“Rain makes grain” — this is an old adage of Iowa farmers.

But the perfect pattern of rainfall during Iowa’s spring and summer seasons is usually hard to come by.

This time of year, rainfall is a mixed blessing, said Lance Gibson, associate professor of agronomy. Rain is certainly necessary for the crops to grow, but during the planting season — usually April 20 through May 15 — too much rainfall can be harmful, resulting in low yields, Gibson said.

“Real intense rain is a negative because it washes the soil away, and [the soil] is not real protected this time of year,” he said.

“The ideal situation is to have a partially dry spring, having good moisture in the summer, but the climate tends to be just the opposite. In the same season, we can go from too much rainfall to not enough.”

The location of Iowa makes for an unusual climate, with large temperature changes from day to day. As such, daily forecasts in Iowa can fluctuate dramatically, causing unpredictable weather predictions and headaches for most of the state’s farmers.

William Gallus, associate professor of geological and atmospheric sciences, is figuring out what is going wrong with many of the state’s forecasts. Gallus is running computer models and using data to create a probability forecast that could be more accurate than those used by many meteorologists.

He said summer weather everywhere is hard to predict, but Iowa, for special reasons, may be even worse.

The main reason is that almost all of Iowa’s rain comes from thunderstorm systems.

“Near the mountains or oceans, thunderstorms are generally triggered by winds colliding down at the ground, and we can usually see it,” Gallus said. “But in Iowa, many thunderstorms happen at night, and, when they happen at night, they’re not usually being created by air colliding at the ground, but instead, something happening a couple thousand feet up in the sky.”

Most farmers care more about weather in the long term, and rainfall prediction in a matter of days is important when applying fertilizer, he said.

Improper application of fertilizer can lead to pollution of streams and waters.

Gibson said this situation could happen if heavy rainfall occurs over a freshly fertilized field, causing the fertilizer to change into a form that can move through the soil and out into the water through tile systems.

He said most farmers wait a while to apply fertilizer after spring planting. But if the farmers are tight on time, and if it rains into June, they cannot apply fertilizer at all.

Gallus said better forecasts for rain would help address that problem.

The main method for forecasting the weather is to run large computer models that use physical laws relating to the atmosphere to give an idea of what is happening. There isn’t enough specific weather data to start the model, though, which means having to make approximations, making for a less accurate forecast, Gallus said.

By collecting and analyzing these computer models and studying weather forecasts, he hopes to uncover the underlying issues with inaccurate weather predictions, he said.

“It’s working, but it’s a slow process. We’re making baby steps forward,” he said.