Local Winds:Why Does the Wind Blow?

Objectives: To Know: 

O1 How atmospheric pressure is measured and varies with height

O2 How the pressure gradient, Coriolis force, and friction affect air motion.
O3 The wind direction found for high and low pressure cells in the northern and southern hemisphere.

Did you Know?

  1. It's impossible for a hurricane to cross the equator, because it would lose its rotational energy.
  2. The Coriolis effect pulls your car 1500 ft to the right for every 100 miles you travel.  Friction between tires and the road prevents this from happening. 
  3. The highest winds on earth typically occur in gusts, like the 231 mile per hour wind recorded on Mount Washington.  This is the world record wind officially, but radar found gusts in a 1991 Oklahoma tornado at 287 miles per hour.

Misconceptions:
M1: A river and wind both flowing north are moving in the same direction.
M2: Water always goes down the toilet bowl the same way in the Northern Hemisphere. 

OBJECTIVES

O1 How atmospheric pressure varies with height and is measured

O2 How the pressure gradient, Coriolis force, and friction affect air motion at the surface and aloft

O3 The wind direction found for high and low pressure cells in the northern and southern hemisphere.

MISCONCEPTIONS

M1: A river and wind both flowing north are moving in the same direction.

M2: Water always goes down the toilet bowl the same way in the Northern Hemisphere. 

INTRODUCTION

What is air pressure?

What is wind?

Sustained wind vs. gust

How is wind direction measured?

Compared to a river? 

O1 How is atmospheric pressure varies with height and is measured?

How pressure readings can vary with altitude

The accumulation of air over a surface causes:

The decrease of air over a surface causes:

It takes a shorter column of cold, more dense air to exert the same surface pressure as a taller column of warm less dense air.

Upper air low pressure cell

Upper air high pressure cell

 

How is pressure measured?

Barometer: mercury vs. aneroid

1013=29.92"=1 ATM

Adjusting for station elevation

10 mb per 100 m

Isobars

O2 How the pressure gradient, Coriolis force, and friction affect air motion
at the surface and aloft

Pressure Gradient Force

Difference in pressure/distance

Strong pressure gradient (tightly packed isobars)

Weak pressure gradient (widely spaced isobars)

Coriolis Effect

In the northern hemisphere

In the southern hemisphere

What effects this deflection

Wind speed

Distance from equator

Friction

Friction layer

Friction decreases wind speed which decreases Coriolis effect

When balancing Coriolis, friction, and pressure gradient, wind crosses isobars at
30 toward the center of a low pressure and 30 away from the center of a high pressure cell

Geostrophic Winds

Aloft, Coriolis force balances the pressure gradient force

Winds flow parallel to isobars rather than crossing the isobars as at lower elevations

O3 The wind direction found for high and low pressure cells in the northern and southern hemisphere.

Northern Hemisphere

High pressure cell in the cross section

 

High pressure cell in the plan view

 

Low pressure in the plan view

 

Low pressure cell in the cross section

 

Southern Hemisphere

High pressure cell in the cross section

 

High pressure cell in the plan view

 

Low pressure in the plan view

 

Low pressure cell in the cross section

 



Web Resources to Supplement this Lecture

  1. 1.U.S. WIND STATISTICS: where is the wind, on average blowing hardest in the U.S.? What is the mean direction? Find out here.
  2. U.S. PRESSURE STATISTICS: discover the highest and lowest atmospheric pressure currently reported in the U.S.
  3. HOW AIR PRESSURE AFFECTS YOU: A NASA sponsored site which discusses the importance of air pressure and offers experiments to demonstrate its effects.
  4. UIUC MODULE ON FORCES AND WINDS