Ch 20 - Weather Patterns and Severe Storms

20.1

Air Mass = a large body of air with the same temperature and moisture throughout -Carry the moisture & temperature from where it originally formed EX: Polar Air Mass = frigid temps.

Named according to their source area: Polar = cold temps Tropical = warm temps Continental = formed over land (dry) Maritime = formed over water (moist)

Types of Air Masses that affect the US 1.Continental Polar (cP) = forms over northern Canada, interior of Alaska, and Arctic areas -Bring cold & dry weather in winter, cool & dry weather in summer -Clear skies, little precipitation -Also causes lake-effect snow around great lakes

2. Maritime Tropical (mT) = forms over the warm waters of the Gulf of Mexico, Caribbean Sea, or Atlantic Ocean -Bring warm & wet weather to areas -Cause of high summer temperatures & high humidity -Affects Mid-West & Eastern US -Unstable

3. Maritime Polar (mP) = form over North Pacific or North Atlantic - Affects weather of Western US & New England - Brings low clouds & showers, decreased temperatures - Nor’eastern = cold temps & snow

4. Continental Tropical (cT) = begin in the southwestern US and Mexico during summer - create hot & dry weather systems - Rarely move outside their source region but can cause drought-like conditions in Great Plains AND mild fall weather in Midwest (Indian Summer)

20.2 Happy St. Patrick's Day!

Fronts = formed when 2 different air masses meet - usually bring some form of precipitation - narrow/small size - warmer (less dense) air will always rise above cooler (more dense) air

Types of Fronts: 1.Cold Front = forms when cold, dense air moves into a region occupied by warmer air - Warmer air is forced to quickly rise creating heavy downpours and fast winds - Tall cumulus clouds indicate approach of cold front

- fast moving - bring violent weather - After front has passed, temperatures drop

2. Warm Front = forms when warm air move into an area formerly covered by cooler air

- warm air rises, cools, condenses to form clouds, and frequently creates precipitation - Cirrus clouds indicate approaching warm front - as front nears, cirrostratus & altostratus clouds form, later nimbostratus clouds bring rain

- slow movement - produce slight to moderate precipitation over a large area - creates gradual increase in temperature as front passes - usually brings winds from the south

3. Stationary Front = occurs when 2 similar air masses - surface position of front doesn’t move - produces occasional precipitation

4. Occluded Front = occurs when a cold front overtakes a warm front - warmer air is forced upward creating large clouds

- fronts then merge to create heavy precipitation - very slow moving = precipitation for days

20.3

Thunderstorms = a storm that generates lightning & thunder, hail, gusty winds, and heavy rain - usually a small size but sometimes can stretch for km along a cold front

- can be produced from single cumulonimbus - form during vertical movement of warm, unstable air - usually form in tropics (US gets ~100,000 each year) and only last for 1-2 hrs

Forms by: 1. Cumulus Stage = strong updrafts supply moist air to fuel storm, creating tall clouds 2. Mature Stage = ~1 hr later, precipitation grows too large for winds to support = lots of rain, lightning & hail can also occur during this stage 3. Dissipating Stage = downdrafts dominate, air temperatures begin to cool, storm dies

Within thunderstorms, ice crystals and water droplets high in the cloud are torn apart and later smashed together = creates electricity

Lightning = electricity discharged from a thundercloud –Can heat the air around it to 4X the temp of the sun –Can go from ground to cloud, cloud to cloud, or cloud to ground

The lightning heats the air, causing it to expand rapidly, creating a large sound wave = Thunder –Because speed of light is much faster than speed of sound, you will always see lightning before you hear thunder –To find out how far away a storm is from you, count the seconds between the lightning & thunder AND divide that number by 3

Tornadoes = violent windstorms that take the form of a rotating column of air (Vortex) that extends downward from a cumulonimbus cloud - Small tornadoes have 1 vortex, larger tornadoes have several small vortexes within a larger main one - US reports ~770 tornadoes each year - max wind speed ever = 480 km/hr - Tornado season = April thru June but can occur any time of year

Taken in 1884, this photo is the oldest known picture of a tornado. NOAA Historic Photo Collection

Fujita Scale = scale from 1-5 that determines a tornadoes intensity based on wind speed F0 - winds up to 116 km/hr, light damage F1 - winds km/hr, moderate damage (mobile homes overturned) F2 - winds km/hr, considerable damage (roofs torn off, trees snapped) F3 - winds km/hr, severe damage (walls torn from foundation) F km/hr, devastating damage (houses gone, cars carried away) F5 - winds above 419 km/hr, incredible damage (everything in path is gone)

Tornadoes Cont… Form by: 1. Vertical cylinder of air develops from an updraft within a thunderstorm 2. Fast moving winds (Jet Stream?) high in atmosphere force cylinder to roll/tilt = creates a Funnel Cloud (Mesocyclone) 3. If pressures within funnel cloud are low enough, air near ground will be drawn into the cloud = air spirals upward around the core of the funnel cloud

 Problem = there is no “magic” pressure where a tornado is guaranteed to form at, so some large storms don’t produce a tornado while smaller storms sometimes do  Makes prediction of tornadoes almost impossible and tracking a tornado very difficult

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Hurricanes = large tropical storms that produce winds of at least 119 km/hr (tropical storms have winds of km/hr) - also known as typhoons, cyclones, and tropical cyclones - heavy rains, strong winds, extensive flooding - can generate up 15 meter storm surge (dome of water about 65 to 80 km wide that moves across the ocean pushed by winds) - most deaths result from drowning - most hurricanes occur between 5-20° N&S latitude - Pacific is much more likely to produce a typhoon than the Atlantic - Winds in Pacific blow typhoons West toward Japan - Winds in Atlantic blow hurricanes West toward E coast of US

Hurricanes Cont… - Hurricane season = late summer into late fall Form by: 1. Begins as a small tropical disturbance of clouds and thunderstorms (low pressures) 2. Warm water heats air above it, causing the air to rise = more clouds 3. A central low pressure zone develops and pulls more warm, moist air inward = spiraling upward winds & rotation of entire storm (counterclockwise), energy builds 4. Storm begins to gather strength & get bigger over water, global wind patterns push storm toward land 5. Once storm reaches land, there is no more warm water to fuel storm, and hurricane eventually dies out

Hurricanes Cont… - Eye = very center of hurricane, precipitation stops and winds decrease - Eye Wall = located just outside eye, large wall of dense clouds, fastest wind speeds of entire storm, heavy rains Saffir-Simpson Hurricane Scale Category 1 = winds km/hr, storm surge of meters Category 2 = winds km/hr, storm surge up to 2.4m Category 3 = km/hr winds, storm surge up to 3.6m Category 4 = winds km/hr, storm surge up to 5.4m Category 5 = winds over 249 km/hr, over 5.4m

Ch 20 Final Assignment THIS IS WORTH 25 POINTS! Pg , 2, 3, 4, 6, 7, 8, 9 = write out both question & answer 10, 11, 12, 15, 18, 25, 28 = write answer only in complete sentences Vocabulary Cold FrontEye Eye Wall Hurricane Occluded FrontStationary Front Storm Surge Thunderstorm Tornado Warm Front