Humidity, Condensation, Clouds, and Fog. Water in the Atmosphere

Transcription

1 Humidity, Condensation, Clouds, and Fog or Water in the Atmosphere

2 The Hydrologic Cycle

3 Where the Water Exists on Earth

4 Evaporation From the Oceans and Land The Source of Water Vapor for the Atmosphere

5 ! Water molecules move in both directions between liquid and gas. Only water molecules are shown in the air. OPEN TOP! More molecules move from liquid to gas than from gas to liquid, and thus there is a net loss of liquid evaporation.! Eventually liquid water will dry up, unless replaced by a source such as precipitation.! Some water molecules that leave the liquid move away from the surface because the container is open.

6 What affects evaporation rate?! Wind speed - The stronger the wind, the more rapid the evaporation. - Why? - Stronger wind over surface blows away vapor molecules from near surface, so they will not re-enter enter liquid surface. - Example - Clothes dry out faster in the wind, we feel colder in the wind with wet clothes

7 ! Temperature of the water The higher the water temperature, the more rapid the evaporation. - Why? - The higher temperature means that water molecules in the liquid are moving faster, and will escape to form vapor. - Example - wet clothes dry faster in the sun

8 ! The humidity of the air (humidity = amount of water vapor in the air) The higher the humidity, the slower the evaporation. - Why? - The more water molecules in the air, the more that go back to the liquid. - Example - clothes dry faster and your perspiration evaporates quickly in dry climates

9 Now Saturate the Air! Put a lid on container so no water-vapor molecules blow away! Water molecules evaporate from the liquid until enough accumulate in the air so that they move back to the liquid at the same rate.! At this point we say that the air is saturated with water vapor it has reached its capacity.

10 The Amount of Water Vapor the Air Can Hold Depends on Temperature! The amount of water vapor that may be evaporated into air is directly proportional to the air temperature. That is, the higher the temperature, the more water vapor the air can hold.

11 For Example! Allow liquid water in a closed container to evaporate until the air is saturated.! Warm the air by 20 F, and evaporation will begin again because the air is no longer saturated. At the new saturation, when evaporation stops, twice the original amount of water will be in the air.! Warm the air another 20 F, and the water in the air will double again when saturation is reached and evaporation stops.

12 So! For about every 20 F change in temperature, the capacity of the air for water vapor changes by a factor of two

13 Condensation to Form Fog, Clouds, Dew How it Happens in the Atmosphere! Condensation occurs mainly when air is cooled and its capacity for water vapor is decreased.! In the atmosphere, water vapor condenses on particles of solid material rather than in the open air.! These particles may be natural or man-made, made, and are called condensation nuclei.! Thus, the condensation that begins each cloud droplet or fog droplet takes place on a solid particle.

14 Humidity Any way of specifying the amount of water vapor in air

15 1. Vapor Pressure Concept - Each gas in the atmosphere contributes its own partial pressure (part of the total). Say the total pressure is 1000 millibars,, and 1% of the air is water vapor. nitrogen (78%) partial pressure = 780 mb oxygen (21%) partial pressure = 210 mb water vapor (1%) partial pressure = 10 mb (Saturation vapor pressure = vapor pressure if the air is saturated)

16 How saturation vapor pressure depends on temperature it takes more water vapor to saturate the air at higher temperatures!!!!

17 2. Relative Humidity Relative humidity tells us how close the air is to being saturated or RH = water vapor content x 100 water vapor capacity RH = actual vapor pressure x 100 saturation vapor pressure

18 Relative Humidity (cont.)! If air contains ½ the amount required for saturation the RH = 50%! If air is saturated, RH= 100%! How does RH change? two ways Change the temperature (capacity for water vapor) Change the amount of water vapor in the air

19 Change of RH that results from daily temperature change

20 Relative humidity and human comfort! The body cools itself through - evaporation from the skin - evaporation from the respiratory track - radiation - conduction and convection! When the body is under heat stress (large sources of heat from metabolism or external sources), evaporation of perspiration from the skin is most important cooling mechanism.! But, evaporation slows down when air is close to saturation (high RH).! Thus, we are uncomfortably hot when the RH is high.

21 3. Dewpoint The temperature to which you have to cool the air to produce saturation

22 Fog! Air becomes saturated and droplets form on condensation nuclei! There is no physical difference between a fog and a cloud, except that we tend to think of a fog as being near the ground! There are different names for a fog, depending on how it forms (that is, what causes the saturation)

23 Fog types

24 Radiation fog! The surface of Earth cools at night because infrared radiation is emitted.! The cool surface cools the air near it.! If the air temperature cools to the dewpoint temperature, a radiation fog forms.

25 Radiation fog (continued)! These fogs tend to form - in low areas (e.g., valleys) because the cool air near the ground drains downhill. - over moist surfaces (e.g., river valleys) because the dew point is higher. - when the sky is clear and air is dry, because more infrared energy escapes to space rather than being absorbed and radiated back to Earth - when the overall airmass is moist, and thus the dewpoint is high - when nights are long in the winter, allowing more cooling - when the winds near the surface are relatively light

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27 Advection fog! When air moves horizontally (advects( advects) ) over a cold surface, the air near the surface cools! If the air cools to the dewpoint, condensation takes place and an advection fog forms.

28 Advection fog (continued)! These fogs tend to form - when air moves over a cold water surface - when the air has a high relative humidity to begin with

29 Mixing fog! Say two volumes of air are unsaturated but have a high relative humidity.! When they mix, the resulting mixture may be saturated.

30 Mixing Fog (examples)! Your breath on a cold day! Condensation trails from jets! Exhaust from automobiles! Steam fog over a warm water surface! Fog forming over snow that is melting on the highway

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33 Fog climatology average number of days a year with heavy fog

34 Cloud Types

35 Cirrus

36 Cirrocumulus

37 Cirrostratus

38 Altocumulus

39 Altostratus

40 Nimbostratus

41 Stratocumulus

42 Stratus

43 Fair-weather cumulus

44 Cumulus congestus

45 Cumulonimbus

46 Lenticular or wave clouds