Why is it that there is heavy frost on the lawn but 20 metres away in the forest there is none?

Answer 1

Frost is frozen dew and dew is the result of excess moisture in the air being deposited on surfaces. The cooler air is the less moisture it can contain, when air cools it has to offload the excess moisture. The colder and more conductive a surface is the more readily dew will form on it (same reason why cold surfaces such as windows, mirrors, tiles etc get condensation on them in a steamy kitchen or bathroom).

Frost will form on the surfaces which *feel* coldest first - you'll have noticed that frost forms first on metal and glass such as automobiles before it forms on the ground.

Also, forests are good at retaining heat. The ground and trees acting like radiators giving off heat that has been absorbed in the warmer weeks and months. The temperature in a forest is more constant than the surrounding open areas - if you go into a forest on a really hot day you'll notice it's cooler and likewise on a really cold day you'll notice it's warmer. Take the temperature inside and outside the forest and see what the difference is.

Answer 2

The dew fell on the trees in the forest - on the lawn there was no such protection, so the dew fell on the grass