Study Finds Rainfall Rates and Amounts Increasing as the Climate Warms

Have you noticed that when it rains it often rains a lot? It’s not your imagination. When a weather pattern supports rain, the probability that heavy rain, and in some cases buckets of excessive rain, to occur, has been shown to be increasing over time. And it’s not just happening here and throughout the Northeast, it’s also happening nationally and across the globe with both rainfall rates and amounts increasing per event leading to increasing instances of flooding and the associated damage, injuries, and loss of life that occur as a result.

Studies show climate change, or more specifically modern day (since the industrial revolution) human induced warming of the oceans and the atmosphere, is largely behind the observed increases in heavy rain events both locally and throughout the world as the water cycle is being supercharged by more available water vapor which leads to the heaviest rainfall events per year getting heavier. We know through the Clausius–Clapeyron relation that a warmer atmosphere creates room for more water vapor through evaporation with every 1 degree F of warming increasing the saturation level by about 4%. In other words, warmer atmospheres have room for more water vapor that can be released as heavier rainfall in cases when a weather pattern supports rain. This is the physics that’s being observed in the real atmosphere.

Since 1950, according to the Fifth National Climate Assessment rainfall data is showing that as the climate has warmed over the last six decades, the change in the total precipitation falling on the heaviest 1% of days has increased throughout the country with the largest increase, 60%, occurring throughout the Northeast. Heavier rainfall events are getting wetter over time. A Climate Central analysis of 144 cities across the country shows that 126 of them, or 88%, have experienced an increase in hourly rainfall rates since 1970 with those rainfall rates on average now 15% higher.

As rainfall rates have been increasing there has been a subsequent increase in the magnitude of flooding events globally. An extreme recent example that has strong climate change finger prints attached to it was the staggering 29” of rain that fell in twelve hours in Rossiglione in Genoa, Italy, on October 4, 2021, a new European rainfall record. It’s a stunningly huge rainfall total that was the result of torrential thunderstorms fueled by an unusually warm atmosphere loaded with high levels of Mediterranean moisture which was unleashed by an advancing low pressure system. The result was catastrophic flooding, loss of life, and widespread damage. Climate change did not cause the event, but likely acted as a threat multiplier making it much worse.

There are plenty of local examples as well with July 2021 being a notable period as a string of flooding events occurred which resulted in millions of dollars in damages. On July 14, 2021, 2”-4” of rain in as little as two to three hours fell across Rensselaer County resulting in major damage to roads and bridges. Fonda was hit on July 19, 2021, with a widespread damaging flooding event ten days later that impacted the southern Adirondacks, Mohawk valley, Glens Falls-Saratoga Region and southern Vermont on July 29, 2021. These were events caused by torrential thunderstorms, supercharged with moisture, in what has become an increasingly humid summer environment across the Northeast making events like this more likely and more frequent.

Synoptic systems (larger than thunderstorms) are also producing more rainfall and with higher rainfall rates. This occurred in August 2024 with the former Tropical Storm Debby coming through with a stream of tropical moisture that generated a short burst of particularly torrential rain late in the day which resulted in significant flash flooding throughout the Capital Region. And we’re observing the same trends in increased rainfall rates and amounts with atmospheric river events as well. Recent examples include huge rainfall amounts on Dec 17-18, 2023 and Dec 11, 2024.

Calculating Rainfall Rates — Climate Central’s Analysis

Climate Central calculated long-term trends (1970-2024) in the simple hourly rainfall intensity index for 144 U.S. weather stations, with the simple hourly rainfall intensity index defined as the total annual rainfall divided by the total hours of rainfall each year (inches per hour). An increase in the simple hourly rainfall intensity index shows an increase in hourly rainfall rates, in other words, more rain falling per hour. Click here for a look at NOAA’s Rain Rate Visualizer videos which illustrate what various hourly rainfall rates look like.

The results of the calculation revealed that 126 of the 144 locations had a 15% increase in hourly rainfall rates since 1970 with two thirds of the 144 sites showing an increase in rainfall rates of 10%. The rainfall intensity at Albany was calculated to be 22% higher now than in 1970.

As the oceans and the atmosphere continue to warm, the expectation is for precipitation extremes to increase globally. This is also expected to be true in regions with decreasing average precipitation, according to the latest reports from the Intergovernmental Panel on Climate Change.

With 2 degrees C (3.6 degrees F) of global warming, much of the U.S. is projected to see increases in precipitation extremes, with the majority (85% or 2,645) of the 3,111 total U.S. counties likely to experience a 10% or higher increase in precipitation falling on the heaviest 1% of days, with a subsequent expected increase in the number and intensity of flash flood events. The planet at this point has already warmed about 1.2 degrees C (2.1 degrees F) since the start of the industrial revolution.

Nationally the upward trend in more downpour days is also already very clear, that’s the number of days (24 hours periods) with rainfall of 1” or greater at an observing station. When looking at all the reporting stations across the country, the Climate Central analysis, based on a methodology by Brian Brettschneider, a climate researcher at the University of Alaska Fairbanks, found a sharp increase in days with 1” or more, 2 inches or more, and excessive 3 inches or more of rain falling in a single day in a data analysis from 1950 through 2017 compared to a long term average.