Humidity, Condensation, and Clouds

1. Humidity, Condensation, and Clouds Chapter 4

2. Circulation of Water in the Atmosphere • From Chapter 1 - water vapor 0-4 % of the air • Yet water in the atmosphere is responsible for many of the things we think of as “weather”: • Clouds • Precipitation • Dew • Frost • Fog

3. Circulation of Water in the Atmosphere • The constant circulation of water through the earth/atmosphere system is called the hydrologic cycle • Simplest form - Water cycles from the ocean to the atmosphere, atmosphere to earth, and back to the oceans • Oceans are the driving force since the earth is about 70% water

4. Hydrologic Cycle Lots of things can happen to water during this cycle • Evaporation • Condensation • Precipitation • Transpiration • Etc….. Very complex

5. Water in the Atmosphere • Brushed over evaporation and condensation earlier - both are important regarding saturation of air • Saturation ?? • Defn. - When air cannot hold any more water vapor at a certain temperature and pressure • Can’t hold anymore?? • For every molecule of water which evaporates into the air, one molecule must condense out of the air • Like a soaked sponge

6. Water in the Atmosphere • Basically, in the atmosphere, at every particular temperature and pressure, there is a limit to how much water vapor can exist in the air - saturation.

7. Unsaturated vs Saturated • Near the surface, water molecules both evaporating and condensing • If we put a lid on top, eventually we would have saturated air (# evaporating = # condensing) Fig 4.2 Fig 4.3 Unsaturated Saturated

8. Saturation • Can water vapor in the air condense into liquid form without a surface to condense upon? • NO • Need to have a surface such as salt, soot, dust, smoke, etc. • These surfaces are called condensation nuclei (CN) • So, will water vapor be more likely to condense onto CN when moving fast or slow? • Slow • If moving fast they can bounce off

9. Saturation • So, water vapor molecules moving at slow speeds are more likely to condense onto CN • Air molecules (or any other molecules) move faster when heated and slower when cooled • So…..important points: • 1) Condensation is more likely in cool air • like at night, ever see dew in the day? • 2) Warm air has a greater capacity for water vapor

10. Humidity • Expresses how much water vapor is in the air • Lots of ways to express humidity. Most common way is relative humidity (more later) • First, vapor pressure - the portion of the total pressure due to the pressure exerted by water vapor alone • High vapor pressure = lots of water vapor in air • Low vapor pressure = little water vapor in air

11. Vapor Pressure • Ex. If the surface pressure is 1000 mb: • 78% is Nitrogen - 780 mb • 21% is Oxygen - 210 mb • 1% water vapor - 10 mb • So the vapor pressure in this example is 10 mb

12. Saturation Vapor Pressure • For any temperature and pressure, there is a maximum # of water vapor molecules that the air can hold. • Saturation vapor pressure - pressure that water vapor molecules would exert if the air were saturated.

13. Saturation Vapor Pressure • Since warm air has a greater capacity for water vapor: • it takes more water vapor to saturate warm air • this means warm air has a higher saturation vapor pressure

14. Humidity • Specifies the amount of water vapor in a volume of air. • In meteorology, we use the term “air parcel” to describe a particular volume of air. • 4 ways to express humidity:

15. Humidity • 1) Absolute Humidity • weight of water vapor / volume of air • 2) Specific Humidity • weight of water vapor / weight of all air in the parcel • 3) Mixing Ratio • weight of water vapor / weight of the dry air in the parcel • 4) Relative Humidity • water vapor content / water vapor capacity OR • vapor pressure / saturation vapor pressure

16. Relative Humidity • Relative humidity is most commonly used • Can be misleading since it doesn’t actually indicate how much vapor is in the air • Given in % • So 50% relative humidity means the air is 1/2 saturated • But does 50% RH in warm air indicate the same amount of water vapor as 50% RH in cool air? • No, since warm air has a greater capacity for water vapor, it takes more water vapor to become 1/2 saturated

17. Humidity • 2 ways to change RH: • 1) Add more water vapor to the air • 2) Change the air’s temperature • If temp stays the same: • Adding water increases RH • Removing water decreases RH • If water content stays the same: • Increasing temp decreases RH • Decreasing temp increases RH

18. Relative Humidity

19. Dew-point Temperature • Defn. - The temperature to which the air would have to be cooled to for saturation to occur. • High dew points = high water vapor content • Low dew points = low water vapor content • Adding water to air increases dew points • Removing water from air decreases dew points • The closer the actual temperature is to the dew point temperature, the more humid it is (feels) • Dew point temperature can NEVER be higher than the actual temperature • Can be equal to actual temp (RH=100%)

20. Dew-point Temperature • Dew point is actually a better indicator of how much water vapor is in the air than RH • Already said that RH tells us how close the air is to being saturated • RH 100% that means the air is saturated…very humid. So does that mean there is a lot of water vapor in the air?? • Depends on temperature • If RH = 100% and the air temp is 75º ….yes, a lot of water • If RH = 100% and the air temp is 10º ….no, dry because the cold air cannot hold as much water vapor. • But dew point is an actual number: • 70º or 72º

21. Humidity and Human (Dis)comfort • How does the body cool itself (like when exercising)? • Evaporation of sweat • This happens easily when air is dry • Dry air has a lot of “room” for water vapor • But what about when the air has a lot of water vapor in it? • Not as much evaporation • Happens when RH is high, dew point is high

22. Humidity and Human (Dis)comfort • Wet-bulb temperature - The lowest temperature that can be reached by evaporating water into the air • If the wet-bulb temperature is low - lots of evaporation (cooling) can occur • If high - not much evaporation or cooling • The closer the wet-bulb temperature is to the actual temperature, the more humid it feels • Heat Index - an “apparent” temperature due to temperature AND moisture

23. Heat Index

24. Measuring Humidity (Psychrometer)

25. Measuring Humidity (Hair hygrometer)

26. Condensation near the Ground • Recall that the air near the ground cools off quickest on calm, clear nights (by conduction) • Result - this air reaches its dew point quickest • Water starts to condense out of air onto surfaces • Cars, grass, trees, etc. • The form the condensation takes depends on temperature

27. Condensation near the Ground • Dew - condensation in liquid form • Forms on nights when temperature is >32º F • Most likely on calm (no mixing), clear (maximum cooling) nights • Frozen Dew - ice form of dew • Frost?? • No. Happens when condensation occurs while temp is above freezing and THEN temp drops below freezing • Frost - frozen from the start • Forms as water vapor changes directly to ice (deposition) • Occurs when the air becomes saturated at temps <32 º F

28. Condensation near the Ground • Haze - condensation on suspended particles • Forms when RH is high enough to allow some water vapor to condense on small floating particles (hygroscopic - water seeking) • As RH goes up, droplets increase in size • When visibility to drops below 1 kilometer (.62 miles), it’s no longer haze….. • Fog - basically a near ground cloud • Forms in one of two ways…...

29. Fog Formation • 1) Cooling of air to saturation • 2) Evaporation of water into the air • Just like clouds, fog droplets need condensation nuclei on which to form • 4 main types of fog…..

30. Fog Formation • 1) Radiation fog (ground fog) • Occurs on clear nights when a shallow moist layer exists w/ a dry layer above • Air near the ground cools quickly and the moist layer quickly becomes saturated • Also helps to have a very slight breeze • This brings more of the moist air in contact with the cool ground • Too strong of a breeze would mix the dry air down and inhibit fog

31. Fog Formation • 1) Radiation fog (ground fog) • Sometimes called “valley fog” since it forms in low lying areas (cold air drainage)

32. Fog Formation • 1) Radiation fog (ground fog) • Doesn’t “burn off” like they say on TV • Air near the ground heats up after sunrise so the air is no longer saturated

33. Fog Formation • 2) Advection fog • Occurs when warm, moist air moves over a sufficiently colder surface • Warm air over snow • Warm air from Gulf of Mexico over us in the winter • Cold ground • Warm air over land moving over colder water • Air is cooled to saturation • Called advection fog because it always involves the movement of air (advection)

34. Fog Formation • Advection fog in San Francisco as air moves over cold water

35. Fog Formation • 3) Upslope Fog • Happens as moist air is forced to flow uphill • Air expands and cools causing it to become saturated • Most common on the east side of the Rockies during winter/spring • No hills in FL = no upslope fog!

36. Fog Formation • 4) Evaporation fog (mixing fog) • Due to the mixing of two unsaturated air masses • This is the only type of fog not involving cooling of the air to saturation • Seeing your breath during winter • Steam over water

37. Annual # of Days with Heavy Fog

38. Clouds • One of the 1st things that comes to mind when we talk about weather • Responsible for • Rain, snow, hail, lightning, thunder, rainbows, etc. • They can tell you much about what’s going on • Come in all shapes/sizes and form everywhere in the troposphere • Thin, thick, low, mid, high, some all water, others all ice, some both, some precipitate, others don’t

39. Clouds • Classification of clouds is actually fairly simple • 4 major cloud groups: • 1) High • 2) Middle • 3) Low • 4) Clouds with vertical development • Just means that their height is at least as great as their width

40. Clouds • All of the major cloud varieties (Table 4.2 on page 93) are named by combining one or more of five Latin terms: • 1) Cirrus - high clouds • 2) Alto - mid-level clouds • 3) Stratus - layered clouds • 4) Cumulus - heaps (puffy looking) • 5) Nimbus - precipitating clouds • Mid level layered cloud? • Altostratus • Layered raining cloud? • Nimbostratus

41. High Clouds • High Clouds - prefix “cirrus” • Bases above 20,000 feet • Usually thin, even transparent • Almost entirely ice (due to height)

42. Cirrus • High - wispy • Strands indicate wind direction at upper levels

43. Cirrocumulus • High - vertically developed • Small rounded “puffs” • Small due to altitude • Sometimes called a “mackerel sky”

44. Cirrostratus • High - layered • Geometry of the ice crystals causes “halos” and “sun dogs” • Can see through these types of clouds

45. Mid-level Clouds • Mid-level Clouds - prefix “alto” • Bases between 6,500-23,000 feet • Mostly water • Higher ones contain some ice

46. Altocumulus • Mid-level - heaped • Individual elements appear larger than those of cirrocumulus since they are lower • Indicate significant moisture at mid-levels

47. Altostratus • Mid-level - layered • Often cover entire sky • Sun or moon just barely visible through them • Don’t produce halos • all water

48. Low-level Clouds • Low-level Clouds - Don’t have a determining prefix • Bases lower that 6,500 feet • Almost always water • Can contain ice/snow in winter

49. Stratus • Layered • Uniform layered cloud which usually covers the entire sky • Can form as fog lifts during the day • No precipitation • If it did start to rain…….

50. Nimbostratus • Layered - precipitating • Usually contain continuous rain/snow • light or moderate • Like the stereotypical cold, rainy winter day • Almost impossible to see where the base is • Cannot see sun/moon through these