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