Properties of the Atmosphere

Heat vs. Temperature  Temperature – measure of how rapidly or slowly molecules move around.  Higher the temperature = faster particle movement Heat- is the transfer of energy that occurs because of a difference in temperature between substances.

Heat vs. Temperature  Heat is the transfer of energy that fuels atmospheric processes, while temperature is used to measure and interpret that energy.  Temperature is measured in:  Degrees Fahrenheit  Degrees Celsius  Kelvins

Temperature Units  In the U.S. Fahrenheit ( o F) is the most common unit to measure temperature.  Celsius (oC) is the unit used in textbook where the difference from freezing to boiling is 100o.  Kelvin (K) is the SI unit for temperature.  Absolute zero, 0 Kelvin, is the coldest anything can be. It is equal to -273oC and is the temperature which molecular motion stops!!

Vertical Temperature Changes  Dew Point: the temperature to which air must be cooled at constant pressure to reach saturation.  Often called the “condensation temperature”  Condensation: occurs when matter changes state from a gas to a liquid The height that condensation occurs is called the lifted condensation level (LCL) and often corresponds to the base of the clouds.

Dew Point If dew point is close to the air temperature then the relative humidity is high

Air Pressure  The Earth’s atmosphere exerts a downward force due to the weight of the atmosphere.  Air pressure – pressure exerted on a surface by the weight of the atmosphere  Units – N/m 2 or mb (1 mb = 100 N/m 2 )  The higher you go in the atmosphere the less air pressure.

Density of Air  Density of air decreases with increasing altitude because the molecules are further apart.  Near sea level density is 1.2 kg/m 3

Pressure-Temperature- Density Relationship  In the atmosphere, the temperature, pressure, and density of air are related.

Air Pressure and Temperature  Warmer air = higher pressure  Particles move faster and exert a greater force

Air Pressure and Density  At the same temperature, air with higher density exerts more pressure than air with lower density.  More gas particles means more collisions.

Temperature and Density  At the same pressure, warmer air is less dense than cooler air.  Heat causes particles to move faster and spread apart.

Temperature Inversion  An increase in temperature with height in an atmospheric layer  Called a temperature inversion because the temperature/altitude relationship is inverted or turned upside down.

Cause and Effects of Temperature Inversion  Land is not radiating thermal energy to lower layers of atmosphere  Example – cold, clear winter night when air is calm  Leads to fog, low-level clouds, and trapping pollutants close to the Earth’s surface

Wind  Wind – movement of air; caused by cold, dense air rushing towards warm, less dense air

Wind and Pressure Difference  Air moves from regions of higher density to areas of lower density  Air pressure increases as density increases, so air moves from areas of high pressure to low pressure as well

Wind Speed and Altitude  Wind moves slower near the Earth’s surface due to friction  Wind speed is measured in:  Miles per hour (mph)  Kilometers per hour (km/h)  At sea – in knots (1 knot = 1.85 km/h)

Humidity  Humidity – amount of water vapor in the atmosphere at a given location  Two ways to express – relative humidity and dew point

Relative Humidity  Water molecules are constantly evaporating into the atmosphere and condensing to form clouds and precipitation.  If rate of evaporation > rate of condensation then the amount of water vapor increases  Relative humidity – The ratio of water vapor in the air to how much water vapor that volume of air is capable of holding  Relative humidity is expressed as a percentage (100% when fully saturated)