atmosphere rain fall

1. Moist Processes
ENVI1400: Lecture 7

2. ENVI 1400 : Meteorology and Forecasting 2
Water in the Atmosphere
• Almost all the water in the atmosphere is
contained within the troposphere.
• Most is in the form of water vapour, with some
as cloud water or ice.
• Typical vapour mixing ratios are:
~10 g kg-1 (low troposphere) (can be up to ~20 g kg-1)
~1 g kg-1 (mid troposphere)

3. ENVI 1400 : Meteorology and Forecasting 3
METEOSAT Water vapour image : 041019 – 1200 UTC

4. ENVI 1400 : Meteorology and Forecasting 4
METEOSAT visible image : 041019 – 1200 UTC

5. ENVI 1400 : Meteorology and Forecasting 5
Typical cloud water contents are:
cumulus (early stage) : 0.2 – 0.5 g m-3
cumulus (later stage) : 0.5 – 1.0 g m-3
cumulonimbus : 3 g m-3 (>5 g m-3 observed in
very strong updrafts)
alto-cumulus : 0.2 – 0.5 g m-3
stratocumulus / stratus : 0.1 – 0.5 g m-3

6. ENVI 1400 : Meteorology and Forecasting 6
Sources and Sinks
Sources:
– Evaporation from
surface: requires
energy to supply latent
heat of evaporation –
sunlight, conduction
from surface (cools
surface).
– Evaporation of
precipitation falling
from above: latent
heat supplied by
cooling of air
Sinks:
– Precipitation: rain,
snow, hail,…
– Condensation at the
surface: dew, frost
• N.B. Most of the water in
the atmosphere above a
specific location is not
from local evaporation,
but is advected from
somewhere else.

7. ENVI 1400 : Meteorology and Forecasting 7
Buoyancy Effects
Water in the atmosphere
has important effects on
dynamics, primarily
convective processes.
– Water vapour is less dense
than dry air
– Latent heat
released/absorbed during
condensation/evaporation.
• molecular weight of water
= 18 g mol-1
• mean molecular weight of
dry air ≈ 29 g mol-1
water vapour = 0.62 air
A mixture of humid air is
less dense than dry (or
less humid) air at the
same temperature and
pressure

8. ENVI 1400 : Meteorology and Forecasting 8
Latent Heat
Latent heat of evaporation
of water
Lv ≈ 2.5 MJ kg-1
large compared with specific
heat of dry air
Cp ≈ 1004 J kg-1 k-1
Evaporation of 1 gram of
liquid water (=1 cm3) into 1
cubic metre of air:
latent heat used ≈ 2500 J
cools air by ≈ 1.9 K.
Similarly latent heat is
released and air warmed
when liquid water
condenses out – e.g. as
cloud droplets.

9. ENVI 1400 : Meteorology and Forecasting 9
Condensation Conditions
Temperature is reduced to
below dew point.
Two most common mechanisms
for cooling are:
– Contact cooling : loss of heat to
a surface colder than the
overlying air, e.g. following
advection over a cooler surface,
or due to radiative cooling of the
surface at night.
– Dynamic cooling : adiabatic
lifting results in very efficient
cooling of the air. (see below)

10. ENVI 1400 : Meteorology and Forecasting 10
Adiabatic lifting may occur on
many scales:
– Largescale ascent along a warm
or cold front (100s of kilometers)
– The rise of individual convective
plumes to form cumulus clouds
(~100m to ~1km)
– Forced ascent over topographic
features (hills, mountains) to form
orographic cloud (~1km to >10s
km).
– Gravity waves above, and
downwind of mountains (few km).
Radiative cooling
(non-adiabatic process)
• Direct radiative cooling of the air
takes place, but is a very slow
process.
• Once cloud has formed, radiative
cooling of the cloud droplets (and
cooling of surrounding air by
conduction of heat to drops) is
much more efficient.
Radiative cooling  reduced
saturation vapour pressure 
more condensation  higher cloud
water content.

11. ENVI 1400 : Meteorology and Forecasting 11
Addition of water vapour, at
constant temperature,
raising humidity to
saturation point.
– Will occur over any water
surface. Since temperature
decreases with altitude,
evaporation into unsaturated
surface layer can result in
saturation of the air in the upper
boundary layer.
– Cold air moving over warmer
water can sometimes produce
‘steam fog’ : common in the
arctic, and observed over rivers
and streams on cold mornings.

12. ENVI 1400 : Meteorology and Forecasting 12
Mixing of two unsaturated air
masses as different temperatures
such that final humidity is above
saturation point
The Temperature and vapour
pressure resulting from mixing is
are averages of the initial values
in proportion to masses of each
being mixed
e.g.
Tmix = T1*M1 + T2*M2
M1+M2
T1 Tmix
T2

13. ENVI 1400 : Meteorology and Forecasting 13
Adiabatic Lifting
• As a parcel of air is lifted, the
pressure decreases & the parcel
expands and cools at the dry
adiabatic lapse rate.
• As the parcel cools, the
saturation mixing ratio
decreases; when it equals the
actual water vapour mixing ratio
the parcel becomes saturated
and condensation can occur.
• The level at which saturation
occurs is called the lifting
condensation level.
Lifting
condensation
level
Saturation mixing ratio
equal to actual water
vapour mixing ratio of parcel
Dew point
at surface

14. ENVI 1400 : Meteorology and Forecasting 14
• If the parcel continues to rise, it
will cool further; the saturation
mixing ratio decreases, and
more water condenses out.
• Condensation releases latent
heat; this offsets some of the
cooling due to lifting so that the
saturated air parcel cools at a
lower rate than dry air.
• The saturated (or wet)
adiabatic lapse rate is NOT
constant, but depends upon
both the temperature and
pressure.

15. ENVI 1400 : Meteorology and Forecasting 15
• The high the air temperature,
the greater the saturation mixing
ratio, and the more water vapour
can be held in a parcel of air.
• Because the gradient of the
saturation vapour pressure with
temperature increases with
temperature, a given decrease
in temperature below the dew
point will result in more water
condensing out at higher
temperatures than at low, and
hence more latent heat is
released.
• Thus the wet adiabatic lapse
rate decreases as the
temperature increases.
T
Q1
T
Q2

16. ENVI 1400 : Meteorology and Forecasting 16
The Föhn Effect
0 m
100 m
200 m
300 m
400 m
500 m
Lifting condensation level
Unsaturated air cooling
at -0.98°C per 100m
Saturated air cooling
at -0.5°C per 100m
10°C
Unsaturated air warming
at +0.98°C per 100m
9.02°C
8.04°C
7.06°C
6.08°C
5.58°C
5.08°C
6.54°C
7.52°C
8.50°C
9.48°C
10.46°C
11.44°C
The different lapse rates of unsaturated and saturated air mean that air flowing
down the lee side of a mountain range is frequently warmer than the air on the
upwind side. In the Alps this warm dry wind is called the Föhn, in American
Rockies it is known as a Chinook. The onset of such winds can result in very
rapid temperature rises (22°C in 5 minutes has been recorded) and is
associated with rapid melting of snow, and avalanche conditions.
4.58°C
5.56°C