Cloud Formation: Lifting Processes

Atmospheric Lifting In order for air to form clouds, the air must be lifted and rise in altitude There are 4 types of Atmospheric Lifting – Orographic – Convective – Convergent – Frontal

Orographic Lifting Air is forced over a barrier such as a mountain – Windward Side: wet and cold – Leeward side: dry and hot Causes a “rain shadow” – dry area on the leeward side of a mountainous area (away from the wind). The mountains block the passage of rain-producing weather systems and cast a "shadow" of dryness behind them. i.e. Sierra Nevada, Rocky Mountains, Cascades

Convective Lifting Air lifting due to local surface heating – The surface is heated from the Sun’s radiation causing the air to warm and rise – Causes rain and thunderstorms i.e.- Hadley Cell along the equator

Convergent Lifting Air flows towards areas of low pressure, forces air to rise Causes thunderstorms i.e.- Florida

Frontal Lifting Air lifting occurs along boundaries of different air masses – Cold Fronts – Warm Fronts Cool air is denser than warm air cool air sinks down and warm air rises – Create different types of weather

Stable vs. Unstable AIR Air stability is the measure of its temperature at given heights or altitudes. Cool air is denser so it wants to sink and warm air wants to rise to create stable conditions Stable air does not rise Unstable air will rise freely to create stability Meteorologists focus on unstable air – produce the weather systems due to movement of air and water vapor. Unstable air is associated with various ways of lifting air as discussed in the previous slide.

Stable vs. Unstable AIR

Humidity and Saturation Humidity is a general term for the amount of water vapor in air. Relative humidity (RH) = water vapor in air as a ratio expressed as % –Example: 32% or 100% RH = 100% = clouds form = condensation level (dew point) Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure. When saturated, warm air contains more water vapor than cold saturated air.

ELR – Environmental Lapse Rate Lapse Rate = speed of temperature decrease with altitude through the atmosphere (in this case the troposphere)

Adiabatic Processes process where NO HEAT is added or removed to the system As altitude increases in the troposphere, temperature naturally decreases Air is denser near surface. As the air rises, it cools as it expands The rate at which it cools is called the Adiabatic Lapse Rate – Two Types Dry Adiabatic Cooling Saturated (Wet) Adiabatic Cooling

Adiabatic Processes Dry Adiabatic Cooling: Does not contain water vapor Air rises and expands as temperature decreases in troposphere Saturated (Wet) Adiabatic Cooling: -Occurs when Relative Humidity = 100% -Occurs when water vapor forms a cloud

LCL & Dew Point LCL = the lifted condensation level is the height at which the relative humidity (RH) of an air parcel will reach 100% when it is cooled by dry adiabatic lifting. Condensation and cloud formation occur at the LCL Air rises and contains water vapor Through adiabatic cooling, the temperature and pressure decrease. This causes the air parcel to expand as it cools as it rises. At a certain altitude in the troposphere the rising air condenses. This is called the ‘lifting condensation level’. The Dew Point is when the temperature of water vapor condenses into water droplets and forms a cloud This is why a cloud appears to be flat at the base