Download Lecture 1 (9/9) METR 1111

Weather
Temperature
Temperature is the average motion of
the molecules of a substance
(measured with a thermometer)
There are 3 different temperature
scales:
Fahrenheit – used to record
surface temp in US
Celsius – used overseas & upper
air obs.
Kelvin – used in scientific
formulas
Humidity
 Humidity is the amount of water vapor
in the air.
 There’s several ways to measure
humidity, but the most useful for
meteorologists is the dew point
 Dew point temperature is the
temperature at which the air would
become saturated (and dew would
begin forming)
 High relative humidity occurs when the
temperature is close to the dew point
Pressure
Atmospheric pressure is the force per
unit area exerted against a surface by
the weight of the air above it.
(measured by barometer)
Pressure
If the amount of air molecules
above a surface increases, pressure
increases too
If the amount of air molecules
above a surface decreases, pressure
decreases too
High pressure is usually associated
with fair wx and Low pressure with
stormy wx.
There are several different pressure
units
Variables influencing the
atmosphere…
pressure:
High Pressure
Low Pressure
Region where
air has piled up
Region where
air has
stretched out
Warmer and
dryer than
surrounding air
Tends to rise
and cool, vapor
condenses into
clouds
Pressure
These units include:
Inches of Mercury (Hg)
Pascals (Pa)
Millibars (mb)
Meteorologists use
millibars most often
Pressure Conversions
 Standard sea level pressure is the
average pressure on Earth at sea level.
 Standard sea level pressure is
=29.92 in Hg=1
atm=1013.25 mb
=101325 Pa=101325 N/m^2
Winds around High’s
&Low’s
Because of the Coriolis force
(discussed in a later course)
winds in the Northern Hem:
blow counterclockwise (or
cyclonically) around Low
Pressure
And blow clockwise (or anticyclonically) around High
Pressure
Winds around High’s and Low’s
If the wind is at your back in
the Northern Hemisphere, low
pressure is always on your left!
(Remember Low pressure =
Left!)
Wind
Wind has 2 different measurements
(speed and direction)
Wind speed is measured with an
anemometer
Wind direction is measured with a vane
Vane
Anemometer
Variables influencing the atmosphere…
winds:
Due to differential
heating of the
earth’s surface
Warm air rises
Cooler air flows in
Units of Wind
Wind speed can be measured in
mph, knots, or meters per second
1 knot = 1.15 miles per hour
1 m/s = 2.24 miles per hour
Wind Direction tells us FROM
WHERE the wind is blowing
Ex: A SW wind means wind is FROM
SW
Wind Direction
Wind direction is measured in
degrees (like a compass, not like
the degrees in math!)
0°=North, 90°=East, 180°=South,
270°=West
N
0 or 360
270
90
180
Meteorology
Latitude and Longitude
Latitude and Longitude lines divide up
the Earth into a usable coordinate
system
Lines from N to S = meridians of
Longitude
Longitude measures how far E or W a
location is from the Prime Meridian
(through Greenwich, England)
Values of Longitude
go from 180°W to
180°E
Latitude
Lines from E to W (parallel to
equator) are meridians of Latitude
Latitude measures how far N or S a
location is from the equator.
Values of Latitude go from
90°S to 90°N
Norman, OK is located at
35.22°N latitude
97.45°W longitude
The global energy balance also varies with
longitude, priamrily due to the differences of
land, water and elevation
Global Air Circulation Patterns
Ocean Currents
 What causes them?
 Wind drag. Friction with the wind drags the
ocean surface to produce a current. The semipermanent position of the subtropical high over
the north and south Pacific and the north and
south Atlantic help drive the anticyclonic gyre of
water in each basin.
 Thermohaline circulation. Density variations in
water arise from differences in temperature
(warmer=> less dense) and salinity (more
salty=>more dense). More dense water tends
to sink relative to less dense water, giving rise
to vertical circulations known as thermohaline
circulations.
 Interactions of ocean currents with bottom
topography and lateral boundaries
 Other ocean “weather”, i.e. ocean waves,
vorticies, etc.
How do ocean currents affect
weather and climate?
 Oceans store and transport heat. The high heat
capacity of water makes it an excellent
mechanism to store the sun’s energy and
transport it from one place to another
 Oceans store liquid water and pump vapor into
the air as a key link in the global water and
energy cycle.
 Ocean storage and release of heat is a key
forcing mechanism for weather
 Oceans absorb (release) atmospheric gasses,
such as oxygen and CO2.
 Absorption of CO2 by oceans is the most
important sink of CO2 from the atmosphere
 Amount of CO2 that the oceans can hold is
inversely proportional to temperature, i.e.
colder water holds more CO2 and warmer
water (think global warming here) holds less.
 Oceans are a major source of oxygen for the
atmosphere due to photosynthesis of
microscopic plants in the ocean.
Basic Ocean Circulations
Thermohaline Circulation
What is a front?
At a frontal boundary, some or
all of the following changes
occur:
Temperature change
Change in dewpoint
Wind Shift
Pressure trough
Cold Fronts
A cold front is a boundary where cold,
dry air is sliding under warm, moist air.
A typical temperature change with a
cold front is about 20°F but varies
greatly
“Fronts” = a clash of air
masses
Cold Front
 digs in
underneath a
warm air mass
 Warm air rises,
mixes with
colder air, &
condenses into
clouds
More on Cold Fronts
 Ahead of a cold front, winds are usually
south to southeastrly winds. Behind a
cold front, winds usually from the north
or northwest.
 A cold front is denoted on a surface
map like this
 Filled triangles point
towards direction
of movement
3-D Picture of Cold Front
Notice the warm air is lifted up and
over the cold air and forms clouds.
Warm air is less dense than cold air
so it rises where they meet.
Warm Fronts
A warm front is a boundary where
warm, moist air is advancing against
cooler, drier air.
Temperature change averages 15°F
but varies.
More on warm fronts
Ahead of a warm front, winds are
usually east or northeasterly winds.
Behind it, south to southwesterly
winds prevail.
 A warm front looks like this where
the half circles point toward the
direction it’s moving.
Stationary Front
Boundary between warm, moist air
and cold dry air in which neither air
mass is advancing.
Indicated by alternating blue
triangles and red half circles.
Often happens when front loses it’s
“punch” or is blocked by high
pressure.
Occluded front
A occluded front, or occlusion, occurs
when a warm air mass and a cool air
mass come in contact with each other,
making boundaries at both the ground
as well as aloft.
Often associated with showers or
snow(in winter) and is the beginning of
the end to a storm.
Indicated by triangles and half circles
on the same side.
What are isolines?
Isolines (also called isopleths)lines that connect equal values
of a variable
In meteorology, we frequently
use:
- isobars (pressure)
- isotherms (temperature)
- isotachs (wind speed)
Isolines
Isolines make it easy to see
different values on a map
Allow us to interpolate values at a
point
Can pick out different features
like fronts, jetstreams, drylines,
pressure fields, etc.
How can I draw an isoline?
Tropical Meteorology
Anything between about 30N and
30S in latitude
Or between Tropic of Cancer
(23.5N) and Tropic of
Capricorn(23.5S)
Fuzzy definition
Radiation budget > 0 in tropics
Job of tropics = export heat
poleward
Tropical Meteorology
Very difficult compared to mid-latitudes
Can’t just measure pressure and get an
approximate wind field
Geostophic Balance - doesn’t happen in
in tropics
Effects of Earth’s rotation not a big
player (Coriolis force is small)
Try to measure convergence (winds
coming together and divergence- winds
pulling apart) to deduce areas of
upward motion
Tropical Cyclone Classification
 TC’s form from a wave or disturbance
 Trop disturbance = any area of “disturbed
weather” ex: stalled out front
 Trop wave = elongated area of low pressure
(just a trough). Go from East to West
(direction of winds at these latitudes).
 Trop depression = closed low (trough pinches
off)
 Trop storm = 39-73mph, hurricane=74+
Strength Versus Intensity
Intensity = core region (center to
100 km)
An increase in intensity = stronger
max winds or lower minimum
surface pressure
Intensity can change quickly
Strength = outer part of core
Associated with area weighted
average wind speed outside of core
Eyewalls & Rainbands
Region of clouds/intense rain/strongest
winds that seperates sinking air in eye
from rest of storm
Typically not vertical (45 deg slope =
common)
Rainbands = stronger areas of rain with
higher clouds between them
Sometimes, tornadic supercells will
form in rainbands as storm hits land
(b/c of added friction)
Severe Storm
A thunderstorm must have one or
more of the following to be
considered a severe storm: (NWS
classification)
Winds 58 mph (50 knots) or more
Hail 3/4” in diameter or larger
Tornado
Supercell Thunderstorms
 A supercell thunderstorm is a t.s. with
a deep rotating updraft (mesocyclone)
 Updraft elements usually merge into
the main rotating updraft and then
accelerate rapidly
 Flanking updrafts "feed" the supercell
updraft, rather than compete with it
 Small percentage of all t.s.’s are
supercells but they cause the majority
of damage
Diagram of a Supercell
A Look from the SE
El Nino
Original definition: Annual warming
of water off of the Coast of Peru
around Christmas. “El Nino”
referred to the Christ child.
Since early 1980’s new definition:
Warm water occurring off of the
Peru and reaching out to the
dateline that tends to occurr
periodically every 2-5 years.
El Nino
El Nino
Sea Surface Temperature Anomaly
La Nina
New term coined recently to
refer to the opposite of an El
Nino when the Equatorial Pacific
is colder than normal and the
SOI is negative
Cross-section
showing
change in
ocean
temperatures
for 1997 El
Nino
development
ENSO sea surface height anomaly
La Nina sea surface height
anomaly
USA impacts of El Nino and La Nina:
temperature
USA impacts of El Nino and La Nina:
precipitation