Chapter 5 - Condensation: Dew, Fog, and Clouds

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Explain how dew, frozen dew, and visible frost each form.

Dew forms when the air cools to the dew point. Frozen dew forms when air cools to the dew point, and dew forms, and then the air temperature drops below freezing, freezing the dew droplets. Frost is when the dew point is below freezing, so the water vapor in the air changes directly into ice, through deposition.

Distinguish among dry haze, wet haze and fog.

Dry haze is when there are dust or salt particles in the air that hinder visibility.

Wet haze occurs when water vapor condenses on the particles, which happens at a relative humidity of around 75%.

Fog is when the visibility of the air drops to less than 1km, and the air is wet with tiny floating water droplets, as a cloud resting near the ground.

Why is fog that forms in industrial areas normally thick?

Because there is a lot more condensation nuclei in the air for water vapor to condense onto. Since there are many more particles, there are many smaller drops of water in the air, making the fog seem thick.

How can fog form when the air's relative humidity is less than 100%?

Because hydroscopic condensation nuclei seek out water and vapor condenses on them even when the relative humidity is less than 100%. Enough of these particles in the air can lead to fog.

Name and describe 4 types of fog.

Acid fog - fog that forms polluted air can turn acidic as the tiny liquid droplets combine with gaseous impurities, such as oxides of sulfur and nitrogen.

Advection fog - when moist air moves over a cold surface, its temperature lowers to the dew point and fog is produced.

Advection-radiation fog - when the cold surface that the moist air from advection flows over has gotten to that low temperature through radiational cooling.

Upslope fog - moist air that flows up a hill or mountain. As the air rises, it expands and cools; if sufficiently moist, fog forms.

Evaporation (mixing) fog - when 2 unsaturated air masses mix together, and the resulting mixture is moist enough and the temperature below the dew point, fog can occur. 2 types, steam fog and frontal/precipitation fog. Steam fog is when the cold air is over warm water, water evaporates, and adds more water to the air, raising the dew point. If the dew point reaches the temperature, fog can occur. Frontal fog is when warm rain falls through a layer of cold moist air, the water from the raindrop evaporates into the air, which can saturate it.

Radiation fog - this fog forms near the ground, due to the ground losing heat through radiation, which results in cooler air at the ground. The cooler air near the ground results in a lower saturation vapor pressure, and fog is more likely to form.

What conditions are necessary for the formation of radiation fog?

Clear night so as not to block earth's outgoing radiation, warmer air layer above cooler moist layer at surface, light breeze to help conduction.

Why do ground fogs usually 'burn off' by early afternoon?

Because as the sun warms the ground the air temperature rises. The rising air temperature evaporates the air and since the air is warmer, condensation is less likely to occur.

List as many positive consequences of fog as you can.

California valley fog helps keep temperatures cool during the summer and fall for fruit and nut trees which require winter chilling. The fog acts as a blanket, preventing sunlight from warming up the air, and at night helps keep the temperature from dropping too low.

Advection fog provides moisture to the coastal redwood trees, which collects on the needles and branches, then dripping to the ground to be absorbed by the redwoods shallow root system.

List and describe 3 methods of fog dispersal.

Cold fog can be cleared by dropping several hundred pounds of dry ice into it, which freeze some of the fog droplets, which grow larger and fall to the ground; while the other fog droplets evaporate (since the air is no longer saturated).

Injecting hygroscopic particles (those that attract water vapor) into the fog helps the fog form into larger droplets, which improves visibility, and lets them fall to the ground as a light drizzle. This method is expensive and doesn't clear the fog for a long time.

Warm the air enough so that fog droplets evaporate and visibility improves (tried in LA International airport, deemed too smoky, expensive and wasn't very effective).

Use helicopters to mix the air. The chopper flies across the fog layer, and the turbulent downwash created by the rotor blades brings drier air from above the fog into contact with the moist fog, which helps to evaporate the fog. This only works on thick fogs, with relatively low liquid water content.

How does radiation fog normally form?

Forms when the air near the ground is moist, and the ground cools overnight through infrared radiational cooling (so the skies should be clear, as clouds relfect it back to earth). The cooling of the air lowers the temperature to the dew point, and fog forms.

What atmospheric conditions are necessary for the development of advection fog?

War m moist air flowing over a cold/cool surface. Must be air movement, unlike radiational fog (which can form in still air).

How does evaporation (mixing) fog form?

Forms when 2 unsaturated air masses mix together. When cold air moves over a warm body of water, water vapor evaporates, and raises the amount of water in the air. This in turn raises the dew point, which if raised to the temperature of the air, leads to the formation of fog.

Clouds are most generally classified by height above the earth's surface. List the major height categories and the cloud types associated with each.

High clouds: cirrus, cirrostratus, cirrocumulus

Middle clouds: altostratus, altocumulus

Low clouds: stratus, stratocumulus, nimbostratus

Clouds with vertical development: cumulus, cumulonimbus

List at least 2 distinguishable characteristics of each of the 10 basic clouds.

1. Cirrus - thin, wispy clouds blown by high winds into long streamers called mare's tails. Usually move from west to east across the sky, indicating prevailing winds at their elevation and generally point to fair, pleasant weather.

2. Cirrocumulus - small, rounded, white puffs that may occur individually or in long rows. When in rows have a rippling appearance. Seldom cover more than a small portion of the sky.

3. Cirrostratus - thin clouds, sun and moon can be seen clearly through them. Produce a halo effect around the sun/moon. Thick cirrostratus clouds usually form in front of an advancing storm.

4. Altocumulus - composed mostly of water droplets, rarely more than 1 kilometer thick. Gray puffy masses, sometimes rolled out in parallel waves or bands. Usually, 1 part of the cloud is darker than another.

5. Altostratus - gray or blue-gray cloud composed of ice crystals and water droplets. Often cover the entire sky. The sun/moon may be dimly visible in thin sections of the cloud; but they usually block enough sun so that shadows are not visible of the ground.

6. Nimbostratus - dark gray, wet looking cloud associated with near continuous rainfall/snow. Darker gray than previous clouds, cannot usually see the sun/moon through it.

7. Stratocumulus - low lumpy clouds that appear in rows, patches, or as rounded masses with blue sky visible between the cloud elements.

8. Stratus - uniform grayish cloud that often covers the entire sky; resembles a fog that does not reach the ground. Sometimes a light drizzle. Very low.

9. Cumulus - vertically developing cloud formation. Base appears light white to light gray and on a humid day may only be 1 kilometer above the ground. Detached, lots of blue sky between each cloud. Dome or tower shaped top.

10. Cumulonimbus - thunderstorm cloud, dark base, very low (600 meters), top can extend upward to the tropopause, over 12 kilometer high. Can occur as an isolated cloud or part of a line or 'wall' of clouds.

Why are high clouds normally thin?

Because the air is quite cold and 'dry'.

Why are they (high clouds) composed almost entirely of ice crystals?

Because the air is quite cold and 'dry'.

How can you distinguish altostratus from cirrostratus?

Cirrostratus form a halo around the sun/moon, while altostratus do not. Also, most of the time altostratus clouds usually diffuse enough light to prevent shadows from being visible. Altostratus can be distinguished by its gray color, height, and how dim it makes the sun seem.

Which cloud is associated with lightning?

cumulonimbus

Which cloud is associated with heavy rain showers?

cumulonimbus

Which cloud is associated with mackeral sky?

cirrocumulus

Which cloud is associated with mares' tails?

cirrus

Which cloud is associated with halos?

cirrostratus

Which cloud is associated with light continuous rain or snow?

nimbostratus

Which cloud is associated with hailstones?

cumulonimbus incus

Which cloud is associated with anvil top?

cumulonimbus incus

Why does a broken layer of clouds near the horizon often appear as overcast?

This is because at a distance the open spaces between the clouds is hard to see. This usually leads to an overestimation in cloud cover.

How do geostationary satellites differ from polar-orbiting satellites?

Geostationary satellites are much higher and orbit the earth at the same rate that it rotates at, allowing it to watch the same spot on the earth's surface the entire time. Polar orbiting satellites loop around the earth at much lower altitudes and pass over both poles on their course. Each pass they make provides images/data to the west o the previous pass.

Explain why visible and infrared images can be used to distinguish high clouds from low clouds?

To determine the height, infrared can be used as higher clouds are usually giving off less radiation (since they are colder) than lower clouds which are warmer and thus give off more radiation.

Explain why visible and infrared images can be used to distinguish thick clouds from thin clouds?

Visible imaging can be used to determine the thickness of clouds as thick ones reflect a lot of light, while thin ones allow most of it to pass through.

Why are infrared images enhanced?

They are enhanced to allow for more contrast between things of nearly the same temperature. This can allow the humans to see details that they could not see in an unenhanced image.

Name 2 clouds that form above the troposphere.

Nacreous clouds form in the stratosphere at altitudes above 30 kilometers.

Noctilucent clouds are sometimes seen in the stratosphere at altitudes above 70 kilometers.

List and explain the various types of environmental information obtained from satellites.

1. images of clouds/weather activity.

2. radiometers - which detect radiation that is emited by clouds.

3. imager - an advanced radiometer which provides much better resolution.

4. sounder, a special radiometer that gives a more accurate profile of temperature and moisture at different levels in the atmosphere.

5. can get vertical profiles of temperature/moisture content.

6, cloud thickness as well as height. Precipitation radar, infrared scanner, microwave imager all help to provide a 3 dimensional image of clouds/storms along with the intensity and distribution or precipitation. Also lightning discharges and earth's 'energy budget'.

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