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Lecture on Weather

Lecture on Weather

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Lecture on Weather

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  1. Lecture on Weather Dr. Leila M. V. Carvalho Dept. Geography, UCSB

  2. Self motivation… PhD in Meteorology at the University of Sao Paulo (USP) Born in Sao Paulo City, Brazil Previous employment (until 2008): professor at USP at the Department of Meteorology


  4. Well, when I think of Meteorology these are the images that come to my mind…

  5. Classic definition: Meteorology is the earth science dealing with phenomena of the atmosphere (especially weather)

  6. In reality METEOROLOGY deals with many problems related to atmospheric sciences such as: • Atmosphere composition (present, past and future) • Solar and Earth Radiation; energy transfer and balance • Atmospheric Physics, precipitation processes, snow, hail, cloud formation and dynamics, lightning • Turbulence and processes in microscale (~ cm ) • Air pollution and impacts • Remote sensing of clouds and precipitation • Climate and weather dynamics • Climate change • Weather forecast • Climate forecast and predictability. • Weather and climate modeling • Etc…

  7. A few important concepts : 1) atmospheric pressure Definition: is defined as the force per unit area exerted against a surface by the weight of the air molecules above that surface. Unities: millibar (USA) or hPa (hectopascals used in scientific publications = 100N/m2) P2 P1 > P2 P1

  8. We should also know that pressure and density decrease exponentially with height : rapidly at first and slowly at higher altitudes P1>P2 P2 P1

  9. 4,322m 1000m Δp/ Δz = 100mb/km If you are climbing a mountain from the sea level to about 1000m, you will notice that pressure will decrease about 100mb/km = 10mb/100m (10mb/328 ft)

  10. Δp/ Δz = 50mb/km Δz=1000m If we were to climb between 9-10km (a little bit above the Everest), pressure fall would be only 50mb/km (Notice that a mountain at this height should have much more snow than this one)

  11. Surface pressure: • It is clear that the terrain varies considerably in most regions of our planet • It is important to compare surface pressure from place to place • This is because differences in pressure in a given level are essential for the horizontal movement of the air (wind) • For this purpose, we apply equations to know what should be the pressure if a particular location was at sea level. • When we look at a map with sea level pressure we can then compare pressure among locations. P2<P1 P2 P1 It is possible that when reduced to sea level, P1~P2

  12. The basics to understand winds is to keep in mind that… Wind is air in movement Air always move from high to low pressure regions That is why we plot maps with lines with same pressure at surface (isobars) to localize where regions with high and low pressure exist High pressure (H) Low pressure (L)

  13. These are surface analysis for Jan 10/2012 – solid yellow lines: isobars (lines with the same sea level pressure) H- High pressure systems DBZ- radar reflectivity – related to precipitation L – Low pressure systems

  14. Training concepts: Arrows represent winds and colors temperature (F). Try to identify patterns of sea level pressure (relatively high and low pressure systems)

  15. Low and high pressure systems

  16. Tridimensional view Northern Hemisphere Clouds and rain

  17. A few lessons I have to learn • 1) Low pressure systems are ‘weather makers’. They are often related to strong winds, rain, snow, hail and eventually tornados (depending where you are). • 2) High pressure systems are related to fair weather and dry conditions

  18. Rules of the thumb: 2) When a low pressure system approaches Southern California, we will experience southerly winds (can be strong, depending on the relative position of the low pressure system) and mild (warm) temperatures, possibly rain and high humidity. 3) After the low pressure system moves inland, we should experience a change in wind direction (northerly winds) turning into westerly winds. Drop in temperature can be observed as well. Northern Hemisphere

  19. Low pressure systems are often associated with ‘convective clouds’ and rain, eventual hail, snow storms, thunderstorms, strong winds, lightning. •

  20. Cloud Development and Forms

  21. Four mechanisms lift air so that condensation and cloud formation can occur: • Orographic lifting, the forcing of air above a mountain barrier • Localized convective lifting due to buoyancy • 2. Frontal lifting, the displacement of one air mass over another • 3. Convergence, the horizontal movement of air into an area at low levels

  22. The upward displacement of air that leads to adiabatic cooling is called orographic uplift (or the orographic effect). When air approaches a topographic barrier, it can be lifted upward or deflected around the barrier. Downwind of a mountain ridge, on its leeward side, air descends the slope and warms by compression to create a rain shadow effect, an area of lower precipitation.

  23. Impact on vegetation Yellow pine forest at 4,000' on west slope of Sierra Great Basin sage at 4,000' on floor of Owens Valley east of Sierra

  24. Convection produced by surface heating Differences in surface characteristics result in differential heating Usually shows strong diurnal cycle peaking in mid-afternoon

  25. Cumulus clouds develop in summer east of San Diego - combination of topography and heating 10 min later

  26. Regional to Large Scale Fronts are transition zones in which great temperature differences occur across relatively short distances. When cold air advances toward warmer air (cold front), the denser cold air displaces the lighter warm air ahead of it (a). When warm air flows toward a wedge of cold air (warm front), the warm air is forced upward in much the same way that the orographic effect causes air to rise above a mountain barrier (b).

  27. When a low-pressure cell is near the surface, winds in the lower atmosphere tend to converge on the center of the low from all directions. Horizontal movement toward a common location implies an accumulation of mass called horizontal convergence, or just convergence for short.

  28. The Basic Cloud Types High clouds - cirrus, cirrostratus, and cirrocumulus Middle clouds -altostratus and altocumulus Low clouds -stratus, stratocumulus, and nimbostratus Clouds with vertical development - cumulus and cumulonimbus

  29. High clouds are generally above 6000 m (19,000 ft). The simplest of the high clouds are cirrus, which are wispy aggregations of ice crystals.

  30. Cirrostratus clouds are composed of ice but tend to be more extensive horizontally and have a lower concentration of crystals. When viewed through a layer of cirrostratus, the Moon or Sun has a whitish, milky appearance but a clear outline. A characteristic feature of cirrostratus clouds is the halo, a circular arc around the Sun or Moon formed by the refraction bending) of light as it passes through the ice crystals. This is a photograph of cirrostratus clouds near Moab, Utah.Click on image for full size version (66K JPG)Courtesy of Anne Pharamond

  31. Cirrocumulus are composed of ice crystals that arrange themselves into long rows of individual, puffy clouds. Cirrocumulus form during episodes of wind shear, a condition in which the wind speed and/or direction changes with height. Wind shear often occurs ahead of advancing storm systems, so cirrocumulus clouds are often a precursor to precipitation. Because of their resemblance to fish scales, cirrocumulus clouds are associated with the term “mackerel sky.”

  32. Altostratus clouds are the middle-level counterparts to cirrostratus. They are more extensive and composed primarily of liquid water. Altostratus scatter a large proportion of incoming sunlight back to space. The insolation that does make its way to the surface consists primarily or exclusively as diffuse radiation. When viewing the Sun or Moon behind altostratus, one sees a bright spot behind the clouds instead of a halo.

  33. Altocumulus are layered clouds that form long bands or contain a series of puffy clouds arranged in rows (midlevel clouds (2000-7000m up) . They are often gray in color, although one part of the cloud may be darker than the rest and consist mainly of liquid droplets.

  34. Low clouds have bases below 2000 m. Stratusare layered clouds that form when extensive areas of stable air are lifted. Usually the rate of uplift producing a stratus cloud is only a few tens of centimeters per second, and its water content is low. Low, layered clouds that yield light precipitation are called nimbostratus.

  35. Stratocumulus are low, layered clouds with some vertical development. Their darkness varies when seen from below because their thickness varies across the cloud. Thicker sections appear dark, and thinner areas appear as bright spots.

  36. Cumuliform clouds are those that have substantial vertical development and occur when the air is absolutely or conditionally unstable. Fair-weather cumulus called cumulus humilis, do not yield precipitation and evaporate soon after formation.

  37. Intensely developed clouds are cumulus congestus. They consist of multiple towers, and each tower has several cells of uplift. Their strong vertical development implies they form in unstable air.

  38. Cumulonimbus are the most violent of clouds and produce intense thunderstorms. In warm, humid, and unstable air, they can have bases just a few hundred meters above the surface and tops extending into the lower stratosphere. A cumulonimbus is distinguished by the presence of an anvil composed of ice crystals formed by high winds of the lower stratosphere. It can be associated with hail, strong winds and lightning and in some circumstances they can be associated with tornados and downbursts!

  39. Cumulonimbus formation Santa Barbara, Feb 26, 2011

  40. Cumulonimbus formation Santa Barbara, Feb 26, 2011 Upper level Divergence of air Possible Strong Downdrafts

  41. New Convective cells formed as cold air descends from decaying storms Santa Barbara, Feb 26, 2011

  42. New cell develops and goes through the same cycle

  43. Storm close up in Santa Barbara, Feb 26, 2011

  44. Waves formed by the passage of air over a topographic barrier can lead to the formation of lenticular clouds.

  45. Banner clouds form atop isolated mountain peaks.

  46. Mammatus clouds are found on the margins of cumulonimbus clouds, are formed by downdrafts, and can be distorted by complex motions. They may be indicative of tornados (not always)

  47. Kelvin-Helmholtz clouds look like breaking waves in the ocean. After wind blows up and over a barrier, like a mountain, the air continues flowing through the atmosphere in a pattern that looks like a wave. These clouds form when there is a difference in the wind speed or direction between two wind currents in the atmosphere. foothills of the Rocky Mountains. Courtesy of Benjamin Foster/UCAR

  48. Supercell Storms

  49. Example of supercell in CA VIS VIS