Chapter 4 - Humidity

Question 1

What happens with the ABSOLUTE HUMIDITY when a parcel of air with water vapor rises, expands and cools?

Answer 1

The total quantity of water vapor in a parcel of air will stay constant
but the volume of the parcel will increase therefore leading to the decrease of absolute
humidity.

Question 2

What happens with the RELATIVE HUMIDITY when a parcel of air with water vapor rises, expands and cools?

Answer 2

Cooling causes the relative humidity to increase.

Question 3

What happens with the ACTUAL VAPOR PRESSURE when a parcel of air with water vapor rises, expands and cools?

Answer 3

Reduction in total air pressure occurs because of expansion, so the
actual vapor pressure decreases because it is a fixed amount of that total pressure.

Question 4

What happens with the SATURATION VAPOR PRESSURE when a parcel of air with water vapor rises, expands and cools?

Answer 4

Saturation vapor pressure: Because of the cooling of the saturation vapor content in the
air, the saturation vapor pressure will decrease.

Question 5

What are the primary factors that influence evaporation?

Answer 5

wind and temperature

Question 6

How are evaporation and condensation related to saturated air above a flat water surface?

Answer 6

Under saturated conditions, for every molecule that evaporates, one must condense.

Question 7

Why do hot, humid summer days, usually feel hotter than hot, dry summer days?

Answer 7

The main source of body cooling is through evaporation of perspiration. When both the temperature and relative humidity is high and the air is nearly saturated with water vapor, body moisture does not evaporate, but instead collects on the skin as perspiration.

Question 8

Explain why the air on a hot, humid day is less dense than on a hot, dry day.

Answer 8

At the same temperature and at the same atmospheric level (pressure), humid air weighs less than dry air. This is due to the fact that water vapor molecules weigh less than the dry air molecules that they are replacing. (The molecular weight of water vapor = 18, while the molecular weight of dry air = 29).

Question 9

Why is cold polar air described as “dry” when the relative humidity of that air is very high?

Answer 9

The air in the polar region is considered dry as the dew point temperature is low. Low dew point indicates low water vapor content. Since the temperature is close to the dew point the RH becomes high.

Question 10

Would you expect water in a glass to evaporate more quickly on a windy, warm, dry summer day or on a calm, cold, dry winter day?

Answer 10

More evaporation on a windy, warm and dry summer day. This is because dry and warm air enhances the evaporation since it has a greater capacity of holding water vapor. Wind make the air above the glass to move away which minimizes the risks of the above air to become saturated with the vapor.

Question 11

Suppose the dew point of cold outside air is the same as the dew point of warm air indoors. If the door is opened, and cold air replaces some of the warm inside air, would the new relative humidity indoors be (a) lower (b) higher (c) same as before.

Answer 11

(b) higher.

The relative humidity would be higher. If the dew point is quite a bit lower than the warm indoor air, the colder outdoor air would be closer to the dew point. Since the new colder indoor air is closer to the dew point, the relative humidity would increase.

Question 12

What is absolute humidity?

Answer 12

Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3).

Question 13

What is relative humidity?

Answer 13

Relative humidity also measures water vapor but RELATIVE to the temperature of the air. It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature. (or actual vapor pressure/saturation vapor pressure x 100 )

Question 14

What is specific humidity?

Answer 14

Specific humidity is a ratio of the water vapor content of the mixture to the total air content on a mass basis.
i.e. mass of water vapor/mass of air

Question 15

What is mixing ratio?

Answer 15

mass of water vapor/mass of dry air

Question 16

What is saturation vapor pressure?

Answer 16

The pressure that the water vapor molecules would exert if the air were saturated with vapor at a given temperature.

Question 17

What is actual vapor pressure?

Answer 17

Expresses the amount of water vapor in terms of the amount of pressure that the water vapor molecules exert.

Question 18

What means that air is supersaturated?

Answer 18

When the relative humidity is above 100%.

Question 19

What is diffusion?

Answer 19

Diffusion is the somewhat random migration

of water-vapor molecules through the air toward existing hydrometeors (water droplets and ice crystals).

Question 20

Describe the solute effect.

Answer 20

When a hygroscopic particle (i.e. salt) replaces a water molecule in the droplet, the saturation vapor pressure decreases.

Question 21

What is the critical radius? What is the critical radius in a water droplet?

Answer 21

Critical radius is the minimum size that must be formed by atoms or molecules clustering together (in a gas, liquid or solid matrix) before a new-phase inclusion (a bubble, a droplet, or a solid particle) is stable and begins to grow. Formation of such stable “nuclei” is called nucleation.

Critical radius water droplet = 0.1 µm.

Question 22

Describe the curvature effect.

Answer 22

Smaller droplets need larger saturation vapor pressure than a flat surface. To keep the water molecules in equilibrium with the environment the air must be supersaturated.