Tag Archive for: temperature

Is Precipitation Increasing with Temperature?

Data provided by the National Weather Service (NWS) shows that precipitation appears to be increasing with temperature in Houston and Harris County. A reader recently asked whether there was a correlation. The hypothesis: in this climate, if temperature increases, then so will evaporation and rainfall. Eighty to 130 years worth of data at different locations show both variables trending up. But most scientists would consider the coefficient of correlation weak to non-existent.

Behind the Theory

The theory is plausible from several perspectives.

  • Warm air holds more moisture than cool air. Warm air also rises. As it cools at higher altitudes, precipitation forms. Think “afternoon thunderstorms on hot summer days.”
  • Precipitation also forms when warm and cool fronts collide.
  • It often forms when warm moisture-laden air streams in from the Gulf.
  • Hurricanes form in the hottest parts of the year.
Distribution of hurricanes by month during the last 100 years.

But the question concerned correlation, not causation.

Other outside factors could reduce precipitation, such as droughts triggered by changes in Pacific Ocean currents. Those who remember the drought from 2011 to 2014 may also remember how hot it was.

But looking at 80 to 130 years of data highlights long-term climate trends. That “evens out” the influence of individual events.


NWS plotted all available historical data for precipitation and temperature on line graphs and then superimposed trend lines. The graphs show official data from two sources: Houston-Hobby Airport and the “City of Houston.”

I put City of Houston in quotes because the the official City-of-Houston data is currently collected at Bush Intercontinental Airport. But the location has bounced around. So the “City” isn’t one location, but many:

  • Cotton Station (July 1881 – September 1909)
  • Stewart Building at Preston and Fannin (September 1909 – February 1926)
  • Shell Building at Texas and Fannin (March 1926 – August 1938)
  • Federal Building at Franklin and Fannin (August 1938 – May 1969)
  • Intercontinental Airport (June 1969 – Present)

We have less data for Houston-Hobby because Hobby Airport didn’t exist until 1927. That’s when someone turned a 600-acre pasture into a landing field. The City of Houston purchased the field in 1937 and expanded it.

With those qualifiers, see the charts below. Both temperature and rainfall vary from year to year. But rainfall shows extreme variance. Regardless, in all four graphs the trend lines slowly increase.

Houston-Hobby Airport

Mean temps at Hobby increased from 69 to 73 degrees – a 4 degree increase between 1931 and 2022.
During roughly the same time period, precipitation increased approximately 9 inches from about 48 to 57 inches. Also notice the extreme range – from less than 30 to more than 80 inches.

City of Houston Data

The City of Houston data covers a wider time period. Within that, the location varied as noted above. The big jump was from downtown to Bush Intercontinental Airport in 1969. Generally speaking, as you go farther north from the coast, precipitation decreases. But the difference is less than an inch. Atlas 14 shows that a 100-year, 24-hour storm is 17.6 inches at Hobby, 17 inches downtown, and 16.9 inches at Intercontinental.

City data indicates mean temp has increased roughly 4 degrees in last 120 years.
During roughly the same years, precipitation increased about 5-6 inches. Here, the range was even more extreme. From less than 20 inches to 80 – a 4X difference.

So the change in where the City collects official data actually worked against the hypothesis. And it shows.

Summary of Trend Differences

Summarizing the key differences:

  • As the temp trend line increased 4 degrees at Hobby, precipitation increased 9 inches.
  • As temp increased 4 degrees at various City locations, precipitation increased 6 inches.

Low Coefficient of Correlation

Jimmy Fowler of the National Weather Service’s Houston/Galveston office calculated the coefficients of correlation between the two series of data at each location.

For Hobby, the coefficient of correlation is only .03. The City’s is slightly higher at .11.

Jimmy Fowler, Meteorologist, National Weather Service

For those who didn’t study statistics in college, the coefficient of correlation tells you how much one variable changes in response to another.

  • A perfect positive correlation is 1.0. Example: population growth and food consumption.
  • A perfect negative correlation is -1. Example: hours worked and free time.

In both cases, one unit of change in the first variable accounts for an equal unit of change in the second. But most correlations fall between the two extremes with different degrees of strength.

The chart below indicates how scientists would characterize correlation coefficients of .03 and .11. Both are considered “very weak” or having “no association.”

correlation strength
By Wayne W. LaMorte, MD, PhD, MPH, from the Boston University School of Public Health website.

Fit of Trend Lines to Data

So, if the trends are all up, why is the co-efficient of correlation so low? Part of the answer has to do with those R2 (R squared) values you see at the bottom of the charts. They show the data doesn’t conform to the trend lines very well. Temperature fits moderately well. But precipitation shows extreme variance.

A perfect fit (1.0) would show all the data points on the line. As a rule of thumb, 0.8 (80%) or higher is considered a good fit. But the R2 values in these trend lines range from 0.03 to 0.5.


You can read into this data whatever you want depending on your point of view. Climate change advocates might see proof in the consistent slope of the trend lines that warming temperatures and more precipitation are related. A deeper dig into the data reveals the correlation is weak at best and possibly non-existent. Other factors may be at play and influencing the data.

To demonstrate causation, you need to show a directional relationship with no alternative explanations. But with weather, you have a multitude of alternative explanations.

Remember that weather is global and that we looked only at Houston in this instance.

However, a friend who traded weather-related derivatives before retirement tracked hundreds of temperature sites. He found they all trended warmer over time. But he believed the variance resulted primarily from changes in surrounding ground cover, i.e., replacement of natural ground cover with concrete – also known as the urban heat island effect.

He also tracked variance to changes in measurement locations (as with Houston).

Finally, remember that some of the hottest and coldest places on earth get very little precipitation. The Sahara and the North and South Poles are all considered deserts based of the amount of precipitation they get.

Net: I find the similarities in the graphs interesting enough to keep digging. As my friend suggested, it would be interesting to find the coefficient of correlation between population growth and temperature change. I won’t leap to any generalizations at this point.

Posted by Bob Rehak on 12/30/2022 with thanks to Jeff Lindner, Harris County Meteorologist and Jimmy Fowler of the National Weather Service.

1949 Days since Hurricane Harvey

NOAA Adopts New Normal: Hotter, Wetter

NOAA has updated its climate statistics for the U.S. Every 10 years, the agency comes up with a “new normal” based on the last 30 years of data. The most recent 30 years, compared to previous averages, shows that our climate is getting hotter and wetter (at least in the Houston area).

Screen capture showing new “normal” temperature and precipitation for Houston Intercontinental Airport. Blue line represents precipitation. Orange = ave. minimum temp. Red = average temp. And dark red = average max temp.
Same data in tabular format.

The “normals” help farmers, energy companies, water managers, transportation schedulers and others whose businesses depend on weather plan their activities. That includes your local TV weather casters who constantly compare what they predict for tomorrow with what has happened in the past.

What’s Normal – From 30 years Down to The Hour

The NOAA stats come in annual, seasonal, monthly, daily and even hourly tables. Because the normals have been produced since the 1930s, they also help put current weather in a historical context.

The New York Times produced a series of animated “heat” maps that show changes in temperature and precipitation for those 30-year windows from the 1930s to today. Heat maps in this sense do not refer to temperature but to colors that reflect temperature or precipition differences. Hotter colors like red and orange reflect increases. Cooler colors like green and blue reflect decreases.

Choose Your Start/Stop Points Carefully

Curiously, the animations show the U.S. getting both hotter and colder through the decades. Likewise with wetter and drier. You can clearly see alternating cycles of hot and cold, wet and dry. As cycles come and go, where you chose your start and stop points lets you support or disprove your favorite climate change hypothesis.

The change is especially drastic between the new normals and the previous ones, from 2010. “Almost every place in the U.S. has warmed,” Dr. Michael Palecki told the Times. He manages the project at NOAA’s National Centers for Environmental Information.

Palecki claims the world has warmed by more than 1 degree Celsius (about 1.8 degrees Fahrenheit) since 1900, and that the pace of warming has accelerated in recent decades.

However, if you compared 1921-50 with 1941-70, you might think the world was cooling. The same goes for large parts of the county with dry/wet cycles. Although the Houston region has experienced increasing wetness on a fairly consistent basis, you can see drought ebb and flow through other parts of the country.

If you use 1900 as your start point and today as your stopping point, Palecki says the world has warmed by more than 1 degree Celsius (about 1.8 degrees Fahrenheit) and that the pace of warming has accelerated in recent decades.  The precipitation maps show the Southwest becoming increasingly drier, while the Central and Eastern parts of the country are getting wetter.

These two “heat” maps show the change in average annual precipitation and temperature during the previous 30-year reporting period and today’s.

Says NOAA, “Most of the U.S. was warmer, and the eastern two-thirds of the contiguous U.S. was wetter, from 1991–2020 than the previous normal period, 1981–2010. The Southwest was considerably drier on an annual basis, while the central northern U.S. has cooled somewhat.”

For More Information

Check out the fascinating NY Times article or go straight to NOAA for far more detailed information.

Like all NOAA statistics, they are publicly available. You can even customize your own data searches based on time and location.

Posted by Bob Rehak on 5/13/2021 based on information from The NY Times and NOAA

1353 Days since Hurricane Harvey