Project Atmosphere American Meteorological Society Weather series: The Coriolis Effect Debra L. Zolynsky

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The Coriolis Effect

Forces Governing Wind Forces acting on air parcels, which either initiate or modify motion, are the consequence of: 1. Air pressure gradients 2. The centripetal force 3. The Coriolis effect 4. Friction 5. Gravity

1. Air Pressure Gradients

1. Air Pressure Gradient Force This force exists whenever air pressure varies from one place to another. Can arise from contrasts in air temperature, Differences in water vapor concentration, or Diverging and converging winds.

1. Air Pressure Gradients Air pressure gradients are always measured in the direction of greatest change, that is, perpendicular to isobars. Air pressure gradient is steep where isobars are closely spaced, and weaker where isobars are farther apart.

1. Air Pressure Gradient Force The force that causes the air to move as the consequence of an air pressure gradient is known as the pressure gradient force and it always acts directly across isobars and towards low pressure.

Air (just as water) flows from areas of high pressure to areas of low pressure.

2. Centripetal Force Newton’s first law of motion: an object in straight-line,unaccelerated motion remains that way unless acted upon by an unbalanced force. When driving around a curve at high speed, you feel a force pushing you outward. What you are feeling is the tendency for your body to continue in a straight path as the vehicle follows a curved path.

2. Centripetal Force A centripetal force operates whenever air parcels follow a curved path. The centripetal force arises from an imbalance in other forces operating in the atmosphere.

3. Coriolis Effect The turning of moving objects’ paths as seen from our vantage point on Earth is the Coriolis effect. The effect of the Earth’s rotation on horizontally moving objects is greatest at the poles and zero at the equator. It always acts perpendicular to the direction of motion: left in the southern hemisphere, and right in the northern.

3. Coriolis Effect More later…

4. Friction Friction is the resistance that an object or medium encounters as it moves in contact with another object or medium. The friction of fluid flow (either liquid or gas) is called viscosity. Eddies are visible swirls of fluid. The rougher the surface of the Earth, the greater the eddy viscosity. Turbulence is fluid flow characterized by eddy motion. Turbulence is experienced as gusts of wind.

5. Gravity Gravity always acts directly downward. For this reason, gravity does not modify horizontal wind. Gravity influences air that is ascending or descending, such as in convective currents. Gravity is responsible for the downhill drainage of cold, dense air.

Joining Forces Forces interact in the atmosphere to control the vertical and horizontal flow of air, that is, the wind. These interactions result in. 1. Hydrostatic equilibrium. 2. The geostrophic wind. 3. The gradient wind, and. 4. Surface winds (horizontal winds in the atmosphere.

Explanation The air parcel moves from high pressure to low pressure directly across isobars. As the air parcel accelerates, the Coriolis force strengthens and causes the air parcel to turn gradually to the right always remaining at right angles to the parcel’s direction of motion. Eventually, forces attain a balance, known as geostrophic equilibrium, so that the geostrophic wind blows at a constant speed in a straight path parallel to isobars with the lowest air pressure to the left of the direction of air motion.

Questions?