How do I use the forecast to decide the right day to spray pesticide?

Spray on a calm dry day when the forecast shows no rain for at least the next several hours, ideally a full day, so the chemical stays on the leaf long enough to work. Check three things in the forecast before you spray. First, rain. Avoid spraying if rain is likely within a few hours, because most sprays need a rain free period to dry and be absorbed, and a downpour soon after wastes your money. Second, wind. Spray when wind is light, in the early morning or late evening, so the spray does not drift onto other fields or onto you. Third, temperature. Avoid spraying in the hot afternoon, because high heat causes fast evaporation and can scorch leaves. The Mausam and Meghdoot apps give hourly and short range forecasts that help you find a calm rain free slot. If a systemic spray is rained on within an hour, you may need to repeat, so it is worth waiting for a clear window.