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

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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.

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.

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