Heavy Rain and Climate Change

Rain comes, rain goes, but lately, when it rains it doesn’t just pour — it pours like never before. The reason? Man-made climate change.

It’s a fact that the atmosphere is warmer so it can hold more water vapor. That means that when it does rain, it can rain more heavily. Climate change is intensifying hurricanes, because ocean waters are hotter, and that’s the power source for hurricanes. The jet stream is more “wavy” and tends to get stuck in place because of changes in atmospheric circulation caused by the disappearance of Arctic sea ice, and when heavy rain systems hang around longer, they dump more rain.

It’s all real, and it all came together in Houston, Texas during hurricane Harvey. I found three weather stations in Houston with reasonably long records of daily rainfall amounts that include the time hurricane Harvey hit. One of them is Hobby airport in Houston, and here’s the data:

Note that the day with the most rainfall was August 26, 2017 — during Hurricane Harvey. They were hit with over 306 mm of rain (over a foot!) in one day.

Then there’s Westbury in Houston:

They got their heaviest on August 27, 2017, enduring over 323 mm (again, over a foot!) in one day.

Then there’s North Houston:

Their heaviest day was not during hurricane Harvey, it happened on June 9, 2001, at 268 mm (about 10.5 inches). Hurricane Harvey only brought a one-day rainfall of 263.7 mm (about 10.4 inches) on August 27, 2017.

Does that mean that at least some part of Houston has had a worse rain event before? NO. It does not.

Sometimes rain events last longer than just one day. At Hobby airport, hurricane Harvey didn’t just bring the heaviest rainfall day on record, it also brought the 2nd-heaviest and the 5th-heaviest — all three in a row.

I tallied the cumulative total rainfall during consecutive rainfall days, to see what the load was during each rainfall event whether it covered only one day or more than one in a row. For Hobby Airport, it looks like this:

At Hobby airport, Harvey beat the 2nd-heaviest rain day by a noticeable amount. But the rain event that was hurricane Harvey beat the 2nd-heaviest by a giant margin. In a consecutive string of rainy days, hurricane Harvey dumped 940 mm of rain on this location. That’s over three feet of rain — from one rain event.

Here are the rain event cumulative totals for Westbury:

Once again, the “significantly heaviest day” was part of the “ridiculously heaviest event” with 796 mm (over 31 inches) of rain.

How about North Houston? This:

Note that although hurricane Harvey didn’t bring the single heaviest rain day at the North Houston station, it did mark the heaviest rainfall event, more than twice as much as any other on record. That’s slightly more than 701 mm (27.6 inches) of water.

Any one of the listed climate change impacts makes heavy rain heavier, making flooding worse. When they all come together, destruction becomes disaster becomes mega-disaster.

My message to the people in Houston: You saw it with your own eyes. When you go to the voting booth next month, remember it.

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11 responses to “Heavy Rain and Climate Change

  1. rhymeswithgoalie

    Often for an individual storm event, the difference between flooding and not flooding might be whether the storm dumps its rain in a single watershed or across boundaries with other watersheds, such that the runoff is split into multiple streams (i.e., a few kilometers distance for the same rain volume can have radically different outcomes). In Harvey’s case, the rain was parked over many watersheds for several days.

  2. Yep. The one that really got me was the Toronto flood of 2013, because it smashed the precip record from Hurricane Hazel, back in 1954–with nothing more than a thunderstorm.


    • It was actually two back-to-back thunderstorms, Doc, about 10-15 min apart. On my street we got through the first one OK, even though it was a deluge and the street looked more like a river. But it filled the storm sewers and catch basins to capacity, so when the second deluge came it had no where to go, turning the street, and my basement, into a lake.

      • Appreciate the first-person account! I used to live in the area–first Willowdale, then right next to the Dundas West station, then out in Sutton. Now I’m a South Carolinian, but I still follow cbc.ca pretty regularly.

  3. We already have a trend in the US for more precipitation most strongly in the upper midwest and northeast :

    Add on top of that additional the variance and voila, precipitation extremes.

  4. science deniers just need to get their feet wet and they will figure this stuff out. Couple of weeks to go on the biennial US referendum on truth, science and public policy. Hoping for the best, bracing for disappointment. AGW is producing dramatic weather events and it’s going to get harder and harder for red staters not to embrace the facts on the ground. I think that is turning point for US policy and action: when the R party says we have to do something about AGW.

  5. Hyperactive Hydrologist

    Might be of interest:


    Temperature scaling studies suggest that hourly rainfall magnitudes might increase beyond thermodynamic expectations with global warming1,2,3; that is, above the Clausius–Clapeyron (CC) rate of ~6.5% °C−1. However, there is limited evidence of such increases in long-term observations. Here, we calculate continental-average changes in the magnitude and frequency of extreme hourly and daily rainfall observations from Australia over the years 1990–2013 and 1966–1989. Observed changes are compared with the uncertainty from natural variability and expected changes from CC scaling as a result of global mean surface temperature change. We show that increases in daily rainfall extremes are consistent with CC scaling, but are within the range of natural variability. In contrast, changes in the magnitude of hourly rainfall extremes are close to or exceed double the expected CC scaling, and are above the range of natural variability, exceeding CC × 3 in the tropical region (north of 23° S). These continental-scale changes in extreme rainfall are not explained by changes in the El Niño–Southern Oscillation or changes in the seasonality of extremes. Our results indicate that CC scaling on temperature provides a severe underestimate of observed changes in hourly rainfall extremes in Australia, with implications for assessing the impacts of extreme rainfall.

  6. John Nielsen-Gammon

    For those interested in an analysis of probabilities of Harvey, the best study so far IMHO is open access: http://iopscience.iop.org/article/10.1088/1748-9326/aa9ef2 with correction: http://iopscience.iop.org/article/10.1088/1748-9326/aaa343 . Oversimplified summary: in the past, you might see one of these a bit less than once per interglacial.

  7. John Nielsen-Gammon, fat-tail statistics for you.