Flood: The Journey Of A Raindrop — Multiplied A Few Quadrillion Times


Home / Flood: The Journey Of A Raindrop — Multiplied A Few Quadrillion Times
By Sunday, the plume of moisture that had parked over eastern Texas had moved east. Water vapor analysis courtesy of NOAA.

By Sunday, the plume of moisture that had parked over eastern Texas had moved east and the flooded areas could begin to dry out. Water vapor analysis courtesy of NOAA.

The recent floods in Texas represent a very modest amount of water by the standards of the total amount of rain that falls over the entire earth — or the United States.

Even in Texas, where the May rainfall was about twice the long-term average, that amount is easily absorbed by the porous soil, lakes, reservoirs and drainage systems.

But the distribution and local intensity of the rainfall caused some loss of life and massive destruction of property. Let’s find out why.

The Scale Of Precipitation

One of the measures of ‘average’ is called the mode: The thing that happens most often. Over the entire globe, except in a few isolated places (none of them in Texas), the daily mode of precipitation is — ZERO. On most days it doesn’t rain. On the other hand, sometimes when it rains, it pours. When it pours for a long enough time in the same place, the water may have to go where it normally doesn’t.

Measuring Precipitation

Precipitation is formally recorded in inches of liquid water. This isn’t a very accurate term, scientifically, because an inch is a linear measure and liquid should be measured as a volume. An inch of rain is defined as water covering a given area of surface to a depth of an inch. When meteorologists measure rainfall, they just collect it in a little jar called a rain gauge and measure its depth.

Yearly precipitation totals in major cities vary from less than ten inches in Phoenix to over 60 inches in Miami. An average month in an average place in the United States receives about 3 inches of rain or melted snow per month. In an average month of May, some places in southeast Texas get about five inches of rain. This year, more than three times the average amount fell in the metropolitan areas of Dallas and Houston.

How Many Raindrops?

Let’s calculate the number of raindrops that fell on Texas in May. Before you read on, try to guess the answer.

A back of the envelope calculation yields the following result: one square mile = four billion square inches. The state of Texas comprises about a quarter of a million square miles, which gives us a nice round number for the area of Texas as one quadrillion square inches.

The volume of a large raindrop is about one-one hundredth of a cubic inch. Texas is a big state, so we will assume only big raindrops fell there in May. That would mean that to cover the state of Texas with one inch of rain would require 100 quadrillion raindrops. Since an average of 8 inches fell over the state in May this year, that means 800 quadrillion raindrops fell on Texas in May. Might as well round it to an even quintillion. Since this was about twice the normal amount, Texas normally gets about 400 quadrillion raindrops in May.

Author’s note: This calculation depends of course on the size of the envelope and the number of martinis consumed. Trust me; it’s pretty close.

The purpose of this exercise was to show that a lot of raindrops fall on the surface of the earth. Citizens and governments have organized in such a way as to avoid having houses and roadways constantly inundated. We build houses on high ground and provide drainage systems for roads and city parks.

This flood scene is from 2009 near Atlanta. Photo courtesy of NOAA.

This flood scene is from 2009 near Atlanta. Photo courtesy of NOAA.

But once in a while too many raindrops try to crowd into the space allotted for them, and they pile up or spill over into yards, underpasses, and other urban, suburban, and even rural areas where they are not welcome.

A Raindrop’s Journey

Let’s follow the path of one of those quintillion-or-so raindrops — let’s call him Tex — into the flood. At first, there was a flow of air northwestward from the Gulf of Mexico. As this southeast wind persisted, molecules of water continually evaporated into the air.

This can be considered the start of the ‘hydrologic cycle.’ Eventually, a ‘plume’ of moisture covered eastern Texas and adjacent areas.

A stagnant weather pattern above the plume, featuring modest divergence of air flows at upper levels, induced a gentle rising of air in the lower layers. As the air rose, it cooled, and since cooler air cannot hold as much water vapor as warmer air, some of the water vapor molecules condensed into water droplets, first small ones, then, as they bumped into each other and stuck together, bigger ones — some Texas-sized.

Tex was born. By the time Tex had been carried on the southeast winds to northeast Texas, the gentle lifting was not strong enough to keep Tex aloft, so he fell to the ground, as did many of his friends. No big deal; it rains often, and the drops either sink into the ground or roll to lower basins of water: Rivers, lakes, and reservoirs.

Normally weather moves from west to east. But the weather pattern that produced Tex, and which Decoded Science named Government for its sluggish movement, stayed nearly stationary for the better part of May. Tex was joined by millions of his friends, and rainfall that could be accommodated by existing lakes, reservoirs and drainage systems was followed by more rainfall. Eventually, there were enough drops to  overwhelm parts of the system.

Tex’s Journey Is Not Over

Tex is in the part of the hydrologic cycle on land, headed for the sea. Graphic courtesy of NOAA.

Tex is in the part of the hydrologic cycle on land, headed for the sea. Graphic courtesy of NOAA.

Tex happened to fall in a rural part of northeast Texas. The ground where he fell was already saturated by previous rain. So he took the easiest path, which landed him in a little stream. He washed down the stream into larger streams and eventually into the Trinity River, which runs through Dallas and Fort Worth.

Even though the rain has stopped, Tex and many of his friends are still being joined by other drops that meandered into the Trinity. So even though most urban flooding is subsiding, the river will be above flood stage for several more days.

Eventually Tex will ride down the river into Trinity Bay, near Houston, and then into the Gulf of Mexico. Someday, the individual molecules that comprise Tex will be torn from the surface of the ocean, and the hydrologic cycle will begin again.

A One-Off? Or Evidence Of Climate Change?

There is no accepted metric for weather stagnation, but most meteorologists agree that the movement of weather systems breaks down more often these days. Witness the snowy winter in Boston and now the floods in Texas.

Many models of the atmosphere that incorporate global warming suggest that weather patterns like Government will become more common, and we can expect more extremes of heat, cold, rain, and snow.

Leave a Comment