Pressure and Density and the Temperature Distribution

Met 10

Introduction

– Temperature, density and pressure are some of the most important variables in meteorology

– Knowing all of these quantities at all times and places would make understanding and forecasting weather events much easier

– Linked together via the gas law• pV = nRT

– Pressure differences cause winds, which are caused by the uneven heating at the surface!!!!

– So, the uneven heating of the surface causes the weather!

Pressure - Gravity causes the atmosphere to exert a downward force on

the surface.- Simply put, it’s the weight of air above you.- Pressure falls off rapidly w/ height, so it doesn’t take long until

air is very thin.- E.g. What’s the pressure in Lake Tahoe and Death Valley??

Look at a place’s elevation.- Pressure does not change as much in the horizontal

- However, these changes are the high and low-pressure systems that may bring certain types of weather events

- So these are most important to meteorologists- It’s the pressure differences we care about, since these are what

cause winds!

Pressure is commonly

measured using a Barometer.

Pressure extremes

Density

- # of air molecules per volume

- Physically, it is mass per unit volume- E.g. population density

- Density falls off rapidly w/ height, so it doesn’t take long until air is very thin.

Vertical structure

of pressure

and density

Pressure changes with height

Temperature Definition: Averagre speed of molecules

Faster speed is synonymous with warmer temperatures, and vice versa.

Can also be thought of as energy. – Hotter objects have more energy associated.– Colder “ less energy”

• E.g. Warmer areas are areas of more energy. Changes in temperature/energy can lead to

changes in pressure and volume, via the gas law. pV=nRT

Temperature Scales

• Fahrenheit: created 18th century physicist• freezing point of water = 32°• Boiling point = 212°• 0° is the lowest temperature he could reach using Salt, Ice and

Water• F = 9/5C + 32

• Celsius: Used worldwide• Made 0° the freezing point• 100 ° boiling point• C = 5/9(F - 32)

• Kelvin: Absolute temperature scale• At 0 K, all molecular movement stops!• K = C + 273.15 °

Thermometers Thermometers work because substances

(like mercury) expand and contract with changes in temperature.

Warmer temperature, fluid expands Colder temperature, fluid contricts

Mercury not used much anymore

Three temperature scales:Three temperature scales:•KelvinKelvin•CelsiusCelsius•FahrenheitFahrenheit

•What does temperature What does temperature mean physically?mean physically?

•What does 0 K mean?What does 0 K mean?

Vertical temperature structure

• 4 levels– troposphere, stratosphere, mesosphere, thermosphere

• we look mostly at the troposphere, where weather happens– 80% of all mass, nearly all vapor, clouds/precipitation

• a little at the stratosphere, where ozone is at max and explains warming in stratosphere

– In stratosphere: Ozone layer: O3 absorbs solar energy and waves.

– lapse rate: How temperature changes with height• Not constant in the atmosphere• Typically, it cools 3 - 5 degrees per 1000 ft• Depends on how much moisture is in the air

Climatological temperature variability: Latitudinal

• A place’s climate depends on it’s latitude– What’s latitude?– Poles, mid-latitudes, tropics?

• Polar/high latitudes: 60-90º• Mid-latitudes: 30-60º• Tropical/low latitudes: 0-30º

• The temperature differences (gradient) between poles and equator are twice as large in their winter season.– This causes mid-latitude weather systems like

cold/warm fronts and high and low-pressure areas

Precipitation Types

• Rain

• Snow

• Sleet

• Hail

• Feezing Rain