Torrential Rain 

Flooding after Torrential Rain in Illinois

Flooding after Torrential Rain in Illinois

by Eli Cohen and
Armaan Shah


Rain is often considered an inconvenience, or at most a side-effect of a powerful storm, but heavy rainfall can cause serious damage and destruction. Common consequences of heavy (or “torrential”) rain are flash floods and landslides, which can decimate houses and flood entire neighborhoods. Even more concerning is the increasing prevalence of these events, which many scientists link to climate change. Before delving into the damage that torrential rainfall events can cause, let's first explore the characteristics and causes of these events.

What is torrential rain?

The classification of heavy rain has not been standardized throughout the world. Although there are a few commonly accepted rates of rain that qualify as heavy rain, the rate may vary based on a classification that better suits the climate in that geographic location. Rain is generally classified as "heavy rain" when falling at a rate of greater than or equal to 7.6mm of water per hour. However, New Zealand classifies heavy rain as any rainfall equal to or above 100mm in 24 hours [6]. In contrast, the U.S. northeast identifies heavy rain to be any rainfall equal to or above 50mm in 48 hours - only one fourth that of New Zealand [3].

Both spatially and temporally, heavy rain can be very difficult to predict. Generally, heavy rain takes place on a small spatial scale (less than 100km) and over a short timespan (less than 12 hours) [4]. Because heavy rain can cause natural disasters, the difficulty in forecasting heavy rain events is essential.

Why does torrential rain occur?

The factors that cause torrential rain are similar to those that cause regular rain. As with any rain storm, the necessary conditions are moisture and atmospheric instability [10]. Warm air holds more moisture than cooler air, which is why it rains so often in the tropics (for example, the Amazon jungle). As temperature increases, an air mass can hold exponentially more moisture (water vapor); a warm air mass can hold much more moisture than a cool one. Because moisture is a necessary ingredient for rainfall, a warm air mass can lead to longer, heavier rains than a cool one [4].

Atmospheric instability simply refers to air masses moving up and down (also known as "vertical development") due to the temperature of the air masses compared to that of the air around it. An air mass is absolutely unstable when, as it rises, it cools slower than the air around it. This means it will always be warmer than the air around it, and thus will continue rising until conditions change. An air mass is conditionally unstable when it cools faster than the air around it unless it is saturated; thus it will only rise if it becomes saturated, and tends to stay where it was displaced [9].

Thus for rain to occur, an air mass with a lot of moisture must rise vertically until the water vapor condenses
and falls as rain. Intuitively, a torrential rainfall would occur when there's enough moisture to fall for a long time at a quick rate. Scientists have characterized torrential rainfall in this way using an "ingredients-based" approach [1]. This relies on two equations:

P = RD               R = Ewq

These state that the total precipitation (P) is directly proportional to the average rainfall rate (R) and the duration of the rainfall (D). Furthermore, we can examine the “ingredients" for R: a high precipitation efficiency (E), a large ascent rate in an updraft (w), and a large mixing ratio of the air (q). Finally, the rainfall duration depends on the size and speed of the weather system, as well as on the rainfall intensity (how "hard" it rains).

The effect of climate change on torrential rain events

Recall that warm air masses can hold more moisture, and thus are often conducive to heavy rainfalls [3]. As climate change warms the planet, air masses all over the globe are indeed becoming warmer and the total amount of water vapor on Earth is steadily increasing. In fact, researchers Rudolf Clausius and Benoît Paul Émile Clapeyron defined this relationship mathematically: for every degree Celsius the Earth warms, the total amount of water vapor the air can hold increases by about 6% [4]. Since scientists have recorded constant relative humidities (amount of moisture relative to saturation) around the globe, the amount of water vapor is also increasing at approximately the same rate.

Unfortunately, researchers recently confirmed the increase in torrential rainfall events [2]. The prevalence of these events seems to correlate with the increase in water vapor in the Earth's atmosphere. However, it is important to note that there are many variables to account for when hypothesizing these correlations. The same researchers conceded that other factors, such as increased land use, could also be influencing the increased prevalence of torrential rainfalls [4]. Despite this, the number of these events will likely continue to increase as long as the Earth continues to warm, and the consequences may be severe.

The consequences of torrential rain

Heavy rainfall events are frequently the cause of landslides and flash floods. In the case of landslides, loose soil or shallow set rocks are often dismantled by the considerable and rapid amount of rain in heavy rain [7]. Those dismantled parts of the earth quickly build up and can pick up speed and size depending on the slope of the terrain and the amount of material loosely bound to the earth. Geographic locations with more open land and less forestry are even more susceptible to landslide events. The impact of landslides can be catastrophic in the loss of life, the destruction of infrastructure, and costs in the hundreds of millions.

Flash floods can also occur due to heavy rainfall events. In urban areas, heavy rain can overflow storm drains and flood roads and buildings. Because urban areas have fewer locations where the ground absorbs water there is increased runoff, making cities common locations for flash floods. Areas near rivers and low water crossings are also at risk of flash flood, as well as, locations with saturated soil. The unpredictability and speed of flash floods induced by heavy rain lead to destruction and more loss of life than tornadoes, hurricanes, or lightning [8].

Works Cited

1. Doswell, Brooks, Maddox. Flash Flood Forecasting: An Ingredients-Based Methodology. American Meteorological Society. Volume 11.
2. National Climate Assessment. Our Changing Climate.
3. Cornell Climate Change. Are recent heavy rain events due to climate change?
4. Schumacher, Russ. Heavy Rainfall and Flash Flooding. Oxford Research Encyclopedias
5. Donegan, Brian. America's Heaviest Rainfall Records. The Weather Channel.
6. Yang, Ed. Extreme weather - heavy rainfall. NIWA.
7. Mountjoy, Joshu. “Rainfall-Triggered Landslides." NIWA.
8. NOAA National Severe Storms Laboratory: Flood Basics
9. American Meteorological Society. Conditional Instability. Glossary of Meteorology.
10. Digital Atlas: Stability & Cloud Development. Idaho State University.

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Eli Cohen and Armaan Shah created this while students in the Department of Earth and Planetary Sciences at Northwestern University