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Chapter 9. Atmosphere and Severe Weather. Learning Objectives. Understand Earth’s energy balance and energy exchanges that produce climate and weather Know the different types of severe weather events
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Chapter 9 Atmosphere and Severe Weather
Learning Objectives Understand Earth’s energy balance and energy exchanges that produce climate and weather Know the different types of severe weather events Know the main effects of severe weather events, as well as their linkages to other natural hazards Recognize some natural service functions of severe weather
Learning Objectives, cont. Understand how human beings interact with severe weather hazards and how we can minimize the effects of these hazards
Energy Force Push or a pull Measured in Newtons (N) Magnitude measured by how much a body is accelerated Work Done when force is applied to an object that moves it a certain distance in the direction of the applied force Measured in Joules (J) Power Rate at which work is done Measured in Watts (W)
Energy, cont. The ability to do work Measured in exajoules (EJ) 1018 to describe global energy Types Potential - stored energy Kinetic - energy of motion Heat - energy of random motion of atoms and molecules
Heat Transfer Conduction Transfer through atomic or molecular interactions Two bodies in contact with one another Convection Transfer through mass movement of a fluid Hot air rises displaces cool air which falls Creates a convection cell Radiation Transfer through electromagnetic waves
Earth’s Energy Balance A general equilibrium between incoming and outgoing radiation Earth intercepts only a fraction of the sun’s radiation Sun’s energy drives hydrologic cycle, ocean waves and currents, and global atmospheric circulation Nearly all of the energy available at Earth’s surface comes from the sun Exception, heat from Earth’s core that drives plate tectonics
Heat Energy Energy transferred between two objects at different temperatures Sensible heat Heat that is monitored by a thermometer Latent heat Energy necessary to cause a change in state Example: latent heat of vaporization is energy necessary to change liquid water into water vapor
Electromagnetic Energy Most of the sun’s energy is electromagnetic Described by the wavelength Distance from one crest to the next All wavelengths are part of electromagnetic spectrum Only a small part of this is visible
Energy Behavior Redirection Reflection back to space by clouds, water, land Scattering disperses energy is many directions Transmission Energy is passed through atmosphere Absorption Alters molecules or causes them to vibrate Some of this may be re-emitted to space
Energy Behavior, cont. Temperature depends on amount of energy absorbed or reflected Reflection depends on albedo Describes the reflectivity of surfaces Dark woodlands reflect 5 percent to 15 percent Light grasslands reflect 25 percent Absorption Energy that is not reflected is absorbed Different objects absorb different wavelengths Hotter objects radiate energy more rapidly and at shorter wavelengths
The Atmosphere Gaseous envelope that surrounds Earth Composed mostly of nitrogen and oxygen Smaller amounts of water vapor, argon, carbon dioxide Water vapor Important for cloud formation and circulation Comes from evaporation off of Earth’s surface Humidity describes amount of moisture in atmosphere at particular temperature Relative humidity is the ratio of water vapor present to the amount that saturates the air Increases at night because of cooler temps, decreases during the day due to heating
Structure of the Atmosphere Troposphere All of Earth’s surface is within this layer Upper boundary is tropopause Temperature decreases with increasing altitude Clouds are present at the tropopause Figure 9.7
Clouds Made from very small water droplets or ice crystals that condense from the atmosphere Cumulus – puffy fair weather clouds Cumulonimbus – Tall, dark storm clouds
Weather Processes: Atmospheric Pressure and Circulation Atmospheric pressure also called barometric pressure Weight of a column of air above a given point Force exerted by molecules on surface In the atmosphere, pressure decreases with increasing altitude Nearly all of the weight of the atmosphere is in the lower atmosphere
Weather Processes: Atmospheric Pressure and Circulation, cont. 1 Changes in air temperature and air movement are responsible for horizontal changes in pressure Temperature influences pressure because cold air is more dense and exerts greater pressure on surface Global variations in temperature cause global winds At equator, air is warm and low in density Creates low pressure zones at the equator Air rises, condenses, forms clouds and rain Cooler, drier air sinks at latitudes around 30° causing deserts
Weather Processes: Atmospheric Pressure and Circulation, cont. 2 Air movement can cause changes in pressure Convergence occurs when air flows in increasing pressure Divergence occurs when air flows out decreasing pressure At surface, air moves from surface high pressures (H) to low pressures (L) Air at low rises into atmosphere and then diverges in the upper atmosphere A surface low is often associated with a high aloft and vice versa Jet streams Narrow, fast moving jets of air caused by low pressures near the top of the troposphere
Unstable Air Tendency of air is to remain in place Atmospheric stability Air parcels resist movement or return to original spot after they move In unstable air, parcels are rising until they reach air of similar temperature and density Air is unstable when lighter, warm or moist air is overlain by denser cold or dry air Some air sinks and some air rises
Fronts Air masses do not mix, Fronts are the boundary between cooler and warmer air masses Cold front when cold air is moving into warm air Warm front when warm air is moving into cold air Stationary front where boundary shows little movement Occluded front where rapidly moving cooler air overtakes another cold air mass wedging warm air in between
Hazardous Weather: Thunderstorms Most occur in equatorial regions Most common in the afternoon or evening hours in spring or summer Three conditions necessary Warm and humid air in lower atmosphere Steep vertical temperature gradient such that the rising air is warmer than the air above it Cold air over warm air Updraft must force air up to the upper atmosphere
Thunderstorm Development Moist air is forced upwards, cools and water vapor condenses to form cumulus clouds Cumulus stage Moisture supply and updrafts continue, clouds grow A continuous release of latent heat from condensation warms the surrounding air causing the air to rise further Expanding the cloud into colder air causes water droplets to freeze Larger snowflakes fall and melt as raindrops Large droplets grow until they cannot be supported by updrafts
Thunderstorm Development, cont. Mature stage Downdrafts and falling precipitation leave the base of the cloud Updrafts and downdrafts are present Cloud continues to grow until it reaches the top of unstable atmosphere (tropopause) Storm produces heavy rain, lightning and thunder, and occasionally hail Dissipative stage Upward supply of moist air is blocked by downdrafts Thunderstorm weakens, precipitation decreases, and the cloud dissipates Most are air mass thunderstorms and do little damage
Severe Thunderstorms National Weather Service, classified severe if winds > 93 km (58 mi.) per hour, or hailstones > 1.9 cm (0.75 in), or generates a tornado Necessary conditions Large changes in vertical wind shear differences in wind speed and direction Greater the wind shear, the more severe the storm High water vapor content in lower atmosphere Updraft of air Dry air mass above a moist air mass
Severe Thunderstorm Types Mesoscale convective systems (MCSs) Most common type Very large clusters of self-propagating storms in which downdrafts from one creates a new storm Downdrafts come together to form outflow boundaries curved lines of thunderstorms that may travel long distances Squall lines Long lines of individual storm cells common along cold fronts Updrafts form anvil-shaped clouds extending ahead of the line Downdrafts surge forward as gust front in advance of precipitation Can develop along drylines Fronts with differing moisture content
Severe Thunderstorm Types, cont. Supercells Smaller than MCSs and squall lines, but more damaging Extremely violent and spawn most tornadoes Last from 2 to 4 hours Downbursts from thunderstorms can create: Derechos Strong, straight-line windstorms Wind gusts can be tornado strength Cause fallen trees, power outages, injuries, fatalities Microbursts Hazard for aviation
Hail Hard, round, irregular pieces of ice originating from thunderstorms Contain rings due to adding coatings during updrafts Hail moves up and down in lower part of the storm adding layers of liquid water which then freezes Cause mostly property damage
Hazardous Weather: Tornadoes Usually spawned by severe thunderstorms 1992–2002, killed 57 people/year Defined by vortex extending downward from the cloud and touching the ground Called funnel clouds when it does not touch ground Form where there are large differences in atmospheric pressure over short distances Figure 9.18b
Tornado Development—Organizational Stage Vertical wind shear causes rotation to develop within the storm Strong updrafts in advance of the front tilt the horizontally rotating air vertically Known as a mesocyclone Updrafts at rear of the storm lower part of the cloud Wall cloud Wall cloud rotates and funnel descends
Tornado Development—Mature Stage Visible condensation funnel extends to ground Moist air drawn upward In stronger tornadoes, smaller whirls may develop within tornado Suction vortices Responsible for the greatest damage Figure 9.18d
Tornado Development—Shrinking and Rope Stage Shrinking stage Supply of warm air is reduced and tornado begins to thin More dangerous because wind speeds increase as diameter decreases Rope stage Downdrafts cause tornado to move erratically and disappear
Tornado Classification Classified according to damage that they produce using Enhanced Fujita Scale (EF) Waterspouts Tornadoes that form over water Develop beneath fair weather cumulus clouds as a result of wind shear
Hazardous Weather: Blizzards Severe winter storms with large amounts of falling or blowing snow, High winds Low visibilities for extended period of time Whiteout – Extremely low visibility In United States: winds > 56 km (35 mi.) per hour, visibilities < 0.4 km (0.25 mi.) for at least 3 hours In Canada: winds > 40 km (25 mi.) per hour, visibilities < 1 km (1.6 mi.) for at least 4 hours Wind chill – wind cools skin, evaporates moisture, reduces time it takes for frostbite to form
Causes of Blizzards Interaction between upper-level low pressure trough and surface low pressure Colorado and coastal storms derived from moist ocean air Alberta Clippers are drier with less snow and cold temperatures Nor’easters on East Coast have hurricane force winds, heavy snows, intense precipitation, and high waves
Ice Storms Prolonged periods of freezing rain Upon contact with cold objects, rain immediately freezes to form a coating of ice Develop during winter on the north side of a stationary or warm front Three conditions for freezing rain Ample source of moisture Warm air over shallow layer of cold air Objects on land close to or at freezing
Fog A cloud in contact with ground Form by air cooling to condensation or adding water to cooled air through evaporation Cooling At night heat radiates from land Warm air blows over cold water Humid air rises up a mountain side Evaporation Cold air flows over warm body of water Warm rain falls through cool air
