(Bloomberg) -- Eleven months ago, Houston had a deadly flood.This week, the city had another.

Events like these are often called “ 100-year floods,” and thatcan be misleading. The U.S. government began using the term in the1960s to describe a flood that has a 1% chance of happening in anygiven year, not a chance of happening only once a century. It’sstatistical probability — and that can change over time.

“Over the span of 30 years, which is the length of many people’smortgages, there is a once in four chance it is going to happen,”said Mari Tye, a project scientist in the mesoscale and microscalemeteorological laboratory at the National Center forAtmospheric Research in Boulder, Colorado. “Over 100 years,there is a 67% chance.”

And that doesn’t take into account other conditions that canalter the outlook, including changing climate or theeffects of El Nino or La Nina.

‘Moving target’

“That’s why the 100-year event is such a moving target, especiallyin an urban environment,” said Chuck Watson, director of researchand development at Enki Research, which develops tools to measurehazards. “Someone builds a couple of parking lots, and you justturned a 100-year event into a 70-year event because of theimpervious surfaces.”

Asphalt doesn’t soak up rain water; it just sends it somewhereelse, such as into the house next door. When you add in naturalclimate cycles, the results are further skewed, Watson said fromhis office in Savannah, Georgia. One of the influences of El Ninois to send more rain across the southern U.S.. In a situation likethat, the chances of a catastrophicflood might rise to one in 20.

At least seven people died in Houston in this week’s rain,according to CNN. The bulk of the downpour was Monday, when a dailyrecord 9.92 inches (25 centimeters) fell at George BushIntercontinental Airport, the National Weather Service said. Someareas received more. At least 100,000 customers lost power, thecity’s light rail was shut and water was over the banks of morethan half of its 22 bayous and creeks, which help with floodcontrol.

Even worse

“If you get that much rain, there is no place for the water to go,”said Jill Hasling, who founded the Weather Research Center in the Texas city.

The current flood is worse in some ways than the one last May,Hasling said in an interview at the American MeteorologicalSociety’s Hurricanes and Tropical Meteorology conference in SanJuan, Puerto Rico. Parts of the city that were dry a year ago areinundated now.

That flood damaged more than 2,500 homes and killed more than 30people in Texas and Oklahoma, according to reports at the time. Alittle more than 35% of the state had been suffering some level ofdrought, but by the first week of June the share had dropped toless than 1%, the U.S. Drought Monitor reported.

Related: Why don't property owners have floodinsurance?

This reveals another problem with trying to quantify extremeevents — things can always get worse, which makes it difficultto come up with a worst-case scenario.

“With all our records, we don’t know what the most extreme is,because they are rare,” Tye said. “You make an estimate of theprobability and then another storm comes along that is worse.”

As for Houston, Hasling has some advice: “There’s more than oneflood a year in Houston. If you live in Houston, buy flood insurance. If youare not in the flood zone, buy it anyway; it will just becheaper.”