Water makes up only small fraction of atmosphere (0-4%) but is extremely important for understanding atmospheric processes. Not only because of its radiative properties but also because of its role in heat transport.
Nearly all water in Earth’s hydrosphere is in the ocean (97%) with very little in the atmosphere (0.001%), but there is an immense amount of water cycled through the atmosphere. The movement of water between reservoirs is known as the hydrologic cycle.
WATER’S CHANGES OF
STATE.
Water has the unique ability to change between any of the states (solid, liquid or gas) under terrestrial conditions. These changes require heat to be absorbed or released.
Evaporation: liquid -> gas (vapor) [requires heat]
Condensation: vapor -> liquid [releases heat = heat for evaporation]
Melting: solid -> liquid [requires heat]
Freezing: liquid -> solid [releases heat = heat for melting]
Sublimation: solid -> gas (e.g., shrinking ice cubes)
Deposition: gas -> solid (e.g., frost)
The above involve release/absorption of latent heat (latent mean hidden) as the heat exchange does not involve change in T. This is in contrast to transfer of sensible heat in which there is a change in T.
HUMIDITY [amount of water vapor in the air].
There are several measures of humidity:
Vapor Pressure = pressure contributed by vapor pressure alone. The maximum vapor pressure that can exist is called the saturation vapor pressure. When air is saturated evaporation = condensation.
The
amount of water required for saturation depends on temperature (but not
pressure) -> more water required for
higher temperature
Absolute Humidity = density of water = mass of water / volume
As an air parcel moves around changes in T and p cause the volume to
change, and hence absolute humidity can change without water being added or
removed.
Specific Humidity (q) = mass of water in unit mass of all air (units g/kg).
q = mv / m = mv / (mv + md)
Specific humidity is not affected by changes in p or T.
Mixing Ratio (r) = mass of water in unit
mass of dry air (units g/kg).
r = mv / md
Relative Humidity (RH) = ratio of actual water content compared with amount required for saturation (at that temperature).
RH be more easily measured than r (or absolute humidity), but is T dependent. However conversion between RH and r is easy: RH = r/rS x 100 % (rS = saturation mixing ratio)
RH can be changed by either
· changing amount of moisture: as water vapor is added the RH increases until saturation (RH=100%)
· changing the temperature: as parcel is cooled then the saturation mixing ratio decreases, and hence RH increases (without change in amount of water).
Temperature at which saturation occurs just by cooling is called the dew-point temperature. If continued cooling RH remains at 100% by condensation occurs … important for formation of clouds.
FIGURE: mixing ratio vrs T, pg 135 D et al.
The saturation mixing ratio versus temperature graph can be used to determine the relations between the above quantities. For example,
(1) T=30 OC and RH=50%, what is water vapor content?
[Question asked for precipitation predictions]
RH=50% means r is half way between r=0 and r=rS
@ 30 OC rS = 28 g/kg -> r = 0.5*28 = 14 g/kg
(2) T=30 OC and dew-point temperature DT=19 OC, what is the RH (or r)?
move up from r=0 and T=19 OC to r=r S curve to get actual r
i.e., r=14 g/kg
move up from r=0 and T=30 OC to r=r S curve to get saturation r
i.e., r S=28g/kg
RH=r/r S x 100% = 50%
FIGURE
Higher RH does not mean more water vapor. In particular cold places can have high RH but very low water vapor content, while warm places can have low RH but a lot more water vapor.
For example,
@ -10OC RH = 100% -> r = 2 g/kg
@ 25OC RH = 25% -> r = 20 g/kg
so “dry” air at 25 OC has ten times the water as “wet” air at -10 OC.
The RH can be changed by
1) daily T variations
2) movement of air from one place to another
3) vertical movement of air
There are two requirements for condensation:
(1) air must be saturated, and
(2) must be a surface on which water vapor can condense.
The surfaces on which this generally happens are those of tiny particles called condensation nuclei, e.g., microscopic dust, smoke, salt particles. These particles are very important because even if RH=100% there would be no clouds without them.
Condensation in the atmosphere results in the formation of Clouds, Fog, Dew, and Frost.
CONDENSATION IN
THE ATMOSPHERE
– visible aggregates of minute drops of water or tiny crystals of ice.
– Classified into several different types depending on height (high, middle, or low) and form (cirrus, cumulus, and stratus).
Fog
– A cloud with its base at the ground
– Different types depending on formation mechanism, e.g., radiation (radiative cooling), advection (warm, moist air over cold surface), upslope (orographic lifting), and evaporation (moisture from evaporation) fog.
Dew
– Condensation on objects that have radiated sufficient heat to lower T than dew-point of surroundings
– Frequently on grass as transpiration of water by blades raises RH.
Frost
– Forms when dew-point is less than freezing, and vapor converted directly to ice crystals (deposition).
Jet
aircraft engines expel large quantities of hot, moist air. As this air mixes
with surrounding air contrails (condensation trails) form.
Whether or not contrails form, and how long they last, depends on the ambient air and how near it is to saturation.
The contrails form a short distance from the aircraft as mixing has to take place before they form.
Note that the aircraft emissions also contain condensation nuclei, which increase cloud formation potential.
Contrails are examples of cloud formation by mixing …
FIGURE: `mixing line’, pg 180 D et al.
Temperature changes in which heat is not added or removed are called adiabatic temperature changes.
These result from expansion (cooling) or compression (heating)
of air. Compression (expansion) increases (decreases) motion of molecules and hence the temperature increases (decreases).
[Everyday example of heating via compression is a hand tire pump.]
As parcels rise -> lower pressure -> expand -> cool.
As parcels fall -> higher pressure -> compress -> heat.
For dry air the change in temperature with height is approximately 10OC/km … the dry adiabatic lapse rate (DLR).
If parcel rises high enough T falls below dew-point, and condensation can occur … this altitude is the condensation level.
Above condensation level rate of cooling decreases because cooling by expansion is partially offset by release of latent heat associated with condensation. This slower rate is the wet adiabatic lapse rate (WDR), and is approximately 5OC/km
What
depends whether a parcel rises, falls, or remains at a constant level? i.e., ,
what determines the stability of air?
Important as determines when air rises above condensation level (and forms clouds).
Parcel
is stable if returns to original position after a small displacement,
and unstable is moves away after displacement.
A lifted parcel will
rise if warmer than surroundings (as less dense) … unstable.
fall if cooler than surroundings (as more dense) … stable.
So stability of atmosphere is determined by the variation of temperature with altitude, called the environmental lapse rate (ELR).
3 regimes:
1. Absolute Stability: ELR < WLR
2. Absolute Instability: ELR > DLR
3. Conditional Stability: WLR < ELR < DLR
In (1) a parcel rising at DLR or WLR is always cooler than environment, and hence stable.
In (2) a parcel rising at DLR or WLR is always warmer than environment, and hence unstable.
In (3) a parcel rising at DLR is stable but unstable some distance above condensation level (when rising at WLR).
What processes can cause air to rise (and hence lead to condensation)?
1. Convective Lifting
Localized surface heating can produce air warmer than surroundings -> “thermals”. Produces only short-lived precipitation.
2. Orographic Lifting
Mountains acts as barriers to the flow, and can cause air to ascend. This produces large condensation/precipitation on windward side but little on leeward side. … “Rainshadow Desserts”
3. Frontal Lifting
Masses of cold and warm air often collide to produce a front. This can than produces the same effect as orographic lifting (warm air rises up front).
4. Convergence
Surface convergence (e.g., low pressure systems) leads to uplift (as air can’t go down).
Why do some clouds produce precipitation and not others?
Cloud droplets are tiny (~20x10-6m)
è fall at very slow rate (48hrs to fall 1km)
è evaporate before they reach the ground (typically evaporate within a few meters).
For a raindrop to be large enough to reach the ground it must contain ~ a million times the water of a single cloud droplet.
Clouds consist of many billions of these droplets, and all `compete’ for available water, so growth by condensation is slow.
So another mechanism is required. Two such mechanisms proposed:
1. Bergeron Process
2. Collision-Coalescence Process
· Rain
· Snow
· Sleet
· Glaze
· The movement of water between reservoirs in the Earth system is known as the hydrologic cycle.
· The transfer of (latent) heat occurs through processes that change the state of water, i.e., evaporation, condensation, melting, freezing, sublimation, and deposition.
· There are several, related, measures of humidity: vapor pressure, absolute humidity, specific humidity, mixing ratio, and relative humidity. When air is saturated the relative humidity is 100%.
· Condensation occurs when air is saturated and there a particles on which water can condense (so called condensation nuclei).
· There are four types of condensation: clouds, fog, dew, and frost.
· In dry air the rate of cooling, due to expansion, with height is 10C/km (dry adiabatic lapse rate), whereas for wet air the cooling is less, because of heat release by condensation, at around 6C/km (wet adiabatic lapse rate).
· The stability of an air parcel depends on its temperature relative to the ambient air, and the stability of the atmosphere depends on the vertical change of temperature profile compared with the dry and wet adiabatic lapse rates.
· There are four main process that cause air to rise (and lead to condensation): convective, orographic, frontal, and convergence lifting.
· Precipitation occurs when droplets or crystals gain sufficient size are formed to fall.