Why is it better to water your lawn in the early morning instead ...

[ghost!]

Why is it better to water your lawn in the early morning instead of the afternoon?

Question 2

What is saturation vapor pressure and why does it increase with increasing air temperature?

[whitcassie]

Answer to Question 1

Relative humidity changes throughout the day, which influences evaporation from soil and vegetation. If you water your lawn in the afternoon, when temperatures are typically higher and the relative humidity is lower than in the early morning, much of the water will evaporate quickly from the lawn and not soak into the ground. If you water in the early morning when relative humidity is higher, evaporation will be reduced resulting in more effective watering.

Answer to Question 2

As opposed to actual vapor pressure, which indicates the air's total water vapor content, saturation vapor pressure describes how much water is vapor is necessary to make the air saturated (when total evaporation equals total condensation and no net loss or gain of vapor molecules occurs) at any given temperature. In other words, saturation vapor pressure is the pressure that the water vapor molecules would exert if the air were saturated with vapor at a given temperature. When air temperature increases, it takes more water vapor to saturate the air.