Lately, we have been seeing nights where temperatures drop below freezing by the morning, but we wake up to no sign of frost here in mid-Missouri. Why is that? What makes frost form on some cold mornings and not the others? There are many factors that play into the formation of frost, not just below freezing temperatures.
First of all, for frost to develop there has to be moisture present in the low levels of the atmosphere. Frost starts out as water molecules suspended just above the surface. Many times this moisture is sourced from the evaporation of the soil, which is why we typically see frost form on grass. If there has been a long dry period within the area, the soil will lack moisture, which makes it difficult for frost to develop.
Even if moisture is present and temperatures are below freezing by morning, that doesn't always guarantee that we will be waking up to frosty conditions. Another factor that plays a big role in frost formation are winds. If we see breezy conditions the atmosphere gets 'mixed', which means the more humid air on the surface combines with the drier air aloft. As the atmosphere is stirred, super-cooled droplets (frost) are prevented from forming at the surface.
Clouds also play a big role in situations like this. On a clear night we see more radiational cooling, which allow temperatures to drop more than they would on a cloudy night. You can think of the clouds like a blanket that keep the surface warm. As the temperatures drop below freezing, frost will form instead of dew, but the temperatures have to drop all the way to the dew point. When temperatures cool below freezing and to the dew point, this is when the air is 100% saturated and the droplets become supercooled and condense.
Don't be confused if some mornings you wake up when temperatures are above freezing and there is in fact frost on your lawn. Surface temperatures are taken a few feet above the ground. This makes for a warmer reading than what the actual temperature is right above the surface where the frost forms.