Humidity

Question 1

Draw the ‘change of state cycle’

Answer 1

Things to remember:

Heat given out (bonds to be made)
GAS —- condensation —- LIQUID —- freezing —- ICE
(with sublimation from gas — ice)

Then oppose this for Heat taken in (bonds to be broken)

Question 2

What is Vapour Pressure (VP)?

Answer 2

A component of total pressure, due to water vapour (hPa)

Question 3

What is Absolute Humidity (AH)?

Answer 3

Mass of water vapour in a volume of air (g/m^3)

Question 4

What is Relative Humidity (RH)?

Answer 4

Percentage of water vapour compared to the saturation level (%)

This depends almost entirely on temperature

Question 5

What is the Humidity Mixing Ratio (HMR)?

Answer 5

The mass of water vapour in a mass of dry air (g/kg)

Question 6

What is the Dew Point (Td or DP)?

Answer 6

The temperature that air must be cooled to, in order to become saturated.

Note: DP must always be equal to or less than Air Temp.

Question 7

What is the Specific Mixing Ratio (SpMR)?

Answer 7

The amount of water vapour in a total mass of air (g/kg)

Question 8

Describe the relationship between the following at saturation:
VP, HMR, SMR, SVP, Td, Ta

Answer 8

VP = SVP (Saturated Vapour Pressure)
HMR = SMR
Td = Ta

Question 9

What is the relationship that links Temperature, Dew point and Relative humidity?

Answer 9

T - Td ≈ (100 - RH) / 5

Or

RH ≈ 100 - 5(T - Td)

Question 10

Draw a graph to show the relationship between saturation and temperature

Answer 10

A curve positive of a gradually increasing gradient - as temperature increases

Question 11

Give the equation to calculate Relative humidity (%)?

Answer 11

RH = (VP/SVP) x 100

OR

RH = (HMR/SMR) x 100

Question 12

What happens to relative humidity as Ta and Td get closer together?

Answer 12

RH will be high, and clouds will begin to form

Question 13

How do we measure humidity?

Answer 13

With a wet and dry bulb thermometer
The wet bulb is wrapped in a saturated muslin sleeve, and this distilled water evaporates (absorbing heat)
This creates a difference in temperature between the wet and dry thermometer, known as the ‘Wet bulb (temperature) depression’

Question 14

What is the effect of temperature on Relative humidity?

Answer 14

As Absolute humidity (capacity) will increase with temperature.
The percentage of the total that the vapour represents will DEcrease

Question 15

What effect does Diurnal variation have on RH?

Answer 15

Since RH and temperature oppose each other

When the day is coldest (sunrise) - RH is at its highest
When the day is warmest (mid-afternoon) - RH is at its lowest