Drawing the Wind

Geostrophic Balance

When the PGF is equal to the Coriolis Force

• We call this geostrophic balance

• Winds flow parallel to straight isobars

Drawing the Force Balance Diagram

Step 1: Draw PGF (always high to low)

Step 2: Draw Coriolis

• In Geostrophic Balance, Coriolis will always be opposite of PGF

Step 3: Draw the Geostrophic Wind

• In the Northern Hemisphere, Coriolis is always to the right and low pressures are always to the left of the wind.  

Step 4: Do your checks!

• Is Coriolis deflecting wind to the right?

• Are low pressures on the left? 

The Surface Wind

At the surface, Friction slows down the wind. 

• Therefore, the Coriolis force is reduced. 

  • • PGF is now stronger than Coriolis. 

    • No longer in geostrophic balance!

• Winds blow across isobars towards lower pressure. 

Drawing the Force Balance Diagram

Step 1: Draw out your force balance diagram as if you were in Geostrophic Balance.  

Step 2: Coriolis weakens when Friction is applied, also weakening the wind. 

Step 3: The wind gets pulled toward PGF.

Step 4: Friction is drawn 180° from the wind.

Step 5: Since Coriolis is always 90° to the right, it rotates with the wind.

Step 6: Do your checks!

• Is Coriolis deflecting wind to the right?

• Are winds blowing towards low pressure/ away from high pressure?