Atmosphere: Pressure and Layers

• The atmosphere is a layer of gases surrounding a body, with its density decreasing with height.
• Earth's weather primarily occurs in the troposphere, the lowest atmospheric layer.

Atmospheric Pressure

• Atmospheric pressure is the force per unit area exerted by a liquid or gas.
• Pressure decreases as height increases because pressure at any level is due to the weight of the air above it.

Troposphere

• The troposphere is the lowest layer of the atmosphere, bounded by the tropopause.
• It is characterized by a positive lapse rate and varies in height from about 8 km at the poles to 16 km over the equator.
• Temperature decreases with height at a rate of 0.6°C per 100 meters within the troposphere.
• Often considered the "effective" atmosphere

Tropopause

• The tropopause is the boundary between the troposphere and the stratosphere.
• Its height ranges from approximately 8 km over the poles to 16 km over the equator.

Intertropical Convergence Zone (ITCZ)

• Also known as the doldrums, the ITCZ is an oceanic region between the northeast and southeast trade winds.
• Characterized by light, variable winds, calms, heavy rains, thunderstorms, and squalls. -The ITCZ moves north and south with the sun, staying within roughly 5 degrees of its average position, and is a very low-pressure area where winds converge.

Trade Winds

• Constant winds

Composition of Dry Air

• Dry air, within 16 km of the surface, consists of:
  • 78% nitrogen
  • 21% oxygen
  • 1% argon, carbon dioxide, helium, ozone, etc.
• Excluding carbon dioxide, the gases in dry air maintain a constant proportional composition.

Water Vapor

• Water vapor is the amount of moisture in the atmosphere.
• Varies due to temp changes, evaporation, condensation, precipitation, dust, salt, and particulate matter.

Precipitation

• Precipitation includes rain, drizzle, sleet, snow, hail, dew, hoar frost, rime, glazed frost, and wet fog.
• Cloud, fog, and mist are not classified as precipitation.

Humidity

• Humidity refers to the amount of water vapor present in the air.

Absolute Humidity

• Absolute humidity is the water vapor content of the air at a specific time and location.
• Measured in mass per unit volume, such as grams per cubic meter (g/m³).
• Higher air temperatures allow for greater saturation values of absolute humidity.

Relative Humidity

• Relative humidity is the ratio of the actual water vapor content to the maximum possible amount at the same temperature.
• Expressed as a percentage.

Saturation

• Air is saturated when its relative humidity reaches 100%.
• Air can become supersaturated with respect to ice.

Supersaturation

• Supersaturation occurs when the absolute humidity exceeds its saturation value at the existing temperature, resulting in a relative humidity greater than 100%.
• Condensation nuclei are typically present in the atmosphere, preventing significant supersaturation.

Lapse Rate

• Lapse rate is the rate at which temperature decreases with increasing height.
• The average lapse rate is 0.6°C per 100 meters.
• The actual lapse rate can vary from day to day and place to place.
• Can also be negative, causing temperature to increase with height. This is known as temperature inversion

Environmental Lapse Rate (ELR)

• The average rate of temperature decrease with height in the atmosphere, typically 0.6°C per 100m.

Unsaturated Lapse Rate (DALR)

• The rate at which dry (unsaturated) air cools as it rises, which is 1°C per 100m.

Saturated Lapse Rate (SALR)

• The rate at which saturated air cools as it rises, which is o.5°C per 100m.

Cloud Formation

• Temperature or height where clouds form is the dew point.

Buys Ballot's Law

• In the Northern Hemisphere, if you face the wind, low pressure will be to your right.
• In the Southern Hemisphere, if you face the wind, low pressure will be to your left.
• This law does not apply within 5 degrees of the equator.

Isobars

• Isobars are lines connecting points of equal barometric pressure.
• Spaced at intervals of one or more hPa.
• Isobaric patterns help identify pressure systems like depressions, anticyclones, and ridges.

Gradient Wind

• Gradient wind is a hypothetical wind in the free atmosphere that flows parallel to curved, equidistant isobars if they were stationary.

Pressure Gradient

• Pressure gradient is the change in pressure with distance, measured perpendicular to isobars.
• A greater pressure gradient results in a stronger wind.

Atmospheric Pressure

• The weight of air above a certain level defines atmospheric pressure.
• Pressure decreases with increasing height.
• Average surface pressure at sea level is 1012 hPa.

Coriolis Force

• A deflecting force caused by the earth's rotation.
• Acts on moving objects, deflecting them 90 degrees to their motion.
• Deflects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Ineffective within 5 degrees of the equator.

Geostrophic Wind

• Wind that blows parallel to straight, equidistant isobars in the free atmosphere.
• Moves under the influence of horizontal pressure force and the Coriolis force.

Geostrophic Wind Speed

• Measured using scales on synoptic charts.
• Measure the perpendicular distance between isobars and using a scale dependent on the latitude.

Causes of Wind

• Horizontal movements of air occur due to pressure differences.
• Air moves from areas of high pressure to areas of low pressure.

Adiabatic Process

• Adiabatic means without gain or loss of heat; thermally insulated.
• Increased pressure causes compressional heating, while decreased pressure causes expansional cooling.
• No heat is exchanged between the air and its surroundings during temperature changes.

Adiabatic Heating and Cooling

• Temperature changes due to pressure and expansion/compression.
• Increased pressure leads to compressional heating, while reduced pressure leads to expansional cooling.

Adiabatic Processes in the Atmosphere

• Atmospheric pressure decreases with height.
• Rising air cools adiabatically due to reduced pressure.
• Sinking air warms adiabatically due to increased pressure.
• No heat exchange occurs between the air and its environment.

Cloud Formation

• Rising unsaturated air expands and cools adiabatically.
• If the ascent continues, the air reaches its dew point and becomes saturated.
• The base of the cloud appears at the condensation level, where the dew point is reached.