Sun angle, day length, and overnight temperatures

In the last few days, summer has made a last ditch effort at a comeback. Before ceding completely to fall, the temperatures have climbed back into the upper 80’s. What is the difference between an upper 80’s day at the end of September versus July or August? It comes down to sun angles/incidence/intensity, day lengths, and overnight temperatures.

July and August are similar enough to generalize the two together as a summer day and compare that to September, which we will describe as a fall day. When we have a day in the upper 80’s to 90 degrees or hotter in the summer that sunlight is hitting our location on the planet at its most direct and its intensity is strongest. This means that the least amount of heat/light/energy is lost through reflection and absorption by the atmosphere. Therefore the suns energy is the most intense. When compared to fall, the suns energy is hitting at a steeper angle and more is impacted by the atmosphere and clouds. So while the temperature rises to a similar degree, it takes longer to achieve that number. Relating this to the golf course, the grass is subjected to less intense heat overall, but even when intensities are equal, it is for a shorter amount of time. So the grass plant has a sprint instead of a marathon. A few hours of warm temperatures are manageable.

The time factor is where day lengths become important. The sun rises later in the day and sets earlier. Therefore, the overall window the sun has to heat the turf is smaller. This heating time partially drives the amount of water lost from the soil and grass plants. If the sun starts heating later and ends earlier, immediately we know that there is less time for water to leave the system. So any water we apply overnight or early in the morning, will last later into the day. Similarly, as it starts to get darker earlier in the evening, the time the sun has to shine its brightest and most intense decreases. In the fall, the warmest part of the day is still near 3pm, but instead of having 4-5 more hours of daylight and intense sun, we now have only 2-4 hours, or less. Accordingly, if we monitor the turf between 1:30-4pm, applying any water as necessary, we can be more confident that water won’t be lost in the remaining daylight hours. In contrast, water applied at that time in summer, may be lost, or as the sun’s intensity continues, other areas may continue to lose more water.

Finally, and possibly most important, are overnight temperatures. In the summer, overnight “lows” can stay in the upper 70’s or even low 80’s. This is especially difficult on the greens. Generally, the turf can handle the heat stress of a hot day when it has a chance to recover overnight. When the temperatures are too warm the turf cannot adequately cool itself and recover from the stress of the day. Moreover, many of the disease issues we face are exacerbated by the warm overnights. The warmth and humidity are perfect for fungus and trouble for the turf. However, once we reach fall, the overnights temperatures start falling to more comfortable levels. The lower temperatures allow the plant to properly transpire and perform metabolic processes to heal and defend against all stress. Now, even when we have a warm day, the turfgrass plants are able to recuperate and prepare for the next day.

These are some of the keys to difference between summer and fall. By understanding these factors, the Grounds department is able to push the turfgrass a little harder so that the conditions of the course are ready for the busy golf calendar of the fall.

Etiquette Reminder of the Month

After filling your divot with sand, either on a tee or fairway, please smooth the sand pile out so our mowers don’t cut through the sand.

See you on the course!

Joe

jvillegas@bwrc.org