• The Atmosphere
• Composition
• Vertical structure
• Heat transfer
• Atmospheric moisture
• Atmospheric circulation
• Weather and climate

• The earth’s atmosphere

• The earth’s atmosphere
• is central in physical geography because of its roles in
• Heat transfer throughout the globe
• Water vapor transport
• Determination of weather and climate

• Composition of the atmosphere
• Gaseous envelope
• Comprised of a mixture of
• Various gases
• Water vapor and ice crystals
• aerosols
• tiny suspended particles of solid or liquid

• Composition of the atmosphere
• a mixture of various gases
• two gases make up 99% of the total volume of the atmosphere
• nitrogen (N₂) = 78%
• oxygen (O₂) = 21%
• percentages of these two gases are fairly constant up to about 80km
• water vapor (H₂O) concentrations vary greatly
• vertically and horizontally

• Composition of the atmosphere
• a mixture of various gases
• carbon dioxide (CO₂)
• occupies a small percent of volume of atmosphere
• ~0.036%
• varies vertically and horizontally
• has been increasing since industrial revolution

• Composition of the atmosphere
• a mixture of various gases
• others, in small amounts
• methane
• nitrous oxide
• chloroflourocarbons
• sulfur dioxide
• ozone

• Vertical structure of the atmosphere
• several atmospheric characteristics vary with changes in altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties

• Vertical structure of the atmosphere
• air density and air pressure
• air density
• is determined by the masses of the component atoms and molecules and the amount of space between them
• density is a measure of the amount of matter in a given volume
• is greatest near earth’s surface
• because there are more atoms and molecules in a given volume of air

• Vertical structure of the atmosphere
• air density and air pressure
• air pressure or atmospheric pressure
• results from the force exerted by constantly moving air molecules as they bump against objects
• since air molecules are matter, they occupy space and have mass
• pressure at any level in the atmosphere can be measured in terms of
• the total mass of air above any point

• Vertical structure of the atmosphere
• air pressure and air density
• air pressure or atmospheric pressure
• average or standard atmospheric pressure at sea level is ~14.7 pounds/in²
• 1013.25 mb (millibars)
• 29.92 in. Hg (inches of mercury)
• if more air molecules packed into the same column, air would be more dense & pressure would increase
• fewer air molecules would result in lower pressure

• Vertical structure of the atmosphere
• air pressure and air density
• air pressure or atmospheric pressure
• the number of air molecules present in atmosphere decreases with increases in altitude
• density decreases with increased altitude
• pressure decreases with increased altitude

• Vertical structure of the atmosphere
• air density and air pressure
• initially decrease rapidly with altitude
• because air near earth’s surface is compressed
• about 10 mb/100 m increase in altitude
• decrease more slowly with altitude
• 1013 mb at 0 km
• 500 mb at ~ 5.5 km
• 100 mb at ~17.0 km
• 50 mb at ~22.0 km
• 10 mb at ~32.0 km

• Vertical structure of the atmosphere
• several atmospheric characteristics vary with changes in altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties

• Vertical structure of the atmosphere
• air temperature
• has a more complicated vertical profile than density and pressure
• which decrease with altitude
• is a measure of the "heat" of the air
• which is a measure of the average speed of movement of the air molecules

• Vertical structure of the atmosphere
• air temperature
• normally decreases from earth’s surface up to altitude of ~11 km
• because atmosphere is heated primarily from below by the transfer of heat energy from the surface
• lapse rate
• rate at which air temperature decreases with height
• average or standard in lower region of atmosphere is ~6.5°C per 1000 m

• Vertical structure of the atmosphere
• air temperature
• lapse rate
• average or standard in lower region of atmosphere is ~6.5°C per 1000 m increase in altitude or elevation
• occasionally, temperature may increase with increases in height in lower region of atmosphere, a condition called a
• temperature inversion

• Vertical structure of the atmosphere
• air temperature
• most common parameter used to define atmospheric layers
• troposphere
• stratosphere
• mesosphere
• thermosphere

• Vertical structure of the atmosphere
• air temperature
• troposphere
• from 0 km to ~ 11 km
• characterized by air temperature decreasing with height
• region in which "weather" occurs
• ends at point where temperature stops decreasing with height
• a boundary called the tropopause

• Vertical structure of the atmosphere
• air temperature
• stratosphere
• from ~11 km to 20 km
• air temperature remains constant with altitude
• from ~20 km to ~50 km
• air temperature increases with altitude
• from ~ - 50°C to ~0°C
• resulting in a temperature inversion

• Vertical structure of the atmosphere
• air temperature
• stratosphere
• temperature inversion is attributed to ozone
• reaches its maximum concentrations in stratosphere
• it absorbs energetic UV solar energy
• some of the absorbed energy heats the stratosphere

• Vertical structure of the atmosphere
• air temperature
• mesosphere
• separated from stratosphere by boundary called stratopause
• from ~50 km to ~85km
• % of N and O about same at this level as at sea level
• but there is much less air
• atmospheric pressure is ~1 mb at 50 km

• Vertical structure of the atmosphere
• air temperature
• mesosphere
• air temperature decreases with height
• partly due to there being so little ozone to absorb solar radiation
• molecules of air lose more energy than they are able to absorb
• cooling continues up to ~ 85 km
• where temperature reaches its lowest average value, ~ - 90°C

• Vertical structure of the atmosphere
• air temperature
• thermosphere
• separated from mesosphere by boundary called mesopause
• from ~85 km to several hundred km
• temperature increases with altitude because oxygen molecules absorb solar radiation
• actual temperature varies greatly depending on solar activity

• Vertical structure of the atmosphere
• exosphere
• upper limit of atmosphere
• at the top of the thermosphere
• at an altitude of ~500 km
• some atoms and molecules from this region
• escape earth’s gravitational pull
• shoot off into space

• Vertical structure of the atmosphere
• several atmospheric characteristics vary with changes in altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties

• Vertical structure of the atmosphere
• gaseous composition
• homosphere
• region below thermosphere where gaseous composition remains fairly constant
• from 0 km to ~85 km
• a well-mixed region
• ~78% N & ~21% O

• Vertical structure of the atmosphere
• gaseous composition
• heterosphere
• thermosphere & above (>85km)
• not well-mixed
• heavier atoms and molecules, such as N & O, tend to settle to bottom of layer
• lighter gases, H & He, float to top

• Vertical structure of the atmosphere
• several atmospheric characteristics vary with changes in altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties

• Vertical structure of the atmosphere
• electrical properties
• ionosphere
• above ~60 km
• an electrified region
• with fairly high concentrations of ions and free electrons
• atoms lose electrons and become positively charged when they can’t absorb all the energy transferred to them by collisions or solar radiation

• Heat transfer in the atmosphere
• occurs via a process called
• convection
• transfer of heat by the mass movement of a fluid (such as water and air)
• takes place because fluids can move freely and it is possible to set up currents within them
• leads to a cycle of heated air rising and cooled air descending
• called convective circulation

• Heat transfer in the atmosphere
• convective circulation
• certain areas of earth’s surface absorb more heat from the sun than others areas
• result in uneven heating of the air near the earth’s surface
• heated air expands and becomes less dense
• expanded , less dense air rises and transfers heat energy upward

• Heat transfer in the atmosphere
• convective circulation
• after warmed, expanded, less dense air rises
• cooler, heavier, more dense air flows toward surface to replace the rising air
• upon closer exposure to the warm surface
• the cool air heats up, expands, becomes less dense and rises
• and the cycle is repeated

• Heat transfer in the atmosphere
• convective circulation
• this vertical exchange of heat called convection
• the rising air "bubbles" (or masses of warmed air) are known as thermals
• as the warmed air rises, its temperature eventually decreases and it sinks to surface where it can replace rising air
• producing a convective circulation or thermal cell

• Heat transfer in the atmosphere
• convective circulation
• any air that rises will expand and cool
• any air that sinks is compressed and warms
• wind
• refers to the horizontally moving part of the circulation

• Atmospheric moisture
• several processes and principles interact to determine the manner in which moisture enters, moves about in, and leaves the atmosphere
• evaporation of water from surface into atmosphere
• transport of water vapor through the atmosphere
• precipitation, return of water to surface

• Atmospheric moisture
• in the lower atmosphere, water exists in all three phases of matter
• liquid - water
• gas - water vapor
• solid - ice
• various atmospheric conditions govern the change of water from one phase to another

• Atmospheric moisture
• water changes phases in the atmosphere
• sublimation
• changing from solid (ice) to gas (water vapor) phase without passing through the liquid phase
• evaporation
• changing from liquid to vapor phase
• condensation
• changing from vapor to liquid phase

• Evaporation
• process by which water molecules escapes surface & enters atmosphere as water vapor
• energy is required for this to
• ultimately comes from radiant energy from sun
• escaping molecules carry heat with them
• evaporation tends to be a cooling process
• rate of evaporation if affected by amount of moisture already present in a mass of air
• affected by wind, temperature and humidity

• Evaporation
• rate is affected by amount of moisture in a mass of air
• amount of water vapor present in a quantity of air can be specified in several ways
• absolute humidity
• relative humidity

• Evaporation
• absolute humidity
• most direct measure of air’s moisture content
• the weight of water present in a given volume of air
• is affected by air temperature
• warm air is able to contain more water vapor per unit volume than cool air

• Evaporation
• relative humidity
• ratio of the amount of water actually present in a quantity of air to the amount that could be held by the same air if it were saturated
• does not by itself indicate actual moisture content of air
• when temperature of moist air changes, so does its relative humidity
• temperature at which the relative humidity becomes 100% is called the dew point

• Transport
• water is transported by the atmosphere in the form of water vapor

• Precipitation
• water leaves the atmosphere and returns to earth’s surface in the form of precipitation
• which can take the form of any of the phases of water
• liquid - rain
• gas - fog
• solid - ice
• on average, the amount of water that leaves the atmosphere is equal to the amount that enters the atmosphere

• Precipitation
• several factors govern the process by which water leaves the atmosphere as precipitation
• temperature
• humidity
• dew point
• presence of condensation nuclei

• Precipitation
• condensation
• when moist air is cooled to its dew point
• the water vapor contained in the air is no longer in stable equilibrium and it tends to condense
• dust particles in the air serve as condensation nuclei
• collection centers for water molecules
• promoting the growth of water droplets to a size large enough to be stable

• Adiabatic processes
• combining aspects of vertical structure of atmosphere, heat transfer, moisture content and circulation
• adiabatic cooling
• adiabatic heating

• adiabatic cooling
• process by which the temperature of a rising mass of air decreases
• air mass heated by earth’s surface, rises
• as the warm air mass rises it expands
• air pressure decreases with altitude
• as it expands, it does work against surrounding atmosphere
• expenditure of energy causes temperature to decrease

• adiabatic heating
• process by which the temperature of a descending mass of air increases
• air mass adiabatically cooled, descends
• as the cool air mass descends it compresses
• air pressure increases with decrease in altitude
• as it compresses, surrounding atmosphere does work against it, pushing inward
• this input of energy causes temperature to increase

• lapse rate
• normally, air temp. decreases with altitude
• ~6.5°C per 1000m increase in altitude
• dry adiabatic lapse rate
• adiabatic cooling in absence of condensation
• ~10.0°C per 1000m increase in altitude
• moist adiabatic lapse rate
• adiabatic cooling in presence of condensation
• varies with moisture content of the air
• ~5.0°C per 1000m

Global Environments: The Atmosphere Air Quality Issues

• Air quality
• Acid deposition
• Global Climate Change

Global Environments: The Atmosphere Air Quality Issues

• Air quality
• can be degraded by
• natural events
• volcanic gases & particulate emissions
• dust from wind-borne soil particles
• gases from decomposition of organic material
• anthropogenic (activities of humans)
• automobile emissions
• industrial process emissions
• factory smoke

Global Environments: The Atmosphere Air Quality Issues

• Air quality
• degradation caused by human activities is considered to be air pollution
• related to
• the number of people living in an area
• the kinds of activities the people are involved in
• when we put material into the air we don’t get rid of it
• we just dilute it and move it out of the immediate area

Global Environments: The Atmosphere Air Quality Issues

• air pollution results in
• reduced aesthetic value of scenery
• human health problems
• damage to ecosystems
• due to harmful effects on plant and animal life and reduced water quality
• international conflicts
• damage to structures
• increased costs
• resulting from attempts to meet air quality standards

Global Environments: The Atmosphere Air Quality Issues

• air pollution incident harms human health, an example
• occurred in the valley town of Donora, PA in 1948
• pollutants from zinc plant and steel mills became trapped in the valley
• due to a temperature inversion
• forming a dense fog
• within five days
• 17 people died
• 5,910 people became ill

Global Environments: The Atmosphere Air Quality Issues

• air pollution
• extremely poor air quality common in megacities of developing countries
• such as
• Mexico City, Beijing, Seoul, Cairo
• due to
• open fires
• large numbers of poorly maintained vehicles
• poorly regulated industrial plants

Global Environments: The Atmosphere Air Quality Issues

• air pollution
• air pollutants in the US accumulate due to the predominant west to east winds
• as an air mass moves from west to east
• each population center adds its pollutants to the total load

Global Environments: The Atmosphere Air Quality Issues

• Primary air pollutants
• globally, 5 major types of materials
• are released into atmosphere in sufficient quantities to pose a health risk
• these are called primary air pollutants
• carbon monoxide
• hydrocarbons
• particulates
• sulfur dioxide
• nitrogen compounds

Global Environments: The Atmosphere Air Quality Issues

• Primary air pollutants
• can interact with one another in the presence of sunlight to form
• secondary air pollutants
• ozone
• other PANs (peroxyacyl nitrates)

Global Environments: The Atmosphere Air Quality Issues

• carbon monoxide (CO)
• produced when organic materials (gasoline, coal, wood) are incompletely burned
• single largest source is automobiles
• automobile CO emissions have ¯
• increased fuel efficiency
• use of catalytic converters
• emissions still a problem because of
• increase in number of automobiles
• increase in number of kilometers driven
• next largest source of CO is smoking tobacco

Global Environments: The Atmosphere Air Quality Issues

• carbon monoxide (CO)
• is not a persistent pollutant
• natural processes convert it to other compounds that are not harmful
• air can be cleared of CO if no other new CO is introduced to atmosphere
• is toxic to humans and other animals
• because it is strongly attracted to hemoglobin in blood
• which will take up CO nearly 250 times more rapidly than O₂

Global Environments: The Atmosphere Air Quality Issues

• Hydrocarbons (HC)
• produced by evaporation from fuel supplies or from incomplete combustion of fuels
• are a group of organic compounds consisting of carbon and hydrogen atoms
• sources of HC emissions
• major
• internal combustion engine
• additional
• oil refineries and other industries

Global Environments: The Atmosphere Air Quality Issues

• Hydrocarbons (HC)
• modifications to automobile engines help reduce HC emissions
• recycling some gases through the engines
• using higher oxygen concentrations in the fuel-air mixture
• using valves to prevent escape of gases
• catalytic converters burn exhaust gases more completely

Global Environments: The Atmosphere Air Quality Issues

• Particulates
• minute pieces of solid materials dispersed into the atmosphere
• produced by a variety of processes
• smoke particles from fires
• asbestos from brake linings and insulation
• ash from industrial plants
• farming
• construction
• desertification
• volcanoes

Global Environments: The Atmosphere Air Quality Issues

• Particulates
• can produce variety of problems
• "visual" pollution caused by smoke
• affect health by acting as centers for the deposition of moisture and gases
• respiratory problems
• miners & workers exposed to dust
• asthma
• cancer from carcinogens such as asbestos
• sulfuric, nitric and carbonic acids

Global Environments: The Atmosphere Air Quality Issues

• Sulfur dioxide (SO₂)
• produced when sulfur-containing fossil fuels are burned
• organisms from which coal and oil were produced contained sulfur, some of which was incorporated into the fossil fuel
• reacts with water and other materials to form sulfur-containing acids
• when those acids become attached to particles that are inhaled
• they are very corrosive to lung tissue

Global Environments: The Atmosphere Air Quality Issues

• Sulfur dioxide (SO₂), some incidents
• in 1306, Edward I of England banned the burning of "sea coles" in the city of London
• these were coals from the seashore that were high in sulfur content

Global Environments: The Atmosphere Air Quality Issues

• Sulfur dioxide (SO₂), some incidents
• in 1952, London was covered with a dense fog for several days
• during which time the air didn’t mix because of temperature conditions
• factories continued releasing smoke & dust
• producing "smog"
• within a few weeks 4,000 people died
• deaths attributed to high level of sulfur compounds in the smog

Global Environments: The Atmosphere Air Quality Issues

• Nitrogen oxides (NO and NO₂)
• produced when N & O in air react with one another as a result of combustion
• several compounds have N & O combinations
• most common are
• nitrogen oxide (NO)
• nitrogen dioxide (NO₂)
• mixture of NO and NO₂ is called NO_x
• nitrogen oxides are noteworthy because of the role they play in production of
• secondary air pollutants (photochemical smog)

Global Environments: The Atmosphere Air Quality Issues

• Nitrogen oxides (NO and NO₂)
• primary source of nitrogen oxides is the automobile engine
• catalytic converters help reduce amount of nitrogen oxides released from the internal combustion engine
• increased automobile traffic has resulted in significant NO_x levels in many metropolitan areas

Global Environments: The Atmosphere Air Quality Issues

• Secondary air pollutants
• compounds that result from interaction of various primary air pollutants
• photochemical smog
• a mixture of pollutants resulting from interaction of nitrogen oxide with UV light (sunlight)
• two secondary air pollutants, most destructive components of photochemical smog
• ozone
• peroxyacyl nitrates (PANs)

Global Environments: The Atmosphere Air Quality Issues

• photochemical smog
• ozone & peroxyacyl nitrates (PANs)
• both are excellent oxidizing agents
• react readily with many other compounds, including those in living systems, causing destructive changes
• ozone
• destroys chlorophyll in plants
• injures lung tissue in animals
• PANs
• eye irritants

Global Environments: The Atmosphere Air Quality Issues

• Typical photochemical smog event
• morning traffic produces lots of NO
• N₂ + O_{2 ®} 2NO
• NO reacts with molecular oxygen from atmosphere to form NO₂, gives photochemical smog its reddish-brown haze
• 2NO + O_{2 ®} 2NO₂
• later in morning, NO₂ reacts with UV in sunlight to form atomic O
• NO_{2 ®} UV light ® NO + O
• molecular oxygen in atmosphere reacts with atomic oxygen to form ozone
• O₂ + O _® O₃

Global Environments: The Atmosphere Air Quality Issues

• Typical photochemical smog
• some areas have more trouble with photochemical smog than others because of their climate and geography
• Los Angeles, Salt Lake City, Phoenix
• each of which is ringed by mountains
• prevailing winds are from west
• cool air flows into valley, pushes warm air up, becomes trapped between two layers of cool air, creates thermal inversion
• cool air can’t move out of valley because of mountains

Global Environments: The Atmosphere Air Quality Issues

• Acid deposition
• the accumulation of potential acid-forming particles on a surface
• acids can result from
• natural causes
• vegetation
• volcanoes
• lightning
• human activities
• burning coal
• internal combustion engine

Global Environments: The Atmosphere Air Quality Issues

• Acid deposition
• combustion processes produce sulfur dioxide and oxides of nitrogen
• which are converted to sulfuric acid and nitric acid
• in the presence of water, and oxidizing agents
• such as ozone, hydroxyl ions, hydrogen peroxide

Global Environments: The Atmosphere Air Quality Issues

• acid-forming reactions are classified as
• wet (precipitation)
• reactions occur in the atmosphere
• acids come to earth in precipitation
• acid rain
• acid dew
• acid snow
• dry (deposition)
• particles related to acid settle onto a surface
• reactions occur when these materials mix with water

Global Environments: The Atmosphere Air Quality Issues

• Acid rain
• is the collective term used to refer to all the acid forming processes
• regardless of how the acids are formed or deposited
• wet or dry
• pH of "normal" rain
• between 5.6 and 5.7
• slightly acidic due to
• formation of carbonic acid when carbon dioxide reacts with atmospheric moisture

Global Environments: The Atmosphere Air Quality Issues

• Acid rain
• scope of the problem
• global
• reports of high acid rain damage
• Canada, England, Germany, France, Scandinavia, US
• can have concentration of acid a thousand times higher than normal
• a rain in NH, 1969, pH=2.1 ~ lemon juice
• rain in NE US and Ontario has average pH=4.0-4.5

Global Environments: The Atmosphere Air Quality Issues

• Acid rain causes damage to
• human-made materials
• buildings and monuments made of limestone can be eroded when
• sulfuric in acid rain reacts with calcium carbonate
• metal surfaces can be corroded
• ecosystems
• terrestrial
• aquatic

Global Environments: The Atmosphere Air Quality Issues

• Acid rain causes damage to
• ecosystems
• terrestrial
• forests in NE US have sustained significant tree death and reduction in tree vigpr
• aquatic
• as lakes become more acidic there is progressive loss of many kinds of organisms
• healthy lakes have pH ~6.0
• lakes with pH~4.5 are nearly sterile

Global Environments: The Atmosphere Global Warming & Climate Change

• Emission of gases to the atmosphere
• increasing due to human activities
• activities
• burning fossil fuels
• deforestation
• industrial processes
• gases
• carbon dioxide
• chlorofluorocarbons
• nitrous oxide
• methane

Global Environments: The Atmosphere Global Warming & Climate Change

• During 1980s, scientists became concerned that the earth may be getting warmer
• in past 100 years, earth’s
• average temperature has ­ 0.3-0.6 ° C
• sea level has ­ 10-25cm
• 1999 was warmest year on record
• strong correlation between temperature increase and amount of "greenhouse gases" in atmosphere
• human activity increases amounts of these gases
• changes in global climate patterns are occurring

Global Environments: The Atmosphere Global Warming & Climate Change

• Areas of concern related to global warming and climate change
• human health effects
• rising seas levels
• disruption of water cycle
• changing forests and natural areas
• challenges to agriculture and food supply

Global Environments: The Atmosphere Global Warming & Climate Change

• what causes global warming?
• several gases in the atmosphere are
• transparent to light
• allowing sunlight to penetrate atmosphere and be absorbed by earth’s surface
• absorb infrared radiation
• sunlight energy that is absorbed by earth’s surface is re-radiated as infrared radiation
• which is absorbed by the gases

Global Environments: The Atmosphere Global Warming & Climate Change

• what causes global warming?
• gases in the atmosphere
• allow heat to enter (as sunlight) but retard the loss of heat
• producing a process called the greenhouse effect
• the atmospheric gases contributing to the effect are called greenhouse gases

Global Environments: The Atmosphere Global Warming & Climate Change

• what causes global warming?
• greenhouse gases
• most important are
• carbon dioxide
• chlorofluorocarbons (CFC)
• methane
• nitrous oxide

Global Environments: The Atmosphere Global Warming & Climate Change

• Carbon dioxide most abundant greenhouse gas
• produced as waste product of
• cellular respiration in living organisms
• energy production
• atmospheric levels have been increasing steadily for 40 years
• reducing emissions of this gas would be a step toward slowing global warming
• increase efficiency of energy usage
• increase amount of carbon dioxide removed from atmosphere
• via photosynthesis which requires it

Global Environments: The Atmosphere Global Warming & Climate Change

• Methane
• enters atmosphere primarily from biological sources
• released by several bacteria abundant in wetlands and rice fields
• released by bacteria in guts of termites and ruminant animals (such as cattle)
• some enters from fossil fuel sources

Global Environments: The Atmosphere Global Warming & Climate Change

• Nitrous oxide
• minor component of greenhouse gases
• enters atmosphere primarily from fossil fuels and fertilizers

Global Environments: The Atmosphere Global Warming & Climate Change

• Chlorofluorocarbons
• present in atmosphere in minute quantities but are extremely efficient greenhouse gases
• about 15,000 times more efficient at retarding heat loss than is carbon dioxide
• are entirely the result of human activity
• refrigerant gases in refrigerators and air conditioners
• cleaning solvents
• propellants in aerosol containers
• expanders in foam products

Global Environments: The Atmosphere Global Warming & Climate Change

• Chlorofluorocarbons
• in the 1970s, were linked to the depletion of the ozone layer in the upper atmosphere
• located in the stratosphere
• absorbs most ultraviolet radiation that is potentially damaging to life
• reactive chlorine is released when CFCs are destroyed after reaching the upper atmosphere
• can then enter into reactions that deplete ozone in the upper atmosphere
• leading to an increase in amount of UV radiation reaching earth

Global Environments: The Atmosphere Global Warming & Climate Change

• Chlorofluorocarbons
• the Antarctic Ozone Hole
• first reported in 1985
• occurs in spring