Dwight Silverman

The SCW Q&A: First, the bad news; obscured by clouds; shuttered; humidity is relative; predicting it.

Welcome to the “just in the nick of time” April edition of the SCW Q&A. Each month Eric and Matt tackle the most interesting questions you’ve thrown at them. You can leave more questions in the comments here, or hit the Feedback link in the blog’s sidebar. We’ll also scan our social media channels (Facebook, Instagram, Threads, X/Twitter, Mastodon, Bluesky) for queries. And we’ll try to get the Q&A posted sooner in May! 

– Dwight

Q. Could this be is our last front until Sept/Oct? April is coming to a close soon, and I can’t recall many pronounced cooldowns in May.

A. Let’s get the bad news out of the way first. We are very near the end of the road when it comes to cool fronts in Houston. However, it is not unheard of to see cooler nights in May. That is not to say cold. If I squint into the distance, I can see a decent chance of a front around the May 11-13 period, so a little less than two weeks from now. That’s a long way out to have some confidence, but I’m modestly hopeful we may have one final fling with drier air before sultry summer settles upon us for months on end.

–Eric

Q.  I had read before the event yesterday that clouds sometimes clear during an eclipse due to the cooldown. Is that what happened yesterday in Houston when we had a thinning of clouds enough to see the crescent?

A. This is a great question. The answer is yes. During the eclipse in Houston, we had an issue with various sorts of clouds. We had higher level cirrus clouds, mid-level clouds, and low-level clouds. Most of the low-level clouds were cumulus clouds. What usually causes cumulus clouds to form? Heating. You heat the ground during the daytime, the heated air parcels rise, cool, condense, and you get clouds. Well, as you begin to block out the sun during an eclipse, you begin to reduce that heating mechanism. And in time, a lot of those cumulus clouds (or what we often call “fair weather clouds”) will dissipate. The animation below is from the 2017 eclipse, and you can see how the cumulus cloud field over Missouri and Illinois really dissipated as the eclipse unfolded.

A satellite loop of the 2017 total eclipse over Missouri and Illinois. (University of Wisconsin CIMSS)

This is likely what “saved” much of Texas this year. There were still plenty of clouds higher up, which are less impacted by daytime heating and more driven by physical storm systems. But by virtually eliminating a layer of lower cumulus clouds, you had one less obstruction to view the sun, which periodically broke through the higher clouds.

–Matt

Q. How far inland would you recommend having hurricane shutters to board up if a storm comes inland? I find myself trying to calculate what would be the strongest type of hurricane that we could feel in order to decide if I want to invest in shutters, but can’t find any info. Would you guys know?

A. We’re not storm damage experts, but whether you should have hurricane shutters depends upon several factors. First of all, the most important factor is whether you’re close to the coast, as winds do die down significantly as a storm moves inland and encounters friction with trees, homes, high rises, and other structures.. The second factor is the proximity of stuff, like trees and unattended deck chairs and you name it. The most common reason windows break is due to flying debris. So if there’s lots of things around that you can imagine flying into your windows, that’s a risk factor.

Maximum sustained winds from Hurricane Ike in 2008. (NOAA)

I’ll be honest with you. I’ve lived through flood storms in Houston. I’ve lived through surge storms. But I’ve not really experienced a wind storm. Hurricane Ike was the closest thing, but it barely brought Category 1 winds to parts of Houston. This was bad enough, knocking power out to the city for up to two weeks. At the time I lived in Clear Lake and we had to get a new roof. I guess what I’m saying is that it’s not just your windows, it’s also your roof. And a prolonged period without power.

I guess what I would advise is that, if you live south of Interstate 10, and can afford it, hurricane shutters are not a bad idea. You will probably never need them, but if it proves peace of mind, that is not a bad investment in an uncertain world. Make sure they’re easy to put up on short notice.

–Eric

Q. On your weather app I notice that the humidity (sorry I mean HOUmidity) can be low in the evening and then go way up in the morning, coming back down again as the day goes on. Why?

A. Relative humidity (RH) is just that, relative. Dewpoint temperature is the temperature to which you’d need to cool the air to saturate it. In other words, the higher the dewpoint, the damper the air is. Dewpoint can fluctuate, but it’s usually more stable than temperature. So, let’s say you have a dewpoint of 72°. The morning low is 75° in this example. That means your morning relative humidity at 75/72 would be 90 percent. Obviously, you warm up during the day, so with a high of 88 degrees but a steady dewpoint still sitting at 72, your RH at 88/72 has dropped to 59 percent. As the air cools around sunset bringing the temperature relatively closer to the dewpoint, the humidity rises again. It limits the usefulness of RH. This is why we flout other metrics like dewpoint to drive home how humid it actually feels.

–Matt

Q. After your earlier answer, I’m confused by the description of what “percent chance of rain” represents. I was taught in my meteorology classes that probability of precipitation in weather forecasts means the probabilistic chance that it will rain at the point specified in a given time period. For example, if I enter my zip code to get a National Weather Service forecast for my area today, a 40 percent chance of rain means each point within my zip code has a 40 percent chance of seeing rain, not that 40 percent of the land area within my zip has a chance of seeing rain. Insights? Or is this just splitting hairs?

A. It’s more or less splitting hairs. But this is a fun topic so why don’t we dive a little deeper. Let’s go directly to the National Weather Service, where we find this delightful introduction to its “probability of precipitation” explainer:

The probability of precipitation forecast is one of the most least understood elements of the
weather forecast.

I just love the “most least” phrasing. A perfectly confusing introduction to a confusing topic. Anyway, fundamentally you are correct that it is a point forecast rather than an areal forecast. But it is a point forecast based on an areal forecast. If that makes sense. Which it probably doesn’t. And that’s ok.

There are two factors that go into making a “probability of precipitation” forecast. First there is the forecaster’s certainty that precipitation will form or move into the area, and this is multiplied by the areal coverage of the precipitation that is expected. Here are two examples for how one might derive a 40 percent chance of rain:

(1) If the forecaster was 80 percent certain that rain would develop but only expected to cover 50% of the forecast area, then the forecast would read “a 40 percent chance of rain” for any given location.

(2) If the forecaster expected a widespread area of precipitation with 100 percent coverage to approach, but he/she was only 40 percent certain that it would reach the forecast area, this would, as well, result in a “40 percent chance of rain” at any given location in the forecast area.

Does that clear things up? Probably not.

But given the general inaccuracy of precipitation forecasts, I think it’s fine to think of a “40 percent chance of rain” as either a point forecast or, more generally, that about 40 percent of an area will receive rainfall during a given period of time. This is not an exact science, it’s just a forecaster making a most best guess of things.

– Eric

The SCW Q&A: Eclipsing, hurricane vs. typhoon, climate changed, thunder calc, bad forecasts!

In this second edition of our new Q&A feature, Eric and Matt tackle an excellent set of questions from you, our most excellent readers.

We do this monthly, and if you’re dying to know something weather- or SCW-related, leave us a comment here or on our many, many social channels – Facebook, Instagram, Threads, X/Twitter, Mastodon, Bluesky (we’re everywhere!) – or hit the Feedback button in the blog’s sidebar.

Let’s get to it.

Q: What does the forecast look like for the solar eclipse on April 8?

A. We have received more questions about the total solar eclipse next month than almost anything that I can remember. And for good reason, it is going to be spectacular—very likely the most dramatic celestial event that most all of us will see in our lifetimes. I cannot wait.

However, in regard the forecast we need to exercise a little bit of patience. Unless we see a very strong signal for high pressure over the state of Texas, confidently forecasting clear or partially clear skies is not something that can be done too far in advance. I plan to make an initial forecast next Thursday, and then we’ll provide regular guidance after that.

– Eric

A preview of things to come: The 2017 total solar eclipse as seen from Casper, Wyoming, by a team of European Space Agency astronomers. (Credit: ESA)

Q: I’ve been noticing that the term hurricane and typhoon are being used interchangeably and frequently by weather forecasters – both on tv and in the paper. I don’t recall this happening in the recent past. Why is this happening and do the terms mean the same thing?

A: The answer depends on where they’re talking about. No, really. Both terms describe the same phenomenon: What we call a hurricane, folks in East Asia call a typhoon. According to the American Meteorological Society’s Glossary of Meteorology, the word “typhoon” originates either from Cantonese (t’ai fung: “great wind”), Arabic (tufan: “smoke”), or Greek (typhon: “monster”). Aristotle described it as such in his text Meteorologica to mean a wind-containing cloud. You can read more about the etymology (and debate) of “typhoon’s” origins on the Wiki page.

Meanwhile, “hurricane” probably derives from “Hurakan,” the Mayan god of wind, storm, and fire. The thought is that the indigenous Taino peoples told Columbus and other Spaniards of this deity, and the name became associated and stuck to what we call hurricanes today.

If you travel to the Indian Ocean or Australia, these storms will be referenced as “cyclones.” I was taught in grade school that Aussies referred to them as “willy willys.” That is not the case. “Willy willy” actually refers to a dust devil which is like a miniature tornado that can form in clear weather (another topic for another day).

But, the bottom line: Hurricane, typhoon, cyclone, tropical cyclone, cyclonic storm are all basically interchangeable at the highest level. Hearing it called one of those names can narrow down for you where it actually occurred. There are occasionally other regional references you may hear, such as “Medicane,” which is essentially a Mediterranean version of a hurricane.

– Matt

Q. In southeast Texas, how has the climate changed over the last 10,000 years? Was this area drier, wetter, more trees, more prairie? How did this effect how people lived?

A. The Texas State Historical Association has a great website with answers to questions like this. I encourage you to visit there for the full details. But the big picture is that 10,000 years ago Texas (and the rest of the present-day United States) were emerging from the last great ice age. At that time, when glaciers reached their greatest extent, average temperatures were 10 to 15 degrees cooler than today. But by 10,000 years ago things were getting back closer to what conditions were like today, although Texas has steadily gotten a bit warmer and drier over recent millennia.

As for how people lived in Houston, how did they do it without air conditioning?

– Eric

Q: When you hear thunder, sometimes it sounds like it is right over you and other times it seems far away. As a meteorologist, are you able to point/pinpoint a specific area in the sky where the thunder is coming from?

A: So the first question to ask is “what is thunder?” Thunder is the audible response you hear when lightning heats and expands the air around the bolt. Remember, lightning is hot, with a temperature in the bolt of up to 50,000°F, almost as hot as last summer in Houston. Just kidding. Sort of. One reason the thunder sounds like it may be right over you is because it could literally be right over you, or at least nearby. In that case, you’re hearing the thunder “right over you” because the lightning strike occurred within a mile or less of your location. I’ve found that usually the loudest thunder I hear is when lightning hits within about a half-mile or so of where I am.

Now, there are other possible reasons for really loud thunder when lightning is nowhere near you. If we have an inversion in place in the atmosphere, or when it’s actually warmer above our heads than at the ground, sometimes those sound waves from the thunder can propagate farther along or just have more impact. So lightning can strike miles away, yet you still hear loud thunder.

Map of lightning strikes on July 28, 2020, in the Houston area. Yikes! (Lightningmaps.org)

So, like anything, it’s complicated! Everything from the air temperature at the ground to the air temperature a few thousand feet up can impact the sound of thunder. Fortunately, we are blanketed with lightning detection sensors today, so we almost always know where lightning strikes or comes from in near-real time. And there’s always the Lightning Distance Calculator. But just remember, if you can hear thunder, it’s time to get inside!

– Matt

Q. Why are models and meteorologists so bad at predicting rain? Apps always say its going to rain and not a drop. This has been happening in the past three years. Something is off.

A. And I took that personally. Seriously, I share your frustrations. A big part of it is that rain can be a very localized phenomenon. Pure misery is watching a radar app during a drought and hopefully seeing heavy showers in the next neighborhood over, but they stop half a mile from your location. Well, guess what, a forecast for rain showers verified for that neighborhood, but failed for yours. Imagine the challenge for us when we must forecast rain chances for a metro area 100 miles across, with varied climates from the coast the piney woods. And all you want to know is whether it will rain at your house.

That’s why you’ll often see a 30 percent chance of rain or a 50 percent chance, or whatever. That means that, for a given forecast area, the percentage of that area expected to see rain—it could be a sprinkle or a deluge—is 30 or 50. It’s a probabilistic answer because we cannot tell you definitively that it will rain. (Except during Hurricane Harvey, of course. One could confidently predict a 100 percent chance of rain at that time).

There are other factors: Even high resolution models cannot account for the physical processes that occur on a small scale, of a few miles or less, that determine whether rain showers develop or dissipate. Finally, it does not help that there are no regular, local weather balloons in Houston.

– Eric

The SCW Q&A: Frost, pollen, predictions, blue skies, and swag!

One of the best things about Space City Weather is the people who read it. Anyone who writes regularly wants to feel like the audience is paying attention – and it’s even sweeter when they’re talking back. SCW is more than just Eric and Matt: It’s you, too.

And you have questions. Lots of them. We enjoy turning your queries into posts, which happens a lot. Now, it’s going to happen more. We’re starting a monthly SCW Q&A feature, in which Eric and Matt will tackle a handful of questions you’ve asked them recently.

Got a question of you want answered? Leave it as a comment here or on one of our many social channels (we’re everywhere!), or hit the Feedback button on the blog’s home page.

Q. Do you think we’ve seen the last chance of frost? I’m itching to get plants in the ground.

A. We received a couple of questions along these lines. We are now past the “average” date of the final freeze for pretty much all of the Houston metro region, except for a few outlying areas like Conroe and Montgomery. So historically the city’s odds of seeing another freeze are below 50 percent.

But I think they’re far lower than this. We’ve clearly entered a more spring-like pattern, and there is no sign of a super-strong front in the next 10 to 14 days. So I’d say our chances of seeing another freeze in the Houston area this season are 10 percent, or less. It could happen. The city has recorded freezes in early April before. But it’s very unlikely.

–Eric

Q. When do you expect that the yellow pollen will be back in Houston again? Thanks!

A. It’s baaaaaack! Allergy season is now in full swing. Usually, we’ll see little bumps in pollen through January and February as we get periodic milder weather. But we never see the full force of pollen arrive until usually the last week of February. It appears that began last Friday. In fact, if you look historically, you can see this in the City of Houston’s tree pollen count. The chart’s a bit messy because data is not taken on weekends or holidays. But you can see the general trend.

The daily tree pollen count in Houston from 2017 through last week shows that allergy season typically peaks around mid-March here. 2022 was a notable exception with a massive peak in late March.

Since 2017, Houston’s peak pollen day has occurred in March, varying between the 6th and the 31st and averaging around St. Patrick’s Day. Pollen typically builds in the first half of March, peaks, and then slowly tails off before a more dramatic drop heading into April. Notice how on the chart above 2021 stands out for how short pollen season was, thanks in part to the mid-February freeze. With a potentially extended spring this year, we probably won’t get so lucky. Whatever the case, the next three to five weeks will probably be rough for those with pollen allergies.

–Matt

Q. You’ve talked about how it’s silly to predict hurricanes too early. But at what point does a season prediction start to become credible, in terms of timing?

A. This is a really good question. Seasonal forecasting for hurricanes is a challenge, whether you do it in January, March or May. This is part of my day job in the energy industry, and it remains the most elusive in terms of consistency, particularly when you get into other variables like temperature or rainfall for a season. But from the standpoint of hurricanes, seasonal forecasting is actually not half bad when you begin to see outlooks emerge in March. Colorado State University issues one of the most publicized hurricane outlooks, and this year’s first iteration comes out on April 4th. Anything prior to that can be hit or miss.

Raw modeling of hurricane season forecasts is occasionally good, occasionally bad. For humans, trying to predict El Niño vs. La Niña and the amplitude can be a challenge this time of year because of the inherent “spring predictability barrier.” That’s one of our best signals for determining directionality of a hurricane season (busier or quieter). But some years are easier than others. The current thought among most meteorologists I know and follow, as well as my own independent thinking, is that we are almost certainly heading into a La Niña this summer. That, combined with continued exceptionally warm oceans globally (a global marine heat wave) seems to imply that virtually no one should be forecasting a quiet hurricane season this year. We wrote about this in detail last week at our companion site The Eyewall.

But as far as firm concrete numbers? Late March and early April offers a viable first guess, with some adjustments in May and even June. That said, what no one is really, consistently good at on a seasonal scale is predicting where storms will go. So regardless of forecasts, it’s important to remember to prepare for every hurricane season as if it will be the one that impacts you.

–Matt

Q. I have a question regarding how clear Houston appears following a cold front. I imagine it’s because the wind blows the smog and pollution away. But what’s the science behind that? Does it also have something to do with our humid climate? 

Well, you have pretty much answered your own question!

First let’s characterize the haze that typically makes for less clear air. Haze is caused when sunlight encounters tiny pollution particles in the air. Some of this light is absorbed by particles, and some is scattered away before it reaches you. More pollutants mean more absorption and scattering of light. In Houston we have plenty of pollutants in the air, which increases the likelihood of haze. But weather plays a role as well.

Fronts typically bring blustery conditions, and these winds clear out air pollution at the surface. In addition, warmer air in Houston tends to be more humid. So think about a warm, summer day in Houston: It’s humid, winds are typically light and its partly to mostly sunny. These are ideal conditions for the formation of haze. Conditions in the wake of a good front: cooler and drier air, and windy conditions, are kryptonite for haze.

–Eric

Haze is caused when sunlight encounters tiny pollution particles in the air. (US EPA)

Q. I love the site and love the new(-ish?) logo! Any chance you’ll get that on a coffee cup at some point?

A. I’m glad you like the new design! It was time for a refresh. The original design, depicting the space shuttle carrier aircraft flying over the city of Houston, dates to October 2015. At the time I was in a rush to create the site in a couple of hours. I grabbed the image because it was a free NASA photo and it captured both the elements of “Space City” and “Weather.” But it wasn’t professionally done. I love the new look, and especially because it features the International Space Station, which is flown right here from Houston.

As for coffee cups, that’s a great suggestion. We’ll be sure and add those to our annual fundraiser. That takes place in November when we encourage people to buy merchandise and donate to support the website and our app.

–Eric

Darker and wider: Introducing the Space City Weather app 2.0

Editors note: We’re thrilled to release the latest and greatest version of our app today. As always it’s free, and we can offer this because of your generous contributions to our annual fundraiser in November. I want to thank Dwight Silverman for shepherding these changes, and Hussain Abbasi for his fine programming work. Here’s Dwight with more information.

When we first introduced the Space City Weather mobile app for iOS and Android devices two years ago, we sought to make it useful, simple and Houston-focused. The design was clean, the information clear, and the approach non-intrusive. There were no ads, and unlike most mobile apps, it did not track you.

We gave it some tweaks last year, adding more area cities, rain percentage chances, a live National Weather Service radar page, a Fahrenheit/Celsius toggle and more. Most of these new features were by request—we really consider y’all to be partners in our development of the app, and we thank you for your continued input.

But there were two high-demand features that we had yet to add—dark mode and tablet view. Well, you asked—and asked and asked and asked again—and this year, we have delivered.

The new Space City Weather app features both a tablet orientation and dark mode – and you can even have both at the same time!

Version 2.0 of the Space City Weather app is now live in both Apple’s App Store and the Google Play Store. As always, it’s free—and still ad-free and tracker-free.

Here’s what’s new:

  • Dark mode. We know that fans of a dark display are passionate about it, so we took our time to do it right, complete with ADA-compliant colors. For now, it’s triggered based on your system settings. When your device is set to dark mode, the SCW app goes dark as well. At the moment there’s no manual toggle, but that’s coming in a future update.
  • Tablet mode. When you install the SCW app on an iPad or Android tablet, it expands to fit the space, with a different layout for horizontal and vertical orientations. If you have a newer Mac with an Apple Silicon processor in it, you can install and run the iOS version of the app in tablet mode. You can click and hold on the lower right corner of the window and drag it to adjust the orientation. It also is designed to display as a tablet on horizontally folding phones, such as the Samsung Galaxy Z Fold family. You can launch it in traditional smartphone view from the narrow, external cover screen, and when you open the phone, the app appears in tablet mode. Nifty!
  • Design. Our stellar developer Hussain Abbasi has done a fantastic job polishing up the visuals. The layout for stories now puts the photo to the right of the headline on the main page, making it easier to read at a glance, and the current story now has a heavier typeface. A gradation line has been added between hourly temps and rain changes, to visually indicate how quickly temperatures are changing. The whole app has gotten many visual improvements, and we think it looks great.

If your device is set to automatically download updates, you will have version 2.0 soon, or you may already have. If not, head to your respective app store and grab it.

The traditional smartphone view also gets some design tweaks, including new cards for current conditions and readability improvements.

We’re particularly excited about this release, because we feel we’ve now got a mature, truly useful app that will help Houstonians keep up with local weather in style, and on whatever platform they choose. We think you’ll love it, too.

Of course, because it’s a big update, there are going to be bugs. If something doesn’t seem to work right, please let us know by sending an email to [email protected]. Please include as many details as you can, including what device you’re using and the version of its operating system.

Again, thanks for your ideas, and please keep them coming. We appreciate your support!