Get Ready For Some Cold Weather…And Different Types of Jet Streams

We have some big swings coming our way weatherwise. Saturday will bring mostly sunny skies and warm temperatures (highs in the lower 60s in Reno) before a cold front drops in Sunday, bringing gusty winds, cooler temperatures and a chance of rain turning to snow.  Scattered snow accompanies even colder temperatures on President’s Day Monday with the high barely clearing the freezing mark. Conditions dry out Monday afternoon and Tuesday, but temperatures remain very cold through the first half of the week.

hadley ferrel polar cells

We’ve been talking about the jet stream. And even though we often talk of the jet stream as a single entity, there are really four of them typically circling the globe. There’s a polar and a sub-tropical jet stream in each hemisphere, each generally following a sinuous path from the west to the east. The polar jets are lower in the atmosphere (generally 25-35,000’ altitude vs. 35-60,000’ for sub-tropical jets), and they tend to be stronger in the winter when the contrast of the polar and tropical airmasses is the greatest.

low level jet stream

And at times you can get other types of jet streams, including one in the summer that goes the opposite direction (east to west) down in the tropics, and low-level jet streams can affect the strength of winter storms hitting the west coast. That’s one kind of jet stream on which the land does have a large effect.



Cold Hit Next Week…And How Does Land Affect Jet Streams?

Temperatures will warm up quickly over the next couple of days as a ridge of high pressure moves into the area… but then they will fall even quicker as we head into the end of the weekend and the first of next. Sunshine Friday and Saturday will be accompanied by highs in the 50s to low 60s, but the cold front coming in Sunday will drop temperatures back into the 40s, and then plunge all the way into the 30s Monday and Tuesday with a slight chance of some light snow Sunday and Monday. Things stay relatively cool through the rest of the week.

hadley ferrel polar cells

Does land affect jet streams? Land receives and releases heat from the sun more rapidly than the oceans do, and therefore the vertical convective air currents (called Hadley, Ferrel and Polar Cells) which on a large scale help form somewhat stable weather patterns will have differing strengths over the oceans versus over continents. The continents can have an effect on the positioning of large scale persistent weather patterns, which also help determine the strength and position of the jet streams. But in terms of direct interaction between land and jet streams, there’s very little. Since the jet stream is usually found above 25,000’ of elevation, you’d have to get to the Himalayas before the land has any real touch on the jet.


Roller Coaster Temperatures… And Does Land Affect Jet Streams?

While there is no single storm headed our way, the temperatures will roller-coaster somewhat over the next week. After a cold front passes through the area Wednesday night, Thursday will clear out and cool down to the mid-40s. The ridge of high pressure builds back in Friday and Saturday, bumping the high back to the 60 degree mark. Another cold and mostly dry low drops back down late Sunday into Monday, bringing at most some isolated snow and dropping the highs all the way down into the 30s on Monday and Tuesday.

Earth wind patterns, artwork

Joe wondered if the landmasses in the southern hemisphere affected their jet streams. It’s kind of hard to answer that in real simple terms, because if you really want to get down to brass tax, anything anywhere that touches air can have some effect on the chaotic system that is the atmosphere. “If a butterfly in Tokyo flaps its wings, it can cause rain a week later in New York” is an old adage that at least holds some truth in theory, if not being absolutely provable. Having said that, landmasses do have a limited effect on jet streams. I’ll tell you how tomorrow.


Warming for the Weekend…And Does the Southern Hemisphere Have Jet Streams?

While we have a slight chance of seeing a few isolated snow showers Wednesday night, for the most part, the forecast will dry out and warm up as we head into the weekend. A weak low will once again drop down from the north late in the day Wednesday, increasing clouds and winds, but it appears the bulk of that system will pass to our east, with us here in Reno only getting a slight chance of a snow shower.

After it passes, skies will clear out on a cool Thursday (high in the mid-40s) and things will warm back up to the low 60s on Saturday before the next weak storm tries to move in Sunday.

Earth wind patterns, artwork

Earth’s wind patterns, schematic view. Atmospheric circulation in each hemisphere consists of three cells. The Hadley Cell (labelled) dominates the tropical atmosphere and is intimately related to the trade winds, tropical rain belts, subtropical deserts and the jet streams. The Polar Cell (labelled) produces the polar easterlies. The outflow from the Polar Cell creates waves in the atmosphere known as Rossby waves which play an important role in determining the path of the jet stream. The polar cell also balances the Hadley Cell in the Earths energy equation. The Ferrel Cell (labelled) is dependent upon the Hadley Cell and the Polar Cell and just as the trade winds can be found below the Hadley Cell, the westerlies can be found beneath the Ferrel Cell. *** THIS PICTURE MAY NOT BE USED TO STATE OR IMPLY ESA ENDORSEMENT OF ANY COMPANY OR PRODUCT ***

Joe wondered: “Is there a corresponding jet stream in the southern hemisphere? Do large land areas that are not present in the southern hemisphere affect the jet stream?”

Yes, there are corresponding jet streams in the southern hemisphere, and they act much like the ones up here do. They even flow the same way (generally west to east.)

As for the land masses (or lack thereof) in the south affecting the jet, that’s a little more complicated to answer, and I’ll tackle that tomorrow.



Colder Weather Coming…And Other Uses For The Dewpoint

Some changes finally appearing in our forecast, although at this time any precipitation will be pretty sparse, and that won’t come until after the weekend. A dry cold front will drop down from the north, kicking up some stiff north breezes Saturday and dropping our high temperatures to the low 50s. After a recovery to the upper 50s on Sunday, another cold front will come through Monday, dropping our highs further into the 40s with a slight chance of a few rain or snow showers. Temperatures then rebound to the upper 50s by Wednesday.

Dew Point grass

We have been talking about the dewpoint. The dewpoint can give you a baseline for how low your temperatures can go. That’s because from a practical point of view, the temperature will not fall below the dewpoint. It doesn’t necessarily mean if the dewpoint is 25 degrees in the afternoon that the overnight low can’t fall below that number, but in order to do so the air has to lose some of its moisture. It usually does that by condensing some of the water vapor out of the air as the temperature meets the dewpoint. But that will slow down your cooling due to a thing called latent heat.

latent heat graph

Whenever water changes its state from vapor to liquid, it gives off heat in order to do so. This release of “latent heat” slows the cooling of the air. This is why dry air cools so much faster than “wet” air.

latent heat 2

Latent heat is a very important factor in the physical chemistry of our air. The amount of water vapor in the air not only is a factor in producing rain directly, but it changes the thermal makeup of the atmosphere as well. Computer models used in forecasting have to take latent heat into consideration in order to be at all accurate.

Hmmm…that gives me a great excuse for the next forecast that busts. “Latent Heat Error.” I like the sound of that.



Upcoming Changes…And Why Dewpoint is Better!

After one more very warm day to finish off the work week, conditions will start to cool back to more seasonable realms with the passage of a couple of (mostly) dry cold fronts. Friday’s high of 66 in Reno could drop about 15 degrees on Saturday, although the front itself will probably be devoid of any showers. Temperatures recover a bit on Sunday before a second system moves through Monday, lowering temperatures down into the 40s and giving us a slight chance of a shower. By the middle of next week, we should warm back up into the mid to upper 50s.

Dewpoint rel hum graph

I’ve always found the dewpoint is a more useful measure of the moistness of air than relative humidity. If you have a parcel of air where the dewpoint is, say, 20 degrees, the relative humidity can go from 100% at sunrise (if the temperature is 20 degrees) to about 25% in the afternoon when the temperature rises into the 50s… all without changing the amount of water in the air. But the dewpoint only changes if the amount of water in the air changes. It’s one of the reasons that we don’t generally “forecast” the humidity, because it changes so much in any 24 hour period.


Cooling Down Next Week…And How Does Dewpoint Work?

While there are a few signs that the very warm and dry weather pattern might have a few cracks in it come next week, our odds of actually getting any precipitation here in Reno is pretty slight. For the short term, sunny weather and near record high temperatures will stay with us through Friday, with high temperatures climbing into the mid to upper 60s. A dry cold front moves through the region over the weekend dropping our highs into the 50s Saturday and Sunday, and then a second front will drop temperatures even further on Monday, with a slight chance of a rain or snow shower then. Things warm slightly Tuesday and Wednesday back into the mid-50s.

Dew Point grass

Yesterday I said that dewpoint was a better measure of air moisture than relative humidity. Air that has a dewpoint of 20 degrees F needs to cool to that temperature before condensation (saturation) occurs, and dew forms (hence the name). If the dewpoint is 40 degrees F, it means it has more moisture in it than air with a dewpoint of 20.

How is dewpoint more useful than relative humidity? It’s not dependent on the air’s temperature. I’ll explain tomorrow.


Cooler Weekend, But Still Dry…And What’s The Best Way To Measure Dryness?

Sunshine will continue across western Nevada and eastern California through the rest of the work week, after which a dry cold front will drop down out of the north cooling us off for the weekend. High temperatures Wednesday through Friday will range from the low to mid-60s, and will then fall to the low 50s Saturday and Sunday under partly cloudy skies. Temperatures will remain mild through the middle of next week.


Yesterday a viewer asked how we could be drier than Las Vegas. Actually, it is a fair bit drier here than in Vegas. Relative humidity doesn’t really tell the story as much as dewpoint. A typical dewpoint in Vegas is often 10 degrees higher than here in Reno. They generally just have more water in their air, a function of their elevation and proximity to the moister air in Mexico.

Dew Point grass

I mentioned the dewpoint. That’s the temperature at which the parcel of air you are measuring will become saturated (100% relative humidity). The more moisture in the air, the less you have to cool it to become saturated, so it is a much better measure of the amount of water vapor in the air than relative humidity. More on this tomorrow.


Dry Week…And Are We Drier Than Las Vegas?

It looks like the long dry spell will continue through this week at least. The ridge of high pressure off the west coast will continue to strengthen, keeping us sunny throughout the rest of the work week. High temperatures will start near 60 on Tuesday and rise to the mid-60s by Friday. The ridge will shift offshore just a bit over the weekend, which will allow some cooler air to carve back in on the backside of the ridge, dropping our highs back into the 50s and bringing in some clouds. It’s not impossible that it could bring some valley showers, but the odds of that happening this far to the west are slight.


If you think it’s been dry, Steve agrees. He recently moved back to Reno from Las Vegas. “Upon moving back to Reno, I noticed my skin started to “crack” and a couple of minor nosebleeds. I said to myself, “There ain’t no way it could be drier here than the ……”desert”. But apparently it is MUCH drier? Less humidity? How is this possible?”

How is it possible? I’ll tell you tomorrow.





Warm February Following Near Record January… And Why Does It Get Colder As You Go Higher?

After finishing the second warmest January on record, it looks like February is on track to follow suit. A strong ridge of high pressure will stay firmly planted across the west coast keeping our skies mostly clear and our temperatures up in the 60s…not quite record levels, but pretty toasty all the same. The clear skies will allow overnight lows to drop to near the freezing mark through the end of next week. The only possible bump could be late next week as a low pressure center could drop back in from the northeast, but unless it cuts back farther to the west than the models are presently showing, all it will do for us is cool us down a bit.


Why is it colder higher up in the atmosphere? We are, after all, closer to the sun up there. But the difference in the distance is so minute (the top of Mt. Everest is about .000005% closer to the sun than sea level), that it becomes a non-factor. But thermodynamics play a much bigger role. The air pressure drops as you gain altitude. The laws of physics demand that if you reduce the pressure of a gas, it will cool. In a dry (and mixed… we’ll ignore inversions for now) atmosphere, the air will cool approximately five degrees F for every 1,000 feet of altitude gain. In a saturated (inside clouds) atmosphere, the rate drops to about three degrees per thousand feet.