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EATS 1011 3 Laboratory 4: Clouds, Radar and Satellite Images (Total 50 marks) If you have a copy of the textbook, Meteorology Today, please remember to bring it to your lab session. It contains some cloud identification charts which will be useful. Please bring some coloured pencils (e.g. blue, red, green). By the end of the laboratory you should understand: ⚫ cloud identification ⚫ how to interpret visible and infrared satellite images ⚫ how to interpret meteorological radar images 4.1 Cloud Classification and Coverage In the first part of this exercise you will estimate cloud coverage, which meteorologists record in eight. However, you may find it easier to first divide the sky up into half, quarter or eighth sections, estimate the coverage in each section, and then convert to the nearest tenth. You can also use your hands held at arm's length to assist in the estimate of cloud coverage. Note that determining sky conditions toward the horizon can be tricky because scattered or broken clouds often appear as though they completely cover that section of the sky. Clouds on the horizon also tend to appear thicker and darker than they would if they were directly overhead, because the light from a distant cloud travels through more atmosphere and is therefore attenuated to a greater extent. Note that water vapour is invisible, and that it is water droplets or ice crystals that reflect light. Since these “hydrometeors” reflect light uniformly in all parts of the visible spectrum they can appear as any shade of grey or white, depending on their brightness. Thin clouds are translucent and so appear brighter than thicker clouds, which may shade their own lower parts from the sun. Cloud tops in direct sunlight appear bright white, while the bases of those clouds are in the shadows and appear grey. Sky conditions are classified as: ⚫ Clear (CLR): Less than 1/10 covered by clouds. ⚫ Scattered (SCT) or Partly Cloudy: from 1/10 to 5/10 covered by clouds. ⚫ Broken (BKN) or Mostly Cloudy: 6/10 to 9/10 sky covered by clouds. ⚫ Overcast (OVC): more than 9/10 sky covered by clouds. ⚫ Opacity: if the clouds are semi-transparent, they are classified as thin, e.g. thin OVC, or thin BKN. The cloud classification system we use today is based on one proposed by Howard in 1803 and later expanded by Abercromby and Hildebrandsson in 1887. Clouds are divided 1 2 into four major groups according to the height of the cloud base above Mean Sea Level (MSL). These are (1) high, (2) middle (or “alto”), (3) low clouds, and (4) clouds with vertical development. (Note that vertically developing clouds may cover the low, middle and high ranges.) The heights given below are for the base of the clouds in mid-latitudes, but note that cloud bases are generally higher in the tropics (due to warmer temperatures) and lower in polar regions (due to cooler temperatures). Within these four height groups, clouds are classified according to their appearance with Latin words, for e.g., stratus means “layered” while cumulus means “heap” or “pile”, and cirrus means “wispy” or “curly”. Nimbus means “rain cloud” and can be applied to any cloud type that is precipitating. You may wish to refer to the book by Ahrens or Cloud Chart link posted on eClass for pictures of the cloud types described below. 1. High Clouds: base at 5 km to 13 km altitude (in mid-latitudes). Formed mostly of ice crystals (due to cold temperatures at high altitude.) Note that water droplets suspended in air can be supercooled below the normal freezing point, and generally don't freeze until well below –9oC: High clouds are rarely extensive or thick enough to prevent the formation of shadows. ⚫ Cirrus (CI): Thin, white, silky clouds blown by strong winds. Generally have a wispy, fibrous or feathery appearance. Often precede fair weather. ⚫ Cirrocumulus (CC): Appear as small rounded white puffs or rippled rows (like waves). Pattern often resembles “fish scales” and hence referred to as “mackerel”. ⚫ Cirrostratus (CS): A thin, transparent, veil-like, layer cloud which is smooth in appearance. Produces halos around sun or moon. Often precede storms, and may indicate rain or snow is on the way, particularly if followed by middle clouds. 2. Middle Clouds: base at 2km to 7km altitude (in mid-latitudes). ⚫ Altocumulus (AC): Grey and white puffy masses, often in parallel waves or bands. Generally less than 1 km thick, composed mainly of water droplets. These clouds are composed primarily of water droplets and hence have sharper edges than clouds composed of ice crystals (e.g. cirrocumulus). Altocumulus clouds show variations in colour from grey to white, which also distinguishes them from pure white cirrocumulus. Altocumulus puffs generally appear larger than those in cirrocumulus. The appearance of altocumulus in the morning is an indication of high level conditional instability, which favors the development of thunderstorms later in the day. ⚫ Altostratus (AS): A grey (never white), striated veil of cloud, composed of both ice crystals and water droplets. Often hard to distinguish from cirrostratus. The sun may appear through altostratus as a dim round disk, but halos only occur with cirrus clouds (ice crystals). Altostratus is generally thick enough to prevent shadows, whereas faint shadows are usually present with cirrostratus. 3 3. Low Clouds: base at surface to 2km altitude (in mid-latitudes) ⚫ Nimbostratus (NS): A dark grey cloud layer which is thick enough to block the sun, and produces fairly continuous light to moderate (but never heavy) precipitation. The air below nimbostratus is usually quite humid due to the evaporation of precipitation. This often leads to the condensation of ragged irregular shreds of clouds, called low-lying scud. ⚫ Stratocumulus (SC): Ragged or lumpy, fair-weather clouds arranged in rows, patches, or rolls. They range in colour from white to dark grey. The individual clouds appear much larger than altocumulus. May be produced by the breakup of stratus clouds. ⚫ Stratus (ST): An extensive grayish cloud layer with a uniform base and appearance. Stratus often covers the entire sky, and sometimes produces precipitation in the form of drizzle (very light rain). An extensive fog layer is really just a stratus cloud with its base at the ground. 4. Clouds with vertical development: base at 0 km to 4 km altitude ⚫ Cumulus (CU): Detached billowy clouds which, owing to their vertical development, usually have rounder tops, flatter bases and sharper outlines than stratocumulus. Small, fair- weather cumulus clouds may occur in rows or patches, but are generally spaced much farther apart than stratocumulus. Occasionally cumulus clouds have ragged edges, but stratocumulus always occur in closely spaced groups. ⚫ Cumulus Congestus or Towering Cumulus (TCU): These are larger cumulus clouds that possess rising domes or towers with very sharp outlines and a shape that resembles cauliflower. Their sunlit tops are bright white, while their bases are relatively dark. They may occur in isolation, or in rows or lines. They often produce showers. ⚫ Cumulonimbus (CB): Extremely large cumulus clouds with extensive vertical development, often reaching the tropopause, where the top spreads into a flattened “anvil". The tops of cumulonimbus clouds are high enough to reach low temperatures where virtually all water droplets freeze. These ice crystals give the top of a cumulonimbus cloud a fibrous and somewhat fuzzy texture compared to the more sharply defined congestus variety. Lightning, hail, thunder, and tornados are all only associated with cumulonimbus clouds (rare exceptions exist, for example during a snowsquall). Heavy showers are usually, but not exclusively, produced by cumulonimbus. Cumulonimbus may be isolated or form part of a “squall-line" or multi cell thunderstorm. 4 4.1.1 Exercise: Cloud Observations… .............................................................. 10 marks Observe the current sky conditions outside. Use the “View of Sky” box below to mark the regions of the sky occupied by various cloud types (i.e. construct a view of the sky, looking upward). You are not required to provide a detailed sketch of the clouds, just the type and extent. Then record your analysis in the “Sky Conditions” Table. Note that this exercise will be very quick if the sky is clear or completely overcast! 1. Record the date and time in the Sky Condition Table. 2. Divide your “View” box into sections by cloud type, and label each section accordingly (e.g. NS, CU, AC, CLR, etc). Also indicate cloud shape (layered or puffy) and opacity (thin or thick) as appropriate. 3. You can use your “View" box or other means to estimate total cloud coverage in tenths, and record the sky conditions (CLR, SCT, BKN, or OVC) in the Table. Also assess cloud opacity in the sky condition row. 4. Identify and record the prevalent cloud types in the Table. Provide brief statements or sketches indicating shape, extent, opacity and colour (or brightness). 5. Record the strength and type of any precipitation, (e.g. light, moderate or heavy; rain or snow). 5 Cloud Coverage and Sky Conditions Date and Time Cloud coverage (tenths) NB: The entire sky at the time of observation Sky Conditions Main Cloud Type (attach picture(s)) Other Cloud Types (Attach picture(s)) Precipitation 6 0 4.1.2 Exercise: Regional Cloud patterns… ...................................................... 10 marks In this exercise, you will obtain and analyze data from the current surface map (either on the computer or from a print-out), in order to improve your understanding of the clouds in the region. Use the blank map of Eastern North America to record the information specified below. Transfer the station circles from the surface map to the correct location on your blank map. 1. Mark the cloud amounts in each station circle. (i.e. fill in the appropriate amount, 15-20 stations). 2. Sketch a scalloped