Warming Back Up…And Just When Does Sunrise Occur?

The skies should remain mostly sunny Friday with a few clouds popping up in the afternoon through the weekend. Those clouds are unlikely to produce any thunderstorms on Saturday, and only isolated storms are possible Sunday through the middle of the next work week. Temperatures will rise to the mid-90s Friday and will top out near 100 degrees through the weekend.

Sunrise Nevada

If you are a sun-watcher, you may have asked the same question as Ted: “What is the actual definition of sunrise and sunset for astronomical/weather purposes? Is it the exact time the sun appears or disappears over the mountains or the time it would appear or disappear if our location was on a flat plane, i.e., the middle of the Pacific Ocean?”

Sunrise water

Sunrise occurs at the moment top of the sun appears (or would appear) on a theoretical flat horizon, such as you would get if the sun rose over the ocean and you were standing on the shore. Sunset occurs when the last of the sun disappears over that same flat horizon. That means if the horizon isn’t actually flat, sunrise will occur sooner than you actually can see the sun, and the sun will disappear (behind mountains, etc.) prior to the official sunset.

Sunrise diagram

Clearing Up, For Now…And What Exactly Causes Thunder?

The skies around the Reno area should clear out for a couple of days as the offshore flow stabilizes, giving us mostly sunny skies with seasonable warm high temperatures in the mid-90s. By the time we get to the weekend, another weak impulse sets up offshore, which could allow some more isolated thunderstorms to build up in the afternoons through the weekend and into the middle of the next. High temperatures will warm to the upper 90s through the weekend.

Lightning bolt

Here’s a follow-up to yesterday’s question about thunder:  “What are the mechanics of thunder? (I suspect it isn’t some old codger who likes to bowl among the clouds.)”

No… no old codgers. Thunder is caused by the rapid expansion of superheated air. An average lightning bolt has a temperature of about 50,000 degrees F. This causes a rapid expansion of the column of air around the bolt, creating a sonic shockwave that goes outward from the bolt, and then sucks back in as the air rapidly cools. Think of a giant woofer on a stereo speaker, and you can get a general idea.

Thunder can usually be heard at least 10 miles away from its lightning source, and if the conditions are just right, it’s possible to hear thunder more than 20 miles away.



Starting to Stabilize…And Can You Have Thunder Without Lightning?

After a couple of very active thunderstorm days, the pattern will slowly start to stabilize, starting from the west. A weak low pressure center just off the California coast will move onshore, shifting the dynamics to the east. While a Wednesday afternoon isolated thunderstorm in the Reno area isn’t out of the question, most of the activity should move slightly to the east, with partly cloudy skies in Reno proper. Temperatures will rebound to the mid-90s by Thursday and will approach triple digits by the weekend. That extra heat and another weak offshore low will bring back a slight chance of an isolated thunderstorm on Saturday.

Lightning bolt

Over the last few weeks we’ve seen our fair share of thunder and lightning. Deanna, wondered: “You said there is never lightning without thunder, even if it is inaudible from being too far away. OK, I can live with that. My question is: Can there be thunder without lightning?”

Thunder by definition is the sound that lightning makes, so no, you can’t have thunder without lightning somewhere. In all likelihood, the bolt of lightning that Deanna heard (but didn’t see) was either behind a cloud or perhaps behind the mountains.



More Thunderstorms?…And What is This Washoe Zephyr?

With light southerly flow coming around a large high pressure center in the central US, we are finally pulling up some of the monsoonal flow out of Arizona. This will result in mainly afternoon thunderstorms across northern Nevada for at least one more day. Considerable cloudiness Tuesday will keep our high temperatures in the low 90s, and as we go through the week the focus of thunderstorms will shift to the east, with mostly sunny skies returning to western Nevada and the Lake Tahoe area.

Washoe Zephyr

We live in one of the breeziest places in the country… and in the summer, you can almost count on the wind kicking up in the afternoons. The “Washoe Zephyr” that you may have heard about is our local moniker given to the late afternoon thermal winds that swoop through our western valleys. What causes them?

It’s all due to differential heating. In the late hours of the day, the sun drops lower into the western sky. That causes its rays to directly hit the hills on the east side of the valley, heating them up… while the hills on the western side fall into shadow and cool.  The warm air on the eastern side rises, while the cooler west air sinks, and you have a conveyor belt effect that sweeps winds out of the west to the east.


More Heat, Less Smoke?…And The Issue With Airport Temperatures.

Temperatures will rise to or near the triple digit range over the weekend, and lighter winds will settle into the region. Those lighter winds will help ease the thick smoke here in the Reno area, but we will still likely have to deal with hazy skies. The lighter winds will also allow the airmass to destabilize a bit, which could bring some thunderstorms back into the picture starting on Sunday afternoon, and possibly repeating on Monday afternoon. It looks like the record setting streak of 90+ degree days (now at 36 days and counting) will continue for another week at least.


We’ve been talking about how you have to be careful trusting car thermometers because of the heat that is emitted from concrete and asphalt roads, among other things. But Steve wrote in with another issue. “I can understand why no one would want to measure temps on a freeway. To me it seems fairly obvious that one would get a major heat island effect on them. But to use the Airport complex for the official temperature seems to me to provide the same issue.”

My response is I couldn’t agree more. One must be very careful here in Reno looking at temperature records and comparing them to previous years. When they started taking temperatures at the airport, it was surrounded by irrigated alfalfa fields instead of the present day tarmac. That greatly changes average temperatures… especially nighttime lows, but it also affects the highs as well. The “urban heat island” has had a definite influence on the historical temperature records here and other places for a long time.

reno asos

Location of the Reno Automated Surface Observing System

But when it all comes down to it, the reason they have the official readings at the airport is quite simply that they have to have it somewhere. Is there really any spot in the valley that would give you a better reading than where it is now? Not really, because wherever they choose, with all the different microclimates around, it will probably be different by some degree that where you are. So we make do with what we’ve got.


Record Heat Continues…And Why You Have To Be Careful About Car Thermometers (Final)

The hot temperatures will continue as I expect we will set a new record for consecutive days above 90 degrees at the Reno airport. Friday’s expected high of 95 will make the 36th day in a row of plus-90 temperatures, breaking the old of 35 which was reached three times previously. And it looks like the new record will be only grow with another week of 90s and possibly even 100 possible as we go into the weekend. That heat will allow some isolated thunderstorms to creep back into the forecast Sunday and Monday…otherwise mostly sunny skies (with smoke and/or haze from area fires) will stick around.

thermometer Car location

We’ve been talking the accuracy of car thermometers. One of the problems with them is they are located pretty close to the ground. While that may not seem to be an issue when the car is racing along the highway, keep in mind the sun can heat the pavement up to over 150 degrees F, and the closer to that you are, the hotter the reading is. So it is no surprise that car thermometers this time of year consistently give you higher readings than official weather stations.


Smokin’ Hot…And Why You Have To Be Careful About Car Thermometers (Pt 2)

Smoke from the Detweiler Fire in Mariposa County, CA will keep our skies anywhere from hazy to downright smoky over the next couple of days. Otherwise we are under a very stable ridge of high pressure, resulting in sunny skies. Winds will be light in the mornings, with westerlies kicking up in the afternoons. High temperatures will remain in the mid-90s through the rest of the week and through the weekend.

temperature gauge

automobile temperature gauge hot summer

Yesterday I mentioned that a thermometer doesn’t measure the temperature of the air, but rather measures the temperature of the thermometer. The key is getting the thermometer the same temperature as the air. While that may seem a simple thing, it really isn’t. Let’s take the car thermometer. While the instrument itself can be quite accurate, where it’s mounted can be in a very different environment than that in which we live. Even if the sensor is well ventilated, there will be some heat from the engine as well as heat from the sun shining on the hood that can contaminate the reading. In addition to that, the air temperature is supposed to be taken five to six feet off the ground, which makes sense since that’s where most of our heads are. More tomorrow.


Another String of 90s…And Why You Have To Be Careful About Car Thermometers (Pt 1)

We have another dry week ahead, with more clouds coming from fires than from meteorological origins. Temperatures will remain fairly high, peaking in the mid-90s Wednesday through Friday before climbing back into the upper 90s for the weekend. Winds will be light in the morning, and we can expect a typical afternoon Washoe Zephyr in the afternoons.

temperature gauge

On Saturday, we recorded a high of 99 degrees. Steve wrote in: “My Wife has a thermometer in her car. I’m actually fairly impressed with how accurate it’s been over the past two winters, and summers. Saturday while returning home, as we passed through the area along 395, she’d commented that it was 101 degrees.

At 4:35, around Mill St. it read 102 as the maximum temp. My wife just got back from the market in Carson, and she told me it was 103 at Raley’s. Hope this upsets the apple cart some.”

Naw… it’ll take a lot more than that to upset this cart. First of all, you have to understand one thing about thermometers. They don’t give you the temperature of the air…they just give you the temperature of the thermometer. Really. Tomorrow, I’ll explain why that matters.




Heat and Thunderstorms?…And Why Isn’t the Sun Setting Earlier? (Final)


The hot temperatures we have been experiencing the last couple of weeks will continue through the weekend, and the flat 4-Corners ridge of high pressure that has kept our skies mostly clear will dig a little deeper, which will help bring some afternoon cloudiness into our region. For Thursday, it’s unlikely these clouds will develop enough to start any thunderstorms in the Reno area, but isolated thunderstorms are possible south of Highway 50. Those could drift up into our area by Friday, although they will probably be few and far between. Temperatures will remain near the triple digits through the weekend.

Earth's orbit

Yesterday I said since the earth over-rotates a bit every day since we are moving slower in our orbit this time of year. This delays the time that the sun sets each night, offsetting the shortening of the days. Conversely, it takes extra time off the morning, but it will be a while before we take the time off the evening (another week or so from now). As we speed up our orbit that offsetting effect evens out, and both sunrises get later and sunsets get earlier.

In the winter (late December/early January), the opposite happens… the sunrise stays the same while the sunsets start to get later. Give it six months and you’ll see.



100s By the Weekend…And Why Isn’t the Sun Setting Earlier? (Pt 2)

A flat ridge of high pressure will keep western Nevada dry and hot through the rest of the week, with triple digit high temperatures a possibility by the weekend. The skies should remain mostly through the work week, but over the weekend the high heat should be enough to allow some cumulus cloud development on our area, and a slight chance of afternoon thunderstorms moves back into the area.

Earth's orbit

So now that the days are getting shorter, why are we losing more daylight in the morning than the evening? Right now we are just about at the furthest point out from the sun, and as a result we are moving around the sun at the slowest speed. Every day, the earth spins one full rotation PLUS 1/365th of a rotation to make up for how far the earth has orbited the sun (we keep our days straight with respect to the sun’s position relative to the earth). That means that every 24 hour period we over-rotate a bit a bit since we are travelling a little less around the sun during that 24 hours.

Tomorrow, I’ll explain why that makes a difference.