Pressure and Density and the Temperature Distribution

1. Pressure and Density and the Temperature Distribution Met 10

2. 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!

3. 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!

4. Pressure is commonly measured using a Barometer.

5. Pressure extremes

6. 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.

7. Vertical structure of pressure and density

8. Pressure changes with height

9. 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

10. 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 °

11. 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

12. Three temperature scales: • Kelvin • Celsius • Fahrenheit • What does temperature mean physically? • What does 0 K mean?

13. 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

16. 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

19. Precipitation Types • Rain • Snow • Sleet • Hail • Feezing Rain