Precipitation Flashcards
Atmosphere
movement and storage of water is in response to general global circulation and to local vapor pressure gradients
Weather
constitutes the current conditions of atmosphere (pressure, temp, wind, humidity)
Climate
summation of weather overtime
Pressure
result of particles moving about randomly in the atmosphere; force/unit area (lb/in^2 or psi)
Pressure at Sea Level
14.7 psi or 1013.250 dynes/cm^2 (millibar)
Standard Pressure
30 inches of Mercury (barometer)
Pressure We Feel
14.7 lb on every in^2
Column of Atmosphere
weighs 14.7 lb over 1in^2
Water Replacing Mercury
33 ft water
Pressure and Density
decrase with altitude
Density
changes due to varying mixes of substances and gravity
Atmosphere
considered as 400 miles deep
Jet Stream
major transport mechanism for weather patterns and circulation; 500 mb
High and Low Pressures
exist because the atmosphere is a fluid & can pile up, thin down, speed up, slow down, etc.
Winds are Created By
gravity which acts on varying density chunks of atmosphere or differences in pressure (high to low)
Coriolis Effect
an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates; influences movement of atmosphere
High Pressure Rotation
clockwise
Low Pressure Rotation
Counter-clockwise
Good Weather
high pressure systems
Bad Weather
low pressure systems
Significance of Strong Low Pressure
associated with stronger winds (stronger pressure difference = stronger wind)
Barometer
measures pressure; responds to atmospheric pressure
Liquid Barometer
mercury; glass tube is without protection; has to stay verticle
Mechanical Barometer
less precise/accurate than mercury
Why do we use mercury and not water in barometers?
equivalent is 33 ft of water
Temperature
expression of heat, not heat of an object
Temperature Changes With
altitude (not at same rate as pressure & density)
Temperature Drops
may not be uniform at all
Boyle’s General Gas Law
p1v1=p2v2
Charle’s Law
V1/T1=V2/T2
Thermometer
measures temperature; min/max
Thermometer Issues
don’t show records (past data)
Humidity
measure of water stored in atmosphere; expression of water vapor and saturation
Water Vapor
water in atmosphere in gas form
Water Content
extremely variable in space and time
Water
3rd most plentiful substance in well mixed homosphere (4%)
Nitrogen
1st most plentiful substance in well mixed homosphere (78%)
Oxygen
2nd most plentiful substance in well mixed homosphere (21%)
Carbon Dioxide
4th most plentiful substance in well mixed homosphere (<1%)
Water Content Varies With
latitude, season, and altitude
Vapor Pressure
partial pressure of atmosphere due to presense of water droplets; pressure exerted by presence of water; mb or in. of mercury
Saturation Vapor Pressure
maximum the atmosphere can hold
Warmer Air
holds more water
Cooler Air
holds less water
As Air Cools
amount of water doesn’t necessarily change; ability to hold water reduces
Saturation Vapor Pressure is Reached When
temperature drops to the point where the holding capacity of air equals the amount of water in the air
Dewpoint
reached when temperature drops to point where actual vapor pressure equals saturation vapor pressure at constant pressure; condensation forms
Dewpoint is >32 F
condensate will be water (dew)
Dewpoint is <32 F
condensate will be frost
Supercooling
temperature gets below dewpoint without condensation forming
-40 C or F
spontaneous condensation occurs; dust or ice crystals act as condensation nuclei
Fahrenheit (F)
9/5 C + 32
Over Ice
vapor pressure is less than over water; ice crystals are good condensation nuclei
Relative Humidity
expression of how much water atmosphere is holding compared to what could be held
Why does the air in our houses fees so dry in winter?
cold air that seeps into our house has a lower humidity; warm air holds more moisture
Why do we feel so warm on humid days?
air is saturated with water, so sweat evaporates slowly
Adiabatic
cooling without giving off heat
Adiabatic Processes
when air particle is lifted through atmosphere it cools (volume is larger and pressure is less), holds less water, expands, and may not lose heat
Clouds
condensed water/ice droplets; not all provide precipitation
Ice Clouds
appear more “feathery” than that of water vapor
Cloud Basic Scenario
warm air at surface and begins to rise, cools and relative humidity rises until saturation
Cloud Types
cirrus, alto, stratus, nimbo, strato, cumulo
Cloud Cover
Clear <10%, Scattered 10-50%, Broken 50-90%, Overcast >90%
Fog
cloud close to the ground
Radiation Fog
back radiation from Earth cools the surface and layer of atmosphere immediately adjacent to earth; cool clear nights
Advective Fog
warm moist air transported over cool surface
Steam Fog
temperature of lake greater than air above
Fog Drip
water droplets from fog adhere to leaves/needles of trees/others
Fog Layers
flat/horizontal sections
Global Circulation
division at equator; wind moves up and away from equator towards the poles and back again
Each Hemisphere
has 3 cells of circulation; rotating up and away from equator towards poles
Creating Doldrums/Horse Latitude
at 30 degrees North and South latitude colder air sinks
Air at Poles
colder air sinks
Creating Polar Front
at 60 degrees North and South latitude, cold and warm air converge
Doldrums/Horse Latitude
bored; state or period of inactivity, stagnation, or depression
Polar Front
area where cyclonic storms are generated, dry easterlies mix with warm moist westerlies
Easterly/Westerly
refer to which direction the wind is coming from
Jet Stream
cooler and warmer currents produce front with associated pressure gradient; doesn’t cross equator
Jet Stream Winds
produced as air flows from high to low pressure; 18,000-30,000 ft
Precipitation Generally Caused By
lifting a parcel of air to a zone of lower pressure and corresponding pressure, cooling causes dew point, and creates cloud
Cyclonic Storm
large air mass that rotates around a strong center of low atmospheric pressure; cool easterlies brush against warm moist westerlies and motion begins, produced cold and warm fronts, mixing of cold dense air with warm moist air produced precipitation area
Summers Here
quick and intense rains, flash floods
Fall/Spring Here
drawn out rains and cover larger area
Terrain
can cause lifting, which is a major component in precipitation mechanism
Mountains
provide lifting for warm advecting moist air; lift, cooling, snow, go off other side warm and dry
Lake Effect
have their own ecosystem
Convectional Storms
thunderstorms
Thunderstorms
warm moist air rapidly lifts (unstable), air cools, precipitation forms, as it falls it cools the air
Thunderstorm Updraft
warm moist air lifts up
Thunderstorm Downdraft
falling precip cools air; spreads out in all directions at ground, produces gust front
Point of Origin
source/feed of warm moist air; appear to “backbuild” here
Cold Gust Front
acts as cold front; can feed other storms
Why Urban Areas Contribute to Thunderstorms
presence of heat source (pavement reflect heat) and typically darker areas; more flash floods
Rainfall
varies in space and time; amounts vary considerably
Spatial and Temporal Variability
variance of space and time
Raindrop Size
0.4 - 0.7 mm
Raindrop Velocity
> 20 mph
Terminal Velocity of Rain
reached in about 30ft
Raindrop Shape
not the traditional dew drop; dome > bowl > splits in 2
Terminal Velocity
doesn’t speed up anymore
Rain Guage
measures rainfall depth
Issue with Rain Guages
point coverage and interference (trees, wind, etc.)
Radar
radio detection and ranging
Standard Guage
non-recording; large: 20in, small: 7in
Fisher and Porter Tipping Bucket
most common mechanical guage; records measurements
Universal
elite of mechanical guages; vacuum system to reduce wind effect and collect more
Radars in Place Right Now
160
WSR-88D
Weather Surveillance Radar - 1988 Doppler
WSR-88D Purpose
provide coverage for large % of the US
WSR-88D Impacts
energy pulses, Z-R Relationship, calibration, assume drop size and distribution, zero-degree isotherm
Ice Types
hail, rime, sleet, graupel (large)
Ice
result of all lift and picking up moisture, lots of power; difficult to measure, very high reflectivity
Snowpack
compressed and hardened by its own weight; reported in depth
Snow Water Equivalent (SWE)
ratio of 10:1 and density is 10%
Snowfall Most Dense
high water content, high temp
Snowfall Least Dense
low water content, low temp
Measure Snow and SWE
guages/boards or tubes
Why is it difficult to predict snow with radar?
storm’s track and intensity can change it drastically
Spatial and Temporal Variation Issue
doesn’t represent total area coverage,
Duration
length of time rain falls
Intensity
rate of rain fall or amount/duration
Frequency
occurrence/probability; return period
Greater the Duration
greater the amount & lower the intensity
More Frequent the Storm
shorter the duration & less the intensity
100 Year Event
chance of it occurring each year; 1/100th chance of getting that event in 1 year
2 Year Event
event will occur on average every 2 years; 50% probability occurring in any given year
How to Calculate Areal Average
arithmetic, thiessen, isohyetal, radar grid based
Calculate Areal Average: Arthimetic
add all precipitation values and divide (obtain mean)
Calculate Areal Average: Thiessen
find area of each gage (connect gages and split into 3 sections from center of gages); weight has more importance and sums to 1 (all gages = 100%); only do it once and just calculate for weight
Calculate Areal Average: Isohyetal
find area between lines; better method; do it for each storm
Calculate Areal Average: Radar Grid Based
area put onto grid to see concentrated area; need a radar
