4 Humidity & Condensation Flashcards
5 unusual properties of water
despite being so common, water is actually a very unusual chemical:
• the melting and boiling points are relatively high compared to
similar chemicals (eg, CO, CO2
, SO2
, N2O)
• it has a very high heat capacity – the ability to store heat without
changing temperature
• it has an exceptionally high lattice energy and intermolecular
attraction (latent heat)
• it has an exceptionally high dielectric constant –It can store large amounts of electrical charge
• most substances expand as they warm – as water warms from 0 to 4
°C, it contracts
• water is able to dissolve a wide range of organic and inorganic
substances, while at the same time carry them away
• this is the foundation of chemical weathering, which controls
Earth’s climate on long time spans
• liquid water is most dense at 4°C – it becomes less dense as it warms
and cools from this point
•without this property, many lakes would not be able to sustain life
• the high heat capacity and its fluid properties make water and water
vapour an effective mechanism of transferring heat around the planet
Water molecule structure
this structure is very good at absorbing red and infrared radiantion (RYLEIGH SCATTERING)
-the electron affinity of oxygen draws electrons away from the
hydrogen atoms, generating a net positive charge on one end, and a
net negative charge on the other
Evaporation
1.Evaporation:the process by which the molecules in a liquid state spontaneously become gaseous
• direct evaporation: exposed bodies of water (oceans, lakes, rivers,
soils, etc.) evaporate directly into the atmosphere
• indirect evaporation: plants lose water through stomata, known as
transpiration
• often, the processes of direct evaporation and transpiration are
grouped together as evapotranspiration
water is earths ________ which regulates heat
thermostat
transpiration
plants lose water through stomata, known as
transpiration
Evapotranspiration determined by 4 factors
- Energy Availability
- the humidity gradient away from the surface
- the wind speed immediately above the surface
- Water Availability
• on a molecular level, water molecules are
always moving – occasionally, molecules near
the surface escape into the air
• at the same time, water molecules in the air
may move into the water – condensation
• it is the balance between outgoing and
incoming molecules that determines the
process
• of course, as you add energy (ie, increase the temperature), the
molecules move faster and it is easier to escape the water surface,
therefore evaporation increases
• however, air can only hold so many water molecules before it becomes
saturated – at this point, evaporation is balanced by condensation and
there is no net change in the water balance
•therefore evaporation needs a steep humidity gradient
• just like a temperature or pressure gradient, the steeper the
gradient the faster the flow
• of course, if winds are removing the saturated air and replacing it with
dry air, the steep humidity gradient will be maintained and evaporation
will continue
• finally, as the supply of water diminishes, the rate of evaporation slows
• this is not a problem over oceans, but can be for soils and plants
evaporation needs a _____ humidity gradient
steep
Humidity
amount of water in the air
• since water is an atmospheric gas, it contributes its own partial
pressure, more commonly referred to as vapour pressure
• the vapour pressure of air is dependent on temperature and the density
of the water vapour molecules
• if temperature is high, then the molecules are moving faster and
exerting a greater pressure
• if density is high, then the total mass of the molecules is greater,
exerting a greater pressure
• in general, the influence of temperature is small compared to
density
air can only hold so much water vapour before it becomes saturated and condensation occurs
• therefore, there is a maximum amount of water vapour in the air
and also a maximum vapour pressure, the saturation vapour pressure
• since this always represents the maximum density, then saturation
vapour pressure is solely dependent on temperature
–at very cold temperatures the sponge shrinks, warm temperatures increase sponge size
if temperature is high, then the molecules are moving faster and
exerting a _____(greater/lesser) pressure
greater
• if density is high, then the total mass of the molecules is ______(greater/lesser) ,
exerting a greater pressure
greater
T OR F
Density is far more important in humidity then temperature
T
Absolute humidity
Absolute humidity: the instantaneous density of water in the air
-measured in grams per cubic meter
• if the size of the air parcel changes, then the density also changes
• this is problematic, since air parcels are constantly expanding and
contracting
• even if the total number of water molecules stays the same, the
density, and ultimately humidity, will always be changing
Specific Humidity
Specific humidity: the mass of water vapour in a given mass of air
-grams per kg
• this avoids the problem of expanding and contracting air parcels
• also, specific humidity is not affected by temperature, so it is useful when
comparing the moisture content of air in different places
• however, it is affected by air pressure
• specific humidity is closely tied to vapour pressure
• since there is a maximum amount of vapour that air can hold – the
saturation vapour pressure – then there is also a saturation specific
humidity
Mixing Ratio
Mixing ratio: the mass of water vapour relative to the mass of all the other atmospheric gasas
• specific humidity and mixing ratio are very similar
• specific humidity considers the mass of everything in the
atmosphere while mixing ratio considers everything except the water
vapour
• since the total amount of water vapour is relatively small when
compared to the whole atmosphere, the mixing ratio is always very
close to the specific humidity
• similarly, the maximum mixing ratio is the saturation mixing ratio
relative humidity
the amount of water vapour in the air compared to
the maximum amount of water vapour the air can hold at a given
temperature
=(specific humidity/saturation specific humidity)*100
• this is the most widely used measure of atmospheric moisture, and is
dimensionless since it is a ratio
• relative humidity takes into account the fact that the air parcel changes
• if the air parcel warms, the saturation specific humidity increases
and the relative humidity decreases
• if the air parcel cools, the saturation specific humidity decreases
and the relative humidity increases
because relative humidity is heavily dependent on temperature, we
can observe substantial daily and seasonal RH changes
• the diurnal temperature cycle results in a similar diurnal relative
humidity cycle (provided the total amount of water vapour stays the
same)
• if the air parcel ____, the saturation specific humidity increases
and the relative humidity decrease
warms
• if the air parcel _____, the saturation specific humidity decreases
and the relative humidity increases
cools
T OR F
Temperature and relative humidity have opposing patterns, relative humidity becomes highest at night and lowest during the day
T
Dew Point
Dew point: the temperature at which saturation occurs
• despite being measured as a temperature, it is dependent exclusively
on the amount of water vapour in the air
• it is usually a measure of how much you must cool an air parcel
before saturation is reached and condensation occurs
• if the dew point is high, then there is a lot of water vapour in the
air
• when the air parcel cools to the dew point, RH is 100%
-air temp is always more then dew point
-if dew point is close to(high) outside temp, it means that the air parcel is very saturated
• the dew point temperature can never be greater than the actual air
temperature
• as the air parcel cools to its dew point, cooling may continue, but
the air will begin to remove the moisture from the air in the form of
condensation and precipitation
• in this case, the dew point temperature begins to decrease at the same rate as the air parcel temperature
if the dew point temperature of an air parcel falls below 0°C, we refer to
it as the _____ point
frost
3 ways to Saturate an air parcel
1.add water from evaporation or evapotranspiration
ex: when you shower
• hot water from the showerhead evaporates into the air in the room
• raindrops falling from the cloud base will evaporate into the air
immediately below the cloud – this generates a precipitation fog
– eventually so much moisture is added that saturation is achieved,
and condensation begins to form, first on the mirror, then as a fog
.same thing happens under a cloud
- precipitation fog(when you think you can see rain)
- input of water into atmosphere creating new clouds
2.mix cold air into warm air, increases relative humidity and dew point
.when 2 air parcels mix, their properties become shared
ex: plane contrails and steam fog
• typically, the temperature will reach some average of the 2 original
parcels – this could raise or lower the temperature of either parcel
• at the same time, the water vapour in the 2 parcels mix to produce
a new specific humidity
• if this new specific humidity exceeds the saturation specific humidity
of the new air parcel, then it is supersaturated and condensation must
occur
-changing temp and changing relative humidity
-makes them super saturated which it doesn’t like so it tries to move below line
3.Lower temp of air parcel by removing energy
• atmospheric cooling is the most common way of achieving saturation
and is most critical in the formation of clouds and ultimately precipitation
• cloud and fog droplets form when the air becomes supersaturated
• individual droplets may be evaporating, but the rate of
condensation is exceeding the rate of evaporation
• however, these droplets have particular properties that affect their
behaviour
•.they are not flat
•.they do not contain pure water
condensation
the process by which the molecules in a gaseous state spontaneously become liquid
• rapidly moving water vapour molecules may randomly collide with
a surface and bond to it, creating a liquid
• if evaporation from the surface exceeds condensation onto it, then
the air is unsaturated
• if condensation onto the surface exceeds evaporation from it, then
the air is supersaturated
• when condensation and evaporation are balanced, the air is
saturated
Droplet Curvature
• a large droplet has little
curvature, while a small
droplet is highly curved
• the curvature actually
affects evaporation
•Cloud droplets are much smaller then rain droplets
• small cloud droplets
evaporate faster than large
rain droplets
• since smaller droplets evaporate faster, they actually need relative
humidities exceeding 100% to survive – this increases the rate of
condensation to balance the increased rate of evaporation
• if not, the small droplets would not exist for very long
.more curvature=more _______
evaporation
Nucleation
• recall that water vapour has to condense onto something – a mirror, a
water body…
• occasionally, two water molecules will collide in mid-air and coalesce
into a liquid molecule under supersaturated conditions – known as
homogeneous nucleation
• but this produces a very small droplet, which evaporates away very
quickly unless supersaturation was maintained for a long time, which is
not very common
• fortunately, the atmosphere contains hygroscopic materials – aerosols
•.these aerosols attract water molecules, in a process known as heterogeneous nucleation
• the aerosol itself becomes a condensation nucleus
• sometimes, the condensation nuclei is not soluble, but water will stick to
its surface
• these tend to be larger aerosols, and their size reduces the
curvature of the droplet and therefore the evaporation rate
impure water has a ______(higher/lower) saturation point than pure water
lower
• if these impure molecules are found at the outer edge of the droplet,
they will reduce the rate of evaporation
-this means that an impure droplet will exist longer and will promote
growth through further condensation
Condensation Nuclei
• the effects of curvature, impurity, and size tend to balance out, so that
most condensation occurs at a relative humidity of 100%
•condensation nuclei are abundant in the atmosphere and always available
• but not all nuclei are the same
• some are more hygroscopic than others
• some are larger than others
• some will attract water at humidities below 90%
• this produces extremely small droplets, which produce haze
• in general, condensation nuclei are more abundant over the continents
than over the oceans, and also more abundant near Earth’s surface than
higher in the atmosphere
•.this suggests that continental dust is a major type of nuclei
• over the oceans, sea salt is important (and very hygroscopic)
• but other factors are important too
• volcanoes and fossil fuel burning produce sulphur dioxide, which
turns into sulphuric acid, which is very hygroscopic and soluble
• phytoplankton in the upper ocean release dimethyl sulphide
• quantifying aerosols and their sources is an important component of
understanding climate change
Ice Nucleation
• of course, saturation (and condensation/deposition) can occur at
temperatures below 0°C
• this logically means that instead of condensing into water droplets,
the vapour is deposited as ice crystals
• but, while water always melts at 0°C, it doesn’t always freeze at 0°C
•atmospheric water will not spontaneously deposit into ice until -4 degrees
• liquid water that exists in the atmosphere between -4 and 0°C is
known as supercooled water
• just as liquid droplets need to condense onto a condensation nucleus,
ice crystals need to condense onto an ice nucleus
• unlike condensation nuclei, ice nuclei are rare in the atmosphere,
and must have a 6-sided lattice structure similar to a typical ice crystal
• there are no materials that are effective ice nuclei at temperatures
above -4°C
• at temperatures below -4°C, the nucleation of ice becomes more
common, and the likelihood of ice crystals forming increases as the
temperature decreases
• between -10 and -40°C, saturation can lead to ice crystals and
supercooled water – clouds in this temperature range will contain both
• as the temperature decreases, the proportion of ice crystals in the
cloud increases
• below -40°C, saturation leads only to ice crystals, with or without the
presence of ice nuclei
_____ minerals share the 6 sided lattice structure that ice nucleus has
clay
• the most common way of achieving saturation is to simply reduce the
air temperature
• since the amount of moisture in the air will remain the same, the
lower temperature (and hence volume) will lead to a higher humidity
WE CAN COOL AIR IN 2 WAYS
1.Diabatically:energy is removed from the air by conduction, thus
lowering the temperature (2nd Law of Thermodynamics)
• eg, a warm air parcel flows over a cold air parcel
• at the contact between the 2 parcels, there will be an energy
exchange
• 2
nd law states that the warmer air will cool and the colder air will
warm, until an equilibrium is reached
• typically, this exchange is by conduction
•the diabatic process is often responsible for fog
2.Adiabatically:there is no energy removed from the air, but instead
the air expands, thus lowering the temperature (1st Law of
Thermodynamics
- the adiabatic process involves no energy exchange, so:
• this tells us that as air expands, the air itself is doing work (expansion)
and therefore is consuming energy, thus lowering the temperature
• by the same logic, as air compresses, work is being performed on
the air (compression) and therefore is gaining energy, thus raising the
temperature
dry adiabatic lapse rate
dry adiabatic lapse rate (DALR):
• as an unsaturated air parcel (ie, RH < 100%) rises, it will expand and cool
at a rate of 1°C per 100m
• this is known as the
• likewise, a falling dry air parcel will warm at 1°C per 100m
10 degrees for every km you go up(if wet, it is slower)
what is the lifting condensation level?
the altitude at which that air parcel rises high enough, it will cool down to the dew or frost
point, and saturation will occur
Saturated/Wet Adiabatic lapse rate
• if that air parcel rises high enough, it will cool down to the dew or frost
point, and saturation will occur
• the altitude at which this happens is called the lifting condensation
level (LCL)
• the saturated air will continue to rise and condensation will occur
• but, recall that condensation releases latent heat, which warms the
air parcel –the air will continue to cool as it rises, but at a slower rate
• the rate of cooling with condensation occurring is the saturated
adiabatic lapse rate (SALR) or wet adiabatic lapse rate (WALR)
• the SALR is always less than the DALR, due to the release of latent heat which partially offsets the cooling rate • but the SALR is not constant – it varies with temperature • when warm saturated air cools, more condensation occurs and more latent heat is released, offsetting the cooling even more • when cold saturated air cools, less condensation occurs and less latent heat is released, offsetting the cooling less
T or F
the DALR is always less than the SALR due to the release of latent heat
which partially offsets the cooling rate
F
SALR is always less than DALR
Environmental Lapse Rate
background temperature 6.5 degrees for ever km you go up, kind of a mix of dry and wet lapse rate
-The lapse rate of nonrising air—commonly referred to as the normal, or environmental, lapse rate—is highly variable, being affected by radiation, convection, and condensation; it averages about 6.5 °C per kilometre (18.8 °F per mile) in the lower atmosphere (troposphere).
• we know that the bottom of the troposphere is warmer than the top,
and the change in temperature with altitude is the ELR
• even large parcels of air (moving or not) will have an internal ELR
Types of Condensation (3)
Dew:lowering of temperature to the dew point near the surface – favoured under clear skies and no wind – diabatic process -appears as a coating of liquid water on surface
Frost:lowering of air temperature to saturation point when the dew point is below 0°C – diabatic process -appears as a large number of small white crystals on surfaces
Frozen dew:formation of dew as above, followed by cooling to temperature below freezing – diabatic process -continuous layer of solid ice on surface
Types of Diabatic Condensation (10)
Dew: lowering of temperature to the dew point near the surface – favoured under clear skies and no wind – diabatic process -appears as a coating of liquid water on surface
Frost: lowering of air temperature to saturation point when the dew point is below 0°C – diabatic process -appears as a large number of small white crystals on surfaces
Frozen dew: formation of dew as above, followed by cooling to temperature below freezing – diabatic process -continuous layer of solid ice on surface
Fog: usually by cooling of layer of air with light winds – sometimes by evaporating water from falling precipitation or by mixing warm, moist air with cold air – diabatic or adiabatic process large concentration of suspended droplets in layer of air near ground – under extreme cold can consist of suspended ice crystals
Radiation fog:
cooling of air to dew point by longwave
radiation loss – diabatic process
same as above
Advection fog:
cooling of air to dew point as it passes
over a cool surface – diabatic process
same as above
Upslope fog:
cooling of air as it flows upslope –
adiabatic process
same as above
Precipitation fog: Increasing the water vapour content of the air by evaporation of falling droplets – adiabatic process same as above
Steam fog:
mixing warm, moist air with cold air –
adiabatic process
same as above
Clouds: usually by lifting of air and adiabatic cooling concentration of suspended droplets and/or ice crystals in air well above the surface
Dew
• dew: liquid condensation on a surface
•.typically occurs on clear and calm nights
• longwave radiation easily moves away from the surface since there
are no clouds, leading to rapid cooling by conduction of the surface
air
• the lack of wind means there will be no turbulent mixing of warm air
near the surface
• mainly seen first thing in the
morning – the coolest time of day
• as temperatures rise,
evaporation exceeds
condensation and the dew
disappears
Frost
• frost: solid condensation (ie, deposition) on a surface
• the same process as dew formation, but occurs when the dew point is
below 0°C
frozen dew
• forms first as dew, but continued
cooling freezes the water droplets
-black ice on roads is frozen dew
Radiation Fog
•all types of fog are essentially thin clouds that occur at Earths surface
• radiation fog is caused by the loss of longwave radiation on clear nights,
but can be promoted by light winds, which stir the air and enhance
condensation
• most fog dissipates in the morning, since insolation warms the surface and
the air above it – this leads to evaporation of the fog
• the comment that fog “lifts” is
not true – there is no vertical
displacement
• it simply evaporates from
the bottom up, so it only
appears to lift
-• in thick fogs, insolation can be reflected by the fog top, reducing the
insolation reaching the surface and hence slowing the rate of evaporation
-.these fogs can last well into the day, or may last several days in the most extreme cases
Advection Fog
• form when warm, moist air flows over a cool surface
• diabatic cooling will occur, and the warm air will experience a
temperature reduction leading to fog
• since the air is moving,the fog will be carried along, and can travel downwind for a considerable distance
• advection fogs are typically
thicker than radiation fogs, and
therefore may last longer
• this is because the
associated winds mix the air
to high altitudes, causing the
fog to thicken
Upsloope fog
• caused by the adiabatic cooling of air as it is forced to rise over higher
terrain
Climate Change
• it is clear that atmospheric moisture and temperature are closely linked
•.warm air holds more moisture
•.warm water evaporated faster
• we also know that global temperatures have risen over the past century –
this must have an effect on atmospheric moisture
• recall also that water vapour is an effective greenhouse gas
• greenhouse gases keep the atmosphere warm
• but atmospheric warming must lead to more water vapour in the
atmosphere, which must lead to even more warming
• this is a positive feedback – a critical component of Earth’s
climate system
• in fact, it is believed that specific humidity has increased 4% since 1970,
so there is more atmospheric moisture
• but this is accompanied by an increase in temperature,so we have not seen any change in relative humidity
• even though there is more moisture, the air can effectively hold more
• but, will this increase in global atmospheric moisture, coupled with a
positive feedback, be the end of the world?
• no, since negative feedback will kick in
Positive Feedback
• positive feedbacks have the ability to substantially amplify any changes
that may be occurring
• they can also be complicated,meaning that they are hard to understand and predict, and are not easy for climate modellers
• positive feedbacks are a major contributor to recent and potential future
climate change, and have the ability to transform the entire planet,
especially the global energy balance
Negative Feedback
• negative feedbacks are the opposite of positive feedbacks – they
effectively slow down any changes that are occurring
• a warmer atmosphere will radiate more longwave radiation – remember
the radiation laws? – this will offset some of the warming
• also, more atmospheric moisture should lead to more clouds, which are white and have a high albedo
• this should reduce insolation and offset some of the warming as well
• this is known as the cloud-radiative feedback system
• but clouds are also very efficient absorbers of longwave radiation,
trapping the excess energy released by the atmosphere – this acts as a
positive feedback
• clouds are the single-most complicated aspect of Earth’s climate
cloud-radiative feedback system
also, more atmospheric moisture should lead to more clouds, which are white and have a high albedo
• but clouds are also very efficient absorbers of longwave radiation,
trapping the excess energy released by the atmosphere – this acts as a
positive feedback
• clouds are the single-most complicated aspect of Earth’s climate
