Delta T: 

Is becoming a standard indicator for suitable spray conditions. It’s indicative of evaporation rate and droplet lifetime. It’s a relationship between dry and the wet bulb temperature OR air temperature and relative humidity. 

A common spray guideline is to spray when Delta T is between 2 and 8, with caution below 2 or above 10. A Delta T value above 8 is associated with higher temperatures and lower humidity, if the value is lower than 2 it is related to high relative humidity values. 

Summary of Delta T:

Too low Delta T 

Droplet survival will be very long leading to increased potential for drift – avoid spraying with RH>90 to 95%. • Spray poorly absorbed because of dew or fog. • Avoid spraying during calm wind condition and inversion layers, spray drift potential high 

Too high Delta T 

Avoid values greater than 10. • Avoid spraying in air temperatures above 28°C. • Potential impact on both droplet survival and evaporation rate: spray droplet will evaporate in air or off the leaf before it’s absorbed. • Stressful situations for applying herbicides