exam 2

Air Pressure and Winds

Definition of Wind

  • Wind is the movement of air relative to the Earth's surface, primarily caused by the imbalance of forces acting on air molecules.

  • Wind moves in two dimensions: vertical and horizontal.

Understanding Air Pressure

What Causes Air Pressure?

  • Air pressure is caused by gravitational forces acting on air molecules and the weight of these molecules.

Definition of Air Pressure

  • Air pressure is defined as the force exerted over a given area.

  • The standard sea-level air pressure is 1013.25 millibars (mb) or 14.7 pounds per square inch (lb/in²).

Air Movement in the Atmosphere

Causes of Atmospheric Circulation

  • The Earth receives unequal solar radiation at different latitudes, leading to temperature and pressure variations.

  • The Earth's rotation on its axis also influences wind patterns.

  • Flowing air encounters frictional drag as it moves across the Earth's surface, altering its speed and direction.

Forces Acting on an Air Molecule

Pressure-Gradient Force

  • The pressure-gradient force acts when there are differences in air pressure between two locations, resulting in air moving from higher pressure areas to lower pressure areas.

Coriolis Force

  • The Coriolis force describes the pattern of deflection taken by objects not firmly connected to the ground.

  • In the Northern Hemisphere, objects deflect to the right, while in the Southern Hemisphere, they deflect to the left.

Example: Coriolis Experiment

  • To observe the Coriolis effect, experiments can be conducted to visually depict the deflection of moving objects in different hemispheres.

Frictional Force

  • The frictional force operates primarily between 500-2000 meters (1640-6560 feet) above the Earth's surface and is often ignored in upper atmospheric calculations.

Air Movement - Large Scale Wind Systems

Geostrophic Winds

  • Geostrophic winds exist at higher altitudes where a balance between the pressure gradient force and the Coriolis effect allows winds to flow parallel to isobars.

Surface Winds

  • Surface winds, found closer to the ground, experience frictional forces that weaken the Coriolis effect and cause them to move across isobars.

Surface Pressure Systems

Cyclones

  • Cyclones are low-pressure systems where converging air rises vertically at the center.

Anticyclones

  • Anticyclones are high-pressure systems where diverging air moves outward and draws air down into the center.

Air Movement - Small Scale Wind Systems

Sea/Land Breeze Systems

How Does a Sea Breeze Work?

  • Sea breezes occur due to differential heating between land and water, creating pressure gradients that drive air from cool sea surfaces to warmer land.

Mountain/Valley Breeze Systems

  • Mountain breezes occur at night when cooler, denser air sinks down slopes into valleys, while valley breezes occur during the day as warm air rises.

Circulation Patterns of the Atmosphere

The Simple Model

Zonal Flow

  • Zonal flow refers to air movement that flows parallel to lines of latitude.

Meridional Flow

  • Meridional flow refers to air movement that flows parallel to lines of longitude.

The Actual Model

Pressure Systems

  • Equatorial Low - A region of low pressure.

  • Subtropical High - A region of high pressure.

  • Subpolar Low - Another region of low pressure.

  • Polar High - A region of high pressure at the poles.

Circulatory Cells

  • Hadley Cell - Extends from 0-30 degrees latitude.

  • Ferrel Cell - Extends from 30-60 degrees latitude.

  • Polar Cell - Extends from 60-90 degrees latitude.

Wind Patterns Based on Latitude

  • Trade Winds (Easterlies) - Found between 0-30 degrees latitude, moving from east to west.

  • Westerlies - Found between 30-60 degrees latitude, moving from west to east.

  • Polar Easterlies - Found between 60-90 degrees latitude, moving east to west.

Jet Streams

  • Equatorial Jet - Located at approximately 0 degrees latitude, moving from east to west.

  • Subtropical Jet - Located around 30 degrees latitude, moving from west to east.

  • Polar Jet - Found around 60 degrees latitude, also moving from west to east.

Global Circulation

Definition

  • Global circulation refers to the large-scale movement of air through the atmosphere, influenced by differential heating of the Earth's surface, the arrangement of land and ocean, and the Coriolis effect.

Parts of Global Circulation

  1. Differential Heating - Variations in temperature due to uneven solar radiation.

  2. The Three Cells - The Hadley, Ferrel, and Polar cells dictate major wind patterns.

  3. The Coriolis Effect and Winds - The deflection of winds relative to the rotation of the Earth.

Monsoon

  • A seasonal prevailing wind, often bringing significant precipitation to regions such as South Asia.

Circulation of the World Ocean

Ocean Temperature Zones

  • Mixed Layer - Extends to 75 meters (245 feet) and constitutes 3% of ocean volume, with generally constant temperature and salinity.

  • Thermocline - Ranges between 75-1000 meters (245-3300 feet) and is characterized by a sharp change in temperature.

  • Deep Ocean Layer - Found below 1000 meters (3300 feet), making up 97% of ocean volume and generally maintains a constant temperature.

Circulation of the World Ocean

Generation of Ocean Currents

The primary drivers of ocean currents are defined below:

  • Wind - Creates kinetic energy and combines with the Coriolis effect to generate ocean currents post-initiation.

  • Sea Water Density - Variations in temperature and salinity differences lead to distinct density and current formation.

Wind-Driven Gyre Circulation (Surface)

Gyres

  • Gyres are large-scale circular motion systems within the ocean, influenced by prevailing winds and the Coriolis effect, alongside landmass interactions.

  • Key types include Tropical Gyres, Subtropical Gyres, and Polar Gyres.

Upwelling

  • Upwelling is the process of nutrient-rich water rising to the surface, influenced by wind-driven currents and the Ekman transport effect.

Wind-Driven Flow

Deep Ocean Currents

  • Deep ocean currents are controlled primarily by thermohaline circulation, also known as the Global Conveyor Belt, which drives water movement based on density gradients formed by temperature and salinity variations.

Atmospheric Moisture and Water Balance

Measuring Water Vapor

Important Concepts in Measuring Water Vapor

  • Vapor Pressure - The pressure exerted by water vapor molecules in the atmosphere.

  • Dew-Point Temperature - The temperature at which air becomes saturated and cannot hold more water vapor, leading to condensation.

  • Specific Humidity - The ratio of the mass of water vapor to the total mass of the air sample.

  • Relative Humidity - The percentage of water vapor present in the air compared to the maximum amount it can hold at a given temperature.

The Hydrologic Cycle

Overview

Key Components of the Hydrologic Cycle

  • Precipitation

  • Evapotranspiration

  • Runoff

  • Infiltration

  • Groundwater flow

Water Balance Conditions

  • Analyzed based on precipitation and evaporation rates across different regions.

Forms of Precipitation

Types of Precipitation

  • Rain - Produced from ice crystals encountering warmer air layers.

  • Snow - Forms entirely in freezing temperatures.

  • Sleet - Occurs when water droplets pass through a sub-freezing air layer.

  • Freezing Rain - Water droplets that freeze upon contact with surfaces.

  • Hailstones - Formed from ice crystals that accumulate layers while being lifted within moist clouds.

Lifting Mechanisms that Produce Precipitation

Main Types of Lifting

  1. Convergent Lifting - Occurs in systems with low pressure, causing air to converge and rise.

  2. Frontal Lifting - Involves the collision of different temperature air masses, creating fronts (warm and cold).

  3. Convectional Lifting - Caused by spontaneous buoyancy-driven air movement, often leading to thunderstorms.

  4. Orographic Lifting - Occurs when air masses are forced upwards due to geographic features like mountains, often resulting in rain shadow effects.

Weather Systems

Low Latitude Weather Systems

Characteristics

  • Dominated by easterly waves, these intangible disturbances often lead to organized low-pressure systems such as tropical depressions and cyclones.

Mid to High Latitude Weather Systems

Latitudinal Temperature Gradients

  • Strong temperature gradients lead to more vigorous clashes of air masses and the occurrence of creating systems including cyclogenesis and mature mid-latitude cyclones.

Polar Jet Stream

  • A significant feature in mid to high latitude weather systems, impacting cyclogenesis and the development of weather patterns.