TAMPA, Fla. — Frost can be seen in Florida nearly every winter. It forms when temperatures fall near or below freezing allowing water vapor to condense and freeze onto object such as grass, cars, grills, etc. It can be a colorful sight in the morning as the ice crystals glisten under the morning sun.
The formation of frost is fairly straight forward but it can be complex when you dive into the nitty gritty. We are about to scrape off the surface and dive into some of the nitty gritty below followed by an experiment where you will make your own frost!
First, let’s talk about the conditions needed to get frost. Cold temperatures near or below freezing is a given. You also need water vapor in the atmosphere so it can condense and freeze onto something. Florida is known for its humidity, especially over the summer. Thanks to the Gulf of Mexico and Atlantic Ocean surrounding much of the state, there is often an abundance of moisture through the winter months too.
A saturated air mass is key for frost to form. How does an air mass become saturated? It happens when the relative humidity is close to 100%. This occurs when the air temperature is equal to the dew point temperature.
These are two important terms meteorologist use when it comes to measuring how humid an air mass is; temperature and dew point.
The temperature is a measure of how hot or cold something is. When the temperature falls near or below the freezing point of 32 degrees, ice can form.
The dew point temperature is a measure of how much moisture is in the air. Over the summer when the dew point is in the 70s, the air mass feels tropical. That’s when you step outside and it feels like you can wear the weather. During the winter when it is drier and there is less moisture in the air, the dew point temperature can fall down to the 30s, 20s or lower at times.
Let’s say the air temperature is 55 degrees during a winter evening and the dew point temperature is 28 degrees. The relative humidity is only 35% in this case. The air mass is not saturated.
Now the sun sets, the sky is clear, the wind is light and the temperature falls. By midnight, the air temperature now reads 38 degrees and the dew point is now at 29 degrees. The relative humidity now reads 70%. Can we expect frost to form by midnight? No because the air temperature is a bit high and the air mass is not saturated.
By 6am, it is still dark outside and the temperature falls to 31 degrees and the dew point is 30 degrees. The relative humidity in this case would equal 96%. Would we expect frost? Yes because the air is saturated and the temperature is below the freezing point.
This is when crops must be monitored so they don’t die or suffer damage. Additionally, plants sensitive to the cold could be killed by the frost.
Do we see frost every time the temperature falls below freezing? No.
Let’s check out a different scenario when the temperature cools to 30 degrees but the dew point is at 10 degrees. The relative humidity would be about 43% in this case. Would you expect frost? No, because the air mass is not saturated.
You would have to see the air temperature near 10 degrees to have a saturated enough air mass to produce frost.
This is why frost doesn’t form every time the temperature falls near freezing but when it does, it sure can look pretty glistening under the morning sun.
Next, let’s make our own frost and see how it is done for ourselves!
Experiment: Making Frost
Purpose: Create a thin layer of frost by cooling and saturating a small layer of air
What you need:
- Metal soup can
- Stirring rod or spoon
- Water
- Salt
- Ice
Procedure:
1. With an adult, gather a clean, empty metal soup can. We are using metal because it is a good conductor of heat and can cool quickly.
2. Fill the soup can with crushed ice
3. Pour water into the soup can
4. Use the stir rod to mix it up, feel free to add more ice.
5. Make an observation to the condensation already forming on the side of the can.
6. Pour salt into the soup can. You will need a lot of salt so be generous with the pour. If you think you poured too much, pour even more salt! The salt lowers the freezing point of water and will allow the water in the can to become super-cooled (under 32 degrees).
7. Using the rod, stir the salt water and ice solution for about a minute or two.
8. Make an observation to the layer of ice forming on the outside of the soup can. This is the frost!
Results: Frost formed on the outside of the soup can when the temperature fell below freezing.
Conclusion: Small droplets started to form on the outer side of the metal soup can after adding water to the can of ice. Metal is an excellent conductor of heat which allowed the outer side of the can to cool quickly. As a result, the thin layer of air around the soup can began to cool and saturate. This led to condensation on the soup can which was observed by tiny drops of water.
When the salt was added to the mix, the temperature inside the can cooled further, below the 32 degree threshold. As a result, the thin layer of air surrounding the soup can cooled near freezing and the water droplets turned into ice. The thin layer of ice around the soup can represents frost.
It is important to remember that energy flows from hot to cold. The icy mix of salt water inside the can became so cold it started to pull the heat energy from the outside. This is why the thin layer of air surrounding the can began to cool.
This is the same scenario that happens at night. Once the sun sets, there is no more heat source and the temperature begins to cool. The air temperature cools down toward the dew point temperature. When the air temperature and dew point temperature are close, the air becomes saturated and “dew” forms on the grass. If it gets cold enough, the dew can freeze and that becomes frost.