It is September and crops are progressing toward maturity, giving us an opportunity to look at an unusual weather factor that may affect yield in some areas. Causing some concern ― in addition to the freeze vulnerability of corn that emerged from late May through early June ― is an unusually cloudy period the third full week of August.

Much of the central third of the state battled wet conditions during August, while portions of western and eastern Nebraska missed most of these major rain events. With rain comes clouds, but usually in August they are associated with thunderstorms that quickly move through the region. This was not the case for much of the period from August 19 to 25.

Intense cloudiness across central Nebraska was the result of a stationary boundary at the surface that spanned the area from north central North Dakota southeastward through northeastern Oklahoma. This was the dividing line between the western U.S. upper air ridge and an upper air low situated over the upper Great Lakes region. Ripples of energy moved southeastward along this boundary, keeping the region in persistent cloudiness.

The Nebraska State Climate Office’s Mesonet stations picked up on this cloudy period through our solar radiation sensors. Table 1 represents the average solar radiation received during August for all active Mesonet locations. This table gives the average solar radiation received, normal solar radiation expected, the difference between this year and normal, and the dates of the highest and lowest solar radiation values observed.

The majority of solar radiation values falling well below normal were in the central third of the state from the South Dakota border to the Kansas border. The western and eastern thirds of the state experienced a drop in solar radiation of 5%-15%, while the central third experienced average reductions of 12%-22% in monthly solar radiation values. Some periods within the month, as indicated in Table 1, saw much higher drops.

What’s a Langley?

A Langley is an energy term that describes solar irradiance. It measures the total light energy from the entire disk of the Sun that is received at a particular point on the Earth’s surface. One Langley has the equivalent energy of 0.086 watts per square meter.

Delving deeper into the data, solar radiation values across central Nebraska dropped to as low as 84 Langleys on August 24, compared to a normal daily value of 457 Langleys. In layperson terms, Central City received just 18% of its daily average solar radiation. It appears that the most intense period of cloud cover was on August 18. The Harvard Mesonet site (formerly Clay Center) had the greatest weekly loss of solar radiation, averaging 195.5 Langleys below normal or 57% of normally expected solar radiation for the week.

To put Langleys in perspective, readings above 500 Langleys a day in August are common on mostly sunny days. Solar radiation will drop below 300 Langleys on a cloudy day and below 150 Langleys during overcast conditions and light drizzle. Throw in moderate rainfall and/or fog and solar radiation will drop below 100 Langleys.

In a related CropWatch article, the authors note that as solar radiation to the plant deceases, corn growth and development slows, extending the grain fill period. This likely occurred in some areas of central Nebraska in August (Table 2).

Forecast

As we peer into the future, the recent string of warm temperatures likely will hold on for at least another week, although high temperatures will range from the mid 70s to the low 80s. Precipitation chances appear to be frequent, particularly across the northern half of the state as an upper air trough crosses the Northern Plains.

As we approach the final weekend of September, another upper air low is projected to move across the Northern Plains and pull near-freezing to freezing temperatures into the Northern Plains. As of now, the models keep this cold air to the north of us, but they have pushed this air mass as far southward as northeast Nebraska in several model runs the past week. If we escape this cold push without freezing temperatures, it will likely be another 7-10 days before another push of cold air moves southward into the Northern and Central Plains.