Dry Week Ahead… And Why Does It Seem Hotter When It’s Humid?

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A stabilizing westerly flow will return to the region Thursday, knocking out the afternoon thunderstorm development, and bringing back sunny and dry conditions. Afternoon winds will kick up to the 10-15 mph range with gusts to 30 mph. Some haze and smoke is possible in varying amounts. Sunny weather will stay in the region through the middle of next week with highs ranging from the mid to upper 90s throughout.

Humid-autumn-fog

So why does humidity make such a 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.

Sweat airplane

We just get smellier.

 

 

 

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Clearing Up… And How Does The Heat Index Work?

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A few thunderstorms could still pop up in the region Wednesday afternoon, as some monsoonal moisture will stick around for another day, but the trend will be for things to stabilize as we head through the rest of the week. Temperatures will stay in the mid-90s through the work week before rising to the upper 90s over the weekend under clear skies.

Heat Index Chart

We’ve been talking about the Heat Index. 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? I’ll tell you tomorrow.

 

 

Chance of Thunderstorms… And What Is The Heat Index?

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So far it has been a pretty dry summer, with only a few patterns that produce thunderstorms. We may break back into the pattern briefly Tuesday as a light southerly flow could bring up some monsoonal moisture as far north as the Reno area, although it appears the bulk of the action will be in the mountains and to our south. High temperatures will top out in the mid-90s most of the week, before climbing into the upper 90s by the weekend. A slight chance of Tuesday thunderstorms drops out as we will go back to mostly sunny conditions for the second half of the week.

Heat Index Chart

As I mentioned, we have been pretty darn dry this summer. Because we are so arid around here, I don’t get asked the following question that often, but we aim to please:

“Hi Mike… Can you explain the “Heat Index” to me?”

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 tomorrow, I’ll give you some examples.

 

Thunderstorms Ahead?… And What’s Up With All This Heat?

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While the triple digit temperatures will back off slightly (falling to the mid-90s by the first of next week,) various amounts of smoke and haze will still likely plague the region through the next week or so. A few afternoon clouds will begin to build up Saturday and Sunday, and by the first of next week, scattered thunderstorms could move back into the region, bringing a slight chance of getting some much needed rainfall, but also bringing the danger of lightning caused fire starts.

500 mb current

500 mb map Friday Afternoon

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 Great Basin. It’s been a very stable pattern that the globe seems content to hang on to. Not only have we been hot, but as you all know, we have been pretty darn dry as well. Let’s hope that changes soon.

 

 

Will a Little Wind Bring Some Relief?…And Updrafts Role in Lightning

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While the supply of fresh smoke into the region isn’t likely to slow down much, some late afternoon winds on Friday could help clear out some of the smoke in our area. The downside to that is the additional breeze could heat up the fire situation itself. Temperatures remain high, topping out near 100 degrees on Friday before dropping slightly into the mid-90s by the end of the weekend. Dry conditions will stick around through the middle of next week.

lightning diagram

Yesterday I mentioned that it is a separation of charges in the clouds that causes lightning. Not content with only this, Sam came back with: “OK fine, but how does the upward movement of air create a separation of charges? I suppose it’s the same as shuffling across the floor and getting a shock, but the question remains: Why?” Actually, it’s different than shuffling. The collision of hail going down and the rain going up is the crux of the cause. Electrons are ripped off the rising particles, which causes a net positive charge on the tops of the clouds, and a net negative on the bottoms. We are still learning about the exact processes involved, but it is much more involved than just a straight static electricity buildup.

lightning science

 

Near Record Heat…Near Endless Smoke…And How Does Lightning Form?

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Smoke, haze and near to record temperatures will stick around the region for the foreseeable future. Light morning winds and mild afternoon breezes will do little to flush out the smoke already in the region, and the dominating ridge of high pressure is showing no signs of going anywhere anytime soon. Low triple digit highs on Thursday will ease back into the mid to upper 90s by the weekend.

Lightning bolt

So what are the ingredients that go into the creation of lightning (either cloud to ground or cloud to cloud)? What occurs in both cases is the vertical motion of the strong updrafts creates a separation of electrical charges…. Positive on the tops of the clouds, and negative on the bottom. Envision a giant Duracell Battery and you’ll get the general idea. On a cloud to ground strike, the negative charge on the bottom of the cloud induces a positive charge on the ground, and since nature abhors big imbalances like that, there is a discharge to neutralize the electrical potential. The cloud to cloud does essentially the same thing, but the negative bottom of one cloud discharges into the positive top of another.

lightning diagram

 

Triple Digit Heat is Back…And Cloud to Cloud Vs. Cloud to Ground

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The heat will continue in triple digit realms for another couple of days, accompanied by generally light winds, the lack of which will be helpful for the firefighting efforts, but will be unlikely to do much to clear up our poor air quality. A slow cooling trend into the weekend will drop the highs back into the mid-90s by the first of next week, and may allow just enough afternoon breeze to help clear up the air somewhat by the weekend. At this time there doesn’t appear to be much of a chance of getting any showers for the next week, although occasional cloudiness could pop up.

heat lightning

Yesterday we talked about heat lightning. Steve followed up with this: “What is the difference in the “ingredients” that go into the thunderstorm “brew” that ends up producing a storm full of cloud-to-ground bolts vs. a storm producing cloud-to-cloud bolts?”

Lightning bolt

The ingredients are really the same. In fact, most storms that have cloud to ground strikes will also have a lot of cloud to cloud strikes as well. Tomorrow, I’ll talk about just what those ingredients are.

 

Heat and Smoke are Back…And How Can You Have Lightning Without Thunder?

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I wish I could say that there would be some wind to come in to blow all the smoke away, but generally light winds and active fires regionally will likely assure we will have at least some smoke and haze in the region for the next several days at a minimum. And after a bit of a break the heat will get cranked back up to around 100 degrees again through the middle of the week, before dropping slightly into the upper 90s by the end of the week. Skies (above the smoke) will vary from sunny to partly cloudy all week, but there doesn’t appear to be much of a chance for any showers or thunderstorms in the near future.

heat lightning

Joan asks: “One summer evening in Kansas, at dusk, under a clear sky, a large cloud formed. Completely within it, an awesome lightning storm happened with no sound. It lasted a good 20 minutes. I’ve never seen that before or since. Could you tell me what caused it?”

The common term for what she is describing is “Heat Lightning.” There is really nothing mysterious about it. The reason she couldn’t hear any thunder is the lightning was too far away. In the evening hours, a flash of lightning can be seen 60-100 miles away, but thunder usually won’t be audible farther away than 6-10 miles. So silent lightning is actually quite common.

 

 

 

Critical Fire Danger…And Was This The Hottest July Ever? (Conc.)

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Strong winds will accompany a cooling temperature profile to create a very dangerous fire situation through Saturday. A Red Flag Warning is in effect for nearly all of northern Nevada and northeastern California, with winds in the afternoon expected to gust to as high as 40 mph in the foothills and wind prone valley locations. Humidity will likely stay below 15%. On a positive note, high temperatures could actually fall to below average to the low 90s over the weekend, before climbing back up to triple digits by the middle of next week.

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Yesterday, I mentioned that while Reno set temperature records for the month of July, it wasn’t necessarily the warmest July ever. I’ll start by saying that this was a hot month…very hot by any metric. But when comparing temperatures over nearly 150 years, at the very least, the thermometer should remain in the same spot. But Reno’s has moved over a half a dozen times, finally landing at the airport in the 1940s (and it has moved to several different places at the airport itself since then). But perhaps even more significant, the environment at the airport thermometer has changed dramatically, moving from irrigated alfalfa fields 40 years ago to now being planted right in the middle of the airport tarmac. Such a change makes a huge difference, at times creating a 5-10 degree bump in the temperatures at the airport compared with the surrounding undeveloped areas.

Airport

So was this the hottest July or not? Unfortunately, there’s really no way to say for sure. I think it’s safe to say it was the hottest July since they moved the thermometer to it present location between the runways (about 15 years ago), but beyond that, it’s not as objective an answer as you’d like.

High Fire Danger… And Was It Really The Hottest July?

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We do have some cooling weather coming our way this weekend, with Friday’s high temperature dropping to the mid-90s and Saturday and Sunday’s to the low-90s, all under mostly sunny skies. But that cooling will bring breezy conditions, and kick up the fire danger to the Red Flag Warning level Friday and Saturday. Varying amounts of smoke and haze will also visit the area. As we head into the next week, the ridge builds back stronger, and temperatures will climb back to the triple digit mark by the middle of the week, all under mostly sunny conditions.

Thermometer in the sun

You may have heard that Reno recorded the hottest temperatures of any July on record, and indeed it did… a whopping 1.3 degrees above the second warmest July. But does that mean it was the hottest July we’ve had since records were kept? Well, didn’t I just say it was? Actually, no. I said Reno recorded the hottest temperatures, which isn’t exactly the same thing. While that may not make sense, let me explain.

In order to have a “pure and reliable” temperature record, two things have to happen. The location of the measuring device (thermometer) can’t move, and the conditions around said measuring device can’t change. Reno’s fails on both counts. I’ll explain more tomorrow.