<p>I invite correction if I missed any details, the column of air above you, at any one time, at 100% humidity can hold maybe 3-6 inches (75-150 mm) of water depending on temperature and it's likey to rain only a percentage of that, not all of it.   If air remained stationary, which, of-course it doesn't, then we'd get much less downpours.  </p>

<p>The hurricane is continuously spiraling in new wet air with water picked up from the gulf.   It might rain 2-4 inches (50–100 mm) per hour, which over 10 or more hours can give you 30-40 inches (750-1000 mm).   As it rains, the air loses moisture, so the area that gets 30-40 inches (750-1000 mm) smaller in size than the Hurricane.   It's the specific area, land just past the ocean or gulf, to the East or North-East of the eye where the saturated bands of rain come in from over the ocean or Gulf that gets hit the hardest.   </p>

<p>This chart isn't updated, it's a forecast from a day ago, but it gives an idea of the localized high rain fall amounts.  The region that gets the 20 plus inches (500+ mm) of rain is smaller than the Hurricane.</p>

<p><a href="https://i.sstatic.net/lhSXi.png" rel="noreferrer"><img src="https://i.sstatic.net/lhSXim.png" alt="enter image description here"></a></p>

<p>The wind spirals around the eye much faster than the Hurricane itself moves, and it gets saturated to 100% humidity and in a sense, over 100% humidity as it can carry water droplets, as it spirals over the warm waters of the Gulf of Mexico.  When it hit's land, the rain continues but the humidity begins to drop, so the rain falls hardest, shortly after landfall and where the rain bands come from over the ocean, Usually, in the East Coast of the US, this is East or Northeast of the eye.   (The dirty side, as blacksmith calls it).</p>

<p><a href="https://i.sstatic.net/vhFgE.jpg" rel="noreferrer"><img src="https://i.sstatic.net/vhFgE.jpg" alt="enter image description here"></a></p>

<p>Air can't hold 40 inches (1000 mm) of rain.  No where close, but air blowing over a body of water can supply steady rainfall because the air blowing over the gulf is continuously picking up water.   It's new, 100% humidity/saturated and super-saturated air that hits land continuously, raining hard, until the storm passes.    </p>

CC BY-SA 3.0
2017-08-27T17:09:46.317Z
2017-09-03T03:01:06.987Z
2017-09-03T03:01:06.987Z
userLTK
<p>Harvey is almost stationary and is rotating new gulf moisture up the "dirty" side (east of the center). I am outside the main path and have 10+ inches (250 mm) in 30 hours (emptied the rain gauge twice). It has been reported that affected areas have exceeded the "500 year" rainfall level. I guess they need a 1000 year level. The atmosphere is pretty big and does hold that much water.</p>

CC BY-SA 3.0
2017-08-29T00:38:10.040Z
+1 for brevity and capturing the crux of the OP question.
Ale..chenski
CC BY-SA 3.0
2017-08-30T01:19:54.543Z
[Houston has been subject to 500 year floods in 2015, 2016, and now with Harvey, in 2017.](https://www.washingtonpost.com/news/wonk/wp/2017/08/29/houston-is-experiencing-its-third-500-year-flood-in-3-years-how-is-that-possible/?utm_term=.43bf41f064b5)
David Hammen
CC BY-SA 3.0
2017-08-31T14:36:47.437Z
I have lived in the Houston area 30 years ; Nothing remotely like Harvey before , no matter what the media says.
blacksmith37
CC BY-SA 3.0
2017-08-27T15:13:44.523Z
2018-01-28T11:25:16.637Z
2018-01-28T11:25:16.637Z
blacksmith37
<p>The atmosphere can "hold" only maximum 75mm worth of precipitation at any given locality, this is true.</p>

<p>However, this particular hurricane Harvey was a very slowly moving structure. The vortex would carry the warm water-saturated masses from Gulf of Mexico, which essentially collided with colder air from the North, generating rainfall. It is the continuous supply of moisture from warmer saturated area that made the rain nearly continuous at some particular locations.</p>

<p>If the hurricane structure would sit over one area and not dissipate over time, then the evaporation from the Gulf could sustain the moisture level in the air, and the amount of total rainfall (in limited area) could be even higher. The source evaporation rate doesn't need to be unusual, because a relatively large area of gulf/ocean feeds moist air to a relatively much smaller area of rainfall under this particular type of air circulation pattern.</p>

CC BY-SA 3.0
2017-08-30T02:18:37.150Z
@gansub, it doesn't ADD, it summarizes the essence of the effect. This is not a scientific conference to flash numbers. The OP is perfectly aware of moisture holding capacity of raw atmosphere, otherwise he wouldn't ask this question in first place. Apparently the OP doesn't watch TV or Weather Channel, to see the stalled rotating pattern of this hurricane, otherwise he wouldn't ask this question at all. And my answer doesn't have any "claims", it just offers a short and obvious explanation of this abnormal rainfall, without a lecture about Thermodynamics.
Ale..chenski
CC BY-SA 3.0
2017-08-29T19:42:24.603Z
2017-08-30T17:18:07.227Z
2017-08-30T17:18:07.227Z
Ale..chenski
<p>The precipitable water vapor content of the air never exceeds about 3 inches even in the most humid tropical maritime air masses, and in most maritime tropical air masses is typically closer to 2 inches. But Harvey is not limited to the water vapor content in its own region of the atmosphere. Its counterclockwise INspiraling winds (note the emphasis on IN) continually bring in new water vapor being evaporated from the Gulf of Mexico. The counterclockwise INspiraling winds concentrate the water vapor supply from a region much larger than Harvey itself into Harvey's central region. Thus these counterclockwise INspiraling winds act as a funnel, concentrating what would have been a 2 or 3 inch rain over a large area to a 50 inch rain over a much smaller area.</p>

CC BY-SA 3.0
2017-08-29T21:36:29.717Z
2017-08-29T21:36:29.717Z
Jack Denur
<p>Harvey has been using a <a href="https://en.wikipedia.org/wiki/Rainband" rel="nofollow noreferrer">rainband</a> to get water in from the Gulf of Mexico.</p>

<p>Think of the rainband as a pipe, it is piping water from the gulf to the core, and this pipe has a leaking problem, so the water gets dumped on Austin on it's path to the center. </p>

CC BY-SA 3.0
2017-09-03T05:52:34.447Z
Does the term [rainband](https://en.wikipedia.org/wiki/Rainband) really refer to a band that is collecting moisture at one end and producing precipitation at the other end, suitable for a "pipeline" analogy?
uhoh
CC BY-SA 3.0
2017-09-03T18:47:23.440Z
@uhoh no the rainband is feeding moisture from one end and moving it to the core
Pokestar Fan
CC BY-SA 3.0
2017-09-02T13:23:40.137Z
2017-09-03T05:50:17.957Z
2017-09-03T05:50:17.957Z
Pokestar Fan
<p>You were right to question whether the atmosphere really held that much water.  It comes nowhere close!  We use <a href="https://en.wikipedia.org/wiki/Precipitable_water" rel="noreferrer">precipitable water</a> to track this, which is the measure of all moisture in the entire column of air in the troposphere.  We can get good widespread estimates from remote observations.  <a href="http://www.ospo.noaa.gov/Products/bTPW/TPW_Animation.html?product=CONUS_TPW" rel="noreferrer">This animation</a> shows current amounts of precipitable water levels across the US from satellite.  Here is a still from this afternoon (Sunday, August 27) during the Harvey event:</p>

<p><a href="https://i.sstatic.net/fhH55.png" rel="noreferrer"><img src="https://i.sstatic.net/fhH55.png" alt="PWat082717"></a></p>

<p>75 mm is less than 3 inches.</p>

<p>We also get <a href="https://www.britannica.com/topic/in-situ-measurement" rel="noreferrer">in-situ</a> exact measurements worldwide from twice daily radiosonde balloon launches.  Unfortunately none are located right near the locations receiving excess rainfall.  But you can check US sites of current precipitable water measurements any time by going to <a href="http://www.spc.noaa.gov/exper/soundings/" rel="noreferrer">SPC's sounding page</a> and looking for the PW value in the left side of the bottom table.  <a href="http://www.spc.noaa.gov/exper/soundingclimo/" rel="noreferrer">SPC also maintains a climatology page for precipitable water and other values</a>.  You can see there that 3 inches (76 mm) of moisture in the sounding is extremely rare, and no US site has ever had 4 inches (102 mm) of precipitable water.</p>

<p>However, the answer is also not truly found in evaporation rates.  The conditions in strong hurricanes do greatly enhance evaporation rates due to the high wind speeds and warmer waters.  Measurements are actually a bit difficult to come by in such extreme conditions, with challenges in isolating evaporation effects from spray as well as in getting the instruments positioned into such environments (new field campaign: who wants to take the research ship out into the category 5 hurricane!?!).  As <a href="http://onlinelibrary.wiley.com/doi/10.1029/2006JD008303/full" rel="noreferrer">this 2007 study by Trenberth et al.</a> noted:</p>

<blockquote>
  <p>We are unaware of reliable estimates of evaporation in hurricanes, and published measurements do not exist in winds above about 20 m s−1 although some progress has been made in the Coupled Boundary Layer Air-Sea Transfer Experiment (CBLAST) </p>
</blockquote>

<p>However, in that study model analyses suggested that evaporation rates in the core of hurricanes are likely no more than 1-2 inches (25–50 mm) per day.</p>

<p><strong>How is that possible?</strong></p>

<p>It's quite important to notice that heavy rainfall - well over 5 inches (125 mm) - can actually fall in as little as a couple hours almost anywhere, such as in <a href="https://www.weather.gov/oax/event_archive_20150507" rel="noreferrer">this 2015 flood event in Nebraska</a>.  How can that be?</p>

<p>The secret lies in the nature of even the weakest storm: even as a storm is just beginning to form, it begins to draw in air from surrounding areas.  This is NOAA's diagram of a typical developing thunderstorm cell:</p>

<p><a href="https://i.sstatic.net/06I0j.jpg" rel="noreferrer"><img src="https://i.sstatic.net/06I0j.jpg" alt="developing"></a></p>

<p>You can see the curve at the arrows near the bottom, indicating inflow of surrounding air into the storm.  This inflow turns a thunderstorm into more of an engine, processing a continuing stream of incoming air, removing its moisture.  In a single cell thunderstorms in an environment without background winds, the "waste" air will eventually pile up and choke off the influx.  But even in such circumstances, a few inches of rain may fall.  That isn't by using up all of the moisture from the cloud's environment, but instead by using just a portion of the moisture from the reservoir in and around the cloud.</p>

<p>If some upper level winds exist to help exhaust the "spent" air, storm systems can persist for even longer periods of time.  For example, the Midwestern United States commonly sees long-duration late summer heavy rainfall initiated along stationary boundaries in which inflow lacks direct access to significant warm water bodies.</p>

<p>Larger systems that dump huge amount of precipitation over greater areas must pull in a more consistent, stronger inflow of warm, moist air from greater distance.  Examples of this happening include the <a href="https://www.accuweather.com/en/weather-news/what-is-a-pineapple-express/38881189" rel="noreferrer">Pineapple Express</a> for rainfall in California/the southwest US, the <a href="http://www.spc.noaa.gov/publications/evans/kumjian.pdf" rel="noreferrer">low-level jet</a> for spring and summer squall lines in the Plains, <a href="http://www.meteo.co.in/monsoon1.cfm" rel="noreferrer">onshore winds during</a> the Indian monsoon, and air from off the <a href="https://www.e-education.psu.edu/earth107/node/1055" rel="noreferrer">Gulf Stream</a> in Nor'easters.</p>

<p>In all these regional large precipitation events moist air gathered from a great surface area flows into a smaller region.  As the air approaches, it is lifted by the low pressure and its associated features, condenses, and finally falls as rain (or snow).  This process is often termed "moisture convergence".  The Storm Prediction Center also <a href="http://www.spc.noaa.gov/exper/mesoanalysis/" rel="noreferrer">offers plots</a> of localized deep moisture convergence [choose a region, then look under the upper air menu].  The convergence contours, shown in red, really show the piling up of humidity that is causing the heavy rainfall in Harvey:  </p>

<p><a href="https://i.sstatic.net/jA6uk.jpg" rel="noreferrer"><img src="https://i.sstatic.net/jA6uk.jpg" alt="Harvey Convergence"></a></p>

<p>But perhaps to visualize the scale involved in creating a catastrophic largescale flood such as Harvey, this plot, created from a base image from <a href="http://www.pivotalweather.com" rel="noreferrer">pivotalweather.com</a>, best shows the conditions around the storm (from the GFS model):</p>

<p><a href="https://i.sstatic.net/7Ykb7.png" rel="noreferrer"><img src="https://i.sstatic.net/7Ykb7.png" alt="harvey diagram"></a></p>

<p>Basically the atmosphere of the entire Gulf (and beyond) is being pumped into the southeast Texas area.  So although the air can only hold a couple inches (some 50 mm) of water, and evaporation rates are typically only a fraction of an inch (several mm) per day... bringing that together from such a large source region, and focusing it down into one small area... can lead to these awful extreme deluges. </p>

<p><hr>
Addendum: It should also be highlighted that the NHC adds in their <a href="https://www.nhc.noaa.gov/data/tcr/AL092017_Harvey.pdf" rel="noreferrer">report on Harvey</a> that rising motion was also enhanced by a front which had stalled in the area.  Air being advected in by Harvey's flow would naturally rise over that layer of cooler air when moving inland (a process called isentropic lift), which proves particularly efficient in condensing out the (abundant) moisture en masse into rainfall.  Most substantial regional floods require similar existence of a significant broad lifting mechanisms overlaid with such a relentless inflow of warm, moist air.</p>

CC BY-SA 3.0
2017-08-28T14:01:41.243Z
40in in 24hrs is the US record - set in Alvin TX (just south of Houston) back in the 1970s. Another tropical cyclone. Don't know the details, but presume it was an Allison/Harvey type scenario. I lived through Allison (my last day living in Houston was the day it finally came ashore!) and I'm surprised that didn't break the Alvin record.
winwaed
CC BY-SA 3.0
2017-08-28T20:29:58.083Z
@winwaed:  It was [Tropical Storm Claudette](https://en.wikipedia.org/wiki/Tropical_Storm_Claudette_(1979)) that dumped 43 inches (110 cm) on Alvin, TX in 24 hours.  As with Allison & Harvey, it came barely on shore and then stalled.
Michael Seifert
CC BY-SA 3.0
2017-08-29T07:11:54.583Z
"...evaporation rates are only a fraction of an inch (several mm) per day... bringing that together over such a large source area, and focusing it down into one small region...." The very last paragraph seems to capture the essence of the answer. I wish it would come earlier in the answer, ideally at the beginning. Otherwise it is a very nice answer.
NoDataDumpNoContribution
CC BY-SA 3.0
2017-08-30T19:58:41.580Z
Some news sources talk about a threshold temperature that needs to be surpassed for this kind of continuous evaporation to happen. 'When Harvey moved toward Texas, water in the Gulf of Mexico was nearly 2 degrees (1 degree Celsius) warmer than normal, said Weather Underground meteorology director Jeff Masters. Hurricanes need at least 79 degrees F (26 C) as fuel, and water at least that warm ran more than 300 feet (100 meters) deep in the Gulf, according to University of Miami hurricane researcher Brian McNoldy.' http://www.latimes.com/sns-bc-us-sci--harvey-climate-change-20170828-story.html
Zds
CC BY-SA 3.0
2017-08-30T20:02:37.133Z
BTW, I believe this mechanism is closely related to how rainforests work. They collect moisture from larger area due to repeating and systematic wind patterns. The result is rainforest in equator and desert in tropics of cancer and capricorn.
Zds
CC BY-SA 3.0
2017-08-31T02:32:25.877Z
@gansub, always questions of interest.  I'd suggest perhaps not a ton, because most rainfall was already developing earlier in trajectory, nearer the coastline.  Of course, similar to how the Trenberth paper noted that while inner core evaporation isn't important in actual moisture levels, but is important in driving the inflow in the first place, perhaps the enhanced evaporation helped sustain the storm core structure better while over land??
JeopardyTempest
CC BY-SA 3.0
2017-08-31T02:45:28.147Z
@Zds, we generally do list 79-81 degrees as one of the necessary conditions for TC maintenance (though indeed there are certainly interesting cases to overcome that value by other dynamics).  It's not that evaporation is suddenly continuous at that level, it's that convection is strong enough to sustain (see http://www.aoml.noaa.gov/hrd/tcfaq/A16.html for a good little discussion from NOAA).  And indeed, depth of warm is something I believe research has shown is increasingly important too.
JeopardyTempest
CC BY-SA 3.0
2017-08-31T02:48:15.123Z
Absolutely, though, it's similar to the continuing patterns that drive rain forests (and indeed monsoons too).  It's just indeed that the large scale patterns well support those continuing wind regimes, whereas a strong SE flow in Texas in summertime conditions really only occurs under TCs, and those rarely stall in just such a productive positioning as this unfortunately did :-/
JeopardyTempest
CC BY-SA 3.0
2017-09-02T18:13:05.803Z
Excellent answer, although a TL-DR section might be warranted
Timpanus
CC BY-SA 3.0
2018-03-20T16:54:08.473Z
@JeopardyTempest this answer is amazing! Definitely +1 from me! _(also congrats on nearly 120 votes (; )_
Eevee
CC BY-SA 4.0
2017-08-27T17:28:06.247Z
2018-05-26T17:02:12.280Z
2018-05-26T17:02:12.280Z
JeopardyTempest
<p><a href="http://www.foxnews.com/us/2017/08/27/harvey-causes-catastrophic-flooding-in-houston-shuts-down-airport.html" rel="noreferrer">Hurricane Harvey dumped more that 20 inches (500 mm) of rain</a> over a large region, with 40+ (>1000 mm) in some spots... and much more expected.</p>

<p>How is that possible?</p>

<p>Does the atmosphere really hold that much water?</p>

<p>Or is it getting repeatedly evaporated from the ocean and dropped onto the land by the circular winds, implying an enormous evaporation rate while over water?</p>

CC BY-SA 3.0
2017-08-31T06:40:58.093Z
[9 trillion gallons](https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/08/27/texas-flood-disaster-harvey-has-unloaded-9-trillion-tons-of-water/?utm_term=.b51174582d85)
Jan Doggen
CC BY-SA 4.0
2020-10-11T03:44:14.187Z
It was a bit more than a meter. Nederland, Texas received 60.58 inches of rain (over 1.5 meters) from Harvey. I would have responded to this question much earlier, but I just ran across it, and my power was out thanks to Harvey when this question was posted. And then I was busy for months recovering from the flooding caused by Harvey.
David Hammen
<rainfall><tropical-cyclone>
How can a storm drop 40 inches (1 metre) of rain?
William Jockusch
# How can a storm drop 40 inches (1 metre) of rain?

(Asked by: username_0 on 2017-08-27)

[Hurricane Harvey dumped more that 20 inches (500 mm) of rain](http://www.foxnews.com/us/2017/08/27/harvey-causes-catastrophic-flooding-in-houston-shuts-down-airport.html) over a large region, with 40+ (>1000 mm) in some spots... and much more expected.

How is that possible?

Does the atmosphere really hold that much water?

Or is it getting repeatedly evaporated from the ocean and dropped onto the land by the circular winds, implying an enormous evaporation rate while over water?

**Tags:** rainfall, tropical-cyclone

> **Comments:**
> **username_1**: [9 trillion gallons](https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/08/27/texas-flood-disaster-harvey-has-unloaded-9-trillion-tons-of-water/?utm_term=.b51174582d85)  
> **username_2**: It was a bit more than a meter. Nederland, Texas received 60.58 inches of rain (over 1.5 meters) from Harvey. I would have responded to this question much earlier, but I just ran across it, and my power was out thanks to Harvey when this question was posted. And then I was busy for months recovering from the flooding caused by Harvey.  

---

## 6 answers

### Answer by username_9 on 2017-08-27. Score: 135 | Accepted answer

You were right to question whether the atmosphere really held that much water. It comes nowhere close! We use [precipitable water](https://en.wikipedia.org/wiki/Precipitable_water) to track this, which is the measure of all moisture in the entire column of air in the troposphere. We can get good widespread estimates from remote observations. [This animation](http://www.ospo.noaa.gov/Products/bTPW/TPW_Animation.html?product=CONUS_TPW) shows current amounts of precipitable water levels across the US from satellite. Here is a still from this afternoon (Sunday, August 27) during the Harvey event:

[![PWat082717](https://i.sstatic.net/fhH55.png)](https://i.sstatic.net/fhH55.png)

75 mm is less than 3 inches.

We also get [in-situ](https://www.britannica.com/topic/in-situ-measurement) exact measurements worldwide from twice daily radiosonde balloon launches. Unfortunately none are located right near the locations receiving excess rainfall. But you can check US sites of current precipitable water measurements any time by going to [SPC's sounding page](http://www.spc.noaa.gov/exper/soundings/) and looking for the PW value in the left side of the bottom table. [SPC also maintains a climatology page for precipitable water and other values](http://www.spc.noaa.gov/exper/soundingclimo/). You can see there that 3 inches (76 mm) of moisture in the sounding is extremely rare, and no US site has ever had 4 inches (102 mm) of precipitable water.

However, the answer is also not truly found in evaporation rates. The conditions in strong hurricanes do greatly enhance evaporation rates due to the high wind speeds and warmer waters. Measurements are actually a bit difficult to come by in such extreme conditions, with challenges in isolating evaporation effects from spray as well as in getting the instruments positioned into such environments (new field campaign: who wants to take the research ship out into the category 5 hurricane!?!). As [this 2007 study by Trenberth et al.](http://onlinelibrary.wiley.com/doi/10.1029/2006JD008303/full) noted:

> We are unaware of reliable estimates of evaporation in hurricanes, and published measurements do not exist in winds above about 20 m s−1 although some progress has been made in the Coupled Boundary Layer Air-Sea Transfer Experiment (CBLAST)

However, in that study model analyses suggested that evaporation rates in the core of hurricanes are likely no more than 1-2 inches (25–50 mm) per day.

**How is that possible?**

It's quite important to notice that heavy rainfall - well over 5 inches (125 mm) - can actually fall in as little as a couple hours almost anywhere, such as in [this 2015 flood event in Nebraska](https://www.weather.gov/oax/event_archive_20150507). How can that be?

The secret lies in the nature of even the weakest storm: even as a storm is just beginning to form, it begins to draw in air from surrounding areas. This is NOAA's diagram of a typical developing thunderstorm cell:

[![developing](https://i.sstatic.net/06I0j.jpg)](https://i.sstatic.net/06I0j.jpg)

You can see the curve at the arrows near the bottom, indicating inflow of surrounding air into the storm. This inflow turns a thunderstorm into more of an engine, processing a continuing stream of incoming air, removing its moisture. In a single cell thunderstorms in an environment without background winds, the "waste" air will eventually pile up and choke off the influx. But even in such circumstances, a few inches of rain may fall. That isn't by using up all of the moisture from the cloud's environment, but instead by using just a portion of the moisture from the reservoir in and around the cloud.

If some upper level winds exist to help exhaust the "spent" air, storm systems can persist for even longer periods of time. For example, the Midwestern United States commonly sees long-duration late summer heavy rainfall initiated along stationary boundaries in which inflow lacks direct access to significant warm water bodies.

Larger systems that dump huge amount of precipitation over greater areas must pull in a more consistent, stronger inflow of warm, moist air from greater distance. Examples of this happening include the [Pineapple Express](https://www.accuweather.com/en/weather-news/what-is-a-pineapple-express/38881189) for rainfall in California/the southwest US, the [low-level jet](http://www.spc.noaa.gov/publications/evans/kumjian.pdf) for spring and summer squall lines in the Plains, [onshore winds during](http://www.meteo.co.in/monsoon1.cfm) the Indian monsoon, and air from off the [Gulf Stream](https://www.e-education.psu.edu/earth107/node/1055) in Nor'easters.

In all these regional large precipitation events moist air gathered from a great surface area flows into a smaller region. As the air approaches, it is lifted by the low pressure and its associated features, condenses, and finally falls as rain (or snow). This process is often termed "moisture convergence". The Storm Prediction Center also [offers plots](http://www.spc.noaa.gov/exper/mesoanalysis/) of localized deep moisture convergence [choose a region, then look under the upper air menu]. The convergence contours, shown in red, really show the piling up of humidity that is causing the heavy rainfall in Harvey:

[![Harvey Convergence](https://i.sstatic.net/jA6uk.jpg)](https://i.sstatic.net/jA6uk.jpg)

But perhaps to visualize the scale involved in creating a catastrophic largescale flood such as Harvey, this plot, created from a base image from [pivotalweather.com](http://www.pivotalweather.com), best shows the conditions around the storm (from the GFS model):

[![harvey diagram](https://i.sstatic.net/7Ykb7.png)](https://i.sstatic.net/7Ykb7.png)

Basically the atmosphere of the entire Gulf (and beyond) is being pumped into the southeast Texas area. So although the air can only hold a couple inches (some 50 mm) of water, and evaporation rates are typically only a fraction of an inch (several mm) per day... bringing that together from such a large source region, and focusing it down into one small area... can lead to these awful extreme deluges.

---

Addendum: It should also be highlighted that the NHC adds in their [report on Harvey](https://www.nhc.noaa.gov/data/tcr/AL092017_Harvey.pdf) that rising motion was also enhanced by a front which had stalled in the area. Air being advected in by Harvey's flow would naturally rise over that layer of cooler air when moving inland (a process called isentropic lift), which proves particularly efficient in condensing out the (abundant) moisture en masse into rainfall. Most substantial regional floods require similar existence of a significant broad lifting mechanisms overlaid with such a relentless inflow of warm, moist air.

> **Comments:**
> **username_10**: 40in in 24hrs is the US record - set in Alvin TX (just south of Houston) back in the 1970s. Another tropical cyclone. Don't know the details, but presume it was an Allison/Harvey type scenario. I lived through Allison (my last day living in Houston was the day it finally came ashore!) and I'm surprised that didn't break the Alvin record.  
> **username_11**: @username_10:  It was [Tropical Storm Claudette](https://en.wikipedia.org/wiki/Tropical_Storm_Claudette_(1979)) that dumped 43 inches (110 cm) on Alvin, TX in 24 hours.  As with Allison & Harvey, it came barely on shore and then stalled.  
> **username_12**: "...evaporation rates are only a fraction of an inch (several mm) per day... bringing that together over such a large source area, and focusing it down into one small region...." The very last paragraph seems to capture the essence of the answer. I wish it would come earlier in the answer, ideally at the beginning. Otherwise it is a very nice answer.  
> **username_13**: Some news sources talk about a threshold temperature that needs to be surpassed for this kind of continuous evaporation to happen. 'When Harvey moved toward Texas, water in the Gulf of Mexico was nearly 2 degrees (1 degree Celsius) warmer than normal, said Weather Underground meteorology director Jeff Masters. Hurricanes need at least 79 degrees F (26 C) as fuel, and water at least that warm ran more than 300 feet (100 meters) deep in the Gulf, according to University of Miami hurricane researcher Brian McNoldy.' http://www.latimes.com/sns-bc-us-sci--harvey-climate-change-20170828-story.html  
> **username_13**: BTW, I believe this mechanism is closely related to how rainforests work. They collect moisture from larger area due to repeating and systematic wind patterns. The result is rainforest in equator and desert in tropics of cancer and capricorn.  
> **username_9**: @gansub, always questions of interest.  I'd suggest perhaps not a ton, because most rainfall was already developing earlier in trajectory, nearer the coastline.  Of course, similar to how the Trenberth paper noted that while inner core evaporation isn't important in actual moisture levels, but is important in driving the inflow in the first place, perhaps the enhanced evaporation helped sustain the storm core structure better while over land??  
> **username_9**: @username_13, we generally do list 79-81 degrees as one of the necessary conditions for TC maintenance (though indeed there are certainly interesting cases to overcome that value by other dynamics).  It's not that evaporation is suddenly continuous at that level, it's that convection is strong enough to sustain (see http://www.aoml.noaa.gov/hrd/tcfaq/A16.html for a good little discussion from NOAA).  And indeed, depth of warm is something I believe research has shown is increasingly important too.  
> **username_9**: Absolutely, though, it's similar to the continuing patterns that drive rain forests (and indeed monsoons too).  It's just indeed that the large scale patterns well support those continuing wind regimes, whereas a strong SE flow in Texas in summertime conditions really only occurs under TCs, and those rarely stall in just such a productive positioning as this unfortunately did :-/  
> **username_14**: Excellent answer, although a TL-DR section might be warranted  
> **username_15**: @username_9 this answer is amazing! Definitely +1 from me! _(also congrats on nearly 120 votes (; )_  

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### Answer by username_3 on 2017-08-27. Score: 25

I invite correction if I missed any details, the column of air above you, at any one time, at 100% humidity can hold maybe 3-6 inches (75-150 mm) of water depending on temperature and it's likey to rain only a percentage of that, not all of it. If air remained stationary, which, of-course it doesn't, then we'd get much less downpours.

The hurricane is continuously spiraling in new wet air with water picked up from the gulf. It might rain 2-4 inches (50–100 mm) per hour, which over 10 or more hours can give you 30-40 inches (750-1000 mm). As it rains, the air loses moisture, so the area that gets 30-40 inches (750-1000 mm) smaller in size than the Hurricane. It's the specific area, land just past the ocean or gulf, to the East or North-East of the eye where the saturated bands of rain come in from over the ocean or Gulf that gets hit the hardest.

This chart isn't updated, it's a forecast from a day ago, but it gives an idea of the localized high rain fall amounts. The region that gets the 20 plus inches (500+ mm) of rain is smaller than the Hurricane.

[![enter image description here](https://i.sstatic.net/lhSXim.png)](https://i.sstatic.net/lhSXi.png)

The wind spirals around the eye much faster than the Hurricane itself moves, and it gets saturated to 100% humidity and in a sense, over 100% humidity as it can carry water droplets, as it spirals over the warm waters of the Gulf of Mexico. When it hit's land, the rain continues but the humidity begins to drop, so the rain falls hardest, shortly after landfall and where the rain bands come from over the ocean, Usually, in the East Coast of the US, this is East or Northeast of the eye. (The dirty side, as blacksmith calls it).

[![enter image description here](https://i.sstatic.net/vhFgE.jpg)](https://i.sstatic.net/vhFgE.jpg)

Air can't hold 40 inches (1000 mm) of rain. No where close, but air blowing over a body of water can supply steady rainfall because the air blowing over the gulf is continuously picking up water. It's new, 100% humidity/saturated and super-saturated air that hits land continuously, raining hard, until the storm passes.


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### Answer by username_4 on 2017-08-27. Score: 13

Harvey is almost stationary and is rotating new gulf moisture up the "dirty" side (east of the center). I am outside the main path and have 10+ inches (250 mm) in 30 hours (emptied the rain gauge twice). It has been reported that affected areas have exceeded the "500 year" rainfall level. I guess they need a 1000 year level. The atmosphere is pretty big and does hold that much water.

> **Comments:**
> **username_5**: +1 for brevity and capturing the crux of the OP question.  
> **username_2**: [Houston has been subject to 500 year floods in 2015, 2016, and now with Harvey, in 2017.](https://www.washingtonpost.com/news/wonk/wp/2017/08/29/houston-is-experiencing-its-third-500-year-flood-in-3-years-how-is-that-possible/?utm_term=.43bf41f064b5)  
> **username_4**: I have lived in the Houston area 30 years ; Nothing remotely like Harvey before , no matter what the media says.  

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### Answer by username_5 on 2017-08-29. Score: 6

The atmosphere can "hold" only maximum 75mm worth of precipitation at any given locality, this is true.

However, this particular hurricane Harvey was a very slowly moving structure. The vortex would carry the warm water-saturated masses from Gulf of Mexico, which essentially collided with colder air from the North, generating rainfall. It is the continuous supply of moisture from warmer saturated area that made the rain nearly continuous at some particular locations.

If the hurricane structure would sit over one area and not dissipate over time, then the evaporation from the Gulf could sustain the moisture level in the air, and the amount of total rainfall (in limited area) could be even higher. The source evaporation rate doesn't need to be unusual, because a relatively large area of gulf/ocean feeds moist air to a relatively much smaller area of rainfall under this particular type of air circulation pattern.

> **Comments:**
> **username_5**: @gansub, it doesn't ADD, it summarizes the essence of the effect. This is not a scientific conference to flash numbers. The OP is perfectly aware of moisture holding capacity of raw atmosphere, otherwise he wouldn't ask this question in first place. Apparently the OP doesn't watch TV or Weather Channel, to see the stalled rotating pattern of this hurricane, otherwise he wouldn't ask this question at all. And my answer doesn't have any "claims", it just offers a short and obvious explanation of this abnormal rainfall, without a lecture about Thermodynamics.  

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### Answer by username_6 on 2017-08-29. Score: 5

The precipitable water vapor content of the air never exceeds about 3 inches even in the most humid tropical maritime air masses, and in most maritime tropical air masses is typically closer to 2 inches. But Harvey is not limited to the water vapor content in its own region of the atmosphere. Its counterclockwise INspiraling winds (note the emphasis on IN) continually bring in new water vapor being evaporated from the Gulf of Mexico. The counterclockwise INspiraling winds concentrate the water vapor supply from a region much larger than Harvey itself into Harvey's central region. Thus these counterclockwise INspiraling winds act as a funnel, concentrating what would have been a 2 or 3 inch rain over a large area to a 50 inch rain over a much smaller area.


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### Answer by username_7 on 2017-09-02. Score: 3

Harvey has been using a [rainband](https://en.wikipedia.org/wiki/Rainband) to get water in from the Gulf of Mexico.

Think of the rainband as a pipe, it is piping water from the gulf to the core, and this pipe has a leaking problem, so the water gets dumped on Austin on it's path to the center.

> **Comments:**
> **username_8**: Does the term [rainband](https://en.wikipedia.org/wiki/Rainband) really refer to a band that is collecting moisture at one end and producing precipitation at the other end, suitable for a "pipeline" analogy?  
> **username_7**: @username_8 no the rainband is feeding moisture from one end and moving it to the core  

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