To paint or not to paint depends on the temperature, humidity (how much water vapor is in the air), and dew point (the temperature at which the air can no longer hold its water vapor and condensation forms).
Check the weather to ensure that the humidity and the temperatures for the ambient air, substrate, and paint match the paint manufacturer’s recommendations. This is especially important when spraying, or when rolling & tipping a two-part polyurethane topcoat.
Begin painting in the morning on a clear, dry day and avoid painting in direct sunlight or in windy conditions. Ideally, paint in the late morning when dew is not present. Time the application so the final coat of paint has time to dry before temperatures fall and nighttime dew sets in.
If there is any condensation on the uncured surface, it will decrease the glossiness of the finish. For best finish results, time your painting so rain will not affect full cure.
Keeping within the paint manufacturer's guidelines for temperature, humidity, and dew point will help ensure complete drying and maximum gloss.