Temperatures plummet in winter and frost can occur, depending on location, climate, and landscape factors. Clear skies and falling afternoon temperatures are usually the right conditions for frost. Freezes occur in very cold weather and damage is more severe than that of frost.
Farmers are always very conscious of weather conditions and have ways to prepare for severe weather to reduce damage to crops. One of the methods they use is to spray crops with water which may seem like a paradoxical action. To understand why it works, it is necessary to understand more about water and what happens when it changes from a liquid to a solid.
What is frost?
Water vapor in the atmosphere changes to a solid as it reaches freezing point, forming a thin layer on solid surfaces called frost. Frost usually looks like small white crystals when it appears on the ground. Frost can coat objects as they become colder than the surrounding air.
Frost occurs more in low-lying areas because of the fact that warm air rises and cool air sinks because it contains more water molecules. The cool air collects and valleys and front forms. This usually happens overnight when the air temperature is cooler and the frost often melts quickly when the sun rises.
When farmers need to determine the suitability of a crop in a specific farming area, among many other important factors, they also have to consider the potential for frost. Frost can damage crops, destroying plants or fruits. Farmers may have entire fields of crops destroyed by a few frosty nights.
What damage does frost cause?
Cold damage occurs when crops are exposed to low temperatures and the lower the temperature, the more damage occurs. Frost damage may have an effect on the entire plant or affect only a small part of plant tissue, which reduces the overall product quality. A light frost is usually short-lived and may not cause much damage.
Damage can occur directly when ice crystals form inside the cells or indirectly when ice forms inside the plant but outside the cells. The extent of damage due to freezing inside the cells depends on how fast the temperature drops and to which level it cools before freezing.
When ice forms on the outside of leaves at temperatures from 0°C to -2°C, they become dry and brittle. Freezing damage occurs when the temperatures are below -2°C. At this temperature, ice crystals form inside the plant tissue and rupture the cell walls and membranes.
The latent heat of fusion
Farmers who spray their crops with water are taking advantage of the transfer of energy that happens when water changes state. Water contains one oxygen and two hydrogen atoms. The oxygen atom is slightly negative and the hydrogen atoms are positive so they tend to stay together and form hydrogen bonds. When ice melts into liquid, it expends energy as the hydrogen bonds need to break. When placing ice cubes in a glass of water, the ice melts and the original liquid cools. The water cools because it expends energy to accomplish the phase change.
The same thing occurs when water converts into solid ice. Energy is released when hydrogen bonds begin to form and water solidifies. This is called the latent heat of fusion. The ice that builds upon the plants insulates them from the colder surrounding air temperature.
How citrus farmers protect fruit trees
When temperatures go below freezing, fruit on citrus trees can freeze, which affects its quality. When the frost is expected, managers of citrus orchards often spray trees to keep them wet with water near 0 °C. Paradoxically, the cold water effectively provides heat to the trees. There is no temperature change as the water freezes at 0 °C so the heat released is called latent heat because it is “hidden” heat or heat that is not evident. Water in the cells freezes at a little below 0 °C but if trees are watered with water at 0 °C, the heat released as ice forms on the fruit actually prevents damage.
