Frost is most common on clear, cold and sunny mornings — so where does that moisture come from? And what are the similarities and differences between frost and the dew that makes morning grass wet in the summer?
The science of dew and frost all comes down to the invisible water vapor in the air around us. Our atmosphere always has water vapor in it, but the amount of water vapor an air mass can hold depends on the temperature. Warm air can hold more water vapor than cold air.
Some nights, the air will cool enough that some of the moisture it was holding is released and the water vapor becomes liquid water. This process is called condensation. The temperature at which water condenses from the air is called the dew point.
Summer days are often very warm and humid, which means the air is holding a lot of water in vapor or gas form. In the evening, the air starts to cool and continues to cool all night long. As the air cools, it can no longer hold as much moisture.
The frost coats everything with beautiful, sparkling crystals that quickly melt away after the sun starts to make them sparkle even more.
Warm air rises and cold sinks. Therefore, the coldest air is often near the ground. So that is why the grass is often wet with dew on the mornings following a warm and humid day.
Many think frost is simply dew that has frozen. This is not true. That would simply be called “frozen dew,” and is not that common. However, the process of frost formation is very similar to dew formation with one big difference — frost formation skips the liquid phase.
We typically think of the three phases of matter as going in order of gas to liquid to solid. But in some cases (such as frost), the liquid phase is completely skipped. The water vapor (gas) turns directly into a solid. This process is called deposition and it is exactly how frost forms. If the temperature falls to the dew point and that temperature is below freezing, then tiny ice crystals will form on all surfaces that are also below freezing. You can tell the difference between frozen dew and frost by looking closely at the ice. Frozen dew will look like frozen droplets of water while frost will look like many tiny white crystals.
Frost above freezing? Frost can be a bit mysterious. You might see a weather forecast that says: “Tonight, we will have a low of 37 with areas of frost possible by morning.” How is that possible?
It is very possible and common that the temperature at five feet is above freezing, but the temperature right at the ground is below freezing. It is also possible that while the official temperature is above freezing, areas at the bottom of a hill may be below freezing. The official temperature is only the temperature of that one location five feet above the ground. This is why your grass may be covered in frost while it appears to be above freezing. Now you know how those beautiful frost crystals form. Next time you see them, get a closer look and enjoy their beauty — even if it’s when you are scraping that beauty from your windshield!
Science is not messing with you. The science is just being a little sneaky — by following scientific laws. Remember, cold air sinks to the lowest point if the atmosphere is calm. It would also help to understand that the National Weather Service measures the official temperature five feet above the surface at a particular station. You cannot have ice formation at temperatures above freezing, right? Yet that is exactly what appears to happen sometimes.