Weather 101 and beyond

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Weather 101 and beyond

• Edward J. Hopkins
• Dept. of Atmospheric Oceanic Sciences
• Univ. of Wisconsin-Madison
• Midwest Hot Air Balloon Safety Seminar
• Hot Aireventure
• Oshkosh 3 March 2001

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Concerns of Balloonists

• The Weather
• The Terrain (or Surface)

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Quiz

• Ballooning (Fair) Weather is associated with High
  or Low Pressure?
• Which way do winds blow around High pressure?
• Low pressure?

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WIND

• What is Wind?
• Why the wind?
• Review of basic concepts

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ASOS Wind InstrumentsWind Vane (left) Cup
Anemometer (right)
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Aerovane Measures wind speed direction
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BEAUFORT WIND FORCE SCALEModern version,
Source Federal Meteorological Handbook I
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BEAUFORT WIND FORCE SCALE (cont.)
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The wind responds to a Difference in air pressure
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BASIC CONCEPTS Air Pressure (cont.)
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Explaining Differences in Air Pressure

• Low Pressure

• High Pressure

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Display of Pressure Differences on a Weather Map
- Isobars
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Isobars -- lines of equal barometric pressure-
use sea level corrected pressure
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AIR PRESSURE in the Vertical (cont.)
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As a Sidebar Altimetry

• Since pressure decreases at a reasonably known
  rate of 1 mb decrease per 10 meter rise or
  0.01 inch of Hg per 10 feet,
• then pressure altimeters are barometers made to
  read in altitude.
• But...

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Pressure Change with height depends upon
Temperature of column - (It is really the
density!)
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WHY THE WIND? (cont.)

• Reasons for Atmospheric Motions
• Buoyancy Effects or Dynamic Effects

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Daily Heating
Heat Gain
Heat Loss
Daylight
Nighttime
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January Temperatures - Madison, WI (1981-90)
Daylight
Nighttime
Nighttime
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January Wind Speeds - Madison, WI (1981-90)
Nighttime
Daylight
Nighttime
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July Temperatures - Madison, WI (1981-90)
Daylight
Nighttime
Nighttime
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July Wind Speeds - Madison, WI (1981-90)
Daylight
Nighttime
Nighttime
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ENERGY TRANSPORT CONVECTION
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LAPSE CONDITIONSTemperature decreases with height
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ISOTHERMAL CONDITIONS Temperature remains
constant with height
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INVERSION CONDITIONS Temperature increases with
height
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U.S. STANDARD ATMOSPHERE See Fig. 1.9 Moran
Morgan (1997)
Thermosphere
Mesopause
Mesosphere
Stratopause
Stratosphere
Tropopause
Troposphere
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WHY THE WIND? (cont.)

• Reasons for Atmospheric Motions
• Buoyancy Effects or Dynamic Effects

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Air Converging Aloft
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Air Diverging Aloft
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The Surface

• The Obvious
• Obstacles to take-off and landing (e.g., trees,
  power lines, animals)
• The Surface and the Winds
• Affects the Boundary Layer wind flow
• Can produce local wind regimes

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Boundary Layer

• Where we live
• Extends from surface to approximately 3000 ft.
  (1000 m)
• Consists of
• Surface Boundary Layer (30 to 60 ft. includes
  Anemometer Level)
• Ekman or Spiral Layer (above 60 ft. to Free
  Atmosphere)

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Relative Surface RoughnessSource Stull, 1995
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B. EXPLANATIONS of ATMOSPHERIC MOTION

• Practical Problems
• Historical Concepts
• Forces of Motion Newton's Laws

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An example of an equation of motionNASA
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PRESSURE GRADIENT FORCE
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PRESSURE GRADIENT FORCE (cont.)
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ASSUMPTIONS

• For convenience, assume that
• Winds are nearly horizontal
• Atmosphere is in nearly hydrostatic balance
  i.e., air parcels do not accelerate
  upward or downward

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HYDROSTATIC BALANCE CONCEPT See Fig. 9.11 Moran
Morgan (1997)
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HORIZONTAL PRESSURE GRADIENT FORCE
(cont.)Direction is from High to Low pressure!
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HORIZONTAL PRESSURE GRADIENT FORCE (cont.) See
Fig. 9.1 Moran Morgan (1997) Magnitude depends
on isobar spacing!
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LOCAL WINDSFLOW RESPONDING TO PRESSURE GRADIENT
FORCE - LOCAL WINDS

• Assumptions
• Only Pressure gradient force operates
• Results from temperature differences
• Acts for short time short distances.
• Examples
• Sea-Land Breeze Circulation
• Mountain-Valley Breeze Circulation
• City-Country Circulation

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Sea (Lake) Breeze(Graphics from UIUC WW2010)
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VERTICAL PRESSURE GRADIENTS - Dependency on
density (temperature)
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Sea (Lake) Breeze (cont.)
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Sea (Lake) Breeze (cont.)
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Sea (Lake) Breeze (cont.)
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Sea (Lake) Breeze (cont.)
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Sea (Lake) Breeze (cont.)
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Sea (Lake) Breeze (cont.)
(Lake)
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Sea (Lake) Breeze (cont.) See Fig. 12.2 A Moran
Morgan (1997)
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Land Breeze
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Land Breeze (cont.)
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Land Breeze (cont.)
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Land Breeze (cont.) See Fig. 12.2 B Moran
Morgan (1997)
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Mountain Breeze See Fig. 12.14 Moran Morgan
(1997)
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Valley BreezeSee Fig. 12.14 Moran Morgan
(1997)
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Larger Scale Flow

• Observation

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Right with Height
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PRESSURE GRADIENT FORCE
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Reason for the Problem Because the earth turns
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CORIOLIS EFFECT or FORCE (cont.)
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Geostrophic Adjustment
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Geostrophic Wind See Fig. 9.12 Moran Morgan
(1997)
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Flow in Friction Layer
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Variation of Friction Effects with Height
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Right with Height
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Varying effects of Surface Roughness
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Curved Flow
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Features in a Surface Low (Convergence Ascent)
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Features in a Surface High (Sinking Divergence)
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Numerical Weather Prediction
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Numerical Weather Prediction
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Numerical Weather Prediction
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My office Dept. of Atmospheric Oceanic
Sciences hopkins_at_meteor.wisc.edu
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UNSTABLE CONDITIONS Compare Environment with
DALRWarmer parcel continues upward