When frost threatens, why do people spray their tomato plants with water to protect themfrom freezing?

This ensures that the fruits stay warm relative to ambient temperatures hence protect them from frosting/freezing hence protect them from being damaged. This action is pegged on the chemistry of heat of fusion. The sprayed water on the fruits will begin to freeze hence lose heat. The released heat will warm up the fruits that will, therefore, stay warmer compared to the ambient temperatures. The ice around the fruits will also acts as insulators of this heat.

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