1. Atmospheric Humidity

2. Circulation of water in the atmosphere
The Hydrologic Cycle (aka the water cycle)
A model that illustrates the movement and exchange of water among Earth, atmosphere, and oceans.
The Gist of the water cycle – water molecules travel from the ocean to the atmosphere to land and then back to the ocean.

3. Circulation of water in the atmosphere

4. Circulation of water in the atmosphere
Evaporation: the process by which a liquid changes into a gas.
The warmer the water, the greater the rate of evaporation.
Wind also enhances evaporation
Condensation: the process by which water vapor becomes a liquid.
Condensation occurs primarily when the air is cooled.
Condensation nuclei: tiny particles upon whose surfaces condensation of water vapor begins in the atmosphere.
Precipitation: any form of water particles – liquid or solid – that falls from the atmosphere and reaches the ground.
Transpiration: the process by which water in plants is transferred as water vapor to the atmosphere.

5. Circulation of water in the atmosphere
Saturation: An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure.
Additional water vapor would result in condensation.
Unsaturated: air that contains less water vapor (at a given temperature) than possible
Supersaturation : air that contains more water vapor than possible (at a given temperature)

6. The many phases of water
Solid = ice
Liquid = water
Gas = water vapor

7. The many phases of water
Sublimation: the process whereby ice changes directly into water vapor without melting.
Deposition: a process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid.
Deposition Nuclei: tiny particles (ice nuclei) upon which an ice crystal may grow by the process of deposition.
Changes of state (or phase changes) – water only changes its disguise, not its identity.

8. humidity
Humidity: term that refers to the air’s water vapor content (water in the air).
There are several ways to express atmospheric water vapor content, and there are several meanings for the concept of humidity

9. humidity
Absolute Humidity: the mass of water vapor in a given volume of air.
It represents the density of water vapor in the air (water vapor density).
Absolute humidity = mass of water vapor
volume of air
*with the same amount of water vapor in a parcel of air, an increase in volume decreases absolute humidity, whereas a decrease in volume increases absolute humidity. For this reason absolute humidity is not commonly used in atmospheric studies.
Parcel Size
Mass of Parcel
Mass of Water Vapor
Absolute Humidity
2 m³
2 kg
10 g
5 g/m³
1 m³
1 kg
10 g/m³

10. humidity
specific Humidity: the ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.
The specific humidity of an air parcel will remain constant as long as water vapor is not added to or removed from the parcel.
Specific humidity = mass of water vapor
Total mass of air
*the specific humidity does not change as air rises and descends.
Parcel Size
Mass of Parcel
Mass of Water Vapor
Specific Humidity
2 m³
1 kg
1 g
1 g/kg
1 m³

11. humidity
The average specific humidity for each latitude. The highest average values are observed in the tropics and the lowest values in the polar regions.

12. humidity
Vapor Pressure: the pressure exerted by the water vapor molecules in a given volume of air.
Actual vapor pressure: is an approximation of the total vapor content.
High actual vapor pressure indicates large numbers of water vapor molecules, whereas low actual vapor pressure indicates comparatively small numbers of vapor molecules.
Saturation vapor pressure: The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water vapor or ice.
It represents a maximum amount of water vapor that the air can hold at any given temperature and pressure.

13. humidity
Saturation Vapor Pressure increases with increasing temperature. At a temperature of 10°C, the saturation Vapor Pressure is about 12 mb, whereas at 30°C it is about 42 mb. The inset illustrates that the saturation vapor pressure over water is greater that the saturation vapor pressure over ice.

14. humidity
Relative Humidity: the ration of the of water vapor in the air compared to the amount required for saturation (at a particular temperature and pressure).
It’s the Ratio of the air’s actual vapor pressure to its saturation vapor pressure.
RH =
Example:
A change in relative humidity can be brought about in two primary ways:
1. by changing the air’s water vapor content.
2. by changing the air temperature
Actual Vapor Pressure x 100%
Saturation Vapor Pressure 65
73 X 100% = 89%

15. humidity
When the air is cool (morning), the relative humidity is high.
When the air is warm (afternoon), the relative humidity is low.
These conditions exist in clear weather when the air is calm or of constant wind speed.

16. Relative Humidity and Dew Point
Dew Point (Dew Point temperature): The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur.
Frost Point: The temperature at which the air becomes saturated with respect to ice when cooled at constant pressure and constant water vapor content.

17. humidity
Average surface dew-point temperature (°F) across the U.S. and Canada for January
Average surface dew-point temperature across the U.S. and Canada (°F) for July.

18. humidity Comparing Humidity
Relative humidity averaged for latitudes north and south of the equator.

19. humidity Comparing Humidity
Air from the Pacific ocean is hot and dry over land, whereas the Gulf of Mexico is hot and muggy over land.
T - represents the air temperature
Td - the dew point
RH - the relative humidity
*All data represent conditions during a July afternoon at 3pm local time.

20. Relative humidity in the home
Cold Outside Temperatures
Outside Humidity = 100% inside Humidity = 8%
Very low Humidity in a house can have adverse effects on things living inside like plants and animals.
Causes rapid loss of moisture in plants, so that why you need to water more in the winter.
Causes rapid evaporation of moisture from exposed flesh in humans, so that’s why you skin is dry, cracking, and itchy in the winter.
Warm Outside Temperatures
You have a higher relative humidity so it doesn’t have the same effect as winter.

21. Relative humidity and human discomfort
“It’s not the heat, It’s the humidity”
Wet-bulb temperature: the lowest temperature that can be obtained by evaporating water into the air.
Hypothalamus: gland that activates the body’s heat-regulation mechanism, and more than 10 million sweat glands wet the body with as much as two liters of liquid per hour.
Heatstroke: a physical condition induced by a person’s overexposure to high air temperatures, especially when accompanied by high humidity.
Heat Index (HI): An index that combines air temperature and relative humidity to determine an apparent temperature – how hot it actually feels.
Apparent Temperature: What the air temperature “Feels Like” for various combinations of air temperature and relative humidity.

22. Humidity Apparent Temperature & Relative Humidity…aka Heat Index

23. Humidity Measuring Humidity
Psychrometer: Instrument used to measure the water vapor content of the air.
It consists of two thermometers (dry-bulb and wet-bulb).
After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.
Two types of Psychrometers
Sling Psychrometer – whirl around to draw air past the bulbs
Aspirated Psychrometer: uses an electric fan to draw air past the bulbs
Hygrometer: an instrument designed to measure the air’s water vapor content.
The sensing part of the instrument can be
Hair (hair hygrometer)
Plate coated with carbo (electrical hygrometer)
Infrared sensor (infrared hygrometer)