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