Cloud Development and Forms

Chapter 6 Lecture

Understanding Weather and Climate Seventh Edition

Redina L. Herman Western Illinois University

• When air lifts, clouds develop and precipitation can also occur. • There are four mechanisms that cause air to lift. These four mechanisms are reviewed in detail in the slides that follow.

• Orographic uplift: Occurs when a mass of air is deflected over or around a terrain, usually a hill or a mountain. This upward movement of air results in adiabatic cooling. This promotes the development of clouds and precipitation. • Rain shadow: Air compresses as it descends down the terrain and results in little to no precipitation.

• Frontal lifting: Occurs when two air masses converge at the front. This can occur when cold air advances toward warm air (cold front) or when warm air advances toward cold air (warm front). Clouds develop as a result of these two situations.

Cold front example Warm front example

• Convergence: Occurs when there is a horizontal movement of air into a region. When air converges along the Earth's surface, it is forced to rise since it cannot go downward.

• Localized convection: Occurs when differential heating at the surface causes air to lift. The air expands and cools as it lifts, causing cloud development.

• Static stability: Refers to the concept that all air has a tendency to lift. – Unstable: Air will continue to rise if given an initial upwards push. – Stable: Air will resist the upward push and sinks back to the original level. – Neutral: Air will not rise or sink. If displaced, it will rest at the height to which it was displaced. • Environmental lapse rate: Refers to the rate of decrease of temperature with elevation.

• Absolutely unstable • Absolutely stable • Conditionally unstable – These three types of static stability will be explained in detail in the slides that follow.

• Absolutely unstable: This occurs when a parcel of air is lifted and it continues to move upward regardless of saturation. If the environmental lapse rate (ELR) exceeds the dry adiabatic lapse rate (DALR), the air is absolutely unstable.

• Absolutely stable: This occurs when a parcel of air returns to its original location after being displaced. If the environmental lapse rate (ELR) is less than the saturated adiabatic lapse rate (SALR), the air is absolutely stable.

• Conditionally unstable: This occurs when the environmental lapse rate (ELR) is between the dry adiabatic lapse rate (DALR) and the saturated adiabatic lapse rate (SALR). An air parcel become saturated at the lifting condensation level (LCL) and it will become buoyant if lifted to a critical altitude called the level of free convection (LFC).

• The ELR can change when the following occurs: – Heating or cooling of the lower atmosphere – Advection of cold and warm air at different levels – Advection of an air mass with a different ELR

• Heating or Cooling of the Lower Atmosphere – Daytime heating of the Earth’s surface occurs rapidly and leads to a steep ELR near the surface. – The opposite occurs at night as cooling can promote the development of a temperature inversion (air temperatures increase with altitude in the lower portion of the atmosphere).

• Advection of cold and warm air at different levels.

• Advection of an air mass with a different ELR (environmental lapse rate).

• Generalized cloud chart

Cloud classification will be explained in more detail in the slides that follow.

• Cloud Observation by Satellite – (a) Visible, (b) infrared, and (c) color-enhanced infrared satellite images