weather weather boundary separating masses air different principal cause meteorological phenomena surface weather analyses fronts depicted colored half circles depending type air masses separated usually differ temperature humidity cold fronts feature narrow bands thunderstorms severe weather occasion preceded lines dry lines warm fronts usually preceded precipitation fog weather usually quickly passage fronts produce precipitation little invariably wind shift cold fronts fronts generally west east warm fronts greater density air wake cold fronts cold faster warm fronts warm mountains warm bodies water slow movement fronts stationary density contrast frontal boundary line separates regions differing wind velocity known common open ocean classification air masses classification widely accepted form air mass classification air mass classification involves letters letter describes moisture properties continental air masses dry maritime air masses moist second letter describes thermal characteristic source region tropical polar arctic monsoon superior air dry air formed significant upward motion atmosphere letter designate stability atmosphere air mass colder ground labeled air mass warmer ground labeled fronts separate air masses different types origins located lower pressure surface weather analysis surface weather analysis special type weather map provides view weather elements geographical area specified time based information ground based weather stations weather maps created plotting tracing values relevant quantities sea level pressure temperature cloud cover geographical map help find scale features weather fronts surface weather analyses special symbols frontal systems cloud cover precipitation important information example represent high pressure implying fair weather hand represent low pressure frequently accompanies precipitation low pressure creates surface winds high pressure zones symbols frontal zones surface boundaries weather maps depict present weather locations weather map addition areas precipitation help determine frontal type location types different words describe weather frontal zone term describes boundaries instability meaning air rises rapidly boundary cause significant weather changes weaker bringing smaller changes temperature moisture limited rainfall cold cold located leading edge temperature drop analysis shows leading edge gradient normally lies sharp surface trough cold fronts bring heavy thunderstorms rain hail cold produce changes weather twice quickly warm fronts cold air warm air rapidly replaces warm air preceding boundary weather maps surface position cold marked symbol blue line triangle shaped pointing direction travel placed leading edge cooler air mass cold fronts come association low pressure area concept colder dense air dense warmer air depict air lifted frontal boundary cold air underneath warmer air creates strongest winds ground surface phenomenon associated property damaging wind gusts lift form narrow line showers thunderstorms moisture present concept isn accurate description physical processes upward motion produced warm air cold dense air circulation upward forcing warm warm fronts leading edge warm air mass located edge gradient lie broader low pressure cold fronts warm moves slowly cold usually follows cold air harder remove earth surface forces temperature differences warm fronts broader scale clouds ahead warm rainfall gradually increases approaches fog occur preceding warm frontal passage clearing warming usually rapid frontal passage warm air mass unstable thunderstorms embedded clouds ahead frontal passage continue weather maps surface location warm marked red line pointing direction travel formed cold warm usually form mature low pressure areas cold warm fronts curve naturally point known triple point lies sharp trough air mass boundary warm cold cold air mass overtaking warm cooler cool air ahead warm air masses warm air mass overtaking warm warmer cold air ahead warm rides colder air mass lifting warm air wide variety weather found thunderstorms possible usually passage associated drying air mass circulation air brings warm air upward sends drafts cold air downward vice versa depending experiencing clouds associated projection earth surface tongue warm air aloft formed process depression fronts indicated weather map purple line alternating half circles pointing direction travel indicated series blue red junction lines stationary stationary non moving stalled boundary air masses strong replace tend remain essentially area extended periods time usually moving waves normally broad temperature gradient boundary widely spaced packing wide variety weather found stationary usually clouds prolonged precipitation found stationary fronts dissipate days shear lines transform cold warm conditions aloft change stationary fronts marked weather maps alternating red half circles blue spikes pointing opposite directions indicating significant movement stationary fronts smaller scale narrow zone wind direction changes significantly relatively short distance known depicted line red dots dry line similar phenomenon weather dry line boundary air masses significant moisture differences increase north surface highs areas lowered pressure form north south oriented mountain chains leading formation lee trough near surface daylight hours warm moist air dry air greater temperature warm moist air drier air like cold higher altitudes warm moist air dense dry air boundary slope vicinity reversal aloft severe weather possible especially triple point formed cold weaker form dry line seen commonly lee trough displays weaker differences moisture moisture pools boundary warm season focus thunderstorms dry line occur earth regions intermediate desert areas warm seas southern plains west mississippi_river united_states particularly favored location dry line normally moves eastward day westward night dry line depicted national_weather_service surface analyses orange line facing moist sector dry lines surface fronts indicated necessarily reflect direction motion line organized areas thunderstorm activity reinforce pre existing frontal zones cold fronts pattern upper level jet splits apart streams resultant forming point upper level split wind pattern running southeast warm sector parallel low level thickness lines convection strong linear curved called line feature placed leading edge significant wind shift pressure rise weaker organized areas thunderstorms lead locally cooler air higher pressures outflow boundaries exist ahead type activity act additional thunderstorm activity later day features depicted warm season united_states surface analyses lie surface outflow boundaries lines form arid regions result lines depicted surface analyses alternating pattern red dots dash labelled outflow boundaries depicted label outflow boundary precipitation produced fronts principal cause significant weather convective precipitation showers related unstable weather caused air lifted clouds movement cold cold mass warmer moist air temperature differences air masses involved large extreme wind shear presence strong jet stream roll clouds tornadoes occur warm season lee outflow boundaries lead convection moisture available precipitation precipitation created lifting action air moving terrain mountains hills common cold fronts mountainous areas occur advance warm fronts moving northward east mountainous terrain precipitation warm fronts relatively steady rain fog extensive dense occurs pre warm frontal areas fronts produce precipitation clouds moisture present air mass lifted movement fronts generally guided winds aloft quickly cold fronts fronts usually travel northwest southeast warm fronts time warm moves southwest northeast reverse true cold usually moves southwest northeast warm moves northwest southeast movement largely caused pressure gradient force horizontal differences atmospheric pressure effect caused earth spinning axis frontal zones slowed geographic features like mountains large bodies warm water
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