ATOC 3

what determines movement of a weather system

1. For short time intervals, mid-latitude cyclonic storms and fronts tend to move in the same direction and at approximately the same speed as they did during the previous 6 hours 2. Low-pressure areas tend to move in a direction that parallels the isobars in the warm sector ahead of the cold front 3. Lows tend to move toward the region of greatest surface pressure drop, whereas highs tend to move toward the region of greatest surface pressure rise 4. Surface pressure systems tend to move in the same direction a the wind at the 500 mb level. The speed at which surface systems move is about half the speed of the winds at this level

ranking hurricanes

- Saffir-Simpson scales: 1: 75-95 mph, 64-82 knots, very dangerous winds, will produce some damage 2: 96-110 mph, 83-95 knots, extremely dangerous winds, will cause extensive damage 3: 111-130 mph, 96-113 knots, devastating damage will occur 4: 131-155 mph, 114-135, catastrophic damage will occur 5: >155 mph, >135 knots, catastrophic damage will occur

stages of cyclone life cycle

wave cyclones develop along the polar front where there is a large temperature gradient: o a) Stationary Front: separates cold polar air from warm sub-tropical air (warm sector—>warm air mass between cold and warm fronts) b) Frontal Wave: the initial kink that forms on the polar front c) Open Wave: fully developed cyclone d) Mature: pressure drops, winds increase, cold front inches closer to the warm front MOST INTENSE as mature cyclone! e) Advance Occlusion: the cold front has caught up to the warm front leaving cold air at the surface f) Dissipation: without a surface temperature gradient, the cyclone loses energy and gradually dies out

cold front

o a. cold, dry air replaces the warm, moist air The cold dense air wedges under the warm air, forcing the warm air upward the moist, unstable warm air condenses and creates cumulus clouds upper level westerly winds create cirrostratus and cirrus clouds out in front of the cold front b. How is a front located: large changes in temperature over a short distance, change in moisture content of air, shift in wind direction, low pressure regions, cloud and precipitation patterns c. Weather: precipitation along the cold front with potential severe thunderstorms directly along the front Thunderstorms, heavy rain and strong winds occur at the front As the front passes: temperature drops, wind shift from SW to NW, pressure rises, precipitation stops Moves quickly with a speed of approximately 25 knots or more

squall lines

relatively narrow band of thunderstorms that develop in the warm sector of a mid-latitude cyclone (usually 100-300 km in advance of the cold front)

polar front theory

theory that explains the life cycle of mid-latitude cyclones and their associated fronts, majority of weather in mid-latitudes (Florida to Alaska) is generated by mid-latitude cyclones (begin to die out once the front has become occluded)

types of forecasting methods (persistence, analog)

Computers to forecast: Numerical weather prediction, prognostic charts, model output statistics, ensemble forecasting Persistence forecasting: forecasting based on the tendency of weather to remain unchanged for several hours or days Climatological forecasting: forecasting by using the average weather statistics measured over many years Analogue method: forecasting based on the assumption that weather repeats itself. This method involves pattern recognition, predicting the weather by weather types Trend forecasting: forecasting based on the speed and direction of feature such as fronts, cyclones, clouds and precipitation-known as nowcasting Statistical forecasting: forecasts that are made routinely of weather elements based on the past performance of computer models. These forecasts use the model output statistics Probability forecasting: a forecast that provides the probability that something will happen (Ex: "white christmas")

supercell thunderstorms

Made of a single, very powerful cell vertical heights extend to 650,000 ft (20 km) storms persist for many hours diameters reach 12-30 miles across rotation occurs in the updraft due to strong vertical wind shear

types of forecasts (nowcast, short-range)

Nowcast: a weather forecast that is good for a few hours Short-range forecasts: a weather forecast that is good from about 6 hours to a few days (usually up to 3 days) Medium-range forecasts: a weather forecast that is good from about 3 days to 8 days Long-range forecasts: a weather forecast that extends beyond 8 days

satellites

Polar satellites: circle the Earth in a N-S direction Geostationary satellites: placed in orbit over the equator, remain at a fixed point above Earth because they travel at the same rate that the Earth rotates Infrared images: obtained by detecting radiation emitted by objects (dependent on temperature of object) Water vapor images: measures how much water vapor is in the air by detecting the wavelength emitted by water vapor

categories of hurricane destruction

Storm surges: a dome of water 40-50 miles wide that sweeps across the coast near the point where the eye makes landfall. This is caused by the piling up of ocean water by strong onshore winds. (Storm surges are more intense on right side of the eye in the Northern Hemisphere) Wind damage: less severe than a storm surge, it affects a larger area. More than half the hurricanes that make landfall, produce at least one tornado Inland freshwater flooding: heavy rain and flooding can occur a couple of days after the storm makes landfall

what we use to report weather

Synoptic weather maps: give a picture of the weather at a given point in time Meteograms: a chart that shows how one or more weather variables has changed at a station over a given time period Soundings: 2D vertical profile of temperature, dew point, and winds Steady state forecasting assumes that weather systems will move in the same direction and at approximately the same speed as they have been moving

hurricane stages

Tropical disturbance: disorganized array of clouds and thunderstorms, have weak pressure gradients and no rotation, convergence is caused by: lifting at the ITCZ, movement of a trough from the mid-latitudes, easterly waves Tropical depression: as the pressure falls, the thunderstorms grow bigger and start to organize. As air flows toward the low pressure zone, it picks up more energy from the warm sea surface and also starts to rotate due to the Coriolis force. At this stage wind speeds are between 23 and 39 mph Tropical storm: the system takes on a circular stage as it becomes more organized, with a clear center. At this stage winds have wind speeds between 40 and 74 mph and is when the storm receives a name Hurricane: the storm turns into a highly organized hurricane as wind speeds exceed 74 mph. A relatively calm, cloud-free zone called the eye appears at its center, surrounded by spiral rain bands

studying hurricanes

Visible, infrared, and enhanced infrared satellite images radar instruments that have the ability to peer into the storm and unveil its clouds in 3D satellites can provide surface wind information Hurricane hunters: aircrafts that fly directly into the storm carrying instruments such as dropsondes Dropsondes: an instrument that is dropped into a hurricane. As it falls it measures air temperature, humidity, and atmospheric pressure. Using the GPS positive of the dropsondes, winds can be calculated over its path Bathythermograph: an instrument that is dropped into a hurricane. Once it reaches the ocean, it measures the water temperature as it slowly descends below the surface A hook-shaped echo on a radar screen often indicates the possible presence of tornado-producing thunderstorms

difference b/w a watch and warning

Watch: indicates that atmospheric conditions favor hazardous weather occurring over a particular region during a specified time period, but that the actual location and time of the occurrence is uncertain Warning: indicates that hazardous weather is either imminent or actually occurring within the specified forecast area

how we observe weather

Worldwide: 10,000 land observation stations, 7,000 ship observation stations, hundreds of buoy observation stations US: 119 Weather Forecast offices, Federal Aviation Administration and NWS

warm front

warm, moist air replaces cold, dry air (overrunning=the rising of warm air over cold air) moves with a speed of approximately 10 knots (about half of an average cold front), front moves slowly (will take a day or two to pass) In front of the warm front, surface winds are light and variable, temperatures are cold, and high cirrus clouds are overhead. The cirrus clouds will gradually thicken into cirrostratus clouds then altocumulus and altostratus clouds Winds become brisk and out of the SE and atmospheric pressure slowly decreases, precipitation warms (transitions from snow to rain), fog may form at the surface. After the front has passed, the temperatures will increase, the winds shift from southeast to south or southwest, and the atmospheric pressure stops falling. The precipitation stops and most of the clouds clear (some stratocumulus will remain)

tornado formation

- Supercell Tornadoes: Tornadoes that form within a supercell thunderstorm—> Rotation forms from: vertical directional wind shear, vertical wind speed shear, vortex tubes: the wind shear causes the air near the surface to rotate about a horizontal axis. The strong updraft within the developing thunderstorm tilts the rotating tube upward and draws it into the storm This rotating shaft in the updraft lowers the pressure in the updraft and enhances rising motion (adds more fuel to the storm) The updraft is so strong that precipitation cannot fall through it=bounded weak echo region (BWER). Although, the precipitation does get wrapped around the rotating shaft=hook echo The updraft, the counterclockwise swirling precipitation, and the surrounding air create the rear flank downdraft The formation of the rear flank downdraft seems to play an important role in the formation of tornadoes. Although, scientists are still analyzing exactly how these processes work. When the rear flank downdraft hits the ground it may interact with the forward flank downdraft and initiate TORNADO-GENESIS=the formation of a tornado At this point, the relatively cold air of the downdrafts wrap around the rotating column. This causes the bottom of the column to rise slower than the top of the column. This difference results in vertical stretching of the rotating column, resulting in a faster spinning column (just as a figure skater). Rapidly rotating column=TORNADO VORTEX Wall Cloud: An area of rotating clouds that extends beneath a super cell thunderstorm and from which a funnel cloud may appear - Non-Supercell Tornadoes: tornadoes that do not form within the pre-existing wall cloud of a supercell thunderstorm Gustnadoes: relatively weak, short lived tornadoes that form along a gust front. They typically form as a result of strongly converging winds along the edge of a rear-flank or forward downdraft Land-spouts: surface convergence along a boundary, air is rising along the boundary and condensing forming cumulus clouds, along the boundary surface there is horizontal rotation created by winds blowing in opposite directions, if the developing cumulus cloud moves over the region of rotating air then the spinning air column will be drawn up into the cloud causing vertical stretching and faster rotating winds, if the developing cumulus cloud moves over the region of rotating air then the spinning air column will be drawn up into the cloud causing vertical stretching and faster rotating winds Water-spouts: can form over the water or over the land and then

thunderstorms

- a (convective) storm that generates lightning and thunder (form in rising air). Usually produces winds, heavy rain and hail. Form in rising air; forcing mechanisms moving air upward - process: 1. random turbulent eddies lifting small bubbles of air 2. unequal heating at the surface 3. effect of terrain or lifting of air along shallow boundaries of converging surface winds 4. diverging upper-level winds, coupled with converging surface winds and rising air 5. large-scale uplift along mountain barriers or gently rising terrain 6. warm air rising along a frontal zone

convergence

- atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specific region, ADDS mass to a column of air) and Divergence (atmospheric condition that exists when the winds cause a horizontal net outflow of air into a specified region, REMOVES mass from a column of air) Convergence and Divergence at the surface: high pressure at the surface is associated with surface divergence Convergence and Divergence aloft: low pressure at the surface is associated with surface convergence How does convergence and divergence aloft change the surface pressure? Convergence aloft is necessary in order to maintain the high surface pressure and divergence aloft is necessary in order to maintain the low surface pressure. If the isobars (geopotential height contours) have even spacing, the winds will be faster in the region of an upper-level ridge than in a region of an upper-level trough. When upper level divergence of air above a surface low exceeds the surface convergence of air, the surface air pressure will decrease and the storm itself will intensify Upper level convergence usually occurs to the WEST of a trough in the Northern Hemisphere Surface pressure systems tend to move in the same direction as the wind at the 500-mb level

ordinary thunderstorms

- form in regions with rising air parcels and very little vertical wind shear;only lasts about an hour; common in mountainous regions - stages: 1. Cumulus Stage: moist rising air creates cumulus clouds—>as water vapor is changed into liquid or solid cloud particles large quantities of latent heat are released, keeping the rising air inside the cloud warmer than the air surrounding it—>no precipitation yet, updrafts keep water in the cloud 2. Mature Stage: falling precipitation creates a downdraft—>downdraft causes entrainment, which brings drier air into the cloud—>entrainment causes evaporation, which cools the air and enhances the downdraft—>both an updraft and a downdraft exist at this stage—>anvil will form if cloud top reaches the top of the unstable region of air—>where the cold downdraft reaches the surface, the air spreads out horizontally in all directions—>GUST FRONT: the surface boundary that separates that advancing cooler air from the surrounding warmer air 3. Dissipation Stage: begins about 15-30 minutes after the mature stage starts—>the downdraft overpowers the updraft, cutting off the source of warm rising air to fuel the thunderstorm

tornado

- rapidly rotating column of air extending down from a cumuliform cloud that blows a small area of intense low pressure with a circulation that reaches the ground: TOP POSITIVE, BOTTOM NEGATIVE - When the air is dry tornadoes can remain invisible until they reach the ground and pick up dust - parts: Funnel clouds: visible funnel that does not reach the ground Some tornadoes consist of a single vortex, but more commonly smaller intent whirls, SUCTION VORTICES, are contained within the large cell. There tornadoes are referred to as MULTIPLE VORTEX TORNADOES - life cycle Dust-whirl stage: dust swirling upward from the surface marks the tornado's circulation on the ground and a short funnel often extends downward from the thunderstorm's base Mature stage: during this stage, damage is usually most severe as the funnel reaches its greatest width and is almost vertical Decay stage: the tornado stretches into the shape of a rope and dissipates - ideal conditions: (May, June, April) Warm, humid surface air is overlain by cooler, drier air aloft (conditionally unstable conditions) combined with strong vertical wind shear - winds rush towards center then up; mostly <125, severe mostly <220 knots - classified by the Enhanced Fujita Scale: EF0 weak 65-85 mph (65-74 knots)... EF4 violent 166-200 mph (144-174 mph) -forecasted w/ doppler; will look like a region w/ rapidly changing wind directions

weather forecasting

- scientific estimate of the weather conditions at some future time (temperature, humidity, cloudiness, precipitation, wind speed, wind direction - primary factor that determines whether a place will have a dry climate is whether evaporation exceeds percipitation

severe thunderstorms

- severe if contains at least one of: winds excess of 58 mph hailstones larger than 0.75 inches in diameter produces a tornado - unlike ordinary, these are long-lived - processes the key factor is the presence of wind shear the updraft does not remain vertical the preoccupation falls in the downdraft, but not the updraft the updraft continues to strengthen and commonly, OVERSHOOTS into the stratosphere downdrafts at the surface act as a wedge to create a GUST FRONT and life the warm, moist air into the updraft

hurricanes

- tropical cyclonic storm having minimum winds of 119 km/hr (74 mph) or higher; also known as a typhoon (western Pacific) and cyclone (Indian ocean), hurricanes dissipate when they move over a body of water - parts: Eye: a roughly circular area of relatively light winds and fair weather at the center of a hurricane, air in the eye is sinking so there is no air Eye wall: the doughnut-shaped area of intensive cumulonimbus development and very strong winds that surround the eye of a hurricane Spiral rain bands: clouds that align themselves into spiraling bands that swirl in toward the storm's center (hurricane=composed of an organized mass of thunderstorms) - formation: need warm ocean temperatures (at least 80 F) to form, the energy for a hurricane comes from the direct transfer of sensible and latent heat from the warm ocean surface, a cluster of thunderstorms must become organized around a central area of low pressure Cannot form along equator because CF is too small The main source of energy for a hurricane is the warm ocean water and release of latent heat energy by condensation - will strengthen b/c: Sea surface temperatures warmer than 79 F Low vertical wind shear Warm moist air Ocean area along the projected storm track - dissipation: : if they move over colder water and lose their heat source, if the layer of warm water beneath the storm is shallow, if they move over a large landmass where they lose their heat source and encounter friction, if they move into a region of strong vertical wind shear - paths are difficult to predict; North Atlantic usually most westward, North Pacific usually westward away from coast

hurricane warnings vs watches

- watch: an announcement aimed at specific coastal areas that a hurricane poses a possible threat, generally within 36 hours - warning: issued when sustained winds of 119 km/hr or higher are expected within a specified coastal area, in 24 hours or less

tornado watches vs warnings

- watches: alert the public to the possibility of tornadoes over a specified area for a particular time interval. Predict that the organized severe weather event where the tornado will affect at least 10,000 square miles and/or persist for at least 3 hours - warnings: alert the public when a tornado has actually been sighted in an area or is indicated by weather radar. Warnings pertain to a specific county or counties and last for about 30-60 minutes

lightening

-discharge of electricity, a giant spark, which occurs in mature thunderstorms. It can heat the air around it to 54,000 F (5X the temperature of the Sun's surface) - parts: 1. The electrical potential gradient becomes large enough to overcome the air's insulating properties and a flow of electrons-THE STEP LEADER-rushes towards the surface 2. as the electrons approach the ground, a region of positive charge moves up into the air through any conducting object 3. when the downward flow of electrons meets the upward surge of positive charge, a strong electrical current-a bright RETURN STROKE-carries positive charge upward into the cloud - process often repeats a number of times, causing flash - For lightning to occur, an imbalance of positive and negative charges must be present (Charge separation occurs within the cloud, negative cloud base induces a positive charge at ground level)