Meteorology Basics

 

   A. Stability and Vertical Mixing

 

            Stability and Instability

 

Rule 1. Warm air is less dense than cold air.  It is therefore buoyant and rises.

Rule 2. Rule 1 applies when comparing air at the same vertical  level.

Rule 3. Air expands and cools as it rises (adiabatic process).

Rule 4. If air is pushed upward, compare its new cooler temperature with the surroundings.

 

Instability: When lifted (and cooled) air is warmer than its surroundings, it will continue to rise. 

Stability: When lifted air becomes cooler than its surroundings, it will sink or oppose lifting.

 

Unstable air column: warm near surface and much colder aloft.

Stable air column: cool near surface and gradual cooling as you go up.

 

 

       Solar heating and changes in mixing depth, mixed layer

 

During the day, solar radiation heats the surface, which in turn heats the air directly above it.  This warm air will rise until it becomes colder than its surroundings.  As this surface air rises, air above it sinks. This creates a well mixed layer of air from the surface up.

The depth of the mixed layer or mixing depth, increases through the day as more energy is added to the mixed layer.

 

Inversions

Inversions are air columns (or layers) where the air temperature actually increases as you go up.  Inversions are super-stable situations which prevent vertical mixing (the mixing depth remains small)

 

Subsidence inversions occur with anticyclones (high pressure).

Areas of high pressure are related to sinking air.  As this sinking air compresses and warms adiabatically, the air becomes warmer than the air below it.

 

Radiation Inversions typically occur at the surface.

At night, the surface radiates out energy and cools

This leaves the air near the surface colder than the air above it

Often we find radiation inversions in valleys where heavy cold air settles

Radiation inversions are strongest on clear nights.

(Clouds radiate energy back to the surface, keeping it warmer)

Clear night often occur with anticyclones.

During the day, clouds and fog can help maintain a surface inversion by preventing solar radiation form reaching the surface.

 

 

Ascending air: low pressure systems, summertime mixing

 

Air converges towards low pressure areas forcing air upward.

During the summer, greater amounts of solar radiation result in enhanced vertical motion and greater mixing depths.

 

B. Wind Systems at Various Scale

Large-scale (synoptic-scale):  Low vs. High pressure systems

 

Surface winds travel with

Low pressure system:

Counterclockwise and spiraling in towards the low

High pressure system:

Clockwise and spiraling out from the high

 

       Winds are faster for a stronger pressure gradient

 

 

        Urban Wind Systems:

         Wind speeds are reduced because of more friction

          An urban heat island circulation cell occurs during the day

          when the city warms up. 

         Winds are directed towards the city

 

 

            Sea and Land Breezes:

          Sea breeze: during the day, air is directed from the cooler ocean

          towards the warmer land towards land during the day.

          Land breeze: at night, winds blow from the cooler land.

 

          Topography: 

      Mountain breeze occurs at night (cold air drains into valley).

Valley breeze occurs during the day, creating an upslope wind. 
This is particularly strong with a south-facing slope.

           

      Topography also affects winds by creating barriers to wind flow.

 

 

            Interactions between Wind Systems

            Wind systems at different scales interact in complex ways.

            For example, a sea breeze can be reinforced on the coastal
            portion of a city by the urban heat island circulation. This could

            create a wake effect on the opposite side of the city.