Transcript Evaporation, Humidity, Fog, Cloud & Precipitation

EG4508: Issues in environmental science
Meteorology and Climate
Dr Mark Cresswell
Evaporation, Humidity, Fog,
Cloud & Precipitation
Phases of water
• Water (H2O) is the most important
material on the planet
• Water can exist in solid, liquid and gas
phases
• Water molecules are free to move in a
gas, are closer together in a liquid and
are locked in an orderly pattern as a
solid
Phases of water
• As a solid, water forms hexagonal (6sided) crystals we call ice
• In freezing air, if enough energy is
available, ice can change directly into
gas (water vapour). This is called
sublimation
• If a water vapour molecule combined
with ice crystals it is deposition
Phases of water
• Applying warmth (energy) to an ice
crystal means that the molecules
vibrate faster - so much so that they
can vibrate out of their hexagonal
crystal structure - the ice melts
• At the surface of water, some molecules
have just enough energy to break free
from the rest - called evaporation
Phases of water
• Some water vapour molecules with very
little energy can combine with other
water molecules on the surface of water
- called condensing
• If a cover is placed over a beaker of
water, eventually an equilibrium
between escaping and returning water
molecules is reached.
Phases of water
• When this state of equilibrium is
reached the air in the beaker is said to
be saturated with water vapour
• Removing the cover from the beaker
would allow some molecules to be
blown away - so the air would no longer
be saturated and more would have to
evaporate to take their place
Phases of water
• This explains why evaporation occurs
more readily when there is wind than
on a still day
• Temperature also affects evaporation
• Warm water means that molecules have
more energy and speed up. These
molecules are more likely to escape
from the liquid surface
Phases of water
• Higher temperatures lead to enhanced
evaporation
• Conversely, condensation is more likely
to occur when the temperature is
lowered
Hydrological (water) cycle
• Water is in constant motion in the
atmosphere, in the oceans and on land
• Water evaporates over the oceans, then
condenses at altitude to form clouds
• Wind carries clouds over land where they
release their water as precipitation
• Over land, water is released into the
atmosphere by transpiration and evaporation
Linacre et al, 1997
Hydrological (water) cycle
• Evaporation and transpiration over
terrestrial areas accounts for only about
15% if the 1.5 billion billion gallons that
annually evaporate. The other 85%
evaporate over the oceans
• If all atmospheric water vapour were to
condense and fall as rain it would cover
the globe 2.5 centimetres thick
Absolute humidity
• The mass of water vapour in a given
volume (parcel) of air
AH = mass of water vapour / volume of air
• Represents water vapour density usually expressed as g/m3
Specific humidity
• Mass of water vapour compared to the
total mass of the parcel of air (including
water vapour)
SH = mass of water vapour / total mass of air
• usually expressed as g/kg
Vapour pressure
• The air’s moisture content may also be
expressed in terms of the pressure exerted by
the water molecules within it
• Air pressure at sea level is the result of
pressure exerted by all gas molecules
(nitrogen and oxygen included). The total
pressure is equal to the sum of all pressures
from all gases - known as Dalton’s law of
partial pressure
Vapour pressure
• An increase in the number of water vapour
molecules will tend to increase the total
vapour pressure
• Actual vapour pressure indicates the air’s
total water vapour content. Saturation vapour
pressure describes how much water vapour is
necessary in order to make the air saturated
at any given temperature (remember the
beaker hypothesis).
Vapour pressure
• Saturation vapour pressure is the pressure
that the water vapour molecules would exert
if the air were saturated with vapour at a
given temperature
Relative humidity
• A common measure that is often
misunderstood
• It tells us how close the air is to being
saturated
• It is the ratio of the amount of water vapour
actually in the air compared to the maximum
amount of water vapour required for
saturation at that particular temperature and
pressure
Relative humidity
• Put more simply, it is the ratio of the air’s
water vapour content to its capacity:
RH = water vapour content / water vapour capacity
RH(%) = (actual VP / SVP) * 100
Dew point
• The dew point is the temperature to
which air must be cooled (with no
change in air pressure or moisture
content) for saturation to occur
• When the dew point temperature is
reached on a surface, dew, frost or fog
forms
• Lifting condensation level for air aloft
Measuring humidity
• Humidity is measured using a
psychrometer (whirling or clockwork)
• Wet and dry bulb thermometers based
in a Stevenson screen use the same
principle
• Difference between wet and dry bulb
temperatures indicates water vapour
content of the air
Formation of FOG #1
• The process of condensation that forms
fog and clouds is not so simple. It is not
simply the case that saturation (dew
point) must be reached
• There must be airborne particles on
which water vapour can condense
Formation of FOG #2
• Although air looks clean - it never really is. Air
contains many tiny particles (impurities)
• many of these particles serve as a surface on
which condensation can occur
• These particles are called condensation
nuclei
• Some condensation nuclei are very small with
a radius of < 0.2µm (Aitken nuclei)
• Particles 0.2 - 1µm are called large nuclei
• Particles > 1µm giant nuclei
Formation of FOG #3
• As the relative humidity reaches 75 100% (saturation) water condenses
onto condensation nuclei
• As the air cools and becomes more
saturated the droplets of suspended
condensed water get larger until visible
to the naked eye
• We can see these clouds of droplets as
fog
Radiation fog
Formation of FOG #4
• City air (with its extra impurities)
produces a thicker fog as there are
more condensation nuclei
• London often suffered from very thick
fog as a result of pollution and industrial
activity until legislation was introduced
early in the 20th century
City fog – exacerbated by pollutant particulates
Formation of FOG #5
• Fog often forms near the ground on a
natural surface (e.g. football pitch)
• This is exacerbated on clear nights
when radiation leaves rapidly and cools
the ground down and the moist air
directly above it
• This is known as radiation fog
Formation of FOG #6
• Fog is usually seen in low lying areas as
it is denser than the surrounding air and
is pulled to the surface by gravity
• When fog is seen to "burn off" by
sunlight it is actually the heating of the
ground which rises the air temperature
above causing the air to become
unsaturated and the fog dissipates
Formation of FOG #7
• Fog may form when warm moist air travels
over a cold surface. This is known as
advection fog
• Fog may form by the mixing of two
unsaturated air masses - leading to
evaporation and enrichment of water vapour.
This type of fog is called evaporation (mixing)
fog. It forms when cold unsaturated air settles
over warm water from which water may be
evaporating - explains why fogs form over
lakes and ponds in summer
Advection Fog in a valley
Formation of Clouds #1
• Clouds form in a similar way to fog - except
that the process takes place aloft
• In the case of cloud formation, the cooling
required to cause water to condense on
particulate nuclei is due to adiabatic cooling
• Clouds consist of tiny particles of ice or water
droplets (formed around condensation nuclei)
so small and light in weight that impacts from
the air's randomly moving molecules are
sufficient to keep them aloft
Formation of Clouds #2
• Cloud formation may be convectional,
orographic or frontal
• Convectional clouds form when moist air is
carried upwards by the action of vertical
convection (due to solar heating of the
surface). Moist air cools as it ascends until it
becomes saturated. At the point of saturation,
the moist air condenses to form cloud
(Convectional Condensation Level)
Formation of Clouds #3
• Strong surface heating where there is
very moist air can lead to the
development of intense storm clouds so called "anvil shaped" structures that
produce large amounts of precipitation
Formation of Clouds #4
• Orographic cloud forms when moist air is forced
upwards - usually when it flows over a plateau or
mountain. The air forced upslope cools until it
becomes saturated forming clouds near to or above
the mountainous structure (Lifting Condensation
Level).
• The air (now free of moisture) flows down the lee side
of the mountain at a higher temperature as energy
lost during condensation is carried away by the wind.
Rainfall usually occurs on the lee side, forming a rain
shadow on the upslope side.
Formation of Clouds #5
• With frontal cloud formation, moist warm air is
forced above cooler air (the cooler air acting
like a wedge). As this moist air is forced
upwards it cools until the air becomes
saturated and condenses into clouds. The
clouds are usually easily seen as a visible
ridge along the line of an active front.
Formation of Clouds #6
• When winds are forced to converge at low
level (the ITCZ near the equator for example)
air will be forced upwards where it cools and
condenses. This is known as convergence
clouds and explains why the equatorial ITCZ
is visible from space as a zone of active cloud
systems along the equator.
Cloud Types
NAME
ABBR.
HEIGHT (km)
CATEGORY
Cirrus
Ci
6 – 10
HIGH
Cirrocumulus
Cc
6 – 10
HIGH
Cirrostratus
Cs
6 – 10
HIGH
Altostratus
As
3–6
MEDIUM
Altocumulus
Ac
3–6
MEDIUM
Stratocumulus
Ac
<3
LOW
Stratus
St
1–2
LOW
Cumulus
Cu
0.6 – 6
CUMULIFORM
Cumulonimbus
Cb
To the tropopause
CUMULIFORM
Precipitation processes
• We have already seen how important
condensation nuclei are for the formation of
droplets when the air becomes saturated
• To keep a droplet in equilibrium, more water
vapour molecules are needed around it to
replace those that are constantly evaporating
from the surface
Precipitation processes
• Small cloud droplets have a greater curvature
which causes a more rapid rate of
evaporation. As a result of this process
(curvature effect) smaller droplets require
an even greater vapour pressure to keep
them from evaporating away. This requires
the air to be supersaturated - with a relative
humidity greater than 100%. The smaller the
droplet, the greater the supersaturation
needed to keep it in equilibrium
Precipitation processes
• One may ask, how do droplets with a diameter of
<1µm grow to the size of a cloud droplet?
• The answer lies with the cloud condensation nuclei.
Many of these nuclei are hygroscopic (have an
affinity for water vapour)
• Condensation may begin when the vapour pressure is
much lower than the saturated vapour pressure
• This reduces the equilibrium vapour pressure
required and is known as the solute effect
Precipitation processes
• In warm clouds (tops warmer than -15ºC) the action
of collisions between droplets is important
• Random collisions with already large droplets
mediated by salt particles (hygroscopic condensation
nuclei) produce larger droplets when they collide
• Large droplets begin to reach terminal velocity and
collide with smaller droplets in their wake - merging
together in a process called coalescence
• Falling droplets may evaporate on their way down, or
reach the ground as drizzle if the air below is moist
Precipitation processes
The most important factor in the production of raindrops is the
cloud’s liquid water content. In a cloud with enough water, other
factors are:
•
•
•
•
The range of droplet sizes
The cloud thickness
Updraughts of the cloud
Electric charge of the droplets and the cloud itself
• Coalescence may be enhanced where strongly
charged droplets exist in a strong electrical field
Precipitation processes
• In very deep convective clouds the ice-crystal process is an
important factor in precipitation
• Ice crystals may form nuclei upon which other ice crystals may
form
• These are deposition nuclei as water vapour changes directly
into ice without passing through the liquid phase
• The constant supply of moisture to an ice crystal allows it to
enlarge rapidly, it becomes heavy enough to overcome updrafts
and begins to fall
• If these crystals stick together (accretion) the icy matter
(rime) that forms is called graupel (or snow pellets).
Ice crystals
No
precipitation
Altocumulus Cloud
Water and Ice clouds – usually bring precipitation
After 15 – 20 hours
Stratus Cloud
Typically overcast or drizzle conditions
Cumulus Cloud
Associated with gusty winds and heavy precipitation
ITCZ (Inter-Tropical Convergence Zone)
Observing Cloud #1
• Clouds are observed at regular intervals at
ground stations
• The degree of cloud cover in the sky is
expressed as oktas (or eighths)
• A value of 4 oktas would mean that about half
(50%) of the sky was covered with cloud
• A value of 8 oktas means that the entire sky
is cloud-covered (totally overcast)
Observing Cloud #2
• Sometimes it is preferable to estimate
the cloud cover for the three main layers
of the troposphere (low, middle and
high)