# Presentation: 013565

## June 16, 2003  Earth Science Professional Development

**How Pressure effects evaporation, condensation and precipitation**

**Scott Cordero**
**Warning Coordination Meteorologist**
**Memphis Weather Forecast Office**
** ****National Weather Service**

**Notes:**
Self introduction and introduction of topic.

## The Partition Function

## What are we going to talk about?

**Basic Definitions**
**Areas of High and Low Pressure**
**Atmospheric Pressure**
**Basic Equations to tie it all together**

## Definitions

**Evaporation - summarizes all processes that return water back into water vapor**
**Condensation - transition from vapor phase to liquid phase **
**Precipitation - deposition of liquid water droplets and ice particles that are formed in the atmosphere and grow to a sufficient size so that they are returned to the Earth's surface by gravitational settling.  **

## Definitions 

**Evaporation - *****the process of a substance changing from a liquid to a gas by exposure to the air and/or heat***
**Condensation -  *****the physical change of state in which a gas or vapor is transformed into a liquid, as in the formation of water droplets when water vapor cools. ***
**Precipitation - *****water from the atmosphere that falls to the ground in the form of rain, sleet, snow, or hail. ***

## Cloud Formation

**Precipitation is one key to the water cycle.  Rain comes from clouds, but where do clouds come from? **
**Through the process of evaporation and transpiration, water moves into the atmosphere. Water vapors then join with dust particles to create clouds. Eventually, water returns to Earth as precipitation in the form of rain, snow, sleet, and hail. **

## Pressure Areas and the Weather

**High Pressure**
**Low Pressure**

## High Pressure

**Air tends to sink near high-pressure centers, which inhibits precipitation and cloud formation. **
**Usually bring bright, sunny days with calm weather **
**Air flows clockwise around a high-pressure system in the N.H. **
**regions to the east of a high-pressure center often have northerly winds bringing in relatively cold air while regions to the west of a high-pressure center have southerly winds bringing in relatively warm air. **

## Low Pressure

**Low pressure area or cyclone is moving toward your region often when cloudy weather and precipitation usually result as the low pressure area approaches. **
**Low pressure systems have different intensities with some producing a gentle rain while others produce hurricane force winds and a massive deluge. **
**Air rises near low pressure areas. As air rises, it cools and often condenses into clouds and precipitation**

## Low Pressure 

**A steady rain or snow can fall to the north of the **_**[warm front](http://www.usatoday.com/weather/tg/wwfront/wwfront.htm)**__** as warm moist air from the south rises up and over the cold air ahead of the warm front. **_
**Showers and thunderstorms often fire up ahead of the **_**[cold front](http://www.usatoday.com/weather/tg/wcfront/wcfront.htm)**__** in the warm, **__**[unstable](http://www.usatoday.com/weather/wstabil.htm)**__** air. **_
**Usually, showers and thunderstorms ahead of the cold front are much shorter in duration than the precipitation ahead of the warm front. **
**counterclockwise circulation **
**cold air will likely be found to the north and west **
**warm air is most often found to the south and east **

## Review of High and Low Pressure

**Air slowly descends with high pressure**
**Rotation around a high pressure is clockwise**
**Rotation around low pressure is counterclockwise**
**Air rises, condenses forming clouds and precipitation**

## Weather Map

**  **

## When evaporation equals condensation

**  **

## Condensation greater than evaporation

**    **

## Evaporation greater than condensation

**     **

## Atmospheric Pressure

**Definition - Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air molecules above that surface.**
**In other words: force exerted by the weight of the air **

## Air molecules and pressure

**If the number of air molecules above a surface increases, there are more molecules to exert a force on that surface, and consequently the pressure increases. **
**The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure.**
**Atmospheric pressure is measured with an instrument called a barometer, which is why atmospheric pressure is also referred to as barometric pressure. **

## Standard Atmosphere

**That means pressure applied by this air on the unit area would be 14.7 pounds per square inch. Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars. **

**consider a "unit area" of 1 square inch.  At sea level, the weight of the air above this unit area would (on average) weigh 14.7 pounds!**

## Pressure with height 

**The **_**[pressure](http://ww2010.atmos.uiuc.edu/%28Gh%29/wwhlpr/pressure_def.rxml?hret=/guides/mtr/pw/prs/hght.rxml)**__** at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation.**_
**At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels. **

## Atmospheric pressure decreases with increasing height 

**most of the atmosphere's molecules are held close to the earth's surface by the force of gravity. **
**air pressure decreases rapidly at first, then more slowly at higher levels **
**more than half of the atmosphere's molecules are located below an altitude of 5.5 km. **
** **_**[atmospheric pressure](http://ww2010.atmos.uiuc.edu/%28Gh%29/wwhlpr/pressure_def.rxml?hret=/guides/mtr/pw/prs/hght.rxml)**_** decreases roughly 50% (to around 500 mb) within the lowest 5.5 km **

## Practical example

**  **

## Equations to tie things together

**PV = NRT  (ideal gas law)**
- dp/dz = - ρ g  (hydrostatic equation)

## What we talked about

**Basic Definitions – Evaporation, Condensation, and Precipitation**
**Areas of High and Low Pressure**
**Atmospheric Pressure**
**Basic Equations to tie it all together**

## National Weather Service 

**7777 Walnut Grove Road**
**Suite 1**
**Memphis, TN  38120**
**(901) 544-0411**

**internet site:**
**http://www.srh.noaa.gov/meg**

**Thank You!!**

**Scott Cordero**
**Warning Coordination Meteorologist**
**Memphis Weather Forecast Office**
**National Weather Service**