Relative humidity refers to the amount of moisture in the air compared to the maximum amount the air can hold. It is measured as a percentage using instruments like a psychrometer, which measures the temperature difference between a wet-bulb and dry-bulb thermometer, or a hair hygrometer, which uses the stretching of human or animal hair to gauge humidity over time. Absolute humidity refers to the actual amount of water vapor in the air regardless of temperature, while specific humidity is the weight of water vapor contained in a unit weight of air. High relative humidity occurs when air already contains most of the moisture it can hold, making it feel hotter, while low relative humidity means air can still hold more moisture and feels cooler.

2. 1. What is humidity?
2. What is relative humidity?
3. How relative humidity is measured?
4. What is a Hygrometer?
5. How Wet- and Dry Bulb Psychrometer and
Hair Hygrometer measure relative humidity?

11. Absolute Humidity
• Absolute humidity (expressed as grams of water
vapor per cubic meter volume of air) is a measure
of the actual amount of water vapor (moisture) in
the air, regardless of the air's temperature.
• The higher the amount of water vapor, the higher
the absolute humidity.
• The absolute humidity is defined as the weight of
water present in unit volume of moist air
measured in grams per cubic metre.

12. Specific Humidity
• SPECIFIC HUMIDITY refers to the weight
(amount) of water vapor contained in a unit
weight (amount) of air (expressed as grams of
water vapor per kilogram of air).

13. • Humidity refers to the amount of moisture (water
vapor) in the surrounding air.
• Relative Humidity is a measure of the amount of
moisture in the air compared with the amount of
moisture the air can hold.
• Relative humidity is expressed as a percentage of how
much moisture the air could possibly hold at the
temperature it happens when you measure it.
• The "wetter or damper" you feel,, the higher is the
relative humidity. If you feel the air is dry around you,
the relative humidity is low.
Relative Humidity

14. • We are very sensitive to humidity. Sweating keeps
our body cool and maintain its current
temperature.
• If the air is at 90% relative humidity, sweat will
not evaporate into the air. As a result, we feel
much hotter than the actual temperature when
the relative humidity is high.
• If the relative humidity is low, we can feel much
cooler than the actual temperature because our
sweat evaporates easily, cooling the body.
Relative Humidity

15. How relative humidity (RH) is measured?
• Humidity is measured by means of a
hygrometer.
• There are different types of hygrometers.
• The most common hygrometers are Wet- and
Dry- Bulb Psychrometer and Hair Hygrometer.
Lesson 2: Relative Humidity

16. How relative humidity (RH) is measured?
Wet- and Dry Bulb Psychrometer
• It consists of two identical mercury
thermometers,
one of which has a wet cotton or linen wick
around
its bulb.
• Evaporating water from the wick absorbs heat
from the thermometer bulb, causing the
thermometer reading to drop.
• The difference between dry-bulb and wet-bulb
temperatures is compared on psychrometric
charts.

17. How relative humidity (RH) is measured?
Wet- and Dry Bulb Psychrometer
Wet - and Dry Bulb Psychrometer

18. How relative humidity (RH) is measured?
Wet- and Dry Bulb Psychrometer Psychrometric
Chart.
If the difference between
wet and dry bulb is 6º F
and the temperature is
72º F (dry bulb), then the
RH is 54%.

19. How relative humidity (RH) is
measured?
Hair Hygrometer
•Humidity stretches
the hair (human /
animal / synthetic)
while dryness
shortens it.
•The hygrometer
has the job of
stretching a hair
between a fixed
and a movable point
to measure humidity
over time.
Hair Hygrometer

20. How humidity (RH) is measured?
• Other kinds of hygrometers use materials
with electrical resistance that varies with
the amount of moisture absorbed.
• With such hygrometers, a measurement
of electrical resistance can be calibrated
as a humidity measurement.

26. Types of Clouds
• High Clouds Family
• The Middle Clouds Family
• The Low Cloud Family