Deals with water vapour or humidity, clouds, fog, and precipitation.

1. Atmospheric Water
Dr. Akepati S. Reddy
Professor, School of Energy and Environment
Thapar University, Patiala – 147 001
Punjab (INDIA)

2. Atmosphere and Water
• Snow and ice, rain water, clouds and water vapour all influence the
weather and the atmosphere
• Water and land surfaces
– Differential heating and cooling
– Creation of highs and lows
– Creation of winds
• Oceans
– Contribute to 80% of the rainfall
– Supplies pure freshwater and receives relatively salty water
• Polar ice caps and glaciers (rivers of ice)
– Reflection of solar radiation (albedo)
• Evaporation, transpiration (and sublimation) -10% by transpiration
– Rates of evaporation depends on solar radiation, temperature,
humidity and wind
– Transpiration helps plants to stay cool (similarly perspiration keeps
humans and animals cool)
– Evapo-transpiration transfer heat from earth surface to atmosphere

3. Atmosphere and Water
• Water vapour in the atmosphere
– 12900 kM3 of water (0.04% of the fresh water and 0.001% of the total
water) is present in the atmosphere (as vapour and clouds)
– An important greenhouse gas of the atmosphere
– Radiation absorption and greenhouse effect
• Moisture laden air parcels (or winds)
– Transport and redistribute heat
• Condensation (deposition) and cloud (and fog) formation
– Effects on atmospheric stability (dry and wet adiabatic lapse rates and
environmental lapse rates)
– (Long-wave) radiation absorption and (short-wave) radiation reflection
• Precipitation (and deposition)
– Rains, snowfall, sleet and hails
– Storms, tornadoes and hurricanes
• Water turnover time in the atmosphere is 10 days (what is average
rainfall of the earth surface?)
– Total surface of the earth: 510,072,000 km2

4. Total earth surface: 510,072,000 km
2
- Total land area: 148,940,000 km
2
(29.2%) – polar ice
caps and glaciers: 11% of the land area - Total oceans area: 361,132,000 km
2
(70.8%) water)
Atmospheric water 3095 ml3
12,900 kM3 (0.04% of fresh
water or 0.001% of total water)

5. Amount of water present in the atmosphere is 3100 cubic miles
Earth's average annual rainfall is about 100 cm
Water turnover time of the atmosphere is about 10 days?

6. Humidity Variables: Vapour Pressure (e) and
Saturation Vapour Pressure (es)













TTR
L
ee
v
s
11
exp
0
0
e0 is 0.611 kPa
Latent heat of vaporization (2.453 × 106 J/kg
T0 is 273 K and T is temperature Kelvin
Rv is 461 J K-1 Kg-1 (gas constant for H2O)
L is latent heat – slightly a function of
Temperature and given as (2501–645+2.361 T)
Clausius-Clapeyron Equation
Sub-
saturated
air
Saturated
air
Super-
saturated air
• Partial pressure of water vapour (a
gas) in air (a mixture of gases) is
vapor pressure.
• Symbol used is e and units are
pressure units (kPa)
• Equilibrium (saturation) value of
vapor pressure (es) - for unsaturated
air: e < es
• Air can be slightly supersaturated (e
> es) - when there are no surfaces for
condensation.
Over ice surface for temperature <0°C

7. Humidity Variables
Mixing ratio (r or w)
• Ratio of mass of water vapour to the mass of dry air
• It is proportional to the ratio of partial pressure of water vapor (e)
to partial pressure of the remaining gases in the air (P-e).
• Tells you how much water vapor is actually in the air
• Saturated mixing ratio (rs) is where es is used in place of e
eP
e
wr



 ε is rd/rv (0.622 g vapor/g dry air) - gas
constants ratio for dry air to water vapor
e is partial pressure of water vapor
P is pressure of total air
P
e
q


 Ratio of mass of water vapor to mass of
total (moist) air
Specific humidity (q)
Absolute Humidity
d
v
v
P
e
TR
e
 

Concentration of water vapor in air (g/ m3).
It is essentially partial density, and can be found from
partial pressure using the ideal gas law for water vapor

8. Humidity Variables: Relative Humidity (RH)
• Ratio of actual vapour pressure (actual water vapour content) to the
saturation vapour pressure (capacity of air for water vapour) at a
temperature
– RH is low in the atmosphere in summers than in winters but
absolute moisture content is higher in summers than in winters
• Dew point: temperature (Td) to which an ait parcel of vapour
pressure (e) and temperature (T) should be cooled for condensation
to start (for RH to become 100%)
– Frost point/Ice point: Dew point temperature is below freezing point
• Evaporative power of air is indicated by dew point depression (T–Td)
– Dew point depression also indicates the quantity of water present in air
• When water droplets and ice both exist together below the freezing
temperature then the saturation vapour pressure above the liquid
droplets is larger than that above the ice

9. Relative Humidity – Temperature - Dew Point
• Dew point temperature corresponds to the amount of water vapour
in the air – high dew point temperature means more water vapour
and low dew point means less water vapour
• Air with dew point >15°C is typically considered as humid and air
with dew point <5°C is considered as fairly dry
• Difference between the actual temperature and the dew point
temperature is called as the dew point depression
‘f’ is relative humidity (%)
TD is dew point temperature (C)
T is temperature (C)
‘e’ is actual vapour pressure (Pa or mb)
‘es’ is saturated vapour pressure (Pa or mb)

10. Changes with the air
temperature and with the
addition/removal of
moisture to/from air
Higher temperatures have
higher sat. vapour pressure)
Shows diurnal variability
with the air temperature
higher during morning hours
(closer to dew point
temperature) and lower in
the afternoon hours

11. Heat Index
Temperature and humidity together form Apparent Temperature (heat index)
Temperature and humidity are related to human comfort level
Apparent temp. 58 = air at 47C temp. and 40% RH = air at 38C and 65% RH
Heat Index combines heat and humidity
factors.
High humidity reduces evaporation and
the cooling power of perspiration.

12. Condensation and Cloud Formation
• Condensation occurs at ≥100% RH and requires surfaces (natural
and/or manmade) or condensation nuclei (soluble aerosols,
hygroscopic aerosols and insoluble aerosols)
• Nucleation is mostly heterogeneous and also homogeneous
– Heterogeneous nucleation: dust, ash, spores, soot, salt, etc. can be the
condensation nuclei
• Deposition can also occur around ice nucleii
– Homogeneous nucleation: condensation in the absence of nucleii can
occur at super saturation vapour pressures
• At below -40C spontaneous condensation occurs
• Condensation into liquid droplets is much easier than ice crystal
formation
– Vapour pressure over water at a temperature is greater than that over
the ice – hence water evaporates easily from droplets and freeze easily
(forming ice)
• When condensation is occurring, a thin layer of air next to the
surface of the droplet remains super saturated
– Smaller droplets require higher relative humidities to remain as liquid
without completely getting evaporated

13. Condensation and Cloud Formation
• Clouds (collections of small water droplets or ice crystals) are
formed in rising air parcels when their RH becomes ≥100%
– Convection, topography, convergence at lows, lifting occurring at
weather fronts all these rise air parcels
• rising air parcels can get adiabatically cooled, dew point
temperatures can be reached and condensation can be initiated
– Mixing of two moist air parcels at two different temperatures, and
advective cooling (air masses moving over cold surfaces) also initiate
condensation and cloud/fog formation
• Cloud formation starts at Lifting Condensation Level (zLCL) (height of
convective cloud base)
– ZLCL is the level at which saturation first occurs if an air parcel is lifted
adiabatically
– It indicates the height of the cloud base
• Winds and atmospheric stability conditions influence the
movement of rising of air parcels

14. Atmospheric Stability and Rising of Air Parcels
• Dry and Wet Adiabatic Lapse Rates (Dry ALR and Wet ALR) and
Environmental Lapse Rates (ELR)
• Rising air parcels show dry ALR of 9.8C/kM, and once saturated with
water vapour, the air parcels show wet ALR of 5-6C/kM
– Latent heat release during the vapour condensation is responsible for
the lower lapse rates in case of wet ALR
• Atmospheric stability classification into unstable, conditional
unstable, stable and highly stable atmosphere
– determines whether air parcels will rise or not
• Stable atmosphere: in a stable atmosphere, when nudged upward
or downward, an air parcel will return back to its original position
• Atmospheric actual temperature profile (ELR) vis-a-vis Dry ALR or
Wet ALR indicates the stability classification of the atmosphere
– ELR greater than dry ALR indicates unstable atmosphere – most
favourable for the rise of air parcels
– ELR < Wet ALR indicates stable atmosphere – air parcels do not rise
beyond certain limit (air parcel – atmos. temp. difference determines)
– Temperature inversion conditions indicate highly stable atmosphere
– ELR between dry ALR and wet ALR indicate conditional unstability
(unstable for dry air parcels and stable for wet air parcels

15. Atmospheric Stability

16. Clouds
• Clouds are collections of water droplets or ice crystals of size 0.01
to 0.02 mm (size of condensation nucleii is around 0.0002 mm)
– High altitude clouds can have ice crystals rather than liquid droplets
– Low altitude clouds (<1.0 kM and >0°C) have only liquid droplets
– Mid altitude clouds (1 to 5.5 kM and 0°C to -20°C) have mixture of ice
and liquid
– Hygroscopic condensation and varying sizes of condensation nucleii
can result in the droplets of varying sizes
• Terminal settling velocity of the cloud droplets and ice crystals is
too low for th precipitation to occur
• Clouds are classified on the basis of their form and height as cirrus,
cumulus and stratus clouds
– On the basis of height, clouds are also classified as high clouds,
meddle clouds and low clouds
– High clouds (above 6000 meters) are thin and white icy crystal clouds,
and may include cirrus, cirrostratus, and cirrocumulus clouds
– Mid clouds - appear at 2000-6000 m altitude - a prefix alto- is used
(altocumulus & altostratus) –may cause infrequent light snow/drizzle
– Low clouds (below 2000 m) include stratus, stratocumulus, and
nimbostratus

18. Clouds
• Cirrus clouds (curl of hair clouds)
– These are high altitude (~5kM) clouds mostly made up of ice crystals
– These are white, wispy, feathery and thin clouds (look like horse tails)
– Presence of these clouds these mean good weather
• Cumulus clouds (pile clouds)
– These are mid altitude puffy cotton ball clouds with flat bottoms
– These are formed when air parcels rise rapidly into atmosphere
– Small cumulus clouds usually mean fair weather – can however
produce light rains or snow showers and even thunder storms
• Stratus clouds
– These are rain/snow producing low altitude clouds and look like a
blanket (sheets or layers) cover over the sky
– These are low clouds formed when warm air mass is slowly pushed up
over cooler air mass or over a mountain
• Cumulonimbus clouds
– These are formed when wet air parcels rise very high into the sky on
hot days (often reach even stratospheric heights)
– These are sure signs of bad weather - associated with thunder and
lightning - may produce thunder-showers, storms and hails (tornados!)

19. Clouds
• Mammatus clouds
– These are associated with cumulonimbus clouds, and look like a
package of cotton balls squeezed together
– These may have pouch like shapes hanging out at the bottom
• Lenticular clouds
– These are found near mountains and look like lenses or almonds or
flying saucers (alien spaceships)
– These produce small amounts of precipitation
– Orgraphic clouds !
• Altocumulus clouds
– These are usually formed at altitudes lower than the cirrus clouds
– These are usually round shaped and have grey patches or layers
– These seldom make rain
• Nimbus clouds
– These are rainy thick grey masses
– Nimbostratus clouds - main precipitation making clouds
• Contrails: made by high flying jet air planes

20. Fog
• Fog is not physically different from clouds – fog is formed at ground
level and clouds at higher altitudes
• Types of fog
– Radiation fog: Earth cools through radiation emission, the air adjacent
to the earth surface also if cools below dew point, radiation fog is
formed
– Advective fog: Air moving over a cool surface if cools below its dew
point, advective fog is formed
– Mixing fog: Mixing of two unsaturated air parcels (one a cold air parcel
and the other a warm high humidity air parcel) can result in below
dew point and form mixing fog
• Fog formed from the air breathed out in winters
– Upslope fog: Orographic raise along the hill slopes can cool the air
parcels to below dew point and form upslope fog
– Steam fog: Evaporation of warm water into the cold air above can rise
humidity to saturation level and form steam fog
– Precipitation fog: warm rain can evaporate and add moisture into the
surrounding air – increases humidity to saturation level and result in
the formation of precipitation fog

21. Precipitation
• Water droplets of clouds, because of the size (0.01 to 0.02 mm), can
not produce any precipitation
– Gathering together of the droplets (a million droplets!) into rain drops
(about 2 mm size) is needed for precipitation to occur
– Collision and coalescence processes bring about rain drops formation
• Production of raindrops is controlled by the following factors:
– Cloud water content
– Droplet size ranges
– Updrafts of the cloud
– Electric field of the droplets
– Cloud thickness factors
• Ice crystals of higher altitude grow in size while falling through the
mixture of liquid and ice crystals

22. Precipitation
• Types of precipitation: Rain, Sleet and freezing rain, Snow and Hail
• Snow
– Clumps of six sided crystals
• Sleet (ice pellets or frozen rain drops )
– Rainfall through a layer of freezing air near the ground results in sleet
– Freezing rain
– Black ice or glare ice (super cooled raindrops freezing when come in
contact with cold surfaces, roads, roofs, power lines, etc.
• Hails (lumps of ice)
– When ice drops get additional coating of water they become hails
– Hail stone is layered (grows through adding layers)
– Hails come from cumulonimbus clouds
• Dew and frozen dew:
– Condensation of water vapour on the ground objects (surfaces)
– Freezing of the dew (frozen dew)
• Frost
– Deposition of ice crystals on surfaces