Air Pressure and Winds II

RECAP Ideal gas law: how the pressure, the temperature and the density of an ideal gas relay to each other. Pressure and pressure measurements ♦ Constant height charts: isobars ♦ Constant pressure charts: contour lines Note wind direction in NH ♦ Low P -> Cyclones ♦ High P -> Anticyclones

Isobaric chart Constant pressure, P=500 mb, then plot heights: contour lines Pressures on surface maps Heights on isobaric charts ♦ Low Low ♦ High High Pressure Temperature ♦ Low Low ♦ High High Note wind direction here ♦ Different from surface winds ♦ Along the contour lines, no crossing

Newton’s Laws of Motion Newton’s First Law: an object will ♦ stay at rest, or ♦ maintain its motion at a constant velocity and in a straight line as long as ♦ no force is exerted on the object, or ♦ all forces cancel each other Says who? An observer at rest G F

Newton’s Second Law The force (F) experienced by an object is equal to its mass (m) times the resulting acceleration (a). Acceleration: change in velocity (magnitude or direction) Examples of accelerated motion ♦ Speeding up ♦ Slowing down ♦ Making a turn A body with a large mass is more difficult to accelerate (or stop). Objects with large masses are more inertial.

Forces acting in the atmosphere. Gravity force. ♦ Vertical force in a downward direction Atmospheric drag force (friction). ♦ Acts against the motion ♦ Proportional to velocity squared Pressure force. ♦ From high to low pressure regions ♦ The bigger the pressure gradient the larger the pressure force Centripetal force ♦ Acts perpendicular to the wind velocity, towards the center of the curvature. ♦ It changes the wind direction not the magnitude. ♦ Larger velocity and larger curvature (smaller radius) correspond to a bigger centripetal force. Coriolis force: due to the Earth’s rotation

Pressure gradient force Example: two tanks ♦ Tank A: full ♦ Tank B: half-full Pressure at the base of tank A is higher than the pressure at the base of tank B The pressure gradient force causes the water to flow from A to B (from high pressure to low pressure) Analogous phenomenon in the atmosphere

Pressure Gradient Vector: It has a magnitude and direction. Magnitude: how fast the pressure is changing in a given direction. Direction: the direction of the fastest increase of the pressure. Two examples: ♦ Color gradient ♦ Mountain slope

What is the direction of the color gradient?

Topographic maps Elevation contours, index contours (bold), slope angle. Steep slope: dense contours Gentle slope: contours are further apart Elevation -> pressure; slope -> gradient

In which direction is the pressure gradient?

Pressure Gradient Force The pressure gradient force is analogous to the gravity force on a mountain slope ♦ Its magnitude is proportional to the pressure gradient. ♦ Its direction is opposite to the direction of the pressure gradient. It is from HIGH to LOW pressure. ♦ It is perpendicular to the lines of constant pressure (isobars). The closer to each other the isobars are the larger the gradient is and the larger the pressure force is. The pressure force along an isobar is ZERO!

In which direction is the pressure gradient force?

Wind and Pressure Map

Coriolis Force (Effect) It is an apparent force; Due to the rotation of the coordinate system (Earth); It makes a moving object deflect from a straight line even in the absence of any forces acting on it.

The Magnitude of the Coriolis Force The rotation of the Earth ♦ The faster the planet rotates the bigger the force The speed of the object ♦ Bigger V -> bigger effect The latitude: ♦ Min. at the equator ♦ Max. at the poles

The speed of the object The latitude: ♦ Min. at the equator ♦ Max. at the poles Coriolis force as a function of:

The Direction of the Coriolis Force In the Northern hemisphere the deflection is to the right of the direction of motion. In the Southern hemisphere the deflection is to the left of the direction of motion. The winds in the Northern hemisphere will be deflected to the right and in the Southern hemisphere they will be deflected to the left. ♦ Hurricanes spin differently in the Northern and Southern hemisphere

The Coriolis Force and the Earth The Coriolis effect is important when moving over LARGE distances (air plane travel), with large velocities, away from the equator.