Why is the surface of leaves prone to dew or frost in the sunny morning?

The frosting of leaves on a sunny morning is caused by the decrease of temperature at night and the saturation of water in the air.

The so-called frosting or dewing temperature is generally called dew point in thermodynamics.

Dew point is defined as the temperature at which water in air reaches saturation. At this temperature, the excess water in the air changes from gas to liquid.

We know that water presents three different states at different temperatures: gaseous (water vapor when it is greater than or equal to 100), liquid (water when it is less than 100 and greater than zero) and solid (ice when it is less than zero). Most other substances also have gas-solid-liquid three States, but the corresponding temperatures are different.

The water vapor content (absolute amount) that can be contained in the air is related to the temperature: high temperature contains more water, while low temperature contains less water. However, at a certain temperature, the water vapor that air can hold also has a maximum value. If it exceeds this value, it will reach saturation and excess water vapor will be converted into water.

For example, at 30 degrees, 20 degrees, 10 degrees, 0 degrees and-10 degrees, the maximum water content per cubic meter of air is 30.3g,17.3g, 9.3g, 4.8g and 2. 1 g, respectively. Above this value, excess water vapor is converted into water precipitation.

If the water vapor content in the air is 20g at 30 degrees, and the temperature changes from 30 degrees to 23 degrees, you can't see the water vapor changing into water. When the temperature drops to 20, 10, 0 and-10 degrees, the dew (frost) you see is 2.7 and 10.7 respectively. That is, at this time, all the moisture in the air that is greater than the dew point is precipitated.