atmospheric pressure
the force exerted by the excitation and motion of the gas molecules on Earth's surface
Factors influencing atmospheric pressure
Density and pressure- density of a gas is proportional to the pressure on a gas
Temperature and pressure- increase in temperature causes an increase in pressure
Dynamic influences on air pressure- surface air pressure is influenced by the movement of air.
Characteristics of atmospheric pressure
- omnidirectional
- decreases with increasing altitude due to gravity
- varies horizontally which is illustrated by isobar charts
Cause of thermal high pressure
cold air sinking from the poles
dynamic high (cause)
this type of air mass is caused by the physical descent of air from high to low altitudes
Instrument used to measure atmospheric pressure
barometer
Units of atmospheric pressure
psi, inches of mercury, millibaar
PGF causes motion in the atmosphere by movement of air masses from high pressure to low pressure.
Why is pressure gradient force is important to atmospheric pressure and wind?
pressure gradient force (PGF)
Existing anywhere pressure changes with distance, it describes the expected path of wind which flows from high to low pressure, "pushing" the wind from high to low pressure in the direction of the right angle to the pressure isobars.
Coriolis force (CF)
An apparent deflection from the expected path described by PGF which is caused by the rotation of the Earth.
friction force (F)
Only occurring when air is in motion, it opposes CF. It is dependent on the roughness of the surface, the height above the surface, and the speed of the wind. It is exactly opposite the direction of wind.
Forces which control the motion of air
pressure gradient force (PGF), Coriolis force (CF), frictional force (F), gravitational force
How does friction cause the jet stream?
The frictional force is caused by the effect of gravity on air molecules. The upper zones of the atmosphere have less air molecules which reduces the frictional resistance to air flow, allowing winds to move more quickly than those in lower zones of the a
In what directions does the Coriolis effect move?
CE deflection acts at 90 degrees from the direction of the movement, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Frictional layer
the zone of atmosphere between the Earth's surface and an altitude of 1000 meters where most of the frictional resistance to air flow is found.
dynamic high
strongly descending air associated with high surface air pressure
thermal high
very cold surface conditions associated with high surface air pressure
dynamic low
strongly rising air associated with low surface air pressure
thermal low
very warm surface conditions associated with low surface air pressure
isobars
Isolines of equal pressure drawn on a weather map which reveal the horizontal distribution of pressure in the studied region.
pressure gradient
the relative closeness of isobars, which indicates the hortizontal rate of pressure change
wind
horizontal air movement
cause of wind
unequal heating of Earth's surface causes temperature gradients that generate pressure gradients which set wind in motion
geostrophic wind
when wind moves parallel to the isobars because of the balance between the Coriolis effect and the pressure gradient force
air masses
distinct and predictable wind-flow patterns with common temperature, pressure, and air moisture characteristics due to the surface where they are formed
cyclone pressure characteristics
low pressure in the center with maximum air lifting in center creating a convergent system
cyclone wind patterns
converging winds flow in the friction layer cause a low pressure center and wind divergence in upper troposphere. this rotates counterclockwise in N.H. due to right CE deflection and clockwise in S.H. due to left CE deflection
cyclone weather associations
this circulation pattern is associated with clouds and precipitation due to rising air, which expands and cools. precipitation is dependent on moisture content of air
cyclone source areas
- intertropical convergence zone, especially at the equator where there is trade wind covergence
- polar front zone creates extratropical cyclones (ETCs) because of the convergence of westerlies and polar easterlies at polar circles
circulation patterns associated with a high pressure center
anticyclones
low pressure air mass systems
what type of air mass system are cyclones?
anticyclonic pressure characteristics
highest pressure is in the center with maximum sinking of air in center creating a divergent system
anticyclones (source and characteristics)
-convergence winds in upper troposphere cause a high pressure system center with wind divergence in the friction layer
-rotates clockwise in N.H. due to right CE deflection of source winds
-rotates counterclockwise in S.H. due to left CE deflection of sou
anticyclonic weather associations
no precipitation because the source winds originate in the upper troposphere which does not allow for the expansion and cooling of moisture filled winds
anticyclones (sources)
This circulation pattern is caused by both subtropical highs (STHs), which are dynamic highs caused by the convergence of westerlies and trade winds, and polar highs, which are thermal highs caused by the sinking of cold air at the polar caps.
Naming source areas of air masses
-first letter is determined by the surface over which the air mass forms
-second letter (capitalized) is determined by the latitude at which the air mass forms
equatorial latitudes
0-10 degrees N/S
tropical latitudes
located between 10-35 degrees N/S
Where are Polar latitudes?
55-70 degrees N/S
Where are Arctic and Antarctic latitudes?
70-90 degrees N/S
mE
maritime Equatorial
What air mass does not exist and why?
continental Equatorial does note exist because the tropical rainforests found in this region produce a large amount of water.
cT
continental Tropical
mP
maritime Polar
cP
continental Polar
mA
maritime Arctic/Antarctic
cA
continental Arctic/Antarctic
equatorial low pressure belt
this air mass is caused by the constant heat gain from solar energy resulting in convection of the equatorial air, creating low pressure
intertropical convergence zone (ITCZ)
Also known as doldrums, warm Equatorial air converges causes low surface air pressure and weak winds. These updrafts pump large amounts of sensible heat and latent heat into the troposphere.
Hadley cells
-a thermal loop of warm moist air
-located between the ITCZ and STH
-circulates the sensible and latent heat received from solar energy
-caused by convection of the warm air at the equator which rises, cools in the upper troposphere, migrates poleward cre
Subtropical highs (STH)
-also known as horse winds
-large semi-permanent high-pressure anticyclonic cells develop from the descending air of Hadley cells
STH locations
centered at 30 degrees latitude N/S
importance of STH
-source of two of the three major wind systems, the trade winds and the westerlies
-not associated with precipitation or wind due to high surface air pressure
intertropical convergence
-also known as trade winds
-caused by the convergent winds of STHs
-blow mostly east to west (due to CF deflection) towards the equator
trade wind
located between the equator and 30 degrees N/S
trade wind characteristics
-start as warm dry winds and accumulate moisture during oceanic travel
-do not produce clouds or precipitation unless forces to rise by topographical barrier or pressure disturbance
-reliable because of very consistent speed and direction
westerlies
-produced by the convergent surface winds of STHs
-blow from west towards the poles
westerlies location
this general wind system is located between 30 and 60 degrees latitude N/S
importance of the westerlies
-the geostrophic winds of the westerlies blow very prominently from the west
-surface westerlies produce two high-speed jet streams which influence commercial air travel
types of jet streams
polar front jet streams and subtropical jet streams
Rossby waves
the large-north south variation in latitudinal position of the jet streams caused by the changing path of the westerlies
polar highs
high-pressure cells located over both polar regions caused by low temperatures which flows outward from the poles creating the polar easterlies.
polar easterlies
-located between 60 and 90 degrees N/S
-move from the east
-cold and dry but variable
polar front
-located at about 60 degrees N/S
-low pressure convergent zone formed by meeting of the cold polar easterlies and warm westerlies
-rising air produces extratropic cyclones
-associated with cloudiness, precipitation, stormy weather cconditions
subpolar lows
Surface zone of atmospheric low pressure located at about 60 degrees N/S. These low pressure systems are produced by the frontal lifting of subtropical air masses over polar air.
antitrade winds
-geostrophic winds caused by the Hadley cell
-blow in the opposite direction of the trade winds
-in the middle latitudes these merge with prevailing westerlies to form the subtropical jet stream
-blows from the southwest in N.H. and from the northwest in
Ferrel cell
known as the "zone of mixing," it acts as a buffer between the Hadley and Polar cells, and is easily influenced by the location of the jet stream. it is characterized by the movement and location of air masses that cause atmospheric disturbances.
Polar cell
a thermal loop of cool dry air locate between 60 and 90 degrees N/S. works as a heat sink to balance the heat from the Hadley cell. produces the polar easterlies and determines the path of the jet stream.
Monsoon winds
causes the seasonal reversal of winds characterized by a general sea-to-land movement in summer and a general land-to-sea movement in winter
Causes of monsoons
Ultimately caused by the differences in specific heat of land and sea this seasonal weather deviation occurs because solar energy causes temperature differences. The heated topography causes the air pressure to differ over water versus over land because w
Summer (wet) monsoons
Strong thermal low pressure cell generated over land allows high pressure cells generated over the sea to flow over the low pressure cell. The moist air rises, expands, and cools causing convectional thunderstorms.
Winter (dry) monsoons
Decreased solar energy generates a high pressure cell which flows over the low pressure cell on the sea and picks up moisture and energy. The warm air rises, expands, and cools causing precipitation and clouds over the sea and produces dry continental air
Locations of monsoons
South Asia- India and Bangladesh
Far East- South China to Korean peninsula
Northern Australia- Australia
Coast of Gulf of Guinea- Nigeria
Types of local winds
Sea and Land Breezes
Mountain and Valley Breezes
Fohn/Chinook/Santa Ana wind
Characteristics of Sea and Land Breezes
- Diurnal winds
- Sea breeze occurs during the daytime
- Land breeze occurs during the nighttime
- Occur consistently along tropical coastlines and during the summer along mid-latitude coastlines when there is the maximum amount of solar energy being rece
Sea and Land Breezes
Caused by differential heating of land and water surfaces. Land warms up quickly during the day, causing the air to expand and rise creating low pressure which attracts an onshore flow of air from the sea. Nighttime cooling over the land causes high press
Mountain and Valley Breezes
-Daily cycling of airflow caused by the uneven distribution of solar energy on valley floors and mountains.
-Mountain Breezes occur during nighttime.
-Valley Breezes occur during the daytime.
Mountain Breeze
-Occurs at night
-Associated with clear skies as there are no clouds to trap heat
-Mountain slope cools at night more than the valley floor due to elevation, creating a high pressure system which flows downhill as a gravity wind over the low pressure area
Gravity wind
winds that blow downhill due to the pull of gravity
most commonly seen in mountain breezes
Valley breezes
- Occurs during the day
- High sun angle heats up the valley more quickly than the mountain, creating a high pressure air mass on the valley floor
- The high pressure air mass of the valley floor flows upslope as thermals
- Causes clouds to form around th
Thermals
rising column of air caused by surface heating
Foehn, Chinook & Santa Ana Winds
- downslope winds
- originates when a steep pressure gradient develops with a high pressure area on the leeward slope and a low pressure area on the windward slope
- forms a cool, moist windward slope and a warm, dry leeward slope
Causes of Foehn, Chinook & Santa Ana Winds
Cool, moist air rising due to orographic lifting up and over the top of a mountain range or large mountain. The warm moist air rises, expands, and cools adiabatically, resulting in precipitation on the windward side. Now over the mountain, the dry air hea
Foehn (f�hn) wind
- located in the Alps
Chinook wind
this localized occur in the interior West of North America in the Canadian Prairies and Great Plains due to a steep pressure gradient off the Rocky Mountains
Santa Ana wind
- located in southern California
Importance of the Foehn, Chinook & Santa Ana Winds
- these winds can have high speeds, high temperatures, and extreme dryness and often occur during the dry spring and fall. This creates ideal conditions for wildfires by fanning large brush fires that destroy dozens of homes and hundreds of thousands of a
humidity
measure of the amount of water vapor in the air (H2OV)
absolute humidity
the weight of the water vapor in the air, measured as g H2O/cm^3
relative humidity
a calculation of the percentage of actual water vapor in the air to the water vapor capacity of air at a given temperature
calculating relative humidity
RH = (WVP/WVC) X 100%
RH= relative humidity
WVP= water vapor present
WVC= water vapor capacity
saturation
when the air is completely saturated with water vapor, when the water vapor present is all that the air at that temperature can hold
dew point
the temperature at which saturation occurs
condensation
water droplets formed on surfaces due to the temperature equally the dew point
precipitation
rain, snow, hail
sources of atmospheric moisture
evaporation and transpiration
evaporation
the conversion of liquid water to water vapor
controls of evaporation
- water temperature
- air temperature
- clouds
- air humidity
- wind
wind as an evaporation control
- wind lying over the water surface is saturated with water
- the air-water interface occurs where the sticky surface tension of water caused by hydrogen bonding does not allow evaporation when the surrounding air is saturated
- when the wind is still, th
locations of highest evaporation levels
the oceans in the subtropical high areas (STH) because the warm water is highly energetic due to maximum solar energy absorption and the constant removal of saturated wind by the trade winds
Locations of lowest evaporation levels
high latitudes the ice and tundra areas are isolated from liquid water sources close to temperatures of evaporation and minimum solar energy absorption. deserts are geographically isolated from sources of warm water.
transpiration
- the process of plants giving up moisture through their leaves
-controlled by wind, humidity, air temp, and plant adaptations
plant adaptations to increase transpiration and their geographic locations
Found in locations with high temperatures and high humidity, such as tropical rainforests, and have large broad leaves with a shiny leaf surface, which reflects sunlight, causing less heat to be absorbed
location of plants adapted to transpire the least
- locations with high temperatures and low humidity
- deserts
- reducing or eliminating leaves, moving the source of transpiration to the stem,and a milky white thick liquid is formed which keeps the water behind
Cause of precipitation
air rises, expands, and cools to dew point
adiabatic processes
- mechanism for the development of clouds and the production of rain
- occurs when air pressure changes due to a change in the temperature of air as it rises or descends
adiabatic lapse rates (ALRs)
- the rate at which large air masses rise, expand, cool, and descend, based upon the saturation of the air mass
- dry adiabatic lapse rate (DALR)
- wet adiabatic lapse rate (WALR)
DALR
+/- 5.5 degrees F/1000 ft
describes the cooling rate of unsaturated air masses
wet adiabatic lifting rate (WALR)
+/- 3.2 degrees F/1000 ft
describes the cooling rate of saturated air masses
clouds
adiabatic cooling
-warm air rising, expanding, and cooling
-decreased temperature leads to decreased pressure
-rates are described by DALR and WALR
lifting condensation line
the altitude at which the rising air mass cools to the dew point temperature causing condensation and clouds to form
adiabatic heating
-cool air descends and condenses
- increasing temperature leads to increased water vapor capacity
-rates are described by DALR
rainshadow (midlatitude) deserts
- occur when mountains block the path of saturated air, leading to one side getting moisture and the other dry air
- caused by the adiabatic cooling of moist wind off the ocean which expands and cools on the windward slope at DALR.
- once at the lifting c
Location of rainshadow/midlatitude deserts
continental interiors that are a far distance from the ocean can be deserts due to the distance that moisture needs to travel
Great basin desert area
Nevada, Utah, California, Oregon, Idaho
Patagonia desert
Argentina
Central Asian desert
This desert is located in Uzbekistan and Turkmenistan.
Gobi desert
Mongolia
cause of the subtropical desert
- subtropical highs (STHs) cause subtropical deserts by maintaining belts of high pressure
- continuous high pressure stabilizes the atmospheric temperature preventing precipitation
- air with lower water-vapor content descends but the relative humidity d
African subtropical deserts
-Sahara and Arabian (~30 degrees latitude north)
-Kalahari and Namibian (~30 degrees latitude south)
North American subtropical deserts
-Colorado/Sonoran
South American subtropical deserts
-Peru
-Atacama in northern Chile
Australian subtropical deserts
Australia has what type of atmospheric-associated topography?
Types of atmospheric lifting
- convectional
- orographic
- convergent
- frontal
Convective lifting
- caused by the surface heating of air near the ground which rises, expands, and cools adiabatically to the dew point forming cumulus clouds
locations of convective lifting
- occurs in warm regions and warm seasons causing convective showers which have a large amount of rain falling fast for a short period of time
- Midwest (Ohio) and the Gulf Coast
thunderstorms and convective lifting
- if the cumulus cloud forms with the proper humidity, temperature, and stability it will form a thunderstorm
orographic lifting
- caused by topographic barriers that block the horizontal movement of air, causing the air to travel upslope
- when the ascending air is cooled to the dew point it produces orographic precipitation
- only requires a topographic barrier and moist air to m
importance of orographic lifting in rainshadow desert formation
- the forced ascention of the air causes precipitation to fall on the windward side due to the rising of the air to the lifting condensation line.
- the air then descends and is adiabatically heated, drawing moisture into the air mass from the surrounding
frontal lifting
occurs when two unlike air masses meet. the warmer air rises over the cooler air and as the warm air rises it cools to the dew point and produces clouds and precipitation. occurs in the middle latitudes where the cold polar air and warm tropical air meet
front
the zone of discontinuity caused by the meeting of two unlike air masses
convergent lifting
- caused by an over-abundance of air lifting when two air masses meet
- the forced uplift enhances instability and often produces showery precipitation
- found in the intertropical convergence zone (ITCZ)
- is important in the development of hurricanes an
types of storms
extratropical (middle latitude) cyclones, thunderstorms, tornadoes, hurricanes
extratropical cyclone occurrence (spatial and temporal)
- found between the middle latitudes (35 - 55 degrees latitude) and the polar front zone (60 degrees latitude) in the westerlies
- 6 to 15 ETCs can be found in each hemisphere at anytime in scattered but irregular intervals
- ETCs are better developed, mo
extratropical cyclone characteristics
- large low-pressure systems which migrate in association with the jet stream
- convergent counterclockwise circulation patterns in the N.H. attracting cool air from the north and warm air from the south
- the convergence of these air masses creates two f
stages of ETC development
- stationary
- cold front development
- mature
- occluded
stationary ETCs
beginning stage of extratropical cyclones, they are not associated with precipitation and have smooth linear motion on either side of the polar front
development of extratropical cyclones (ETCs)
caused by the convergence of the polar front, maritime tropical front, and jet stream causing frontal lifting. beginning as a stationary front, a wave of cold air mass appears along the front. this develops as a cold low pressure front which then causes t
weather associated with mature ETCs
- the cold front is characterized by a steep slope of discontinuity, cumuliform clouds, heavy precipitation, and strong winds
- the warm front has a gentle slope of discontinuity, stratiform clouds, light precipitation, and weak winds
mature ETCs
stage when the cold front and warm front are well developed and the pressure in the low has decreased severely, leading to intense and severe weather at the cold front
occluded ETCs
- occur when the two fronts come closer together
- the warm front is increasingly displaced, forcing more warm air aloft until the cold front catches up with the warm front
- the warm air is no longer in contact with Earth's surface, forming an occluded f
occluded front
a complex front which occurs when a cold front catches up with a warm front, lifting all the warm air off the surface
occlusion process
a short-lived period of intensified precipitation and wind that occurs until the cold front meets up with the warm front and forces all of the warm air off the surface
thunderstorm occurrence
occur during the summer in middle latitudes due to a monsoon-like effect when the land heats up more quickly in the Midwest than the ocean or Gulf of Mexico, by unstable uplift caused by mountain-valley breezes, in low latitudes in the ITC zone, and assoc
thunderstorms characteristics
- violent convective storms accompanied by thunder and lightening
- usually localized and short-lived
- always associated with vertical air movement, considerable humidity, instability, a towering cumulonimbus (anvil-shaped) cloud, and heavy precipitation
gust fronts and thunderstorm regeneration
- multi-cell storms are a cluster of short-lived single cells
- the cold outflow from each cell combines to form a larger and stronger gust front
- convergence along the gust front often triggers new updraft development
tornado occurence (spatial and temporal)
- 90% occur within the U.S. with most in central U.S.
- 80% are associated with thunderstorms
- occur in the spring, summer, and fall except in the Gulf region where they occur year-round
tornado formation
occur on flat topography and are formed by polar outbursts that cause cold fronts to sweep down the plains of Canada into the U.S. which the Rocky Mountains block and redirect. this outburst creates a strong pressure gradient force and strong upper air wi
Hurricanes
massive tropical cyclones that form in the warm oceans of the tropics and occasionally move poleward into the midlatitudes
hurricane occurence
- occur from June to November in the N.H.
- rarely begin between 8 degrees latitude north/south
- require warm water, minimum 80 degrees F
- most occur on the western sides of the ocean, the Pacific Ocean accounts for 50% of all hurricane locations
types of hurricanes
- tropical depression
- tropical storm
- hurricane
tracks of hurricanes
they follow irregular tracks within the general flow of trade winds and are influenced by high pressure air masses, many follow the isobars of the high pressure
tropical depressions
- tropical disturbances with sustained wind speeds less than 39 mph
- formed in the Atlantic Ocean by easterly waves
tropical storms
- sustained wind speeds between 34 and 63 mph
- named storm
hurricane (characteristics)
circular low-pressure centers with a steep pressure gradient outward from the center, winds greater than 74 mph, and a converging cyclonic wind patter that uses strong winds that spiral inward to pull in warm, moist air
hurricane name
Atlantic Ocean
typhoon
Pacific Ocean
cyclone
Indian Ocean
tropical cyclone names
based upon the body of water upon which they form
baguio
Phillipines
willy-willy
Northern Australia
chubasco
Western Mexico
landfall
where a hurricane comes on land
storm surge
the offshore rise of water associated with hurricanes that is caused by the high winds pressing on the ocean surface
sensecence
when a storm dies due to the lack of energy caused by making landfall
famous U.S. hurricanes
- Galveston killed between 6 and 12 thousand people
- Agnes killed 128 people in 1972
- Andrew was a catagory 5 hurricane which was the most costly until Sandy with $45 billion
- Perfect Storm was a catagory 1 that combined with tropical storm Grace
-Katr
What is the location of the region with the greatest precipitation and why?
Charrapunji, India is located on a windward slope in a monsoon area, and receives both the orographic precipitation as well as monsoon rains. It has a record precipitation of 1,403" per year.
Locations with highest amount of precipitation
- ITC zone
- Monsoon areas
- Upper middle latitudes
- Islands
Why does the ITC zone receive so much precipitation?
The ITC zone is the wettest due to trade wind convergence which causes convective lifting and precipitation year-round, especially daily thunderstorms. The migration of the solar energy due to the tilt of the earth puts the area with the majority of rainf
Why do monsoon areas receive so much precipitation?
Monsoon areas are created by convective lifting during the summer, which causes daily thunderstorms. Charrapunji, India is located on a windward slope, the orographic precipitation gives a record 1,403" per year.
Why do the upper middle latitudes receive so much precipitation?
The mountainous west coasts experience orographic lifting and extratropical cyclone fronts. In the Canadian Rockies Henderson Lake recieves 264" per year. In the southern Andes Mountains of southern Chile, Bahia Felix had a record 325 days with fog.
Why do tropical islands receive so much precipitation?
Tropical islands are influenced by the trade winds. These winds cause precipitation by orographic lifting due to the topographical barriers that the islands represent. Mt. Waialeale on Kaua'i, Hawaii recieves 460" annually.
Where and how much precipitation does the driest area receive?
The Atacama Desert in Chile has an average rainfall of 1" and some mid-desert regions, rain has never been recorded.
regions with the least precipitation
- subtropical deserts
- polar highs