Changing cloud patterns modify the Earth's energy balance, and, in turn, temperatures on the Earth's surface. As we said, clouds form in the atmosphere because air containing water vapor rises and cools. The key to this process is that air near the Earth's surface is warmed by solar radiation. But, do you know why the atmosphere cools above the Earth's surface? Generally, air pressure, is the reason.
The pressure weight , called barometric pressure, that results is a consequence of the density of the air above. At higher altitudes, there is less air above, and, thus, less air pressure pressing down. The barometric pressure is lower, and lower barometric pressure is associated with fewer molecules per unit volume. Therefore, the air at higher altitudes is less dense.
As the total heat content of a system is directly related to the amount of matter present, it is cooler at higher elevation This means cooler air. On California's Marin Headlands, facing away from the Golden Gate Bridge, the August heat hits the cool air from the Ocean, creating a very thick fog that tends to sit low on the ground.
Condensation also occurs at ground level, as this picture of a cloud bank in California shows. The difference between fog and clouds which form above the Earth's surface is that rising air is not required to form fog.
Fog develops when air having a relatively high humidity comes in contact with a colder surface, often the Earth's surface, and cools to the dew point. Additional cooling leads to condensation and the growth of low-level clouds. Fog that develops when warmer air moves over a colder surface is known as advective fog. Another form of fog, known as radiative fog, develops at night when surface temperatures cool. If the air is still, the fog layer does not readily mix with the air above it, which encourages the development of shallow ground fog.
You probably see condensation right at home every day. If you wear glasses and go from a cold, air-conditioned room to outside on a humid day, the lenses fog up as small water droplets coat the surface via condensation. People buy coasters to keep condensed water from dripping off their chilled drink glass onto their coffee tables.
Condensation is responsible for the water covering the inside of a window on a cold day unless you are lucky enough to have double-paned windows that keep the inside pane relatively warm and for the moisture on the inside of car windows, especially after people have been exhaling moist air. All of these are examples of water leaving the vapor state in the warm air and condensing into liquid as it is cools.
Air, even "clear air," contains water molecules. Clouds exist in the atmosphere because of rising air. As air rises and cools the water in it can "condense out", forming clouds. Since clouds drift over the landscape, they are one of the ways that water moves geographically around the globe in the water cycle.
A common myth is that clouds form because cooler air can hold less water than warmer air—but this is not true. As Alistair Fraser explains in his web page " Bad Meteorology ": "What appears to be cloud-free air virtually always contains sub microscopic drops, but as evaporation exceeds condensation, the drops do not survive long after an initial chance clumping of molecules.
As air is cooled, the evaporation rate decreases more rapidly than does the condensation rate with the result that there comes a temperature the dew point temperature where the evaporation is less than the condensation and a droplet can grow into a cloud drop. When the temperature drops below the dew-point temperature, there is a net condensation and a cloud forms," accessed on Sep. You've seen the cloud-like trails that high-flying airplanes leave behind and you probably know they are called contrails.
Maybe you didn't know they were called that because they are actually condensation trails and, in fact, are not much different than natural clouds. I should point out, however, that we can't just substitute dew point and temperature into the equation for relative humidity above and do a simple calculation.
The mathematical connections between condensation rates and dew point, and evaporation rates and temperature are too complex for that, and are beyond the scope of this course.
Still, understanding the basic connections between temperature and evaporation rates, and dew point and condensation rates leads us to the following important lesson learned:. This lesson has many important applications. For starters, it helps us understand the assertion I made in the last section that the potential for evaporational cooling when rain falls is greatest when there's a large difference between temperature and dew point.
When there's a large difference between temperature and dew point, the rate of evaporation is much larger than the rate of condensation relative humidity is low , meaning there's large net evaporation , which causes notable cooling. These concepts also help us understand the conditions needed for cloud formation net condensation. We'll be exploring how net condensation is achieved for cloud formation in the coming sections, but commonly, you may hear some folks erroneously explain cloud formation by saying that "clouds form as air cools because cold air can't hold as much water vapor as warm air.
In the next section, we're going to explore the fallacy of warm air holding more water vapor than cold air. Skip to main content.
An experiment that begins with a container free of water molecules left. In the second step of the experiment, water is added to the container, and the water begins to evaporate. At the same time, water molecules in the gas phase are free to condense back into the liquid. At first, the evaporation rate far exceeds the condensation rate. In the second phase of the experiment, a container at equilibrium left is heated. When water temperature increases right , the rate of evaporation also increases.
In turn, the amount of water vapor in the "air space" above the water increases. As relative humidity increases, so does the dew point. The temperature must increase to increase relative humidity. Dew Point DP Dew Point is the temperature that air has to be cooled to in order to reach vapour saturation. The higher the Dew point, the higher the water content in the air. Dew point is calculated using air temperature and relative humidity.
When air is cooled, relative humidity increases until it reaches a dew point air becomes saturated. Further cooling below the dew point will induce condensation. The dew point of humid air will be higher than the dew point of dry air.
Temperature T When temperatures are high hot , the air in the atmosphere can handle more water vapour than when the temperature is low cold. As a temperature increases, so does the dew point.
When an object is cooler than the air around it, the water molecules in the air come together and stick to its surface, forming a thin layer of water droplets.
Both air temperature and absolute humidity will determine what type of condensation will occur when the air is cooled. Condensation C Condensation occurs when water vapour in the air is returned to its original liquid state.
When relative humidity nears percent, haze particles grow larger and condensation occurs on less active nuclei. As fog droplets increase in size they begin to settle toward the ground. This process is fairly rapid approx.
How then, is fog maintained? Fog forms when air is either cooled cooled below its saturation or dew point or by evaporation and mixing water vapor enters the air by evaporation and the moist air mixes with relatively dry air. Radiation or Ground Fog is a fog produced by the earth's radiational cooling. It forms best on cold, clear nights where a relatively thin layer of moist air close to the ground is overlain by drier air.
The moist, shallow layer does not absorb much of the outgoing infrared radiation. The ground and the air directly above it cool rapidly, creating a surface inversion. The moist, lower layer quickly becomes saturated by this rapid cooling and fog forms. Radiation fogs are most corm-non in late fall and winter due to long nights allows for longer cooling time and cool air. A light breeze of less than five knots enhances the formation of radiation fog.
This occurs because the slight breeze brings more moist air in contact with the cool ground, thus cooling the moist air layer more efficiently. A strong breeze squelches this process by mixing the moist surface air with the drier air above.
You may have noticed that fog tends to dissipate or "burn off" after the sun has been up for a short while. This occurs because some sunlight penetrates the fog and begins to warm the ground thus disrupting the fog creating cycle. Advection fogs form as the result of wind moving moist air from above a warmer surface to a region above a cooler surface. The moist air cools to its dew point after coming in contact with the cooler surface producing fog.
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