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Chapter 18 Review. Place these notes into your Meteorology Notebook. The number of grams of water vapor in the air is called the specific humidity. Example: 16 g/kg.

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Chapter 18 Review

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chapter 18 review

Chapter 18 Review

Place these notes into your Meteorology Notebook


A comparison of the actual amount of water vapor in the air with the maximum amount of water vapor that can be present in the air at a given temperature and pressure is called relative humidity.

The specific humidity of air is 16 grams per kilogram, and the capacity of the air is 80 grams per kilogram, the relative humidity is 20%. Or: The specific humidity of air is 24 g/kg, and the capacity of the air is 80 g/kg, the relative humidity is 30%.



= Relativehumidity

Specific humidity of air in g/kg

Capacity of air in g/kg

24 g/kg

80 g/kg


16 g/kg

80 g/kg


= Relative humidity @ 30%


= Relative humidity @ 20%

Condensation of water vapor in the air occurs when the temperature drops below the point at which the air’s capacity to hold water and specific humidity are the same. (dew point)

Fog droplets remain suspended in air because they are supported by slight movements of air. ad “toward” and vection “to bring”

Advection fog:


Particles that do not usually act as condensation nuclei are ozone particles.

Dust, smoke, salt, sulfur, nitrate particles all act as condensation nuclei.

The temperature at which condensation begins is called the dew point.

If both wet-bulb and dry-bulb thermometers read the same, the air is considered to be saturated.

The moist-adiabatic lapse rate is much slower than the dry-adiabatic lapse rate.

Dry: unsaturated air cools at 10ºC/kilometer

Moist: saturated air cools varying from 5ºC/km to 9ºC/km. p.399

As air rises, the approximate rate of cooling if no water condenses is 10ºC/kilometer.

In a cumulonimbus cloud, heat from condensation keeps the rising air inside the cloud warmer and less dense than the air around it.

When the temperature of a cloud is below -20ºC, the cloud consists almost entirely of snow and ice crystals.

A stratus cloud forms in stable air.

Clouds can form in any altitude in the troposphere.

If a cumulus cloud does not have a continuous supply of moist, warm air, it may evaporate.

(Watch this example)

Meteorologists predict the condensation level and how tall clouds will grow by considering the air temperature at different elevations, the dew point, and the cooling rate of rising air.

Fog is not considered to be a major form of precipitation.

Advection fog will form when warm, moist winds blow over snow covered ground.

(Watch this animation)

Clouds will contain water droplets of various sizes. Some large; some small.

The dark gray layers of clouds that produce a steady rain are nimbostratus clouds.


Air Movement

Condensation Level


Artificial rainmakers where invented and used during WWII to created inclement weather so enemy aircraft could not see their targets (In Theory). Silver iodide was used as condensation nuclei in this process.

Essay #1

What types of clouds will form when a body of warm, moist air rises to a region of dry, stable air? Explain why and how they form.

Cumulus clouds will form.

Cumulus clouds grow vertically.

Warm air rises and cools, causing condensation.

Clouds grow upward until temperature and density of inside of cloud is same as outside air.

Essay #2

Explain how ice crystals grow in a cloud. Under what conditions will ice crystals fall as snow? Under what conditions will the ice crystals form hail?

Upper layers of clouds contain ice crystals and super-cooled droplets. As super-cooled droplets evaporate, the water vapor is deposited on ice crystals. Heavy crystals fall, clumping together to form snow.

If the snow doesn’t fall through a layer of warm air that melts it into rain, it may reach the ground as snow .

Hail forms when a growing ice crystal is kept aloft by updrafts and continues to grow into a large ice clump. Bonus points: As the hail is carried up it then refreezes adding to its diameter. Bonus: Show a drawing depicting this concept.