Atmospheric Pressure and Weather

• Air consists of atoms (nitrogen and oxygen) and molecules (carbon dioxide) in constant motion.
• Particles release heat through collisions, with more energy leading to faster motion and more heat.
• Energized air expands, cooling as particles move apart and collide less frequently.
• Air movement results in wind

Causes of Wind

• Air movement is caused by pressure differences.
• Atmospheric pressure is the weight of air pushing down
• More molecules above a point mean higher atmospheric pressure at that point.
• Elevation, humidity, and temperature affect pressure
• Higher elevations have thinner air, resulting in lower atmospheric pressure.
• Humidity is the amount of water vapor in the air.
• Water vapor is lighter than air, so higher humidity lowers air pressure.
• Energized particles move faster and farther apart and as air expands, it becomes lighter and rises.
• Warm, rising air lowers pressure; cool, descending air increases pressure, thus the sun's uneven heating creates pressure systems.
• Air moves from high to low-pressure systems, producing wind.

Convection Currents

• Convection transfers heat from warmer material through air or liquid.
• Heated air expands, rises and then cools, and falls.
• Rising air leaves space, pulling in more air, creating a cyclical current.
• Convection currents occur when the sun heats surfaces like black roads, causing warm air to rise and cool air to move in.

Pressure Gradient Force

• The Earth's surface has varying atmospheric pressures, creating high and low-pressure systems.
• Uneven heating of the Earth's surface by the sun creates these systems.
• Air moves from high to low-pressure systems as wind.
• Wind strength depends on the pressure difference between systems.
• The pressure gradient force is determined by measuring pressure and distance between systems.
• Greater pressure differences and closer systems result in faster air movement.
• Atmospheric pressure is measured with a barometer in pascals (Pa), usually expressed as hectopascals (hPa).
• One atmosphere is 1,013 hectopascals.
• Pressure gradient force equals the pressure difference divided by the distance.
• The stronger the pressure gradient force, the stronger the wind.
• Measuring the pressure gradient force is fundamental in meteorology for weather forecasting.
• Strong pressure gradient forces indicate severe storms.
• Hurricanes and tornadoes have extremely low pressure, creating strong gradient forces.

Coriolis Effect & Wind

• The Earth's rotation affects air currents.
• The equator is a low-pressure area due to high sunlight, while the poles are high-pressure.
• Air moves from the poles to the equator (high to low); however, it doesn't travel in a straight line.
• In the Northern Hemisphere, winds move counterclockwise from the North Pole to the equator.
• In the Southern Hemisphere, winds move clockwise from the South Pole to the equator.
• The Coriolis Effect acts at a perpendicular angle to the pressure gradient force, creating weather patterns.

Why Does the Coriolis Effect Occur?

• The Coriolis Effect occurs because of the Earth's rotation.
• Points further from the equator rotate more slowly.
• A ball thrown from the equator drifts east, and a ball thrown from the North Pole drifts west.
• Air moving from the equator to the poles drifts east, and air moving from the poles to the equator drifts west.
• Air does not move directly from the poles to the equator.
• The Northern and Southern Hemispheres have three convection cells.
• The Hadley Cell (0-30 degrees latitude) creates the Trade Winds blowing west.
• The Ferrel Cell (30-60 degrees latitude) has winds blowing east or west.
• The Polar Cell (60-90 degrees latitude) is located near the poles.

Friction & Wind Pressure

• The Earth's surface has mountains, trees, buildings, etc.
• These create friction, slowing wind and changing direction.
• Friction reduces the Coriolis Effect, strengthening the pressure gradient force.
• Friction's effects are mostly felt near the Earth's surface.
• Higher in the atmosphere, there are fewer obstacles.

Explore the relationships between temperature, altitude, humidity, and atmospheric pressure. Understand what causes wind and how air particle movement, energy, and heat relate to one another. Learn why weather patterns form.