Chapter Four - Test Two Flashcards
Movement of Water Vapor through the Atmosphere
What cycle?
The Hydrologic Cycle (Fig 4.2)
The Hydrologic Cycle (Fig 4.2)
- Annually 3.80 x 105 km3 of water evaporates/precipitates
- Enough to cover the entire globe with roughly 1 meter of rain
- ____ times more water is carried by the atmosphere over North America than by all the rivers on the continent!
Six
Water’s Changes of States
Water (H2O) is unique in that it is the only substance that can exists in all three phases (states) in the atmosphere
Keep in mind that the substance isn’t changing, only the distance between the molecules and the way in which they interact
Ice, Liquid Water and Water Vapor (Fig 4.3)
Solid (Ice):
the molecules, lacking Kinetic Energy, are locked into a six-sided (hexagonal) crystal lattice, unable to move.
Liquid (Water):
the molecules, having more Kinetic Energy, are now able to slide by one another, but are still in constant contact.
Gas (Water vapor):
the molecules now have enough Kinetic Energy to break all bonds and move about freely as individual molecules.
Whenever water changes state, it either releases or consumes energy
A process called Latent Heat Transfer (Fig. 4.3)
Going from a high energy state to a low energy state ____ energy
releases
-thereby: Warming the atmosphere.
gas→→→____→→→liquid→→→____→→→solid
gas→→→→→→→→→→→____→→→→→→→→→→solid
Condensation; Freezing; Deposition
Going from a low energy state to a high energy state ____ energy
consumes
- thereby: cooling the atmosphere.
There are numerous ways to express the water vapor content of the atmosphere (Fig. 4.6), which as seen earlier comprises between ___% and ____% of its volume;
0.1; 4
Four ways to express the water vapor content of the atmosphere:
- Mixing Ration (MR) [g/kg]
- Relative Humidity (RH) [%]
- Dewpoint Temperature (T d) [F, C]
- Wetbulb Temperature (T w) [F, C]
Mixing Ratio (MR)
The ratio of mass of water vapor in a unit mass of dry air, expressed as:
Grams of water vapor (g) / 1 kilogram of dry air (1 kg or g 1kg -1)
- Values range from:
20 g/kg in tropical regions
While the Mixing Ratio tells us how much water vapor is ____ ____ ____, its also important to know how much water vapor could be in the air, or stated differently, how much is water vapor is necessary to saturate the air
in the air
The amt. of water vapor necessary to saturate 1 kg of air is called the:
Saturation Mixing Ratio (MRs)
Saturation Mixing Ratio (MRs)
This amount is solely dependent upon the air’s temperature, therefore, if you know the air’s T, you know its MRs. This association is provide in both Fig 4.9 and Table 4.1
- Rule of thumb: for every 10ºC increase in T, the amount of water vapor needed to saturate the air (MRs) roughly doubles
The _____ of the air’s actual water-vapor content (MR) compared with the amount of water vapor required for saturation (MRs) which as we just saw depends on the T
ratio
Relative Humidity [RH] MR subscript =
= Water vapor content / Water vapor Capacity
= (MR / MRs) X 100%
Ex: Given: T = 25ºC; MR = 10g/kg
From Table. 4.1 We know that MRs = 20.0 g/kg
Therefore RH = 0.50 X 100% =- 50%
The RH is the most familiar and widely used measure by the public,however its also the most misunderstood, because:
RH tells us how close the air is to being saturated, not how much water vapor is in the air!
When the RH reaches 100%,(i.e. when MR = MRs) the air is ____.
saturated
RH is based on the air’s ____ content, as well its ____
water vapor; temperature
Relative Humidity can change in two ways:
Adding or Subtracting Moisture (Fig. 4.10)
Changes with Temperature (Fig. 4.11) [MOST IMPORTANT]
At constant T, the RH will increase if water vapor is added, until it reaches 100%:
- The addition of more water vapor will result in condensation, as the RH cannot (normally) exceed 100%
Even if the water vapor content remains constant:
- : The RH will increase if the T decreases:
If additional cooling takes place after saturation is achieved, condensation will result, a process that is critical for dew, fog and cloud development
- Even if the water vapor content remains constant:
- The RH will decrease if the T increases.
Natural Changes in Relative Humidity (due to ____ changes)
temperature
Diurnal changes, Advection, Convection
Diurnal changes ==>
important in dew, frost and some fog formation
Advection ==>
important in some fog formation
Convection ==>
important in cloud, precipitation formation
Fig. 4.12 illustrates that diurnal variations in RH are generally controlled by diurnal variations of the air Temperature
Early AM: T is usually lowest, RH is usually highest Late PM: T is usually highest, RH is usually lowest
This is why dew, frost and most fog forms at night
Dewpoint Temperature (Td) Mr no subscript
The temperature to which the air must cool in order to become saturated.
- point at which dew (frost) will form
- RH = 100%
Dewpoint Temperature (Td) Mr no subscript Just as the air’s temperature:
T provides the value of MR s
Dewpoint Temperature (Td) Mr no subscript The air’s dewpoint temperature:
Td provides the value of MR
We can calculate the Relative Humidity (%) if we know:
T, Td
____ tells us exactly how much water vapor is in the air, it is a very useful indicator
Td
Rules of thumb :
T is always -> Td; When T = Td, RH =100%
Td is generally:
- Higher in the summer (500-70 F) compared to winter (0 – 30 F)
- Higher near sources if water
- for Raleigh Td averages:
~to July: MR ~15 g/kg
~30 January: MR~ 3 g/kg
The human body is ____ efficient at cooling itself when the RH is high, because evaporation of perspiration (which as seen earlier, is a cooling process) is ____.
less; reduced
As a result, the NWS developed the: Heat Index
Heat Index
Draws attention to the dangers associated with high heat combined with high humidity.
This Index combines Temperature and Relative Humidity data to arrive at an:
“Apparent Temperature”
As we saw in Ch. 1, more people die because of Heat Waves in an average year than any other meteorological phenomenon (Fig. 3.C).
More than 500 people died in Chicago in 1995 More than 13,000 people died in France in 2003
Hygrometers
Instruments that provide a measure of RH
Hair Hygrometer
uses human hair which expands and contracts with increasing/decreasing
- Must be calibrated often
Electric Hygrometer
contains an electrical conductor coated with a moisture absorbing chemical. Electrical current changes with changing RH.
- Lightweight
- Used on radiosondes
Infrared Hygrometer
measures the amount of infrared energy absorbed [and hence from Kirchhoff’s Law] emitted by water vapor
- Used on satellites
The Ideal Gas Law (also called the ____) tells us that there is an inverse relationship between the volume of an air parcel and its ____.
Equation of State; temperature
The Ideal Gas Law:
Accordingly - when a parcel of air rises:
Its volume increases, there its temperature decreases due to ____
Conversely - when a parcel of air sinks:
Its volume decreases, therefore its temperature increases due to ____
expansion; compression
Dry Adiabatic Rate (DAR) (figs 4.16)
When an unsaturated (i.e. RH less than 100%) parcel of air rises:
- it expands and cools (adiabatically) at a constant rate of:
- 10 c per 1000 meters.
If the rising parcel’s T cools down to the Td, saturation and subsequently condensation occurs (RH now 100%) and:
A cloud is formed.
The height at which the cloud first forms is called the:
Lifted Condensation Level (LCL)
We can estimate the LCL using the SURFACE T and Td (C) and the following equation:
LCL Height (m) = 125 (T - Td)
The drier the air…
The higher the LCL or cloud base.
Wet Adiabatic Rate (WAR)
If a (now saturated) parcel of air continues to rise:
- it expands and cools at a (nearly) constant rate of:
- 5.0ºC per 1000 meters.
The rate of cooling is reduced because condensation releases latent heat energy as we saw earlier.
Processes That Lift Air
Orographic (topographic) Lifting (Fig. 4.18)
Frontal Wedging (Fig. 4.20)
Convergence (Fig 4.21)
Localized Convective Lift (Figs. 4.22, 4.23)
Orographic (topographic) Lifting (Fig. 4.18)
Air ascends up the windward side of mountains
Frontal Wedging (Fig. 4.20)
Air masses often collide, forcing ascension
Convergence (Fig 4.21)
Due to low pressure at the surface (figs. 6.20, 6.22)
Due to sea-breezes
Localized Convective Lift (Figs. 4.22, 4.23)
Buoyant parcels of air rise due to unequal heating of the surface creating thermals
The Leeward side of mountains are generally ____ (and often warmer) than the Windward side (Fig. 4.F).
Where did all of the moisture go?
drier; snowed or rained out on the windward side.
If one side of a mountain is much warmer and drier than the other side, it can lead to the formation of a:
Rain Shadow
