Arizona weather model aids lettuce growers

with better forecasting of icing conditions

A weather model developed by the University of Arizona (UA) at Tucson is providing timely weather forecasts and monitoring information to improve crop efficiency in winter lettuce production.

“The goal of the Lettuce Ice Forecast System is to provide high resolution temperature and ice forecasts and monitoring to help growers better manage and schedule harvest crews when ice forms on lettuce during nighttime freezing conditions,” says Paul Brown, UA biometeorologist and director of the Arizona Meteorological Network.

The UA’s Atmospheric Science Department, under the leadership of Mike Leuthold, created the Lettuce Ice Forecast System model for use in Yuma County, Ariz.

“Weather models are the nuts and bolts behind forecast maps,” Brown says.

About 90 percent of the U.S. winter vegetable supply is grown in Yuma County and neighboring Imperial County, Calif., in this low desert region. Harvested iceberg, romaine, and leaf lettuce in Yuma County totaled about 56,000 acres during the 2008-2009 crop year, according to the National Agricultural Statistics Service.

Ice formation in winter lettuce is common, occurring on average about five times during December and about 10 times each in January and February, says Kurt Nolte, UA’s Cooperative Extension agriculture agent and director in Yuma County, who has worked closely with Brown on the project.

“Growers want to know beforehand about the predicted location and severity of a freeze,” he says.

If ice forms on lettuce, the plant can’t be harvested until the ice thaws in the later morning hours, Brown says. Harvesting iced lettuce can damage cell tissue and create quality issues.

The new UA lettuce ice system has a two-pronged approach. The first is a 48-hour weather forecast available online, which creates a hourly forecast and a “movie” illustrating the predicted hour-by-hour weather change for the next two days.

“Every hour, you can watch the sequence of cold air development and the cool temperatures draining down into the river bottoms online,” Brown says. “During the day you can watch the temperatures warm as the sun comes up.”

The second component focuses on critical real time information gathered from 10 weather monitors located across the county that capture current temperature, humidity, dew point, and wind speed information, which is immediately available online.

While many think of Arizona as a place to escape extreme winter temperatures, nighttime temperatures can drop below the freezing mark.

The just-ended 2010-2011 lettuce season included three significant cold snaps. The coldest recorded ground temperature of 18 degrees occurred Feb. 3 in the South Gila Valley and the South Yuma Valley. The deep freeze caused significant damage to the leaf lettuce crop; in some fields, the entire crop was lost.


Yuma County weather is significantly different due to micro-climates. Irrigated farm land that backs up to mountain ranges and the summer monsoon (rainy) season generate varied moisture levels and temperatures.

Summer can sizzle in the 115 to 120 degree range. Winter daytime highs in the 60s to the 80s are commonplace and ideal for growing high quality winter lettuce.

The just-completed winter lettuce season was the initial test of the lettuce ice forecast system. The UA weather model divided the earth’s surface into layered grid areas. Data were sent to a powerful weather modeling computer and server where complex mathematical equations transformed the data into valuable agricultural weather forecasts.

“It’s a benefit to local growers to know where ice is forecast so the logistics can be arranged for delivering field workers to particular locations,” Nolte says. “If lettuce ice is forecast for the south Yuma Valley for four hours in the morning, harvest crews can be sent to a field unaffected by lettuce ice, or perhaps begin work later in the morning. This enables greater efficiency and can reduce costs for the grower.”

The online forecasting and monitoring information can also benefit lettuce shippers and buyers so they can make more informed decisions on lettuce harvests and purchases, he says.

The UA weather system replaces a National Weather Service system discontinued in Yuma County about 15 years ago. The new system was developed with a three-year, $88,000 specialty crops grant from the Arizona Department of Agriculture.

Weather monitoring stations cost about $3,000, plus an additional $2,000 annually for upkeep and management. Funding for the UA lettuce ice program will run out following the 2011-2012 lettuce season, and new funding dollars will be sought from the private lettuce industry.

Another idea is to expand the weather program beyond the winter months to a year-long program in order to service summer cropping systems.

“We could provide weather services for the melon and cotton industries, especially with the growing interest in expanding cotton acreage,” Brown says. “We can examine the potential benefits and discuss the program with commodity groups for possible financial support.”

Heat stress is a major issue in cotton, Nolte says; excessive heat and humidity can cause cotton flower abortion.

“Accurate weather forecasts and monitoring information could provide growers with timely information that would allow them to pursue possible solutions to generate higher crop yields.”

This summer, Brown will meet with lettuce growers to gain feedback on the first year of operation and to provide training on the web-based program and data interpretation.

He is exploring new ways to disseminate weather information, including text messaging and social media sites, including Facebook and Twitter.

To view the 48-hour weather forecast movie, go online and use the following link:

To view online weather monitor information, use this link: