Atmospheric Moisture and Condensation

Chapters 4 and parts of 5

1

2

Chapter 4:Atmospheric Moisture

• Relevant chapter content‒ Water in the Atmosphere‒ Ways to describe humidity‒ Heat Index‒ Measuring humidity

Chapter 5:Condensation, Dew, Fog and Clouds‒ Basics of condensation‒ Dew and Frost‒ Fog formation and types‒ Clouds and cloud observations

3

Water in the Atmosphere

4

• What is the chemical formula for water?

• What is the shape of ice crystals?

Water ‒ H2O

6

• All phases of water are present in the atmosphere‒ Gas

• Water vapor in the air‒ Liquid

• Cloud droplets, fog, rain‒ Solid

• Ice crystals (in clouds), snow, sleet, hail

• Hydrologic Cycle

7

8

• The term humidity is used to describe the amount of water vapor in the air

• Measured with a hygrometer‒ Many different types

• The amount of water vapor in the atmosphere varies dramatically‒ 0% to 5%

Water Vapor in the Atmosphere

• The maximum amount of water vapor possible depends on the temperature and pressure at a certain location

• Warmer air can “hold” more water vapor than colder air

• When the air contains the maximum amount of water vapor for the current conditions, it is said to be saturated.

9

10

Ways to describe the amount of moisture in the atmosphere• Dew Point Temperature• Absolute humidity• Specific humidity• Mixing Ratio• Vapor Pressure• Relative humidity• Others…

11

Dew Point Temperature (Td )

• Temperature to which air must be cooled in order for saturation to occur

• Td is always ≤ T• It depends solely on the amount of water vapor in the air

• It can be used to forecast minimum temperatures

12

Absolute Humidity• Mass of water vapor in a given volume of air‒ (Water vapor density)

• Units g/m3

• Not very useful in atmospheric studies‒ Volume of parcel of air changes when it goes up or down

14

Specific Humidity and Mixing Ratio

• Specific Humidity‒ Mass of water vapor divided by the mass of all airin the parcel

• Mixing Ratio‒ Mass of water vapor divided by the mass of dry air

• (dry air is total minus water vapor)

• Units g/kg for both‒ grams of water vapor per kilograms of air

• Remain constant when air goes up and down‒ as long as water vapor is not added or removed

• Global distribution of specific humidity• Mixing ratio distribution would have same shape

17

Vapor Pressure

• Pressure exerted by the water vapor present in the air‒ Air pressure is the sum of the partial pressures of all the different gases

‒ Vapor pressure is the partial pressure of water vapor

• Varies approx 1-30 hPa, depending on amount of water vapor present

• (Note: much smaller than average SLP of approx.1,000 hPa)

Vapor Pressure vs. Saturation Vapor Pressure

• Vapor pressure‒ Actual pressure exerted by water vapor

• Moisture dependent

• Saturation vapor pressure‒ The pressure that water vapor would exert if the air was saturated• (if it had as much w.v. as it can “hold” for the current conditions

• Temperature dependent 18

21

Relative Humidity• Ratio of actual amount of water vapor to maximum amount possible

• Expressed in terms of %• Can be calculated using

• Does not indicate the actual water vapor content of the air‒ Tells you how close you are to saturation‒ Tells you how humid it is for a certain temperature

%100pressurevaporsaturation

pressurevaporRH

22

• Factors that must change to increase or decrease relative humidity‒ Amount of water vapor in the air‒ Air temperature

• To increase relative humidity‒ _______ the amount of water vapor‒ _______ the temperature

• Notice that‒ When the air is saturated, T = Td and the RH = 100%

23

• Variation of Relative Humidity During a 24-hr Period

Dew point temperature

• Global distribution of relative humidity

26

• Practice at home‒ Humidity variables

• handout

27

Calculating RH…

• … If you know T and Td

• Calculate the relative humidity if the temperature is 70°F and the dew point is 65°F.

‒ We are going to need saturation vapor pressure (from T) and vapor pressure (from Td)

29

• Calculate the relative humidity if the temperature is 70°F and the dew point is 65°F.

‒ For T = 70°F, SVP = 25.0 hPa‒ For Td = 65°F, VP = 21.0 hPa

‒ RH = VP/SVP x 100% = 21hPa/25hPa*100%‒ RH = 84%

30

Calculating Td

• …If you know T and RH

• Calculate the dew point temperature if the temperature is 80°F and the relative humidity is 60%.

• First need to find the vapor pressure

‒ RH = VP/SVPx100% = 60%=60/100=0.6

‒ VP=0.6*SVP

32

• Calculate the dew point temperature if the temperature is 80°F and the relative humidity is 60%.

‒ RH = VP/SVP = 60%‒ For T= 80°F, SVP = 35 hPa

‒ VP=.6*SVP=0.6*35 hPa = 21 hPa

33

• Calculate the dew point temperature if the temperature is 80°F and the relative humidity is 60%.

‒ RH = VP/SVP = 60%‒ For T= 80°F, SVP = 35 hPa

‒ VP=0.6*SVP=0.6*35 hPa = 21 hPa

‒ A VP of 21 hPa corresponds to‒ Td = 65°F

• Practice at home‒ Relative Humidity calculations

• handout

34

35

Comparing Specific and Relative humidity

Specific Humidity Relative Humidity

36

Relative Humidity and Human Comfort

37

What humans perceive - Review• There is a constant heat exchange between our skin and the environment

• The temperature we perceive depends on our ability to maintain our body temperature (̃98.6 °F)

• If we receive more heat than we emit, we feel hot• If we emit more heat than we receive, we feel cold

• Various atmospheric conditions can affect this ability‒ Different variables affect more or less

• Temperature• Relative humidity• Wind speed• Intensity of solar radiation• Cloudiness• Pressure• elevation

38

Warm Weather Concerns• Heat Index

‒ Combines air temperature and relative humidity to determine how hot it actually feels

‒ NWS uses heat index and apparent temperature interchangeably, although technically apparent temperature uses other weather variables besides RH and T

• Main source of cooling is the evaporation of perspiration‒ If it is too humid, sweat cannot evaporate

• We feel hot‒ If it is dry, sweat evaporates easily

• We cool down

39

Fig. 4-9, p.87

• Current conditions?‒ http://vortex.plymouth.edu

40

• Consider the following:

• Even though the air feels heavy when it is humid,

• Moist air is less dense than dry air• Why?

41

Moist air is less dense than dry air

• Dry air has a molecular weight of 29

• Water vapor has a molecular weight of 18‒ H2O 1+1+16

• For two equal amounts of air, the one with more water vapor will weigh less

• Dry Air

‒ Mass: 29x20 = 580‒ Density 580 per volume

• Humid Air

‒ Mass: 29x16 + 18x4 = 536‒ Density: 536 per volume

42

29

2929

29

29

29

292929 29

29

2929

29

29

2929

29 29

29

29

2929

29

2929

29 2918

2929

29

29

29 29

1818

18

29 29

Humid air is less dense!!

43

Condensation

Dew, Frost, Fog and Clouds

44

Condensation• Process by which water vapor becomes a liquid

• Starts to occur when air reaches saturation (T=Td, RH = 100%)

• Clouds, fog, dew‒ Form when water vapor condenses into tiny droplets

45

Condensation Nuclei• Particles are needed to begin condensation of water vapor in the atmosphere

• If they are present, condensation starts when RH reaches 100%.‒ Clouds can form

• If they did not exist, clouds would not form until RH ≈ 400%.

• Hygroscopic nuclei ‒ very active (water seeking) condensation nuclei‒ Can start tiny droplets of condensation at RH=75%

‒ Causing haze

47

Dew and Frost• Dew

‒ Water condensed onto objects near the ground when their temperature falls below the dew point of the surface air

• Frozen Dew‒ Beads of ice formed when liquid dew freezes• When air temperature falls below freezing after dew forms

48

• Frost‒ Depositions of ice formed when air temperature falls below the frost point• (Dew point is below freezing)

49

http://www.touchingthelight.co.uk/rhinog/1996_104020.jpg

Rime• Forms when supercooled fog droplets freeze onto surfaces‒ (not technically frost)

50

Fog

• Cloud near the ground

‒ formed when air at the surface reaches saturation

51

‒ by cooling to the dew point• radiation fog• advection fog• upslope fog

‒ or by water vapor being evaporated into it• steam fog• seeing your breath in cool/cold air

52

…by cooling to the dew point• radiation fog• advection fog• upslope fog

53

• radiation fog‒ Temperature cools by radiational cooling

• Common in valleys (valley fog)‒ Accompany radiation inversions when there is enough moisture

54

• advection fog‒ Temperature cools by moving warm (moist) air over a cold surface

‒ Ex: San Francisco

55

• upslope fog‒ Temperature cools by being forced up a mountain

56

…by increasing water vapor

• steam fog‒ Need cold air on top of warm water‒ Evaporation occurs from the warm water‒ Surface air gets warm and moist‒ It rises and mixes with colder air‒ Mixture is saturated‒ Fog looks like steam rising from water‒ Also called evaporation-mixing fog

• seeing your breath• (special type of mixing fog)

‒ Warm moist air from mouth‒ Mixes with cold air outside

• Outside air must be close to saturation‒ Mixture is saturated‒ Looks like steam coming from mouth

58

59

• It’s all about temperature and humidity‒ You need temperature to cool down or‒ You need to add enough moisture

‒ such that T = Td

60

Clouds

• Immensely important‒ Bring precipitation; important to life

• Rain, snow, etc.‒ Important to climate issues

• Affect the energy balance of the earth• Transmit, reflect and absorb radiation

‒ Aesthetically pleasing

• Cloud formation discussed in Ch 6‒ form when air rises and cools to Td

61

Cloud Observations

62

Satellites and clouds• Satellite

‒ Package of observing instruments orbiting earth

‒ Commonly used to observe clouds• Visible ‒ detects visible wavelengths

‒ picture of clouds• Infrared ‒ detects infrared radiation given off by earth and clouds‒ Gives information about the temperature of the clouds‒ Coldest clouds are higher

• Water vapor ‒ detects specific wavelengths (also in the IR spectrum) emitted by water vapor‒ Gives information about dry vs. humid regions

‒ Geosynchronous satellites always observe the same region of the earth

‒ Speed of orbit same as the earth rotation speed‒ Example: GOES East and GOES West

• Relevant Textbook readings‒ Circulation of Water in the Atmosphere ‒ pp 90-91‒ The Many Phases of Water ‒ p 91‒ Humidity

• Absolute Humidity ‒ p 93• Specific Humidity and Mixing Ratio ‒ pp 93-94• Vapor Pressure ‒ pp 94-95• Relative Humidity ‒ pp 95-97• Relative Humidity and Dew Point ‒ pp 97-100• Relative Humidity and Human Discomfort ‒ pp 103-105

‒ Computing Relative Humidity and Dew Point ‒ p 102‒ Is Humid Air “Heavier” than Dry Air? ‒ p 106‒ Evaporation, Condensation and Saturation ‒ pp 91-93

‒ The Formation of Dew and Frost ‒ pp 112-113‒ Condensation Nuclei ‒ p 113‒ Haze ‒ pp 113-114‒ More…

63

• Relevant Textbook readings‒ Continued…‒ Fog

• Intro ‒ pp 114-115• Radiation Fog ‒ pp 115-116• Advection Fog ‒ pp 116-118• Upslope Fog ‒ pp 112-113• Evaporation (Mixing) Fog ‒ pp 118-119

‒ Clouds• Intro ‒ p 122• Cloud Observations ‒ pp 131-137

64

• Extra textbook reading‒ Chapter 4

• Vapor Pressure and Boiling ‒ p 96• Relative Humidity

‒ Measuring Humidity ‒ pp 105-107

‒ Chapter 5• Satellites do More than Observe Clouds ‒ pp 136

65