a “dry heat.” How does that affect the Heat Index? I may have given some of it away by mentioning our relative dryness, but the Heat Index is sort of the summer’s equivalent to the wind chill in the winter. Just as wind makes cold temperatures feel even colder, high humidity makes hot temperatures feel even hotter. The Heat Index is an attempt to take humidity into account. The exact formula used to calculate Heat Index would make most of our heads spin, so I won’t bother putting it down.  But let’s try an example:

Let’s say the temperature here in Reno and in Houston is 90 degrees F. In Reno, a dewpoint of 30 degrees is fairly common, which gives us a relative humidity of 12%. The Heat Index for these conditions is 86 degrees… in other words; it “feels” like 86 degrees. But in Houston, you can easily have a dewpoint of 75 degrees (61% humidity), and that gives you a Heat Index (HI) of 100 degrees. An 80 degree dewpoint (72% RH) causes the HI to soar to 107. So the amount of moisture in the air can make more than a 20 degree difference in comfort level.

But just why does the humidity make such a big difference in how hot it feels? That all has to do with how our bodies cool themselves.

When we get hot, our brains (marvelous organs they be) send a signal to our sweat glands to open up and wet down our skin. As the sweat evaporates, it draws heat out of our skin in order to provide the energy necessary to turn liquid into vapor. For the real geeky of you out there, it’s called the heat of evaporation. The faster the sweat (water) evaporates, the faster we cool down. In dry air, water evaporates much quicker than when the air is moist, and the cooling process is much more efficient. When it is muggy, we keep sweating, but since very little of it is evaporating, we don’t get cooler.

We just get smellier.

The overall weather pattern will stabilize starting Friday, as we are headed into an extended dry and sunny period, with temperatures near the norms for early to mid-August. Friday should bring back mostly sunny skies with a high of about 93, a pattern which should last through most if not all of next week.

One of the most oft questions I have been asked lately is “What’s up with all this heat? Where’s it all coming from?”

To be honest with you, I think most of those who ask are just venting their spleens rather than attempting to journey into scientific discovery. Be that as it may, all this heat is coming from a very stubborn and very beefy center of high pressure centered over the west coast. It’s been a very stable pattern that the globe seems content to hang on to. If it is any consolation, the eastern half of the country, for the most part, has been relatively cool, as is usually the case with this type of pattern.

But what’s this we hear about the “Heat Index?” More on that tomorrow.

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It looks like we still have a couple of active thunderstorm days ahead…especially on Wednesday, as a weak low pressure center off the California coast steers up additional moisture up the Sierra and western Nevada. Light winds will mean slow moving systems, which will increase the likelihood of seeing some flash flooding. As we head to the end of the work week, the low moves out and we stabilize the flow, bringing back sunnier conditions through the weekend.

High temperatures will be seasonable, in the upper 80s to low 90s through the weekend.

With a high temperature of 89 recorded at the airport on Sunday, our long record streak of consecutive days of 90+ degrees came to an end at 51. That figure smashed the old record of 35 days by over two weeks. At first blush, it might seem highly unlikely that any record could have been beaten by that much with over 100 years of weather records to draw from, but in this case, the record isn’t nearly as unusual as you might think. Tomorrow, I’ll tell you why.

While the weekend will cool significantly, the level of thunderstorm activity should pick up significantly as well. A weak low pressure center sliding up the coast of California will bring some deep moisture from the south, and the cloudiness will keep our high temperatures down to near 90 degrees, but will also up the chances of showers and thunderstorms. These cells probably won’t be moving very quickly, and therefore could produce localized heavy rainfall and even possible flash flooding. Thunderstorms will be in the forecast through the weekend and into early next week, before a drying trend returns beginning Tuesday.

Lightning is one of the most fascinating and unpredictable of nature’s phenomena. And one of the most misunderstood.

For instance, even though a bolt of lightning looks like it is several feet across, it’s actually barely the width of a pencil. It only appears wider because it is so bright.

On average, there are between 50-100 cloud to ground lightning strikes every second world-wide.

While the thunder from a bolt of lightning can only be heard about 12 miles away, at night under the right conditions lightning can be seen up to 100 miles away.

While most of the time it looks like lightning only strikes once, it is actually a series of strokes in rapid succession (usually 3-4, but sometimes over 20) separated by about 40 ms.

The average lightning bolt is 6-8 miles long, but in 2001, a visual detection system recorded a single lightning bolt that travelled from Waco to Dallas, TX, a distance of 110 miles.

As the temperatures begin to back off a bit (highs should drop to the upper 90s Friday and then fall to the low 90s over the weekend), thunderstorm activity will pick up as a low pressure center moves slowly up the California coastline, steering some monsoonal moisture into the region and adding a little bit of dynamic on the side. Scattered Friday afternoon thunderstorms become more frequent over the weekend. In addition smoke from several large fires in Idaho and Oregon will keep our skies hazy to the next day at least.

More on lightning: On the one hand you might think that it would be easy to figure out just how a bolt of lightning will act. After all, it’s just electricity, isn’t it? And electricity is governed by some pretty well understood equations. But when Ohm wrote his law (V=IR), he was working with circuits in controlled environments. In nature, while the same basic laws apply, the variables V (voltage of the bolt), R (resistance of the air, etc) and I (current of the strike) are all unknown from one lightning bolt to another. It’s very similar to weather prediction. The basic physics of air molecules moving around the earth, and what that movement can do to produce changes in temperature, pressure and moisture content, are pretty well understood. But once you throw in a few gazillion variables that make up the chaotic system that is our atmosphere, it gets a lot tougher to predict how the butterfly flapping its wings will affect Sunday’s picnic.

I expect another high temperature record will fall on Thursday as the intense ridge of high pressure remains over us. A valley high of about 104 degrees Thursday will set another temperature record, and will likely produce an isolated thunderstorm or two through Friday. As we head into the weekend, a deeper flow of moisture from the south will produce more cloudiness, slightly cooler temperatures, and more numerous and intense thunderstorms. It is the type of pattern that could result in flash flooding situations in isolated areas, and if you are spending the weekend out of doors, you should pay attention to the conditions and watch for any warnings that might occur.

Kim wondered if closing an indoor pool during a thunderstorm was necessary. Lightning is very funny stuff, and it’s kind of like dealing with sharks. Once you think you have it all figured out, it turns around and bites you. The problem with being in an inside swimming pool isn’t that you will be a direct target for a lightning bolt, but you could be on an indirect pathway for the electricity (some 300 million volts or so) to get into the ground. Pools (indoor and outdoor) have extensive plumbing and wiring attached to them, and a hit on the building could potentially send a jolt through those pathways back into the pool. Is it a likely occurrence? Maybe not, but a large steel building does make an attractive target. More tomorrow.

Record heat will remain in the region for the next couple of days, as the intense ridge of high pressure stays put. A high Wednesday and Thursday in the range of 105 degrees will set new records for the dates, and all that heat will produce some afternoon cloudiness with a slight chance of an isolated thunderstorm. The odds of thunderstorms increase somewhat Friday through the weekend as more monsoonal moisture works its way up from the south. Temperatures will fall back into the mid-90s through the weekend.

With thunderstorms in the forecast, the following seems timely. Kim wondered: “Is there any logical reason to close an indoor swimming pool during a lightning storm? The pool in question is located on the bottom floor of a high rise building. There is a large window located twenty feet from the end of the pool and the building is a steel frame structure. Both factors have been given as reasons to close the facility. I would be interested to know your opinion on the possible lightning strike danger.”

As is often the case, the answer isn’t cut and dry. I’ll tackle it tomorrow.

We are now in the historically hottest time of year here in the Reno area and this next week will certainly go along with that, and then some. High temperatures will rise to the upper 90s over the weekend, with sunny skies in the morning giving way to a few afternoon clouds (and a slight chance of isolated thundershowers.) As we head into next week, the ridge of high pressure strengthens, and triple digit temperatures will be with us through most of the week.

Yesterday, we talked about how they come up with official sunrise and sunset times. Now if you have ever stayed up late at night wondering just who comes up with the “official” (or who the ubiquitous “they” are), might I prescribe some warm milk and read the following: The U.S. Naval Observatory has been commissioned with the responsibility of computing sunrise/sunset times worldwide. And it’s not really as tough a job as you might think. Since a flat horizon is always assumed, sunrise/sunset times can be calculated at any point on the earth if you know the latitude and longitude. Employ a little astronomy and trigonometry, and any personal computer could handle the task without straining its processors.

But there’s one additional factor they have to take into account, because we see the sun before we geometrically ought to be able. That’s because the atmosphere acts like a lens, and bends (refracts) the light coming in from the sun, allowing us to see it before it would be visible if we didn’t have an atmosphere messing with the optics. While it may not seem that it could make a huge difference, on average it makes sunrise about two minutes earlier, and sunset the same amount later. If you wondered why the days seem longer, now you know.

Sharp eyed observers (with a near obsessive-compulsive eye for detail) will notice that the times are only given to the nearest minute. In an era where we can make astronomical calculations down to the second, why aren’t they any more precise?

US Naval Observatory

The Naval Observatory will be the first to admit there is a degree of inaccuracy to their figures. Changing atmospheric conditions will affect the amount of refraction, which will affect when you can see the sun. And since there are very few places where you have a perfectly flat horizon, it’s unlikely you’ll ever see the sun appear on time anyway.

It’s a perfectly appropriate example of “close enough for government work.”