Climate & Earth System Science. Introduction to Meteorology & Climate. Chapter 04 Lecture 07. Peter Lynch VIS WATER IN THE ATMOSPHERE MAPH 10050

Transcription

1 Climate & Earth System Science Introduction to Meteorology & Climate MAPH Peter Lynch Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Chapter 04 Lecture 07 WATER IN THE ATMOSPHERE MEASURING WATER VAPOUR. PHASE CHANGES IN THE ATMOSPHERE VIS IR 1

2 WV MOISTURE Water vapour constitutes only a small fraction of the atmosphere. Varies from 0% to about 4% Water is probably the most important gas in the atmosphere for understanding atmospheric processes. The source of atmospheric water is evaporation. Crumpled steel electrical transmission towers Canada, January, 1998 Satellite image of clouds over North America, 9 January, 1998 Ice Storm Sequence of events leading to saturation of water vapour in air OBSERVATIONS OF VAPOUR PRESSURE AS A FUNCTION OF TEMPERATURE Six years of hourly observations 2

3 HUMIDITY Humidity describes the amount of water vapour in the air. Humidity is described quantitatively as vapour pressure, absolute humidity, mixing ratio and relative humidity. Saturation is achieved when the number of water vapour molecules leaving a water surface is equal to the number returning from the atmosphere to the water surface. HUMIDITY Saturation vapour pressure is the pressure exerted by the water vapour at saturation. Absolute humidity is the mass of water per unit volume. Units are usually grams per cubic meter (g/m³) ³). Mixing ratio is the mass of water vapour in an unit mass of air. Usually in grams per kilogram (g/kg). Relative humidity is the actual amount of water vapour in the air over the amount of water vapour required for saturation. Climatology of hourly temperature and relative humidity Condensation: Example When the temperature of the air around this web cooled to the dew-point temperature, dew formed, making the web more visible RELATIVE HUMIDITY Relative humidity changes as daily temperature changes. It changes from one location to another. It changes when air moves vertically in the atmosphere. There is a daily variation of temperature and relative humidity However the water vapour content of the air can stay the same. Dew point is the temperature at which water vapour will condense out of the atmosphere. 3

4 Temperature ( C) Relative Humidity Mixing Ratio (g/kg) Vapour Pressure (mb) Sat. vapour pressure (mb) Dew point Temp ( C) Heat index table Dew point depression ( C) Lecture 08 FORMATION OF FOG FORMATION OF CLOUDS CLASSIFICATION OF CLOUDS FOG FORMATION Fog is cloud with its base at or near the ground Fogs result either when air is cooled, or by the addition of water vapour to cause saturation FOG FORMATION Fogs are composed of fine droplets of water suspended in the air near the Earth's surface. The presence of these droplets act to scatter the light and thus reduce the visibility near the ground. The formation of a fog layer occurs when a moist air mass is cooled to its saturation point (dew point). FOG FORMATION This cooling can be the result of: radiative processes (radiation fog) advection of warm air over cold surfaces (advection fog) evaporation of precipitation (precipitation or frontal fog) air being adiabatically cooled while being forced up a mountain (upslope fog). 4

5 FOG FORMATION Another type of fog is the so-called valley fog.. This fog forms when air is radiatively cooled on the slopes of hills. The air becomes denser than its surroundings and sinks down the slope (this is called a katabatic wind). The result is a pool of cold air at the valley floor. If the air is cold enough to reach its dew point, fog occurs. RADIATION FOG VALLEY FOG UPSLOPE FOG PRECIPITATION FOG ADVECTION FOG 5

6 Diurnal Variation of Visibility Lifting Mechanisms that form Clouds A fog layer is reported whenever the horizontal visibility at the surface is less than 1 km. A typical evolution of visibility during a radiation fog episode is shown here: Air raised to the Lifting Condensation Level (LCL) becomes saturated. Orographic lifting Frontal lifting Convection Convergence Four mechanisms that cause air to ascend The Cloud Percy Bysshe Shelley I am the daughter of Earth and Water, And the nursling of the sky I pass through the pores of the ocean and shores; I change but I cannot die. For after the rain when with never a stain The pavilion of Heaven is bare, And the winds and sunbeams with their convex gleams Build up the blue dome of air, I silently laugh at my own cenotaph, And out of the caverns of rain, Like a child from the womb, like a ghost from the tomb, I arise and unbuild it again. CLASSIFICATION OF CLOUDS Major cloud types arranged by altitude. 6

7 CLOUD CLASSIFICATION BASED ON HEIGHT LOW CLOUDS: Below 2 km MEDIUM CLOUDS: Between 2 and 6 km HIGH CLOUDS: Above 6 km CLOUD CLASSIFICATION: STRUCTURE STRATIFORM CLOUDS Occur as horizontal layers with limited vertical extent CUMULIFORM CLOUDS Occur as cauliflower-like heaps with large vertical extent CIRRIFORM CLOUDS Occur as whisps or thin sheets Major cloud types arranged by altitude. HIGH CLOUDS Above 6000 meters Three main types Cirrus - detached clouds composed of delicate icy filaments, have some vertical extent (mares tails) Cirrostratus - transparent cloud veil - produces a halo around the sun or moon. Cirrocumulus - very small cells or ripples - mackerel sky High clouds can portend stormy weather Mackerel scales and mares' tails make tall ships carry low sails Cirrus Clouds Cirrostratus clouds showing halo around the sun 7

8 Cirrocumulus clouds MEDIUM CLOUDS 2000 to 6000 meters. Composed of water droplets Altocumulus - large patches composed of rounded masses or rolls. Altostratus - formless layer of grayish clouds covering all or a large portion of the sky Nimbostratus dense, dark cloud with large vertical extent. Altocumulus clouds Altostratus clouds Nimbostratus clouds LOW CLOUDS Up to 2000 meters. Composed of water droplets Stratus - formless layer of grayish clouds covering all or a large portion of the sky Stratocumulus layers or rolls of cloud with modest vertical extend. Often covers the skys completely Cumulus fluffy heaped clouds. Cumulonimbus Large convective cloud with large vertical extent. 8

9 Stratus Stratocumulus Cumulus clouds Cumulonimbus Major cloud types arranged by altitude. Effect of clouds on temperature Do clouds have a warming or cooling effect? 9

10 My Favourite: Altocumulus Lenticularis PRECIPITATION GROWTH Formation of Precipitation Cloud droplets are typically 10 microns in size. Small raindrops are typically 1000 microns (more than a million cloud droplets to make a raindrop) Raindrops grow by two processes (1) Collision-coalescence warm clouds (2) Bergeron process cold clouds. From droplets to drops CONDENSATION AND DEPOSITION Supersaturation relative humidity can be above 100% without condensation Nucleation droplets usually form around particles condensation nuclei. Condensation nuclei can be hydroscopic or hydrophobic. 10

11 Dust Plume off Mauritania 1 nm 1 mm 1 um Aerosols: particle sizes AEROSOLS & CCNs Particles suspended in the atmosphere Diameters of microns one millionth of a meter. Act as cloud condensation nuclei (CCNs) for formation of water droplets. PRIMARY SOURCES: Sea salt spray Wind erosion Volcanoes Fires Human activity Fig. 4-7, p. 93 DROPLET GROWTH BY DIFFUSION Curvature effect even if air is saturated over a flat surface, it may not be for a curved surface: Collision-coalescence Collision-coalescence process is dominant in warm (tropical( tropical) ) clouds Large droplets have greater fall-speed than small droplets Relative motions allow collision between droplets and coalescence to occur Diffusion effect - big drops grow at the expense of small ones. In convective clouds, dwell-time of droplets may be very long. 11

12 Collisioncoalescence process Collisioncoalescence process Bergeron-Wegener Wegener-Findeisen Process Cold clouds occur in middle and high latitudes Saturation Vapour Pressure In cold clouds, snow and ice crystals are present as well as water droplets Ice crystals grow at the expense of water droplets. Saturation vapour pressure over ice and water Attraction of water vapour to ice versus water 12

13 Bergeron Process Process of Aggregation Extra-tropical tropical rainfall In cold clouds, snow and ice crystals are present as well as water droplets Rain in middle and high latitudes is the result of the melting of the snow- flakes/ice crystals as they descend to temperatures above zero. 13

14 WARM FRONT FORMS OF PRECIPITATION Rain - droplets of water greater than 0.5 mm in diameter. Droplets smaller than 0.5 mm called drizzle. THE EFFECTS OF AIRFLOW OVER A MOUNTAIN Much rain starts out aloft as ice crystals. Snow - ice crystals. If air is cold (low humidity), we get light and fluffy snow (powder). If air is warm than about -5ºC,, then we get wet snow (good for snowballs). Sleet - small particles of ice. Raindrops encounter freezing air on descent. If freezing not complete - freezing rain. Hail - layers of ice form as the hailstorm travels up and down in a strong convective cloud. Rime - formed by freezing of supercooled fog on objects. Passage of air over a Mountain As air ascends mountain it cools adiabatically, clouds form, and precipitation occurs. Above this altitude the relative humidity stays at 100% At the peak of the mountain the absolute humidity is determined by the saturation vapour pressure at -12ºC. As the air descends its absolute humidity remains the same as at the peak 14

15 Passage of air over a Mountain As the air descends it is compressed, so it warms Hence the saturation vapour pressure will increase, and the relative humidity will decrease The net effect of the air ascending and descending the mountain is that the air becomes drier and warmer. On the island of Hawaii, the west side of the coast (westerly winds) has rain forests, the eastern side has deserts. THE EFFECTS OF AIRFLOW OVER A MOUNTAIN Global Annual Rainfall NEXT LECTURE Cloud Recognition Quiz Class discussion on clouds End of Lecture 08 15