Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons
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Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.
It's a warm, spring day, and you're enjoying your weekend camping at the beach. Warm sand, clear skies, blue water and a cold iced tea. You can't help but notice the outside of your iced tea is covered in water droplets. Where did those water droplets come from?
What if I were to tell you they came from the air? You can't see them, but there are millions of tiny water molecules floating around you right now. When they bump into your cold iced tea glass, they cool down and undergo a phase change, meaning they change from water vapor into liquid water or from a gas to a liquid. This phase change is called condensation.
Wow! I spent all that time talking about condensation, and the sun is setting. Better head inside your tent for the night, and I'll see you in the morning!
Welcome back! I know it's early, but you're going to need to hurry out of your tent in order to see some condensation! If you look around, you'll notice there is water everywhere. Take a look! I see some on your tent and some on the plants behind your tent.
It didn't rain last night, so where did all of this water come from? Believe it or not, it actually came from the air and is a type of condensation called dew. Dew forms when the temperature drops below the dew point temperature, which means the temperature has dropped to a point where the air can no longer hold any more water vapor molecules, so they condense.
When the air cannot hold any more water vapor, it is saturated. You may notice that the air became saturated after the temperature dropped. As it turns out, warmer air can hold more water vapor molecules compared to cooler air, so as the air cooled, it could hold less water vapor and therefore became saturated. Now, technically the air isn't 'holding' the water vapor, but realize that more water vapor molecules can be present in warm air versus cold air (without the air becoming saturated).
So, dew is just water vapor from the atmosphere that has condensed into water droplets because the air has become saturated. These water droplets condense onto a cool surface. In this case, your tent and the leaves of the plants.
Fortunately, for you, it isn't below freezing at your campsite, but let's take a moment to look at what can happen if it's really cold! If it is below freezing, the dew can freeze. In some cases, the water vapor can go from a gas directly to a solid in what is called deposition. When this occurs, you have frost, or water vapor that has turned directly into ice crystals. So, how can you tell frozen dew apart from frost? Frost appears whiter with ice crystals whereas frozen dew is more solid and not as white. Oftentimes, people confuse frozen dew with frost, but it's important to note they're formed differently.
Okay, back to your campsite. If you look over the water, you'll notice fog has formed. Fog is a cloud that is close to the Earth's surface. And as you look around, you see fog everywhere! Over the ocean, next to your tent and on the mountains next to the beach. Where did all of this fog come from? Fog forms in the same way as clouds. As you know by now, there are millions of water vapor molecules floating around in the air.
So far, you've seen what happens when those water vapor molecules condense on objects (like your tent, your iced tea glass or the leaves of plants) but what happens when they condense in the air? As mentioned previously, the air can only hold so much water, and when it becomes saturated, the water vapor needs to condense. Well, in the air, the water condenses on condensation nuclei, or tiny particles like dust, pollen, salt, ash or even bacteria that provide a surface for water to condense on. Fog forms when all of these liquid water droplets are suspended in the air.
There are actually quite a few different types of fog, so let's take a moment to explain each one, as well as give an example of where you can find it (other than your campsite!).
Radiation fog occurs on cool, clear nights when the Earth's surface cools, causing the air near the surface to condense. As the Earth's surface cools, the air near the surface will also cool. If you remember, cooler air can hold less water vapor than warmer air, so as the air cools, it becomes saturated, and if condensation nuclei are present, the water vapor will condense, and you'll get fog! Radiation fog is fairly common and is found near the ground.
The fog you witnessed above the ocean is called advection fog. This type of fog occurs when warm air moves across a cooler surface. That cooler surface may be the ground, snow, ice or water.
As the warm air moves across the cold surface, it cools and reaches its saturation point. This may sound similar to radiation fog, but it's a little different. Advection fog is the result of warm air moving over a colder surface, so you may notice advection fog moving horizontally across the ground. Another difference is advection fog can last several days, can occur when it's windy and when clouds are present. You might find this fog over the ocean or other bodies of water. For example, it's often seen along the California coastline.
The third type of fog you witnessed is called upslope fog, which forms when moist air is pushed up a hill or mountain by wind. This is the fog that is near the mountains behind the beach. As the moist air goes up the mountainside, the temperature decreases, so the air cools and can hold less water vapor, thus becoming saturated. As you might guess, you'll see this fog on hills or mountains. For example, it's common in the Rocky Mountains of the United States.
Now, lucky for you, you didn't witness any ice fog or freezing fog on your camping trip! Ice fog forms in polar regions when water vapor freezes into ice crystals. In the interior parts of Alaska, ice fog is fairly common in the winter months.
Finally, there is mixing fog, or evaporation fog. Here, two parcels of air that are not saturated mix together to create a saturated air mass. For example, let's say you have a cold parcel of air that moves over a warm body of water. As the water evaporates, it rises and mixes with the cold air. This causes the warm air to cool and reach its saturation point, thus causing condensation to occur. Although there isn't any mixing or evaporation fog at your campsite, you may have seen it elsewhere. For example, a type of mixing fog called steam fog can be seen near hot springs.
Who knew your beach camping trip would turn into a lesson on condensation? Let's take a moment to review what we have learned, so you can get back to enjoying the beach!
That was probably enough information to ensure you don't get foggy when it comes to condensation knowledge. Get it?
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Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons