1. Air Pressure Astronomy/Meteorology
Chapter 19
Air Pressure
Astronomy/Meteorology

2. Understanding Air Pressure 19.1
Air Pressure Defined
Pressure exerted by the weight of the air above
Exerted in all direction
minutes
Measuring Air Pressure
Barometer – device used to measure air pressure
Bar= pressure
Metron = measuring instrument
Pressure increases  pushes mercury up
unit = millibar
Air pressure at sea level is mbars

3. Factors Affecting Wind 19.1
Air moves from high pressure to low pressure
Wind result of differences in air pressure
Differences in air pressure b/c of unequal heating (temp)
Solar radiation source of wind
Air doesn’t move in straight lines b/c
Pressure differences
Coriolis Effect
Friction

4. Pressure Difference 19.1 Great difference = greater wind
Isobars – lines on a map that connect places of equal air pressure
Pressure gradient – spacing between lines
Close: steep pressure gradient = high winds
Widely spaced: weak pressure gradient = light wind
2 minutes
Driving force of wind
High to low

6. Coriolis Effect 19.1
Describes how Earth’s rotation affects moving objects
Northern Hemisphere = deflected to the right
Southern Hemisphere = deflected to the left
Only Affects wind direction
Strong winds equals strong deflection
Strongest at poles
Basically nonexistent at equator
2 minutes

9. Friction 19.1 Affects wind speed and direction
Only affects a few km from Earth’s surface
Jet streams = fast moving rivers of air that travel between 120 – 240 km/hr from east to west

10. Pressure and Centers and Winds 19.2
Highs and Lows
Cyclones – centers of Low Pressure
Pressure decreases from the outer isobars towards center
Northern Hemi: counterclockwise
Low = Left
Southern Hemi: clockwise
Anticyclones – centers of High Pressure
Pressure increases from outer to inner
Northern Hemi: clockwise
Southern Hemi: counterclockwise

12. Pressure and Centers and Winds 19.2
Weather and Air Pressure
Rising air = clouds and precipitation
Surface Convergence = Cyclonic (low)
Sinking air = clear skies
Surface Divergence = Anticyclone (high)

13. Pressure and Centers and Winds 19.2
Weather Forecasting
Low-pressure centers – can produce bad weather
Lows move W to E
Predicting is difficult

14. Global Winds = Non-Rotating Earth Model 19.2

15. Global Winds = Rotating Earth Model 19.2

16. Regional Wind Systems 19.3 Local Winds – small-scale winds
Caused by either topographic effects or variation in surface (land and water)
Land and Sea Breezes
Land heats and cools faster than water

17. Valley and Mountain Breezes
Air on slopes of mountains heated more during the day than the valley floor
Slope air = less dense  glides up slope to generate a valley breeze
Air cools quickly at night = more dense = sinks into valley

18. How Wind is Measured 19.3 Wind Direction Wind Speed
Prevailing Wind – wind consistently blows more often from one direction
US = westerlies move from west to east
Wind Speed
Anemometer – measure wind speed

19. El Nino and La Nina
El Nino = irregular intervals of 3 to 7 years of warm countercurrents that replace normal cold offshore waters
Affects Ecuador and Peru
La Nina= temperature in eastern Pacific are colder than average

20. Normal Conditions
El Nino Conditions

21.
3 minutes
4 min 30 sec
3 min 15 sec