To take preventive measures against damage or disease by dew, knowing when dew will form is useful. Two methods are discussed here. A "rule-of-thumb" oriented model (Fig. 7.15) will place some of the principles in perspective. Wind speeds are indicated across the bottom from 0 to 8 mph. If the wind is greater than 7 mph, dew is not expected to form. As mentioned, a moist soil will have some effect on dew formation. This is measured by the number of days since last rain. If rain has fallen recently, if no wind is blowing, and the sky is less than half covered by clouds, expect to have heavy dew. If it has not rained in a week and there is a wind of 3 mph, expect moderate dew. If the wind is 5 mph, still moderate dew. But if it has been 9 days without rain, only light dew will occur. Two forecasting methods, one based on the number of days since the last rain and the other on soil moisture, are depicted in Figure 7.15.

Fig 7.15a Dew forecasting chart based on wind speed and length of dry period.
Fig 7.15b Chart based on soil moisture and wind speed.
Study Question 7.2
If the average wind speed is 3 mph and rain occurred 6 days ago, what amount of dew is expected?
Study Question 7.3
If soil moisture is 2.5" below field capacity with a wind speed of 7 mph, what type of dew conditions would be expected?

A case where dew is very significant agriculturally is with peanut leaf spot. A study on peanut leaf spot was one of the early studies done (this one conducted in 1966) on the influence of dew on crop disease. Significant leaf spot on peanuts will greatly reduce the peanut harvest. Peanut growers would prefer not to spend all of their finances on spraying the peanuts. This method of control is environmentally unsound as well. Therefore, the peanut leaf spot is of great concern.

Fig 7.16 Late leaf spot on peanut.

Two important factors when forecasting peanut leaf spot are high relative humidity and high minimum temperatures since both contribute to leaf spot transmission. A method for determining the rate of infection for peanut leaf spot using these factors is displayed in Figure 7.17. The chart combines the hours of relative humidity, which are greater than 95%, with the lowest temperature during a time of high relative humidity. For example 14 hours of leaf wetness and a low temperature of 74°F (23.3°C) would indicate a "rapid infection rate". Knowing the hours of dew and the overnight low temperature indicates "no infection" because it was either too dry or too cold, "high infection rate" or a condition somewhere in between. These levels are usually given a score of 4, 3, 2, 1 or 3, 2, 1, 0 or something similar. When the score for a certain number of nights sums to a particular value, such as "6", people are advised that they need to treat their plants if they want to save them for their intended purpose. It would be economically feasible to treat them and prevent the diseases that are likely developing because of the hours of wetness and the temperature in combination. Those are the things to look for in evaluating plant disease as influenced by wetting or by dew.

Fig. 7.17 Possible infection rate caused by number of hours of dew exposure.
Study Question 7.4
If you had consecutive nights of conditions listed below, would treating peanuts for disease be necessary?
  Night 1 Night 2 Night 3
Minimum Temperature 72 66 78
Hours of 95% relative humidity 8 4 10

The lower atmosphere temperature and humidity conditions necessary for the formation of dew can be summarized in Figure 7.18.

Fig. 7.18 Low-level temperature and humidity conditions which lead to dew formation.

What is the significance of 95% relative humidity? That is assumed to be the condition for dew formation if dew observations are not available. If dew cannot be measured directly, by having a Hughes and Davis dew sensor for instance, how can dew be estimated? Many more places than have dew sensors have hygrothermographs to measure the amount of humidity in the air as well as the temperature. A trace of one of these can be seen in Figure 7.19. Hygrothermographs simultaneously measure the temperature and relative humidity on a continuous trace. Any time the humidity trace goes above 95%, dew is assumed to have formed or precipitation has fallen.

tempchart2small Fig 7.19 Hygrothermograph trace for detecting relative humidity of 95% occurrence. Click the image above for a better view.

Temperatures are depicted in the top chart with humidities on the bottom. Notice how the humidity level raises as temperatures fall at night. The top of the humidity chart indicates 100% relative humidity.

How can there be dew at 95%? First, the humidity is probably measured at 5-6 feet above the ground. Temperatures will likely be colder near the ground producing a higher relative humidity. Second, the humidity will be a little bit higher near the ground because of evaporation from the ground and transpiration from plants. Third, the leaves at night are usually colder than the air. A relative humidity of 95% will likely cause the formation of dew. It has almost become a "rule of thumb" that a relative humidity of 95% will likely be associated with dew. This value is used by horticulturalists, crop protection people, and people who grow especially high value crops such as tomatoes and other things which are very sensitive to diseases and the amount of dew.