1. Chapter 6 Humidity, Saturation, and Stability
Weather Studies Introduction to Atmospheric Science American Meteorological Society
Chapter 6
Humidity, Saturation, and Stability
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point
Cloud forests are forests that are perpetually shrouded in clouds or mist
They are found from m ( ft) in elevation in the tropics and subtropics
Onshore and upslope winds that are warm and humid supply the moisture
Warm air blowing upslope cools through expansion
Expansional cooling raises the relative humidity to saturation and water vapor condenses into low clouds and fog
The tree canopy strips moisture from the clouds and this water drips to the forest floor
Deforestation reduces available moisture and raises air temperature → clouds form less readily and at higher elevations

3. Case-in-Point
If global warming translates into higher sea surface temperatures (SSTs) in the tropics, cloud forests could be affected
Air flowing onshore would be warmer
Greater ascents would be required to produce clouds
Clouds would be at higher elevations
Perhaps even lift off the mountains
Cloud forests are extremely sensitive to climate variations
They may prove to be early indicators of effects of global-scale climate change

4. Driving Question
How does the cycling of water in the Earth-atmosphere system help maintain a habitable planet?
This chapter will tell us:
How the global water cycle functions
Especially as it relates to transference between the Earth’s surface and the atmosphere
How to quantify the water content of air
How air becomes saturated through uplift and expansional cooling
How atmospheric stability affects the ascent of air

5. Global Water Cycle
Assumption – the amount of water in the Earth-atmosphere system is neither increasing or decreasing
Internal processes continually generate and break down water molecules
Volcanoes and meteors (minute amount) add water
Photodissociation of water vapor and chemical reactions break down water molecules
Fixed quantity of water in Earth-atmosphere system is distributed in 3 phases among various reservoirs, mostly the ocean (97.2%) and ice sheets and glaciers (2.15%)
The sun powers the global water cycle and gravity keeps water from escaping to space, causing water to fall from the sky as precipitation and flow to oceans

6. The Global Water Cycle

7. Where is the Water Stored?
Note the small percentage of the total water that is stored in the atmosphere.
Even though small in percentage, this is vital to weather processes

8. The Global Water Cycle Transfer processes 1. Phase changes
Evaporation – more molecules enter the atmosphere as vapor then return as liquid to the water surface
Condensation – more molecules return to the water surface as liquid then enter the atmosphere as vapor
Transpiration – Water that is taken up by plant roots escapes as vapor from plant pores
Evapotranspiration is the total of evaporation and transpiration
Sublimation – ice or snow become vapor without first becoming liquid
Deposition - water vapor becomes solid without first becoming liquid
All 3 phases of water exist in the atmosphere
2. Precipitation
Rain, drizzle, snow, ice pellets, and hail

9. Percent of Precipitation Originating from Land Sources
Ocean evaporation is the origin of most precipitation.

10. Pathways Taken by Precipitation Falling on Land

11. The Global Water Budget
Via precipitation and evaporation, the ocean has a net
loss of water and the land has a net gain.

12. How Humid is it?
Humidity describes the amount of water vapor in the air
This varies with time of year, from day-to-day, within a single day, and from place-to-place
Humid summer air, and dry winter air cause discomfort
Ways of measuring humidity:
Vapor pressure
Mixing ratio
Specific humidity
Absolute humidity
Relative humidity
Dewpoint
Precipitable water

13. How Humid is it? Vapor pressure Mixing ratio Specific humidity
Water vapor disperses among the air molecules and contributes to the total atmospheric pressure
This pressure component is called the vapor pressure
Mixing ratio
Mass of water vapor per mass of the remaining dry air
Expressed as grams of water vapor per kilograms of dry air
Specific humidity
Mass of the water vapor (in grams) per mass of the air containing the vapor (in kilograms)
In this case, the mass of the air includes the mass of the water vapor
Mixing ratio and specific humidity are so close they are usually considered equivalent

14. How Humid is it? Absolute humidity Saturated air
The mass of the water vapor per unit volume of humid air; normally expressed as grams of water vapor per cubic meter of air
Saturated air
This is the term given to air at its maximum humidity
A dynamic equilibrium develops where the liquid water becomes vapor at the same rate as vapor becomes liquid
“Saturation” may be added to various humidity terms
Saturation vapor pressure, saturation mixing ratio, saturation specific humidity, saturation absolute humidity
Changing the air temperature disturbs equilibrium temporarily
Example: heating water increases kinetic energy of water molecules and they more readily escape the water surface as vapor. If the supply of water is sufficient, a new dynamic equilibrium is established with more vapor at higher temp.

15. Variations with Air Temperature of Vapor Pressure Saturation Mixing Ratio

16. How Humid is it? Relative humidity Probably the most familiar measure
Compares the amount of water vapor present to the amount that would be present if the air were saturated
Relative humidity (RH) can be computed from vapor pressure or mixing ratio
RH = [(vapor pressure)/ (saturation vapor pressure)] x 100
RH = [(mixing ratio)/(saturation mixing ratio)] x 100
At constant temperature and pressure, RH varies directly with the vapor pressure (or mixing ratio)
If the amount of water vapor in the air remains constant, relative humidity varies inversely with temperature
See next slide

17. The Relationship of Relative Humidity to Temperature

18. How Humid is it? Dewpoint
The temperature to which the air must be cooled at constant pressure to reach saturation
At the dewpoint, air reaches 100% relative humidity
Higher with greater concentration of water vapor in air
With high relative humidity, the dewpoint is closer to the current temperature than with low relative humidity
Dew is small drops of water that form on surfaces by condensation of water vapor
If the dewpoint is below freezing, frost may form on the colder surfaces through deposition
Dewpoints below freezing are sometimes referred to as frostpoints

19. How Humid is it? Precipitable water
The depth of the water that would be produced if all the water vapor in a vertical column of condensed into liquid water
Condensing all the water vapor in the atmosphere would produce a layer of water covering the entire Earth’s surface to a depth of 2.5 cm (1.0 in.)
Highest in the tropics
Map of precipitable water
at various locations

20. Monitoring Water Vapor
Humidity instruments
Hygrometer
Measures the water vapor concentration of air
Dewpoint hygrometer
Uses a temperature-controlled mirror and an infrared beam
When the mirror temperature reaches a point that condensation forms, the reflectivity of the mirror is changed, altering the reflection of the beam. The temperature is recorded as the dewpoint.
These are common at NWS forecast stations
Hair hygrometer
Relates changes in length of a humid hair to humidity – hair lengthens as relative humidity increases
Hygrograph
Provides a record of humidity variations over time
Electronic hygrometer
Based on changes in resistance of certain chemicals as they absorb or release water vapor to the air

21. Monitoring Water Vapor
The temperature/dewpoint sensor (hygrothermometer) used in the NWS Automated Surface Observing System (ASOS)

22. Monitoring Water Vapor
Sling psychrometer
Wick is wetted in distilled water
Instrument is ventilated by whirling
Wet-bulb and dry-bulb temperatures are recorded
Dry bulb – actual air temperature
Water vapor vaporizes from the wick as it is whirled and evaporated cooling lowers the temp. to the wet-bulb temperature
Important to remember – use the depression of the wet bulb on the chart
This is the difference between the wet and dry bulb temperatures
Aspirated psychrometers do the same thing, but use a fan instead whirling

23. Monitoring Water Vapor
The difference between the dry-bulb temperature and the wet-bulb
temperature, known as the wet bulb depression, is calibrated in
terms of percentage relative humidity on a psychrometric table.

24. Monitoring Water Vapor
The dewpoint can be obtained from measurements of the dry-bulb
temperature and the wet-bulb depression.

25. Monitoring Water Vapor
Water vapor satellite imagery
IR imagery using infrared wavelengths that detect water vapor
Water vapor imagery indicates presence of water vapor above 3000 m (10,000 ft) The whiter the image, the greater the moisture content of the air
This image shows moisture plumes extending from the Pacific Ocean into the central U.S. and in the southeastern U.S. from the Gulf of Mexico and Atlantic Ocean

26. How Air Becomes Saturated
As relative humidity nears 100%, condensation or deposition becomes more likely
Condensation or deposition will form clouds
Clouds are liquid and/or ice particles
Humidity increases when:
Air is cooled; saturation vapor pressure decreases while actual vapor pressure remains constant
Water vapor is added at a constant temperature; vapor pressure increases while saturation vapor pressure remains constant
As ascending saturated air (RH about 100%) expands and cools, saturation mixing ratio and actual mixing ratio decline and some water vapor is converted to water droplets or ice crystals

27. How Air Becomes Saturated
Adiabatic process and lapse rates (review from Chapter 5)
During an adiabatic process, no heat is exchanged between the air parcel and its environment
Expansional cooling and compressional heating of unsaturated air are referred to as adiabatic processes if no heat is exchanged with surroundings
Air cools adiabatically as it rises
Lower pressure with altitude allows the air to expand
Unsaturated ascending air cools at 9.8° C/1000 m (5.5° F/1000 ft) and it warms at the same rate upon descent.
This is called the dry adiabatic lapse rate
Upon saturation, air continues to cool, but at the moist adiabatic lapse rate of 6° C/1000 m (3.3° F/1000 ft) → rate is lower because latent heat released upon condensation partially offsets cooling as parcel rises

28. Atmospheric Stability
Air parcels are subject to buoyant forces caused by density differences between the surrounding air and the parcel itself
Atmospheric stability is the property of ambient air that either enhances (unstable) or suppresses (stable) vertical motion of air parcels
In stable air, an ascending parcel becomes cooler and more dense than the surrounding air
This causes the parcel to sink back to its original altitude
In unstable air, an ascending parcel becomes warmer and less dense than the surrounding air
This causes the parcel to continue rising

29. Stable Air
Note that movement of the parcel upward means it is colder than the surrounding air, so it sinks back down to its original altitude
Also, in movement of the parcel downward, it becomes warmer than the surrounding air, and returns to its original altitude
Stable air inhibits vertical motion

30. Unstable Air
Note that movement of the parcel upward means it is warmer than the surrounding air, so it continues rising.
Also, in movement of the parcel downward, it becomes colder than the surrounding air, and continues descending
Unstable air enhances vertical motion

31. Atmospheric Stability
Soundings
These are the temperature profiles of the ambient air through which air parcels are moving
Soundings (and hence stability) can change due to:
Local radiational heating and cooling
At night, cold ground cools and stabilizes the overlying air
During day, warm ground warms and destabilizes the overlying air
Air mass advection
Air mass is stabilized as it moves over a colder surface
Air mass is destabilized as it moves over a warmer surface
Large-scale ascent or descent of air
Subsiding air generally becomes more stable
Rising air generally becomes less stable

32. Atmospheric Stability
Absolute instability
Occurs when the air temperature is dropping more rapidly with altitude than the dry adiabatic lapse rate (9.8° C/1000 m)
Conditional instability
Occurs when the air temperature is dropping with altitude more rapidly than the moist adiabatic lapse rate (6° C/1000 m), but less rapidly than the dry adiabatic lapse rate
Air layer is stable for unsaturated air parcels and unstable for saturated air parcels
Implies that unsaturated air must be forced upwards in order to reach saturation

33. Atmospheric Stability
Absolute stability
Air layer is stable for both unsaturated and saturated air parcels and occurs when:
Temperature of ambient air drops more slowly with altitude than moist adiabatic lapse rate
Temperature does not change with altitude (isothermal)
Temperature increase with altitude (inversion)
Neutral air layer
Rising or descending parcel always has same temperature as ambient air
Neither impedes nor spurs upward or downward motion of air parcels

34. Atmospheric Stability

35. Stüve Diagrams
Temperature – Horizontal axis, increasing from left to right
Pressure – vertical axis, decreasing upward

36. Lifting Processes - Convective Lifting

37. Lifting Processes - Frontal Lifting
Frontal uplift occurs where contrasting air masses meet – leads to expansional cooling of rising air, and possible cloud and precipitation development
Warm front – as a cold and dry air mass retreats, the warm air advances by riding up and over the cold air
The leading edge of advancing warm air at the Earth’s surface is the warm front
Cold front – cold and dry air displaces warm and humid air by sliding under it and forcing the warm air upwards
The leading edge of advancing cold air at the Earth’s surface is the cold front

38. Lifting Processes – Orographic Lifting

39. Lifting Processes – Convergent Lifting
When surface winds converge, associated upward motion leads to expansional cooling, increasing relative humidity, and possible cloud and precipitation formation
For example, converging winds are largely responsible for cloudiness and precipitation in a low-pressure system
In another example, converging sea breezes contribute to high frequency of thunderstorms in central Florida