Rainfall Erosivity Climbing to its High Point

Historic precipitation records show that the amount of monthly precipitation does not vary much from month to month in the Mid-Atlantic. However, the type of precipitation is quite different between winter and summer.

In fall, winter, and spring, our precipitation is a result of frontal passages and storms, primarily moving from west to east. The air massing ahead of and behind these fronts has different temperatures. Since cold air is heavier than warm air, the warm air rises slowly above the cold air mass. As it rises, it becomes colder, and because cold air can hold less moisture than warm air, the water vapor starts to condense – clouds start to form, and eventually, rain starts. Nowadays, we can see these fronts develop on the radar of our weather apps – they appear as long lines of rainfall.

This process typically results in relatively gentle rain (we are not concerned about snow in this article since it doesn’t cause erosion except when it melts). This affects the 'erosivity' of the rainfall – the small droplets do not fall very fast, and their kinetic energy is small. Therefore, they don't tend to break up aggregates much, and soil sealing is limited. The rain mostly soaks into the soil until it is completely saturated and then starts to run off.

In the summer, however, another process of precipitation is common – giving rise to convectional rainfall. Convectional rainfall is due to intense heating of the soil surface causing thermal convection, making warm air rise quickly into the atmosphere. Again, the warm air cools off as it rises, causing condensation, cloud formation, and eventually, rain. This is often accompanied by atmospheric instability leading to thunderstorms. It shows up on the radar as 'popup' storms that appear in random places (in contrast to large fronts).

This type of rain is much more violent and intense and it is typical of June-August rainfall when it is hot. The large raindrops fall at high speed and have high kinetic energy. When they fall on bare soil, they act as little 'bombs' that destroy aggregates. The soil particles clog up the pore spaces and the soil surface seals up. The soil underneath may still be dry, but the soil cannot take in the water because the surface is plugged up. Therefore, much runoff and erosion can happen.

There is another type of rainfall that also occurs sometimes: rain associated with hurricanes. Hurricanes normally form in the Atlantic off the coast of Africa, the Caribbean, and the Gulf of Mexico due to the heating of the ocean waters, causing evaporation and the rise of warm, humid air masses up into the atmosphere. These may cause hurricanes to form above the warm ocean waters of the Caribbean and the Gulf of Mexico, which move west. Then, they curl up northeast, traveling toward our region.

The first hurricanes typically form in June, but the height of the hurricane season is from August to October. Most hurricanes, however, don’t reach Pennsylvania or have lost a lot of their strength as they move inland, so they don’t usually play a huge role in explaining the average rainfall typology in our region. When they do occur, however, they can cause major damage, for example, hurricane Agnes in June 1972, Floyd in September 1999, Lee in September 2011, or Ida in September 2021.

Erosivity is a property of rainfall that was developed in the USA. It quantifies how much erosion can be expected based on the rainfall characteristics alone. It is predominantly the product of maximum rainfall intensity sustained for 30 minutes during a storm. Because of the predominance of high-intensity storms in July and August, average rainfall erosivity is highest at that time of the year. From a soil management point of view, this is the most important time of the year to keep soil covered so it is protected from erosion.