BESC 320 – Water and Bioenvironmental Science(Week 3; 29 Jan – 2 Feb 2018)

Atmospheric components of hydrologic cycleWater, Weather, Energy Transport

Evaporation, sublimation, transpiration, volcanic release – send water into the atmosphere. (Sublimation – direct change in water phase from solid to gas) Humidity - Amount of water vapor in the airRelative humidity – Amount of water vapor in the air expressed as a percentage of

the maximum amount that can be held at that temperature. Air holds more water at higher temperatures, and higher pressure.

Saturation point – Point where the air contains the maximum amount of moisture given its temperature (i.e., 100% relative humidity). At this point water must condense onto solid surfaces such as atmospheric dust (forming clouds), or terrestrial surfaces (forming dew).

Dew Point - Temperature at which condensation occurs for a given amount of water.

Condensation nuclei – Atmospheric particles that facilitate condensation (e.g.: smoke, dust, salt mist, spores, large molecules like dimethylsulfide, DMS)

Influence of terrestrial topography air sweeps up windward side of a mountain pressure decreases, air cools, saturation point surpassed, moisture

condenses rain falls on the windward side and mountaintop (if it is cold enough at the

higher altitudes the precipitation becomes snow—that is why snow-capped peaks are such a common occurrence)

cool, dry air descends other side of mountain, warms, absorbing moisture from other sources making for very dry leeward sides of the mountain and desert valleys, called a rain shadow.

Another look (more detail):

Colliding air massesA similar phenomenon to the “mountain effect” described above happens when two air masses of different density run into each other:

• Cold air is more dense and will push under warmer (less dense) air • Warm air then gets vaulted into the stratosphere• The vaulting air cools and loses pressure• Saturation point may be exceeded• Condensation (clouds) and precipitation may occur

We speak of this as “frontal weather”. A front is a region of contact between dissimilar air masses. Scandanavian scientists came up with the analogy between weather fronts and war "fronts" of WWI. Dissimilar air masses "battle" at weather fronts, especially at cold fronts. The front is named (warm or cold) for the faster moving of two colliding air masses. Cold fronts are especially likely to produce severe weather.This collision effect can also happen when air masses of like properties collide. The Intertropical Convergence Zone (ITCZ) is the region where NE and SE tradewinds converge near the Equator:

Convection

Hot (less dense) air rises, creating an area of low pressure under the column of rising air.

The low pressure area draws in cooler (denser) air, resulting in vertical convection currents

Simultaneously transports air, water and energy (heat)

Sun not only heats the landscape and air above it, but the heat also vaporizes water, which carries with the rising air. Energy is stored in water vapor as latent heat.

Rising hot air creates an area of relatively high pressure at the top of the convection column

If condensation nuclei are present, temperatures lowers enough, and pressure is high enough, condensation will form clouds and potentially create precipitation

Air flows out of high-pressure zone towards areas of low-pressure, where cool, dry air is subsiding

Also, the risen air will be shedding its energy (radiating heat to space), and will then itself cool

Subsiding air is compressed as it approaches the earth’s surface where it piles up and creates an area of high pressure at the surface

Air flows out of this region back towards low pressure, closing the cycle:

relative left-right

relative left-right

Look at this process at a larger scale. Recall, evaporation is driven by heat (mostly solar) input, which is highest near the equator air at the equator rises, cools, and falls as rain (forming rainforests) cooler, drier air descends along Tropics of Cancer and Capricorn causes evaporative losses and creates deserts

Convection Cells and Prevailing WindsAs air warms at the equator, rises, and moves northward, it sinks and rises in several intermediate bands, forming circulation cells.Surface flows do not move straight North and South, but are deflected due to Coriolis Effect.

Major zones of subsidence occur at about 30° north and south latitude

Where dry, subsiding air falls on continents, it creates broad, subtropical desert regions.

Winds directly under regions of subsiding air are often light and variable.

Doldrums and horse lattitudes Think of the pattern depicted in this

figure and how it is reflected in the former figure.

If the global latitudinal pattern produces north-south gradients, how can there be longitudinal (east-west) patterns in the US and prominently, in Texas?

Energy Balance Solar energy is unevenly distributed Earth’s axis is tilted Compare: CITY Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Bogotá 11:53 11:59 12:05 12:13 12:20 12:23 12:22 12:16 12:08 12:00 11:55 11:52

Barrow 0:00 4:05 9:20 14:13 19:44 24:00 24:00 24:00 14:45 11:03 5:52 0:00

at the equator day length (hence radiation) is consistently 12 hours; off equator things can reach the extremes (complete day or night).

credit: Wikipedia

Energy imbalance is smoothed out by movement of air and water vapor in the atmosphere and by liquid water in rivers and ocean current

Interaction of energy disequilibrium and motion of planet creates large-scale upper airflows known as “Jet Streams”. Usually two over NA. Wind speeds often 200km/hr.

Cyclonic StormsWhen rising air is laden with water vapor, latent energy released by condensation intensifies convection currents and draws up more warm air and water vapor. Intensification sets up a self-feeding synergistic cycle. Low pressure in center of developing storm draws air in, but the path of incoming air becomes curved due to the Coriolis effect. (Quick mythbust: the Coriolis effect does not affect toilet flushes)Storm cell will exist as long as temperature differential exists.

Hurricanes (Atlantic) Typhoons (Western Pacific) Cyclones (Indian Ocean)

Tornadoes - Swirling funnel Rotation not generated by

Coriolis forces. Generated by “supercell”

frontal systems where strong dry cold fronts collide with warm humid air.

Greater air temperature differences in Spring thus more tornadoes.

Monsoon – Seasonal reversal of wind patterns caused by differential heating and cooling rates of oceans and continents, esp. in tropics.

El Niño / La Niña Large pool of warm

surface water in Pacific Ocean moves back and forth between Indonesia and South America.

Most years, the pool is held in western Pacific by steady equatorial trade winds.

Every 3-5 years the Indonesian low collapses and the mass of warm surface water surges back east.

During an El Nino year, the northern jet stream pulls moist air from the Pacific over the US.

Intense storms and heavy rains.

During intervening La Nina years, hot, dry weather is often present.

Pacific Decadal Oscillation – Very large pool of warm water moving back and forth across the North Pacific every 30 years

As emphasized earlier, the water of the Earth ameliorates temperature changes by storing, gradually releasing and transporting heat. This process moderates global temperature. Contrast with the moon (212oF during the day; –200oF at night). This happens in both the atmosphere, and in the hydrosphere per se—the oceans.

Case study I: Wetlands

– Florida Everglades (maybe not so much with the “ever” part)