Delta-T is an index of acceptable spraying conditions based on evaporation rate and droplet lifetime. Best spraying conditions are when Delta-T is between 2 and 8.
Values below 2 generally occur when the air is quite moist, resulting in small droplets surviving for extended periods, increasing the risk of travel to off-target areas. Values greater than 8 occur when the air is hot and dry, causing droplets to evaporate at faster rate. Smaller, more concentrated droplets remain airborne for longer and are more likely to float away. Generally, coarser nozzles will allow spraying in conditions with slightly higher Delta-T ratings.
More information about Delta-T can be found here.