Unit2 Lecture1

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ES 1111: 

ES 1111 Moisture in the Atmosphere

Moisture in the Atmosphere: 

Moisture in the Atmosphere Water is part of a distinct system called the hydrological cycle Water is removed from the surface into the atmosphere by two processes: Evaporation: Water removed off a free water surface, like a lake, river, ocean, or even soil Transpiration: Water released into the air by the stomata in leaves

The Hydrological Cycle: 

The Hydrological Cycle Figure 4.1, Page 56

Evapotranspiration: 

Evapotranspiration Both evaporation and transpiration result in the same thing – water in the atmosphere Because they result in the same thing, we combine the two processes into one word: evapotranspiration (ET)

Rate of Evapotranspiration: 

Rate of Evapotranspiration The rate of evapotranspiration is controlled by: Energy availability Humidity gradient away from the surface Wind speed above the surface Water availability

Water Availability: 

Water Availability An open water surface provides a continuous water source Transpiration can provide water up until a certain limit based upon the plant’s ability to pull water up through its roots and out its stomatae (rate of transpiration)

Potential Evapotranspiration: 

Potential Evapotranspiration Rate that will occur from a well-watered, actively growing, short green crop covering the ground surface Essentially equal to the ET over a large open water surface Rate is controlled entirely by atmospheric conditions Measure of possible agricultural activity if the crop is well-watered Measured by an evaporation pan

Evaporation Pan: 

Evaporation Pan Figure 4.I.1, Page 58

Actual Evapotranspiration: 

Actual Evapotranspiration Amount actually lost from the surface given the prevailing atmospheric and ground conditions Provides information of soil moisture conditions and the local water balance Measured by a lysimeter (difficult to maintain, not many in existence) that weighs the grass, soil, and water above

Lysimeter: 

Lysimeter Figure 4.I.2, Page 59

Global Evaporation Map: 

Global Evaporation Map Difficult to construct due to sparse data Maximum rates are found over subtropical oceans (clearer skies in subtropics than at the Equator) Rates decrease as one goes poleward Land values less than ocean values

Global Evaporation Map: 

Global Evaporation Map Figure 4.2, Page 61

Saturation of the Air: 

Saturation of the Air Saturation refers to the equilibrium condition where the rate of evaporation into the air equals the rate of condensation out of the air (in = out) When the air is saturated, evaporation can still take place, as long as condensation of the same amount also takes place The amount of water vapor present in the atmosphere at saturation depends upon Temperature Ice versus water surface that water enters/leaves Pressure (can be ignored if dealing with same height)

Type of Surface: 

Type of Surface The amount of water vapor that can be present in the atmosphere depends on whether there is a plane of pure water as a surface, or a plane of pure ice Less water vapor can be present in the atmosphere at saturation over an ice surface than a water surface

Measuring the Vapor Content: 

Measuring the Vapor Content There are a number of ways that we can measure and express the amount of water vapor content in the atmosphere: Vapor Pressure Mixing Ratio Relative Humidity Dew Point Precipitable Water Vapor Others (absolute humidity, specific humidity)

Vapor Pressure (e): 

Vapor Pressure (e) Vapor pressure (e) is simply the amount of pressure exerted only by the water vapor in the air The pressures exerted by all the other gases are not considered The unit for vapor pressure will be in units of pressure (millibars and hectopascals are the same value with a different name)

Mixing Ratio (w): 

Mixing Ratio (w) The mixing ratio (w) is the mass of water vapor present in the atmosphere compared to the mass of dry air in a given volume of air Because water vapor is at most 4% of the atmosphere per volume, we use units of grams of water vapor per kilogram of dry air (to avoid decimals)

Saturation Vapor Pressure (es) and Saturation Mixing Ratio (ws): 

Saturation Vapor Pressure (es) and Saturation Mixing Ratio (ws) If we measure the vapor pressure when the air is saturated, we call that vapor pressure the saturation vapor pressure (es) If we measure the mixing ratio when the air is saturated, we call that mixing ratio the saturation mixing ratio (ws) Unless the air is saturated, the vapor pressure and mixing ratio will always be less than the saturation vapor pressure and the saturation mixing ratio The vapor pressure and mixing ratio will only be equal to the saturation vapor pressure and the saturation mixing ratio if the air is saturated

Saturation Vapor Pressure vs. Temperature: 

Saturation Vapor Pressure vs. Temperature Figure 4.3, Page 62

Relative Humidity (RH): 

Relative Humidity (RH) The relative humidity (RH) is calculated using the actual water vapor content in the air (mixing ratio) and the amount of water vapor that could be present in the air if it were saturated (saturation mixing ratio) RH = w/ws x 100% The relative humidity is simply what percentage the atmosphere is towards being saturated Relative humidity is not a good measure of exactly how much water vapor is present (50% relative humidity at a temperature of 80 degrees Fahrenheit will involve more water vapor than 50% relative humidity at -40 degrees) Relative humidity can change even when the amount of water vapor has not changed (when the temperature changes and the saturation mixing ratio changes as a result)

Dew Point (Td): 

Dew Point (Td) The dew point temperature is the temperature at which the air will become saturated if the pressure and water vapor content remain the same The higher the dew point, the more water vapor that is present in the atmosphere The temperature is always greater than the dew point unless the air is saturated (when the temperature and dew point are equal)

Precipitable Water Vapor (PWV): 

Precipitable Water Vapor (PWV) Precipitable water vapor (PWV) is the amount of water vapor present in a column above the surface of the Earth Measured in units of inches or millimeters It represents the maximum amount of water that could fall down to the surface as precipitation if all the water vapor converted into a liquid or a solid Can be measured easily by weather balloons or satellites

Clouds: 

Clouds When the air becomes saturated, water vapor may condense to form solid ice or liquid water droplets in the atmosphere, and this is what clouds are made of The type of cloud that is formed depends on what process led to the air becoming saturated

Cloud Classifications: 

Cloud Classifications Clouds can be basically classified based upon their visual appearance and their height above the ground (which influences whether they are made of ice or water)

Classification Based on Appearance: 

Classification Based on Appearance Clouds that are “heaped up” in appearance are called cumulus clouds Clouds that are flat and featureless in appearance are called stratus clouds Clouds that are very thin and whispy (resemble horse’s tails or flocks of hair) are called cirrus clouds

Classification Based on Height: 

Classification Based on Height Clouds that are low to the ground and are likely to be composed of liquid water only are not given any special prefix Clouds that are in the middle part of the troposphere and are likely to have a mixture of ice and water are given the prefix alto- Clouds that are in the upper part of the troposphere and are likely to only have ice are given the prefix cirro- (except cirrus clouds)

Basic Cloud Classifications: 

Basic Cloud Classifications Appearance: Heaped-Up Flat Height Low cumulus stratus Middle altocumulus altostratus High cirrocumulus cirrostratus

Cloud Classifications: 

Cloud Classifications A cloud that is causing precipitation to fall at the surface is given the prefix or suffix nimbo- or –nimbus Stratus cloud: nimbostratus Cumulus cloud: cumulonimbus Cumulonimbus (a thunderstorm) is not classified as low, middle or high because of its extensive vertical development

Cloud Formation: 

Cloud Formation Once again, clouds form whenever the air reaches saturation. This can happen by any of the following processes: Cooling the air down and causing the temperature to equal to the dew point Adding water vapor to the air and causing the dew point to equal the temperature Mix two different bodies of air together to average out the moisture and temperature, thereby possibly resulting in saturation

Cooling the Air: 

Cooling the Air Air can be cooled in any of the following ways: Air coming into contact with a cold surface Vertical motion in the atmosphere

Contact With a Cold Surface: 

Contact With a Cold Surface If the skies are clear and the wind calm, the surface may cool down rapidly due to the emission and loss of infrared radiation out to space. The surface cools down, and the air in contact with the surface also cools down. Once the dew point is reached, radiation fog forms at the surface If warm, moist air is blown by the wind (advected) over a cold surface and it cools down to its dew point, advection fog is the result

Vertical Motions: 

Vertical Motions While radiation fog and advection fog may be important at some locations (San Francisco and Seattle), vertical motions in the atmosphere is the most common mechanism of cloud formation. Vertical air motions can be caused by: Buoyant (unstable) ascent Forced ascent over sloping terrain Fronts and low pressure storm systems

Parcels: 

Parcels To understand how vertical air motion can result in saturation, it is best to introduce the concept of a parcel A parcel is simply a blob of air that we will move around and study what happens to it You may picture in your mind a balloon or a box of air as the parcel There are three rules to parcels: There is no energy exchange between the parcel and the environment (the parcel is insulated) There is no mass exchange between the parcel and the environment (the parcel keeps the molecules it starts with) The parcel may change shape as needed (so the idea of a rigid box is not as good as a balloon)

Surface Parcel: 

Surface Parcel We can start by forming a parcel right here in the room (picture filling a balloon with air that is from the classroom) The starting temperature of the air in the balloon will be identical to the room temperature. In addition, the pressure and humidity are also identical to that of the outside

Lifting a Parcel Up: 

Lifting a Parcel Up What happens if we take our parcel and lift it vertically upward in the atmosphere? One variable that we know will change in the environment as we go up in the atmosphere is pressure. Recall that pressure always decreases rapidly with height If we lift our balloon up quickly, the pressure inside the balloon will be greater than the pressure outside of the balloon. What happens? The balloon will expand in size until the pressure inside the balloon equals the pressure outside

The Meaning of Temperature: 

The Meaning of Temperature The more scientifically precise definition of temperature is that it is the average kinetic energy of a substance Kinetic energy is the energy of motion. All the gas molecules are zipping around inside the parcel, and they have mass, so the gas molecules also have kinetic energy Some gas molecules are moving faster than others. When we take the temperature of the parcel, we are taking an average of how much kinetic energy there is of all the gas molecules

Expansion Takes Work: 

Expansion Takes Work In physics, work is defined as a force being applied over a distance In order to do work, energy must be expended When our parcel expands after it is lifted, it is doing work by pushing out against the environment a certain distance Therefore, the parcel must use up energy in order to expand. The energy that is available to be used up is the kinetic energy of the molecules. With the kinetic energy being used up, the molecules must slow down With the molecules moving more slowly, the average kinetic energy of the molecules in the parcel decreases, and the temperature decreases

Expansion Cools: 

Expansion Cools If you have ever let air out of a tire and felt it as it exited the tube, it should have felt cold Air leaving a tire feels cold because it is going from a higher pressure environment to a lower pressure environment, so it expands and cools

Adiabatic Processes: 

Adiabatic Processes Notice that our parcel will cool even though heat is not leaving the parcel, it is cooling down due to internal processes Because it does not involve a heat exchange between the parcel and the environment, we use the word adiabatic to describe this process (“without heat”)

Dry Adiabatic Lapse Rate: 

Dry Adiabatic Lapse Rate If a parcel is dry (water vapor is not condensing or depositing out), a dry parcel will always cool at the same rate when it is lifted The dry adiabatic lapse rate is approximately equal to 10˚ C for every kilometer the parcel is lifted For example, a dry parcel at the surface with a temperature of 20˚ C will cool down to a temperature of -10˚ C if lifted up 3 kilometers

Cooling Dry Adiabatically: 

Cooling Dry Adiabatically When a parcel is lifted vertically, the temperature will eventually cool down to the dew point (which also decreases slightly as the parcel is lifted) Once the parcel is saturated, water vapor will condense or deposit out, and recall that that those processes involve the release of latent heat into the parcel The level where the parcel becomes saturated is called the lifting condensation level (LCL)

Saturated Adiabatic Lapse Rate: 

Saturated Adiabatic Lapse Rate Because latent heat is being released into the parcel during phase changes, the saturated parcel will not cool at the dry adiabatic lapse rate any longer The addition of latent heat counteracts the cooling that results from lifting, so a saturated parcel will cool more slowly than a dry parcel The saturated adiabatic lapse rate is not a constant value like the dry adiabatic lapse rate

Lapse Rates and LCL: 

Lapse Rates and LCL Figure 4.6, Page 68

Orographic Lift: 

Orographic Lift Orographic lift is when a parcel is forced to rise upward due to sloping terrain When wind blows towards a mountain, it can not blow through the mountain The wind is forced to rise up as it encounters this mountain This vertical motion results in cooling

Frontal Uplift: 

Frontal Uplift Air parcels can also rise upward in association with cold fronts and warm fronts Cold fronts have a steep vertical slope, so the lifting mechanism is more vigorous Warm fronts have a shallow vertical slope, but they still cause parcels to rise upward

Frontal Uplift: 

Frontal Uplift Figure 9.6(b), Page 157

Buoyancy: 

Buoyancy An air parcel may also rise vertically upward if it finds itself less dense than the surrounding environment If you take a beach ball and try to submerse it in a pool, the ball will shoot back up to the surface once you let go

Ideal Gas Law: 

Ideal Gas Law The Ideal Gas Law (also called the equation of state) relates the pressure (P), density (ρ), and temperature (T) of a gas P = ρ R T, where R is a constant If pressure remains a constant and temperature increases, the density must decrease This explains how a hot air balloon works

Buoyancy and Parcels: 

Buoyancy and Parcels In order to determine if a parcel is buoyant, we must compare the temperature of the parcel with the temperature of the environment Whenever the parcel is warmer than the environment, it will be less dense and continue to rise Whenever the parcel is colder than the environment, it will be more dense and sink down Whenever the parcel is the same temperature as the environment, the parcel will remain

Stability Conditions: 

Stability Conditions Unstable: When a parcel is displaced and it continues rising away from its original position Stable: When a parcel is displaced and it returns to its original position Neutral: When a parcel is displaced and it remains at its new position

Stability and Buoyancy: 

Stability and Buoyancy In order to determine if the atmospheric stability is stable, unstable, or neutral, we must compare the temperature of the parcel with that of the environment If Tp > Te , then unstable If Tp < Te , then stable If Tp = Te , then neutral

Clouds vs. Stability: 

Clouds vs. Stability Cumulus clouds, because of their vertical development, are signs of instability Stratus clouds, because of their lack of vertical development, are signs of stability

Diurnal Variations in Environmental Lapse Rates: 

Diurnal Variations in Environmental Lapse Rates The temperature profile of the troposphere can change throughout the course of a day The temperature usually decreases with height at all levels in the late afternoon There usually is an inversion (temperature increasing with height) that starts to form in the evening and reaches maximum strength at sunrise The temperature near the surface changes the most, whereas the temperature farther up sees less change throughout the course of a day

Diurnal Variations in Environmental Lapse Rates: 

Diurnal Variations in Environmental Lapse Rates Figure 4.10, Page 71

Other Means to Saturate: 

Other Means to Saturate Water vapor uptake can yield saturated parcels. “Steam Fog” over lakes is an example where water vapor uptake over the lake can result in saturation Mixing of two parcels together can also result in saturation. Seeing your breath on a winter’s day and jet contrails are examples of this