1 Weather and Climate Bay Area Earth Science Institute (BAESI) Pressure San Jose State University, January 24,

2 Weather and Climate

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5 Principle forces Q: What drives the weather in the atmosphere? Q: What causes the weather to change? We will develop a foundation for answering these questions through understanding the predominant forces in the atmosphere, namely:    

6 Weather and Climate Principle forces Q: What drives the weather in the atmosphere? Q: What causes the weather to change? We must analyse the predominant forces in the atmosphere, namely:  pressure and pressure gradients,  gravity  rotation of the earth  friction

7 Weather and Climate Pressure  Measured in millibars (mb) or inches of mercury  Surface Pressure:

8 Weather and Climate Pressure  Measured in millibars (mb) or inches of mercury  Surface Pressure: Sea level pressure: 1012 mb or 29.9 in of mercury

9 Weather and Climate What causes pressure to change?  Temperature change  Density change Formally, the equation of state describes this relationship: p=  RT p-Pressure (Pa)  - density (g m -3 ) R - Gas Constant (287 J K -1 kg -1 ) T - Temperature (K)

12 Weather and Climate Pressure Differences  Determines the direction and speed of winds.  Can help explain general circulation of atmosphere.  General guidelines: –High pressure: –Low pressure: Why is this?

13 Weather and Climate Pressure Differences  Determines the direction and speed of winds  Can help explain general circulation of atmosphere.  General guidelines: –High pressure: clear skies/fine weather –Low pressure: cloudy skies/changeable weather Why is this?

14 Weather and Climate

15 Weather and Climate

16 Weather and Climate Pressure Gradient Force  One of the most important forces in meteorology  Pressure gradient: –Dependent on pressure differences between different location. Wind blows from high to low pressure

17 Weather and Climate Pressure Gradient Force

18 Weather and Climate Coriolis Force

19 Weather and Climate The rotation of the Earth  Rockets, migrating birds, and large scale weather systems are all deflected due to the rotation of the Earth.  The Earth’s rotation causes –Rotational movement that deflects the winds.  The Coriolis Force is the name of this rotational force that deflects motion.

21 Weather and Climate Coriolis Force   Affects direction, not speed of object   Maximum at the poles   Zero at the equator   Always acts to the right of the motion in Northern Hemisphere. Always acts to the left of the motion in the Southern Hemisphere.

23 Weather and Climate Geostrophic Wind Northern Hemisphere