lecture 10 march 15, 2010, monday atmospheric...
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Atmospheric Pressure & Wind: Part 1
Speed, Velocity, Acceleration, Force, Pressure
Atmospheric Pressure & Its Measurement
Ideal Gas Law (Equation of State)
Pressure Gradient Force (Horizontal & Vertical)
Isobars (horizontal)
Hydrostatic Equilibrium (vertical)
Isobaric Surface
Isobars vs. Height Contours
Coriolis Effect & Coriolis Force
Frictional Force
Lecture 10
March 15, 2010, Monday
Speed: distance traveled per unit time, a scalar (magnitude
only), positive value only, in units of m s-1, mph, kmph, etc.
Velocity (v) : speed & direction, a vector (magnitude &
direction), both positive and negative (direction, coordinate),
same speed with opposite directions having different velocity,
also in units of m s-1, mph, etc.
Acceleration (a) : change of velocity (either in speed or in
direction) with time, in unit of m s-2, also a vector and can be
both positive (acceleration) or negative (deceleration).
Gravity (g): a particular acceleration in meteorology, related to
the downward gravitational force (G) due to attraction by the
solid Earth, 9.8 m s-2 with small changes with latitude & altitude
Newton’s Second Law: F = ma
F is force (a vector, in unit of N, newton), N = kg m s-2
a = F / m, g = G / m, m is the mass of a body (kg)
Pressure = Force / Area, Pa (pascal) = N m-2
Atmospheric
Pressure
Atmospheric
pressure units in
meteorology:
1 mb (millibar)
= 100 Pa
= hPa (hectoPascal)
1 kPa (kilopascal)
= 1000 Pa
= 10 mb = 10 hPa
Other units:
inch Hg or mm Hg
g x air mass (m) = weight (G)
surface area on Earth
The weight of the column
of mercury balances the
weight of the column of
atmosphere on the same
area which is the cross
section of the tube.
Atmospheric pressure
= g x density of mercury
x height of mercury
weight=g x density x volume
volume = height x area
pressure = weight / area
Measurement of
Atmospheric
Pressure
Mercury
Barometer:
an inverted
tube filled
with mercury.
inch Hg= inches of mercury
mm Hg= millimeters of
mercury
Conversion:
1 mm Hg = 1.3332 mb
1 inch Hg = 33.864 mb
Shown in this picture is a
Fortin Mercury Barometer
located in the Atmospheric
Science Laboratory (Brewster C-203) of the
Department of Geography at
ECU. It is very accurate as it
meets the National Weather
Service Standard.
Measurement of
Atmospheric
Pressure
~ 14.7 pounds per square inch (psi)
Range of Atmospheric Pressure
at the Mean Sea Level
Aneroid Barometers:
a collapsible chamber which compresses proportionally to atmospheric pressure.
Relationships between pressure (p),
temperature (T), and density (): p = R T
R=287 J kg-1 K-1, gas constant of dry atmosphere
(excluding water vapor) of the Earth.
At constant temperatures, an increase in air
density will trigger a pressure increase.
Under constant density, an increase in
temperature will be accompanied by an
increase in pressure.
Pressure is exerted in all directions equally,
not just downward.
Dalton’s Law: total pressure = sum of partial
pressures exerted by different gas molecules.
Vapor pressure is the partial pressure by water
vapor molecules and is a humidity index.
The Equation of State: Ideal Gas Law
Compressibility of atmospheric gases causes a non-linear
decrease in pressure with increasing altitude.
Pressure at P2 < P1 can be simply due to pressure decreasing
with elevation, thus difficult to evaluate horizontal pressure
difference, for example, at the surface.
Recorded surface pressure is reduced to sea level pressure
equivalents to facilitate horizontal pressure comparisons.
Vertical and Horizontal Changes in Pressure
Horizontal pressure differences across space are useful.
By analyzing isobaric maps, pressure gradients are apparent: strong or weak pressure gradients indicated by closely or widely spaced isobars, respectively.
In this weather
map, green lines
are isobars of
sea level
atmospheric
pressure
distribution.
Isobars:
lines of equal
atmospheric
pressure
Mapping Atmospheric Pressure
Forces that drive wind
Pressure Gradient Force (PGF) PGF drives air move from high pressure to low pressure.
PGF is perpendicular to isobars.
Magnitude of PGF increases with pressure gradient.
PGF is great in severe weather events (tornadoes, hurricanes).
weak pressure gradient: low wind speeds
strong pressure gradient: high wind speeds
Vertical Pressure Gradients Average vertical pressure gradients are usually
greater than extreme examples of horizontal
pressure gradients such as hurricane, tornado.
However, vertical wind speed is usually much smaller than horizontal wind speed because…
Hydrostatic Equilibrium The downward force of gravity balances strong
vertical pressure gradients to create hydrostatic equilibrium.
Forces balance and the atmosphere is held static relative in the vertical direction to Earth’s surface.
Local imbalances initiate various updrafts and downdrafts.
The Role of Density in Hydrostatic Equilibrium
Gravitational force is proportional to mass: G = g m
Denser atmosphere experiences greater gravitational force.
A vertical pressure gradient must increase to offset increased gravitational force to maintain hydrostatic equilibrium.
Higher temperature columns of air are less dense than cooler ones due to expansion in volume or increase in depth (height).
For warm or cold air, greater depth for the same pressure decrease in the vertical translates into smaller or larger vertical pressure gradients, which leads to or maintains hydrostatic equilibrium.
Heating
causes a
density
decrease in a
column of air.
The heated
column
contains the
same amount of
air as the cool
one, but has a
lower density
due to greater
depth or height
and thus
smaller vertical
pressure
gradient.
Upper air heights decrease with increasing latitude
Constant pressure surfaces of cooler air will be at a lower
altitude (height) than those of warmer air.
Height contours indicative of horizontal pressure gradient.
Horizontal Pressure Gradient in Upper Atmosphere Upper air horizontal pressure gradients may be determined
using height contour distribution for given constant pressure.
Isobaric Surface
Isobars Height Contours
Coriolis
Effect
Free moving
objects in the
atmosphere
are influenced
by Earth’s
rotation
Path of
missile
Deflected to
the right in
Northern
Hemisphere
South Pole
South Pole
Coriolis
Force
A resulting
apparent
deflective
force
Path of
missile
Deflected to
the left in
Southern
Hemisphere
The Coriolis force is a sine function of the latitude,
maximum at the poles and zero at the equator.
Taking place regardless of the direction of motion.
Coriolis force increases with speed of moving objects.
Overall deflection effect noticeable only on objects with long
periods of motion.
Only changes direction, DOES NOT change speed
Frictional Force
A force of opposition which slows air in motion (wind).
Initiated at the surface and extend, decreasingly, aloft
by atmospheric turbulence.
Important for air within 1.5 km (1 mi) above the ground
surface which is called the planetary or atmospheric
boundary layer (PBL or ABL).
Because friction reduces wind speed it also reduces
Coriolis deflection.
Friction above 1.5 km is negligible, thus atmosphere
above 1.5 km is called the free atmosphere (flow aloft).
Effects of
Frictional
Force on
Winds
near the
Surface
Balance
between PGF,
Coriolis force,
and frictional
force.
“Northern
Hemisphere”
Effects of Frictional Force on Winds near the Surface
Rougher surface
(Urban)
Strong Friction
Low Wind Speed
Smoother Surface
(Lakes & Oceans)
Weak Friction
Medium Wind Speed
Upper Free
Atmosphere
No Friction
Highest Wind Speed
L
H
L
H
L
H
1000 mb 1000 mb1000 mb
1012 mb
1008 mb 1008 mb1008 mb
1012 mb 1012 mb
For the same PGF,
lower wind speed
means smaller
Coriolis force and
greater angle
between the wind and
the isobar.
For the same PGF,
higher wind speed
means greater
Coriolis force and
smaller angle
between the wind and
the isobar.
Wind parallel
to the isobar
“Northern
Hemisphere”
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