GEOG101: Physical Geography
Lecture 09: Monday, February 23, 2009
Topics
1. Land-sea breezes
2. Regional Scale Winds -- Monsoonal Winds
3. Water on Earth
4. Unique Properties of Water
5. Humidity
6. Atmospheric Stability
7. Clouds and Fog
Regional and Local Winds
• Land-sea breezes
Caused by temperature differences between land and sea;
During daytime, the land heats up quickly and form low pressure zone. Wind would flow from sea towards land;
During night time, the land cools down quickly, forming high pressure area. Wind would flow from land towards the sea.
• Regional Scale Winds -- Monsoonal Winds
Monsoon means that wind changes direction with season;
A typical example is the monsoon of Asia: During summer, central Asia land heats up fast, warm and moisture-laden air from the Indian ocean flows toward central Asia, causing heavy rains in coast and mountain areas (Wet Monsoon season). During winter, drier, colder air of the continent blows offshore, causing dry conditions in coast and mountain areas (Dry Monsoon season).
Also found in east-central Africa, and southwest desert of U.S. (during the summer).
A question from last lecture
• What are the differences between Cyclones, Typhoons, and Hurricanes?
All these are tropical cyclones that form destructive storms with high speed;
These names are geographically distinctive;
Large storms around N. America are called Hurricanes
In western pacific Japan, Philippines), such storms are called Typhoons;
In Indonesia, India, Bangladesh, they are called Cyclones
Water on Earth
• Earth's water originated from icy comets and H- and O-laden debris during the formation of the Earth;
• Water reached the Earth's surface by outgassing;
• For water to remain on Earth's surface, the land temperature had to drop below 100 degree C (212 degree F)--possibly 3.8 billion years ago;
• It has been hypothesized that water attained the present volume about 2.0 billion years ago;
• Changes in sea level (Eustasy) occur in Earth history by changes in ice volume
• Larger than 97 percent of the water is stored in the ocean;
• Only 2.78 percent of freshwater available in the earth surface, among which 77 percent are stored in glaciers and ice;
• We have only < 1 percent of usable water, and mostly resides in lakes and subsurface;
• We rely heavily on the < 0.33% of the freshwater;
• Saving and protecting this precious water!
Unique Properties of Water
• Three States (phases) of Water: solid, liquid, gas
• Phase change: water changes from one state to another
Sublimation - Deposition: ice to gas - gas to ice;
Melting - freezing: ice to water - water to ice;
Evaporation (Vaporization) - Condensation: water to gas - gas to water;
Vaporization: water to vapor change at 100 degree C;
Evaporation: water to vapor change at < 100 degree C;
• Latent heat: The quantity of heat absorbed or released by a substance undergoing a change of state (heat of transformation )
Phase change absorbs or releases latent energy;
Why do you feel cold when the ice is melting?
Why do you feel hot before a summer thunderstorm?
Because each gram of solid (ice) -> liquid (water) absorbs 80 calories of latent heat;
Because each gram of vapor -> water releases 540 calories of latent heat;
Read Figure 7.7, p. 184 to understand the basics about the energy exchange when phase changes.
Humidity
• Relative humidity: a ratio (percentage) of the amount of water vapor that is actually in the air compared to the maximum water vapor possible in the air at a give temperature.
Higher temperature -> Lower relative humidity
Lower temperature -> Higher relative humidity
Question: Is the relative humidity at noon (e.g., 11:00 am) higher than that at the early morning (e.g., 5:00 am)?
• Saturation: The air is saturated when the rate of evaporation and the rate of condensation reach equilibrium, or 100% humidity;
In another word, if the actual water vapor in the air = the maximum water vapor that the air can hold at that temperature --> the air is saturated;
If the air is saturated, any addition of water vapor or decrease of temperature will lead to condensation !
• Dew point temperature: a given mass of air becomes saturated at that Temperature and net condensation starts to form droplets;
The air is saturated when the dew-point temperature and the air temperature are the same!
• Vapor pressure: Water vapor adds pressure to the air. The share of air pressure that is made of water vapor molecules is the Vapor Pressure.
Saturation Vapor Pressure: The vapor pressure when the air is saturated.
• Specific Humidity: the mass of water vapor (in grams) per mass of air (in kilograms) at any specified temperature -- (g/kg)
When would be the best circumstances to use the Specific Humidity?
• Maximum Specific Humidity: The maximum mass water vapor possible in a kilogram of air at any specified temperature;
When would be the best circumstances to use the Maximum Specific Humidity?
Atmospheric Stability
• Air parcel: a volume of air, e.g., 300 m in diameter
• Buoyant force: an uplifting force caused by density differences
Cooler air -> lower density -> sinks;
Warmer air -> higher density -> rises;
• Stability: The tendency of the air parcels to change their position;
Stable: tends to remain at its original position;
Unstable: tends to rise upward;
• Adiabatic Processes: Temperature changes in both ascending and descending air occur without any significant heat exchange between the surrounding environment and the vertically moving parcel of air
Expansion -> cools internally
Compression -> heats internally
• Dry adiabatic rate (DAR): Refers to the rate at which “dry” air cools by expansion (if ascending) or heats by compression (descending)
10 degree C/ 1000 m or 5.5 degree F/ 1000 ft
• Moist adiabatic rate (MAR): Refers to the rate at which moist-bearing or saturated air cools by expansion (if ascending) or heats by compression (descending)
6 degree C/ 1000 m or 3.3 degree F/ 1000 ft
• If the DAR or MAR is smaller than the environmental lapse rate, the air parcel would rise, causing unstable condition;
• If the DAR or MAR is smaller than the environmental lapse rate, the air parcel would not move upward, stay at the surface level -> stable condition;
Clouds and Fog
• Cloud Formation Processes
When air parcel rises, it cools and may reach the dew point
Water start condensation around the condensation nuclei (dust, ash, etc)
When temperature is supercooled, ice-crystals starts to form
• If you want to lear how to identify the types of clouds, read Table 7.2 in p. 196, and the photos from page 198 to page 201.