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ow and high pressure systems in the Northern Hemisphere? Low pressure system : clockwise & outward circulation High pressure system : counter-clockwise & inward circulation What is pressure and how does it change with height? Pressure = (Weigh or Force) / Area Units : mb, lb/in2, in Hg Pressure always decreases with height — because there is less atmosphere (weight of air) above. What is an inversion? Inversion : the layer in which the temperature increases with height What is latent heat? Latent heat : energy absorbed/released during a phase change (“hidden” energy) What is advection? Advection : transfer of heat (or moisture) through horizontal movements of air i.e. sea or lake breeze What is saturation? What mathematical relationships are true at saturation? Saturation : the water vapor concentration in the air at which evaporation rate = condensation rate At saturation, RH = 100% What is relative humidity? How can it be changed? Relative humidity : the ratio of vapor pressure (VP) to the saturation vapor pressure (SVP) To increase RH - Add moisture to the air (increase VP) - Cool the air (decrease SVP) To decrease RH - Remove moisture from the air (decrease VP) - Warm the air (increase SVP) What is dewpoint? Why is it useful? Dewpoint temperature : the temperature at which saturation occurs Higher dewpoint always means that there is more moisture in the air. dewpoint temperature = temperature How do clouds form? Most clouds form in rising air. 1. As air rises, it expands and cools. 2. Amount of water vapor in the air doesn’t change as air rises. 3. As temperature of the air decreases, its relative humidity increases. 4. Air eventually rises high (gets cold) enough to become saturated (RH = 100%). 5. When air saturates, a cloud forms. Clouds are made of billions of small water droplets. How do you read a station model? Know the location of the 50 US states, oceans, and the Rocky and Appalachian Mountains. What is a constant pressure surface? Constant pressure surface : where the atmospheric pressure at all points along the surface is constant How are low and high heights related to temperature and weather? Low heights : cold air below that pressure level - As you cool air, it contracts - Analogous to low pressures on a constant height surface High heights : warm air below that pressure level - As you heat air, it expands. - Analogous to high pressures on a constant pressure surface How do you identify ridges and troughs? Ridges : high heights (or pressures) Trough : low heights (or pressures), valley What is the pressure gradient force? In what direction does it act? Pressure gradient : change in pressure/distance (mb/km) - Acts perpendicular to isobars (or height contours) Tightly-packed isobars : steep/strong pressure gradient (big change over a small distance) Widely-spaced isobars : gentle/weak pressure gradient (small change over a big distance) Pressure gradient force : the net force acting on air when differences in horizontal air pressure exist - Directed from higher to lower pressure (or heights) perpendicular to isobars How does the pressure gradient force influence the wind speed? strong PGF = strong winds What is a cyclone? An anticyclone? Cyclone : the center of low pressure - NH : counterclockwise flow - SH : clockwise flow Anticyclone : the center of high pressure - NH : clockwise flow - SH : counterclockwise flow How do you determine the surface wind direction from isobars? Surface winds do not blow parallel to isobars — they always cross isobars at an angle from high to low pressure. What is convergence? Divergence? Convergence : rising air that goes counterclockwise & inward around a low - forms clouds Divergence : sinking air that goes clockwise & outward around a high - prevents formation of clouds How are convergence and divergence related to the development of surface lows and highs? Surface lows - divergence causes surface pressures to fall - air converges into low pressure systems at the surface Surface highs - convergence causes surface pressures to rise - air diverges into high pressure systems at the surface Why is vertical motion important? faster rising air = more interesting weather What is a jet stream? Where are jet streams found? Jet stream : a swiftly-flowing air current typically found near the tropopause (300-200mb) Typically two jet streams per hemisphere - subtropical - polar What is an air mass? How are they classified? Air mass : a large body of air with similar temperature and moisture concentrations in the horizontal Classified by Moisture - Continental (c) : dry (forms over land) - Maritime (m) : moist (forms over water) Temperature - Tropical (T) : warm/hot - Polar (P) : cool/cold - Arctic (A) : frigid cT — dry & hot cP — dry & cold cA — dry & frigid mT — moist & warm mP — moist & cool What is a front? Front : the transition zone between two distinct air masses How are fronts represented on weather maps? Triangles and semi-circles that point to the direction that the front is moving Cold front : winds in cold air blow toward the front Warm front : winds in cold air blow away from the front Stationary front : winds in cold air blow parallel to the front What is a dryline? Dryline : a boundary that separates warm & dry air from war & moist air - Usually represents a zone of instability along which thunderstorms form What is a mid-latitude cyclone? Mid-latitude cyclone : a cyclonic storm that most often forms along a front in middle and high latitudes - a.k.a. extratropical cyclone, depression, low Where is warm air and cold air found with respect to a surface low? - Brings warm air northward - Brings cold air southward How do mid-latitude cyclones influence the weather? - Showers & thunderstorms may form along the cold front - Light-to-moderate precipitation to N of the warm front - Heavy snow (possibly blizzard) to NW of the surface low LECTURE 17 What is the Gulf Stream? Gulf Stream : an intense, warm ocean current in the western North Atlantic Ocean What is the thermocline? Thermocline : rapid temperature decrease with depth a few hundred meters below the surface How does oceanic temperature vary with depth? Temperature decreases with depth. - Warmest water typically found at ocean surface - Thermocline - Deep ocean waters only a few degrees above freezing What is upwelling? How does it affect oceanic temperature? Upwelling : vertical transport of deep ocean waters upward - Results in cooler surface water temperatures What is ENSO? El Niño? La Niña? El Niño : unusual warming of surface waters in the (eastern) Pacific Ocean off South America La Niña : unusual cooling of surface waters in the (eastern) Pacific Ocean off South America ENSO : El Niño Southern Oscillation Southern Oscillation : atmospheric oscillation over the South Pacific Ocean (encompasses both El Niño and La Niña) How does the phases of ENSO affect the weather in the eastern Pacific? The western Pacific? - ENSO typically reaches maximum around Christmas - Occurs every 2 to 7 years Normal Conditions - Surface low pressure near Indonesia, high pressure near South America - Results in trade winds : easterly (NE in NH, SE in SH) winds across the equatorial Pacific - As easterly winds push water from E to W, water is warmed by sunlight - Warmer water to W, cooler water to E - Warm & moist air rises over the western South Pacific Ocean (clouds, heavy precipitation) - Air sinks near the west coast of South America (clear skies, little precipitation) Warm Phase — El Niño - Pressure gradient in the atmosphere weakens across the equatorial Pacific - Easterly trade winds weaken - Warm water flows to E across the Pacific - Walker Circulation reverses - Clouds/rain in eastern Pacific, dry in western Pacific Cool Phase — La Niña - Pressure gradient strengthens across the equatorial Pacific - Easterly trade winds strengthen - Walker Circulation strengthens - Clouds/rain in western Pacific, dry in eastern Pacific LECTURE 18 What is the difference between a hurricane and a typhoon? Hurricane : a tropical cyclone in the Atlantic and east Pacific Typhoon : a tropical cyclone in the west Pacific What is a typical hurricane track in the Atlantic Ocean? What are the names and relative intensities of tropical cyclones in the Atlantic Ocean? Tropical disturbance/wave : cluster of showers and thunderstorms Tropical depression : tropical cyclone with winds less than 34 knots Tropical storm : tropical cyclone with winds between 34-64 knots Hurricane : strong tropical cyclone with winds greater than 65 knots and eye wall development When are hurricanes most likely? Why? Between late August, September (peak), to October What ingredients are required for hurricane formation? Why are these ingredients necessary? - Sea-surface temperatures greater than 26.5 ºC - Deep layer of warm water in the upper ocean - Cluster of thunderstorms - Moist air in the mid-troposphere - Weak vertical wind shear (less than 15 knots) - Latitude poleward of 5 ºC What is the ITCZ? Intertropical Convergence Zone (ITCZ) : a belt of thunderstorms found near the equator NE trade winds in NH converge with SE trade winds in SH What is a tropical wave? Tropical wave : a slow-moving disturbance embedded in the easterly trade winds What is vertical wind shear? How does it affect hurricanes? Vertical wind shear : the change in wind direction or wind speed with height Bad for tropical cyclones (rips them apart) LECTURE 19 How do hurricanes intensify? More heat & moisture (evaporation) = stronger thunderstorms, more divergence aloft, lower pressure at center Lower pressure = stronger PGF, stronger winds, more heat/moisture transfer from the sea What can cause a hurricane to dissipate? - Strong vertical wind shear - If it moves over land (mountains rip the storm apart) - If it moves over cold water (loses source of heat & moisture) - If it remains stationary for too long (upwelling of cold water weakens the storm) What is the eye of a hurricane? The eye wall? What are the weather conditions in each? Eye : center; lowest pressure - clear, calm, warm Eye wall : strongest pressure gradient - strongest winds, heaviest rain, worst weather Where does air rise in a hurricane? Sink? Rise — eye wall Sink — eye How are the left and right sides of a hurricane defined? If you are moving with the storm, your right is the storm’s right & your left is the storm’s left. What are the main hazards associated with hurricanes? - Strong winds - Storm surge - Inland flooding - Tornadoes How does storm motion affect the winds around a hurricane? - Winds are strongest on the right side of the circulation because storm motion & rotational winds are in the same direction. - Winds are weaker on the left side of the circulation because storm motion & rotational winds are in the opposite direction. LECTURE 20 What is the Saffir-Simpson scale? Please do NOT memorize the wind speed table. Saffir-Simpson scale : a 1 to 5 rating based on a hurricane’s maximum sustained wind speed What is storm surge? Where is it most intense? How is it created? Storm surge : a rise in the sea level due to a tropical cyclone - most intense in the right-front quadrant of a tropical cyclone 1. Winds pile up water against the coast. 2. Low pressure allows sea level to rise. How did Hurricane Katrina flood New Orleans? - Storm surge funneled Lake Pontchartrain. - Lake levels rise. - Northerly winds direct lake waters into New Orleans. - Much of the city is below sea level. Where are tornadoes most prevalent with respect to the eye of a hurricane? Right of the eye How have hurricane forecasts improved? Track forecasts have improved greatly; intensity predictions still need work. - Hurricane watch - Hurricane warning LECTURE 21 What is the difference between forced and a buoyant ascent? Forced ascent - Non-precipitating layered cloud or drizzle - Steady rain or snow Buoyant ascent - Cumulus cloud - Hurricane - Ordinary thunderstorm - Sever thunderstorm How does temperature and moisture affect air density? - An air parcel that is warmer than its surrounding air will rise. - An air parcel that is colder than its surrounding air will sink. How does the temperature of air change as it rises and sinks? Why? - Warm air is less dense than cold air. - As air rises, it expands and cools. Know how to read and interpret a sounding. (Lab #2) - Slope is moist/dry adiabatic lapse rate - Environmental temperature — red - Rising parcel temperature — black When rising parcel temperature is to the right of the temperature line, the parcel is warmer than the environment and will continue to rise (unstable). When rising parcel temperature is to the left of the temperature line, the parcel is cooler than the environment and will begin to sink (stable). What are the dry and moist adiabatic lapse rates? Why are they different? Be able to do basic calculations with them. Dry adiabatic lapse rate : the rate at which a rising, unsaturated air parcel cools — 10 ºC - Unsaturated air means temperature != dewpoint - If the DALR is 10 ºC/km and an unsaturated air parcel rises 2km, the temperature will cool by 20 ºC. - If an unsaturated surface air parcel has a temperature of 15 ºC and ascends 1km, the final temperature will be 5 ºC. Final temp = initial temp - change in temp, 5 = 15 - 10 Moist adiabatic lapse rate : the rate at which a rising, saturated air parcel cools — 6 ºC/km - Saturated air means temperature = dewpoint What is the environmental lapse rate? How is it measured? Environmental lapse rate : the rate at which atmospheric temperature decreases with an increase in altitude Define and identify the following from a sounding: LCL, LFC, EL, CAPE, CIN. Lifting Condensation Level (LCL) : the level at which a rising surface air parcel becomes saturated Level of Free Convection (LFC) : the level at which a rising surface air parcel becomes positively buoyant (warmer than its environment) Equilibrium Level (EL) : the level at which a rising surface air parcel s no longer positively buoyant Convective Available Potential Energy (CAPE) : the area between the parcel temperature and environmental temperature when the parcel is warmer than the environment big CAPE = big storm Convective Inhibition (CIN) : the area between the parcel temperature and environmental temperature when the parcel is cooler than the environment How is the lifted index calculated? Why is it useful? (Lab #13) LI = Tenvironmental - Tparcel (at 500mb) - Positive LI : thunderstorms not likely - Negative LI : thunderstorms possible How can the atmosphere be stabilized? Destabilized? Stabilized when - Warm & moist near surface, cold aloft - Temperature cools rapidly with height Destabilized when - Cool/dry near surface, warm aloft - Temperature cools slowly with height To make the atmosphere more stable - Warm the air aloft, cool the surface Under what conditions are thunderstorms most likely? (Lab #13) - Unstable atmosphere - Large CAPE - Small or zero CIN - Negative LI - Late afternoon / early evening hours LECTURE 22 What is a thunderstorm? Thunderstorm : a tall, vertically developed cloud that produces lightning and thunder What is required for all thunderstorms to form? Instability (warm/moist low levels, cool aloft, steep lapse rates, rapid temperature decrease with height) What mechanisms can trigger thunderstorms? How? - Sea breezes - Outflow boundaries (gust fronts) - Mountains - Fronts or drylines What is a severe thunderstorm? Only one of these needs to occur: - Wind gusts greater than 50 knots - Hail with a diameter greater than 1 inch - Tornado What are the stages of thunderstorm development? Cumulus - warm air rises and expands - water vapor condenses and forms a cloud Mature - air diverges outward at tropopause and forms an anvil cloud Dissipation - rain falls into updraft, cools the updraft, and drags rising air downward - updraft weakens and is replaced by downdraft - surging outflow cuts off supply of warm & moist, unstable hair - updraft fully dissipates, which means the thunderstorm dissipates What is an updraft? Downdraft? Cold pool? Anvil? Cumulonimbus cloud? Gust front? Updraft : rising air Downdraft : sinking air (precipitation required) Cold pool : pool of cold air under a storm Anvil cloud : cloud composed of ice particles, formed in the upper parts of thunderstorms Cumulonimbus cloud : thunderstorm clouds Gust front : boundary between cold outflow and warm inflow How does the cold pool form? When cold downdraft reaches the ground, it spreads out, forming the cold pool What best discriminates between ordinary and severe thunderstorm environments? Ordinary — forms in areas with weak wind shear, not near fronts Why do ordinary thunderstorms only last for about an hour? Because they have a weak vertical wind shear which prevents them from becoming very stable and organizing LECTURE 23 What is a downburst? A microburst? What causes them? Downburst : strong downdraft that comes in contact with the ground Microburst : smaller and more intense downbursts Sinking air must be colder than its environment. What are straight-line winds? How do they form? Straight-line winds : intense winds blowing in the same direction (can reach 100mph) They form when downburst hits the ground. - Air spreads out when it reaches the ground, yielding intense straight winds How can one determine if damage was caused by straight-line winds or a tornado? - Winds in a tornado rotate - Straight-line winds cause all debris to fall in the same direction What is an MCS? What are the primary severe weather threats from one? Mesoscale Convection Systems (MCS) : a large complex of thunderstorms Primary severe weather threats - Flooding - Straight-line winds - Occasional weak tornado What are the two components of MCS structure as seen by radar? - Convection zone — heavy rain, strong gusty winds - Stratiform region — large area of light-to-moderate rain found behind the convection zone Why is vertical wind shear important to MCS maintenance? - Prevents gust front from moving too far in front of updrafts - Creates more lift at gust front What is the rear-inflow jet? How does it form? Rear-inflow jet : rain falls into cool dray air near rear of system forms - Evaporation cools air - Air descends, eventually reaching the surface What is a shelf cloud? Where is one likely to be found? How does it form? Formed by warm air being lifted at gust front, rises, and forms a shelf cloud What is a bow echo? A squall line? A derecho? Bow echo : thunderstorm in shape of a bow; prolific wind damage Derecho : especially intense bow echo Squall line : line of thunderstorms that form along a cold front or out ahead of it LECTURE 24 What is the charge separation in a typical thunderstorm? Negative — large ice particles, fall downward Positive — small ice particles, stay higher in clouds Each collision transfers a small charge but this billions of times in a single storms adds up to a large charge What are the three main types of lightening? - Cloud to ground - Cloud to cloud - Intra-cloud What is sheet lightning? “Heat lightning?” Sheet lightning : light that is scattered by clouds and rain (the sky appears to light up) There is no such thing as “heat lightning”… How does a lightning strike occur? 1. The negative particles falling from the cloud are attracted to positively charged molecules on the ground, such as a tree. 2. They collide with positively charged molecules and then become positive on the return stroke back to the ground. What is the relationship between lightning and thunder? - Lightning is always followed by thunder. - Lightning heats air around it, causing the air around it to expand violent, forming a sound wave, which we hear as thunder. What are the safest places to be during a lightning storm? What places should be avoided? Safe places - Indoors - Car Places to avoid - Showers and corded phones - Open fields - Trees - Metal objects - Water What is hail? How is it formed? Hail : frozen precipitation that falls during a thunderstorm Supercooled water suspended in updraft that grow by collecting supercooled water (fast rising air keeps hailstone suspended in updraft) How do hailstones acquire layers? The suspended embryo is suspended in updraft and it grows by collecting supercooled water. What are the two main types of flood and the dangers from each? - Flash floods — occurs rapidly, with little warning - Widespread floods - Coastal floods Why is it dangerous to drive through flood waters? - Your car can be swept away. - You cannot see through the water. and may not be aware of the objects in your way. LECTURE 25 What weather conditions are favorable for the formation of supercell thunderstorms? - Vertical wind shear - Low level jet stream (strong winds from south) - Warm moist air toward the ground - Jet stream aloft has a westerly flow - Low CIN and high CAPE What is a supercell? Supercell : long-lived rotating thunderstorm cells that can last for hours, often isolated (strongest and rare) - produces most hail golf-ball sized and larger - produces nearly all violent tornadoes Why are supercell able to persist for hours while ordinary thunderstorms do not? - Often isolated - Always rotate (rotating updraft) What is a mesocyclone? Mesocyclone : a persistent, rotating updraft within a supercell 5km across How do supercell acquire rotation? - Vertical wind shear - Updrafts tilt this rotation into vertical What is a wall cloud? Wall cloud : a lowered, sometimes rotating, cloud beneath the base of a thunderstorm from which a tornado can develop What are mammatus clouds? Mammatus clouds : little cotton ball ones, sometimes seen hanging beneath the anvil of thunderstorms What is the RFD? How is it visually manifested? Why? Rear-Flank Downdraft (RFD) : region of dry air subsiding on the back of, and wrapping around, a mesocyclone in a supercell thunderstorm - Appears as a clear slot near the updraft base What is an HP supercell? An LP supercell? In which type of supercell are tornadoes most likely? High-Precipitation supercell : significant precipitation near updraft Low-Precipitation supercell : little precipitation everywhere Tornadoes are most common in classic supercells. LECTURE 26 What is a tornado? Tornado : a violent rotating column of air in contact with the ground (typically visible as a funnel cloud) What defines when a tornado has “touched down?” When the funnel makes contact with the ground What is a multiple vortex tornado? Multiple vortex tornado : the downdraft in the center breaks circulation into smaller vortices - Wind field is complex : add tornado motion, rotation of tornado, and rotation of suction vortices What are landspouts and waterspouts? How do they differ from supercell torandoes? - Circulation at surface stretched by updraft - No downdraft or supercell is required - Usually occur in ordinary thunderstorms or MCSs, but still rare - Usually less intense than supercell tornadoes What is the Enhanced Fujita Scale? On what is it based? Please do NOT memorize the wind speed table. Enhanced Fujita Scale : a scale in which the intensity of tornadoes is measured based on the quality of construction Which types of tornadoes are the most deadly? Why? Multiple vortex tornado How can radar be used to detect tornadoes? Doppler radar is able to detect rotation within a thunderstorm. What is the safest place to be during a tornado? Why? - Basement - Inner room - Away from windows What places should be avoided during a tornado? Why? - Windows - Highway overpasses - Mobile homes - Cars - Large rooms