GEOG EXAM 2

Water vapor is considered an energized substance because it contains a great deal of: (A) latent heat. B) ionic bonding. C) sensible heat. D) regenerative bonding. E) ultra violet heat.

(A) latent heat

Atmospheric Stability and Buoyancy (To rise or not to rise - THAT is the question.):

A basic rule of physics is that lower density fluids will always rise above higher density fluids (e.g. oil rises to the top of water). This brief explanation describes buoyancy (or the buoyant force). One might also call it the floating force.

Mercury Barometer:

A mercury barometer consists of a long glass test tube filled with liquid mercury which is turned upside down into an open dish of mercury. Increased atmospheric pressure pushes downward on the mercury in the dish, causing the mercury in the tube to be pushed upward. The reverse is true for decreased pressure.

Dew Point Temperature

The dew-point temperature of air is the temperature that it would have to cool down to in order for it to become saturated (100% full with water vapor). In other words, it is the temperature that it would have to cool down to in order for condensation - DEW - to form. All air has a dew-point temperature, though the actual temperature is usually higher than the dew point temperature (unless you are standing in fog or on a cold, wet lawn in the early morning).

Hydrogen Bonding

The hydrogen atoms in a water molecule have a chemical attraction to the oxygen atoms in nearby water molecules. It is this hydrogen bonding (a form of energy) which holds water molecules tightly together in solid water (ice). When these bonds begin to break, ice melts into liquid water. When these bonds completely break, liquid water evaporates into water vapor. As vapor condenses condenses and then freezes, the hydrogen bonds re-form and strengthen again.

Westerly Winds:

The main key to understanding westerly flow in general is that aloft (1000s of feet above the surface) there is a very steep pressure gradient (high toward low) from equator toward poles. There is also NO friction! This results in very high winds speeds and an extreme coriolis deflection. Instead of flowing perpendicular to the isobars, winds take an extreme 90 degree turn and flow from west to east. This is geostrophic flow...

The Friction Force:

Wind moving along Earth's surface is slowed down by friction. Water, trees, rocks, soil - EVERYTHING - slows down wind, though different surfaces create different friction. Ocean creates the least friction. The friction force acts directly opposite the direction of airflow.

Measuring Atmospheric Pressure

mercury and aneroid barometer

Assume that the dry-bulb reading on your sling pshchrometer reads 30°C. Which of the following wet-bulb readings would indicate the driest air? A) 24°C B) 26°C C) 28°C D) 30°C E) 32°C

A) 24°C

Pressure on Upper Level Maps (height contours):

The pressure aloft (i.e. higher in the atmosphere) is mapped using height contours. In this case, the lines are equal to the altitude where a particular pressure is measured. For example, on a 500mb height contour map, all the lines show how high in the atmosphere you must go to find a 500mb reading (generally around 5600meters or 18,000ft). As a rule, higher values mean higher pressure and lower values mean lower pressure. Troughs refer to zones of lower pressure, while ridges refer to zones of higher pressure. 

The Pressure Gradient Force (PGF):

The pressure gradient force is the force generated by a difference in pressure between two places. The direction of the pressure gradient force pushes perpendicular to the isobars. Closely spaced isobars on a map indicate a strong pressure gradient force (i.e. a high pressure difference) and strong winds. Widely spaced isobars on a map indicate a weak pressure gradient force (i.e. a low pressure difference) and weak winds.

Pressure on Surface Maps (isobars):

Traditionally, centers of high and low pressure are usually clearly marked with H or L. Isobars are lines on a map connecting points of equal pressure; they are usually labeled. The change or difference in pressure between each line is the interval. The pressure at any point along an isobar is identified by the labeled value of that isobar. The pressure between two isobars is a value somewhere between the two labeled values. Because pressure decreases in the mountains, all values are converted to sea level equivalent to create uniform readings.

Precipitation Formation

Bergeron Process (cold cloud process) Collision Coalescence Process (warm cloud process)

Land and Sea Breezes (night vs day):

During a warm day the land surface heats more rapidly than the sea surface, generating a thermal low pressure on land. As heated air rises by convection, a vacuum is created and a cool sea breeze is pulled into fill the void. Facing the ocean, this wind is in your face. The land breeze occurs at night as the reverse happens (the land cools more rapidly, causing the air above it to become cooler and denser. This generates a thermal high pressure on land where cold air sinks and pushes out to sea. Facing the ocean, this wind is at your back.

Heavy airline traffic can lead to jet contrails which cause <______> than normal daytime temperatures and <_______> than normal nighttime temperatures. A. <higher > ...... < higher > B. < lower > ...... < higher > C. < higher > ...... < lower > D. < lower > ...... < lower >

B. < lower > ...... < higher >

The Case of the Temperature Inversion (an example of extremely stable conditions):

A temperature inversion is a brief reversal of the normal pattern where temperature decreases as you move upward away from the surface. Instead of colder, colder colder, etc.; the pattern is colder, colder, warmer, colder, colder... This can occur in valleys at night where cold, dense air drains down the surrounding mountain slopes, settles in the valley, and pushes the warmer air above it. This creates a layer of warm air sitting above cooler air. Conditions like this can trap smoke, factory emissions, and other pollution which simply cannot rise up and escape through the layer of warm air.

Which of the following is NOT true regarding the Bergeron precipitation formation process? A) It generally occurs at lower latitudes in tropical regions. B) It generally occurs at temperatures at or below -15°C (5°F). C) It occurs when supercooled water droplets exist in the same cloud as ice crystals. D) It can ultimately produce rain OR snow at ground level.

A) It generally occurs at lower latitudes in tropical regions.

All of the following are necessary ingredients for the formation of clouds, EXCEPT: A) warm surface air temperatures. B) saturated air. C) adequate water vapor content. D) condensation nuclei.

A) warm surface air temperatures.

Absolute Instability:

Absolute instability (or UNstable conditions) occurs when a rising parcel is warmer (and less dense) than the surrounding air. In this case the parcel IS buoyant and actively rises. Usually conditions are stormy, with tall, vertically developed clouds, intense downpours, thunder, lightning, etc. See slide showing both graphic and illustrated examples of this.

Absolute Stability

Absolute stability (or stable conditions) occurs when a rising parcel is colder (and denser) than the surrounding air. In this case the parcel is NOT buoyant. It resists rising and will often sink instead. Usually conditions are clear, but if it does rain it will usually only be drizzle. See slide showing both graphic and illustrated examples of this.

Aneroid Barometer:

An aneroid is a tightly sealed container which is extremely sensitive to changes in atmospheric pressure. When atmospheric pressure increases, the aneroid compresses (gets smaller). When atmospheric pressure decreases, the aneroid expands (gets bigger). Aneroids can be rigged with various mechanisms to display pressure readings and to convert them to electronic/digital formats.

Anticyclone (a downward pressing, outward spiraling high pressure system):

Anticyclone is a generic term for any outward spiraling, downward pressing high pressure system. In an anticyclone, air is constantly descending and spiraling toward Earth. As the air arriving at the surface moves outward from the high pressure, it is constantly being bumped off course to the right in the Northern Hem and the left in the Southern Hem. This results in clockwise flow in the NH and counter-clockwise flow in the SH. High pressure is associated with clear, dry weather because the downward motion results in adiabatic heating which suppresses clouds.

Dry Adiabatic Cooling (ascending parcels cool at a rate of 10C°/1000meters):

As a parcel of air rises it expands due to a decrease in the surrounding atmospheric pressure. This results in decreased molecular collisions, decreased friction and decreased temperature of the parcel.

Dry Adiabatic Heating (descending parcels heat at a rate of 10C°/1000meters):

As a parcel of air sinks it compresses due to an increase in the surrounding atmospheric pressure. This results in increased molecular collisions, increased friction and increased temperature of the parcel.

COLD surfaces generate HIGH pressure:

As air cools, the air molecules slow down and pack together. This causes the air to compress and become more dense, sink downward, and push outward.

Chinook / Foehn / Santa Ana Winds:

As air moves up the windward slopes of a mountain it often dries out through the precipitation process. Additionally, condensation pumps extra heat into the air. Chinook and foehn winds occur when this air then rapidly moves down the leeward slope, heating adiabatically on its way downhill. Such winds arrive significantly warmer and drier. These winds occur on the leeward slopes of the Rockies, Alps, and most major mountain ranges around the world. In the case of a Santa Ana wind in California, when high pressure develops in the inland deserts and low pressure develops off the California coast, the pressure gradient force drags dry air rapidly downhill toward the coast.

HOT surfaces generate LOW pressure:

As air warms the molecules move faster and spread outward and upward. This causes the air to expand, become less dense, rise upward, and draw in surrounding cooler air.

Polar Front Jet Stream and Rossby Waves:

At the core of the westerly winds along the polar front where temperature contrasts are high, an extremely fast "pipe" of wind - a jet stream - will develop. These can be over 100 miles wide and travel at speeds over 200mph. They generally mark the boundary between cold, polar air and warmer tropical air. Because of land/water contrasts and other differences, the winds will often develop waves or undulations - called Rossby waves. These can bring unexpectedly warm or cold conditions to locations close enough to the polar front to be affected. As with virtually all of the major global wind and pressure systems, the polar front jet will shift northward during our summer and southward during our winter (see slide showing seasonal differences in location).

Vertical Variation:

Atmospheric pressure (and density) is highest at sea level and decreases rapidly toward space. Pressure in space is zero. This is because the pull of gravity is highest at sea level, holding more molecules in place there.

Atmospheric Pressure:

Atmospheric pressure is the force exerted by air molecules, essentially the weight of the air above us. Since the atmosphere is fluid, this force "pushes" in all directions, not just downward. This force is approx. 1kg/cm2 (or 14.7lbs/in2). If we were to take a perfectly vertical column 1cm2 at the base and rising to space, the collective weight would be 1kg. It is important to note that atmospheric pressure is highest at sea level and decreases rapidly toward space. Pressure in space is zero. This is because gravity is holding the molecules in place.

Which of the following is NOT true regarding a parcel of air that is being lifted into the atmosphere? A) Molecular collisions decrease. B) The parcel compresses. C) Friction decreases. D) Temperature decreases.

B) The parcel compresses.

Which of the following instruments can measure both wind speed and wind direction with greatest accuracy? A) wind vane B) aerovane C) spoon quadramometer D) wind sock E) cup anemometer

B) aerovane

As a rule, winds are generally named for: A) the direction or place to which they are heading. B) the direction or place from which they are blowing. C) the approximate latitude of their origin. D) the altitude where they are strongest.

B) the direction or place from which they are blowing.

Jet Contrails:

Because jet exhaust contains both water vapor and particulate matter, it provides the perfect ingredients for cloud formation. The are called contrails. Areas with heavy air traffic can end up producing enough contrails that the increased albedo can lead to lower daytime temperatures and higher nighttime temperatures.

Hydrologic Cycle

Because of its unique qualities, water moves through Earth systems - and carries energy - in massive amounts. Evaporation carries it into the atmosphere, while condensation, freezing, and precipitation drops it back down to the surface.

During which month is Northern Australia most likely to experience heavy, monsoon rains? A) May B) July C) January D) August E) October

C) January

Moving from Earth's surface toward space, what trend will be evident with atmospheric pressure? A) No trend will be evident B) increasing C) decreasing D) decreasing at first, then increasing

C) decreasing

Saturation vapor pressure is reached when the rate of condensation <____________> the rate of evaporation. (A) is greater than B) is less than C) equals D) boosts E) There is NO relationship.

C) equals

The attractive forces between the hydrogen atoms in one water molecule and the oxygen atoms in another molecule are known as: A) water bonds. B) liquid bonds. C) hydrogen bonds. D) dark forces.

C) hydrogen bonds.

As air temperature increases, its maximum capacity to hold water vapor: (A) remains unchanged. B) fluctuates randomly. C) increases. D) decreases. E) There is NO relationship

C) increases.

Which type of rain gauge is most appropriate for collecting and reporting precipitation data electronically? A) simple B) standard C) tipping bucket D) localized E) rain pillow

C) tipping bucket

The Coriolis Force will be strongest at <______> wind speeds and <_______> latitudes. A. < high > ...... < high> B. < high > ...... < low > C. < low > ...... < low > D. < low > ...... < high >

C. < low > ...... < low >

Air pushed downward toward the surface generates HIGH pressure:

Certain conditions can cause air to pile up and collect high above the surface (like a traffic jam). This then results in the air pushing downward toward the surface, resulting in high surface pressure.

Collision Coalescence Process (warm cloud process):

Cloud droplets collide and gather together (coalesce) until drops from that are big enough to fall. When drops become large enough they make break into smaller drops, which will all continue to repeat the process and form more drops.

Upwelling:

Cold currents are often blown and/or deflected away from coastlines by the coriolis force. This causes deeper, colder water to rise up and take its place - a phenomenon called upwelling. Since cold water contains more oxygen and nutrients, such coastlines are often known for their extremely productive and diverse marine ecosystems. This is especially true for the west coasts of North and South America.

Conditional Instability:

Conditional instability occurs when a rising parcel starts out colder (and denser) than the surrounding air, BUT THEN becomes warmer (and less dense) at some height. In this case the parcel resists rising at first, but then actively rises. See slide showing both graphic and illustrated examples of this.

Instruments to Measure Wind

Cup Anemometer (speed) Wind Vane (direction) Wind Sock (speed and direction) Aerovane (speed and direction)

Cyclone (an inward spiraling, upward lifting low pressure system):

Cyclone is a generic term for any inward spiraling, upward lifting low pressure system. As surface air moves inward toward the low pressure, it is constantly being bumped off course to the right in the Northern Hem and the left in the Southern Hem. This results in counter-clockwise flow in the NH and clockwise flow in the SH. The air then rises toward space, continuing to spiral. Low pressure is associated with cloudy, wet weather because the lifting motion commonly results in adiabatic cooling, cloud formation, and precipitation.

What is the average sea level pressure? A) 29.9 mb B) 766.0 mb C) 1000.3 mb D) 1013.2 mb E) 1024.6 mb

D) 1013.2 mb

Which term describes a collection of numerous small high altitude clouds which are puffy in nature and likely composed of ice crystals? A) Nimbostratus B) Altocumulus C) Cirrus D) Cirrocumulus E) Altostratus

D) Cirrocumulus

<_______________> occurs when a rising parcel is always colder and denser than the surrounding air. absolute instability B) regional stability C) conditional instability D) absolute stability

D) absolute stability

Which term describes precipitation which occurs when water droplets fall through a freezing layer of air at the very last minute, become supercooled, and then freeze on contact when they hit the Earth? A) snow B) sleet C) graupel D) freezing rain E) rime

D) freezing rain

Which of the following associations is NOT correct? A) sea breeze — day time B) chinooks — leeward side of mountains C) mountain breeze — night time D) land breeze — blows from the sea onto the land E) katabatic --- extremely cold

D) land breeze — blows from the sea onto the land

If the Pacific Sub-Tropical High Pressure system is found at about 40°North Latitude in July, at approximately what latitude would it likely be found in January? A. 75°South Latitude B. 50°North Latitude C. 85°North Latitude D. 30°North Latitude E. 50°South Latitude

D. 30°North Latitude

Cloud Formation and Wet Adiabatic Cooling (ascending clouds cool at a rate of approx. 5C°/1000meters):

Depending on conditions, some rising parcels will cool to the point that they become saturated. In other words, they will cool down to the dew-point temperature and hit 100% relative humidity. At this elevation - called the lifting condensation level - condensation will lead to cloud formation. Because the process of condensation releases sensible heat into the air, the rising cloud only cools at a rate of about 5C°/1000meters.

Measuring Snowfall:

Depth can be measured with a stick. Water content is measured by collecting a sample and melting it. Snow pillows can be installed to measure the weight of the snow sitting on them; important for measuring snowpacks potential contribution to water supply. LIDAR (remote sensing) now being used to assess snowpack.

Mountain and Valley Breezes (night vs day):

During a warm day the mountain slopes heat more rapidly than the same elevation air above the valley, generating a pressure gradient from the valley up the mountain slope. If you are facing downhill toward the valley, this warm valley breeze is in your face. During a cool night the mountain slopes cool rapidly, generating cold, dense air which begins to drain downslope under gravity. If you are facing downhill toward the valley, this cool mountain breeze is at your back.

Assume that the relative humidity is 50% at each of the temperature conditions below. At which temperature will the air actually hold the most water vapor? A) -5°C B) 5°C C) 15°C D) 25°C E) 35°C

E) 35°C

Which term describes fog which forms as humid air moves horizontally and condenses as it passes over a cold surface? A) Steam Fog B) Upslope Fog C) Frontal Fog D) Radiation Fog E) Advection Fog

E) Advection Fog

Chinook winds, land-sea breezes and Santa Ana winds are examples of ________ circulations. A) microscale B) vortex C) synoptic D) macroscale E) mesoscale

E) mesoscale

Unique Qualities of Water

Exists in massive quantities on Earth's surface Stores and releases huge amounts of heat Changes phase (solid, liquid, gas) readily

Air pulled up and away from the surface generates LOW pressure:

Extremely fast upper winds (jet streams) can cause air to be sucked upward away from the surface. This results in lower pressure at the surface.

Change of State (also called Change of Phase)

Hydrogen Bonding and Latent Heat

Pacific Sub-Tropcial High Pressure:

In July, this large high pressure off the coast of California has shifted almost as far north as 40°N. This pushes the polar front and the path of the westerly winds far enough north that California normally stays very dry in summer. By January this high pressure has shifted almost 10 degrees southward to about 30°N. This allows the polar front and the path of the westerly winds to shift south, bringing winter storms and precipitation from the Pacific.

Latent Heat

It takes energy to break hydrogen bonds and keep them broken, and this energy is called latent heat. It cannot be felt or measured by a thermometer; it is essentially hidden. A net surplus of radiation will provide the energy (the latent heat) needed to break hydrogen bonds (and keep them broken), thus causing ice to melt and water to evaporate. In this way water is absorbing - and hiding - massive amounts of hidden heat. Water vapor is essentially an energized substance (meteorological jet fuel!). When vapor condenses and then freezes, it releases the latent heat and essentially converts it back to sensible heat (the heat we feel).

Scales of Motion

Microscale (gusts, dust devils - blowing for seconds or minutes) Mesoscale (tornadoes, thunderstorms, local winds - blowing for hours to a few days) Macroscale/Synoptic (midlatitude cyclones, hurricanes - blowing for days to weeks) Macroscale/Planetary (trade winds, westerly winds - blowing for weeks+)

Bergeron Process (cold cloud process):

Most common in mid to high latitudes. Cloud temps must be -15°C (5°F) or lower. At this temp supercooled liquid droplets coexist with ice crystals. At these temps vapor easily skips the liquid phase and deposits directly onto the ice crystals. This drops the RH of the air, causing the droplets to evaporate, thus feeding more vapor to the ice crystals. Eventually the ice crystals become large enough to fall under gravity. Often they will melt before hitting the ground, simply falling as rain.

Katabatic Winds:

Mountains and highland areas can develop extremely large, cold air masses in winter. Katabatic winds can occur when these cold, dense air masses spill downhill and pick up significant speed. Though they heat adiabatically, they can still arrive bitter cold (and dry). Katabatic winds are common at the edges of Greenland and Antarctica. Other examples include the mistral in the French Alps and the bora in the Balkans.

Lifting Mechanisms (What causes the air to go up anyway?)

Orographic Lifting (Crash air into mountains and it goes UP!) Frontal Lifting (Crash warm and cold air together, and warm always goes UP!) Convective Lifting (Heat air and it goes UP!) Convergent Lifting or Convergence (Discussed later...)

Relative Humidity (RH = what the air is holding ÷ what it could hold x 100)

Relative humidity is a relative term; it compares what the air IS holding to what the air COULD hold when completely saturated. It is stated as a percent. FORMULA: RH = [actual mixing ratio] ÷ [saturation mixing ratio] x 100 Because air's saturation mixing ratio (i.e. its capacity to hold water vapor) increases as temperature increases, 50% relative humidity in cold air means something very different than 50% relative humidity in warm air.

Measuring Rainfall

Simple Rain Gauge: Consists of any container left in the rain. Drop a ruler in and measure the height of water. Standard Rain Gauge: Has a larger opening which funnels water to a smaller calibrated collecting tube. This allows for smaller increments of rainfall to be measured. Tipping Bucket Gauge: These gauges are equipped with two tiny buckets positioned on either end of a small teeter totter. When the first bucket fills up, it tips and empties under the weight of the water. That automatically positions the second bucket to begin filling. Each "tip" is equivalent to 0.01 inches of rain. They are easily fitted with digital connections to allow the date to be automatically recorded and reported.

The Coriolis Force:

The Coriolis Force is an apparent deflection of wind and ocean currents caused by the rotation of the Earth. Essentially, when a force pushes something (wind, water, missiles, etc) toward a "target", Earth's rotation causes the "target" to be moved, making it appear that the moving object or substance turns off course...

El Nino, La Nina, and the Southern Oscillation

The Southern Oscillation refers to periodic changes which occur every few years in pressure patterns on either side of the Pacific Ocean (see above graphic). These changes, in turn, result in significant ocean temperature anomalies - deviations from average ocean temperatures at particular times and places. An El Nino event will occur when trade winds weaken and allow warmer than normal water to collect along the Pacific coasts of North, Central, and South America. This leaves SE Asia drier than normal and many locations near the "warm pool" wetter than normal. A La Nina event will occur when the trade winds re-strengthen and cause an exaggeration of the "normal" conditions. This leaves SE Asia wetter than normal and many locations in North, Central, and South America drier than normal. A multitude of other global impacts are just now being studied.

Other Major Sub-Tropcial High Pressures:

Though not obvious on many global pressure maps, most of the world's major anticyclones (i.e. subtropical high pressure zones) shift similarly, giving midlatitude west coast locations dry summers and wet winters. This is essentially the main cause behind Mediterranean climate zones (discussed later). 

Winds Aloft and Geostrophic Flow (jet streams):

Thousands of feet above the surface, there is no friction and faster wind. At mid to high latitudes in places where pressure gradients are stronger, wind speed is high and the resulting coriolis force is very strong. In such conditions the PGF and the coriolis force are equally strong. This results in winds traveling parallel to the isobars (straight-line flow) instead of perpendicular. Such winds are often called jet streams.

Surface Wind:

The speed and direction of surface winds are a combination of the PGF, coriolis, and friction forces. The general results is that wind flows generally toward centers of low pressure but diagonal to the isobars.

Anticyclones Create Gyres:

There are five major anticyclones (the outward spiral flow associated with sub-tropical high pressure cells) over the world's oceans. These winds push/drag surface waters, creating massive circular ocean currents called gyres. The resulting flow helps redistribute heat from the equator toward the poles. Major WARM currents move warm water along EAST coasts toward the poles... (know the currents by name and location) Major COLD currents move warm water along WEST coasts toward the equator... (know the currents by name and location)

Impact of Water Vapor Changes on RH:

There is a direct relationship between changes in water vapor content and RH As the actual water vapor content increases, the RH increases. As the actual water vapor content decreases, the RH decreases.

Impact of Temperature Changes on RH:

There is an inverse relationship between changes in temperature and RH As the temperature increases, the RH decreases. As the temperature decreases, the RH increases.

Mixing Ratio

This is a common measure of humidity, expressed as grams of water vapor per kilogram of dry air (g/kg). In contrast with absolute humidity, it is not affected by changes in air pressure or temperature.

Psychrometer (or Sling Psychrometer)

This is an instrument that uses two normal thermometers, though one of the thermometers has a tiny wet "sock" covering the bulb. This is referred to as the wet-bulb thermometer (the other is the dry-bulb thermometer). The drier the air is, the faster water evaporates from the wet bulb thermometer. This leads to evaporative cooling and causes the wet-bulb reading to decrease compared to the dry bulb. The greater difference between the two thermometers, the drier the air.

Urban Effects

Urban heat island can help promote convectional cloud development, especially in summer. Urban precipitation has been seen to increase as much as 20% Precipitation is also increase downwind from large cities because pollution increases condensation nuclei and promotes cloud formation and precipitation.

Vapor Pressure and Saturation

Vapor pressure is defined as the part of the total atmospheric pressure which is exerted specifically by water vapor. In totally dry air, the vapor pressure is zero! This means that water molecules will easily evaporate into the air. This will cause the vapor pressure to rise and - while evaporation continues - will lead to some water molecules beginning to condense back to liquid water. When the number of molecules evaporating equals the number condensing, saturation vapor pressure is reached. As stated above, saturation occurs when the number of molecules evaporating equals the number condensing. As air heats, its saturation vapor pressure increases. In other words, as air heats its total capacity to hold water vapor increases. Simply put, warm air can hold more water vapor than cold air.

Condensation Nuclei:

Water vapor actually needs a surface to condense onto. On the ground this can be virtually anything (a window, grass, rocks, trees, etc) In the air this can take the form of smoke, dust, volcanic ash, pollen, salt particles, etc. Without condensation nuclei available, air can become supersaturated with an RH above 100% before it actually condenses.

Weather Radar

Weather radar transmits radio waves sideways across the landscape and then reads the signal which is bounced off the precipitation back to the radar unit. (called backscatter) Weather radar can detect both the rate and areal extent of precipitation.

The Orographic Effect

When air collides with mountain it is lifted upward (in other words, it has no way to go but up and over) The lifting air cools adiabatically on its way upslope. This commonly leads to condensation, cloud formation, and precipitation. When winds commonly come from the same direction, they are called prevailing winds. The side of the mountain facing "toward" the oncoming wind is called the windward side, while the side where the wind is "leaving" is called the leeward side. Rainshadow deserts are found on leeward sides of mountains, because air masses almost always arrive on the leeward side drier than they started (because rain on the windward side dries out the air).

Pressure Tendency Generalizations

When pressure rises↑, clearing weather is generally expected... When pressure drops↓, stormy weather is generally expected... 

Prevailing Winds:

Winds are always referred to based on where they are coming FROM! When winds generally come from the same direction for part of all of the year, they are referred to as prevailing winds. In much of North America prevailing winds come from the west (called the westerly winds).