Questions: Pressure Systems and Surface Winds

Question 1

In the Northern Hemisphere, surface winds around a low-pressure system (cyclone) spiral in which direction?

• A. Clockwise and outward
• B. Clockwise and inward
• C. Counterclockwise and outward
• D. Counterclockwise and inward

Answer: (D)

Low-pressure systems have a pressure gradient force pointing inward (toward the center of low pressure). The Coriolis effect deflects this inward-directed flow to the right in the Northern Hemisphere, producing counterclockwise rotation. Near the surface, friction prevents full geostrophic balance, causing winds to cross isobars slightly inward toward the center (convergence), which forces air to rise, cool, and produce clouds and precipitation.

Question 2

At altitude (above the boundary layer), winds blow parallel to isobars because the Coriolis force and pressure gradient force are in balance. Near the surface, this geostrophic balance holds equally well.

• T. True
• F. False

Answer: False

Near the surface, friction with the ground slows the wind. Slower wind means less Coriolis deflection (since the Coriolis force is proportional to wind speed), so the balance between the pressure gradient force and Coriolis breaks down. The pressure gradient force wins a partial contest, causing winds to cross isobars at an angle toward lower pressure — typically about 15–30° from the isobar. This inward spiraling is why surface lows produce convergence and rising air.

Question 3

Why does a high-pressure system produce clear, dry weather rather than clouds and rain?

Model answer: In a high-pressure system, air subsides (sinks) from altitude toward the surface. As this air descends, it is compressed by the increasing atmospheric pressure below it, which causes it to warm adiabatically. Warmer air can hold more water vapor, so relative humidity drops and clouds evaporate or fail to form. The dry, stable descending air suppresses convection, resulting in clear skies.

A common misconception is that high pressure itself physically prevents clouds from forming. The actual mechanism is adiabatic warming of descending air. Conversely, in low-pressure systems, converging surface air is forced to rise, cools adiabatically, and eventually reaches the dew point — producing condensation, clouds, and precipitation. The key in both cases is the vertical motion of air, not the pressure value itself.