ATMOSPHERE - WHAT HAPPENS IF AIR PRESSURE CHANGES?

Why do my ears pop?  If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure decreases.  This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit. As you climb higher, air temperature decreases. Typically, air temperatures decrease about 3.6° F per 1,000 feet of elevation.

ATMOSPHERE - AIR PRESSURE

Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. Since there's a lot of "empty" space between air molecules, air can be compressed to fit in a smaller volume.

When it's compressed, air is said to be "under high pressure". Air at sea level is what we're used to, in fact, we're so used to it that we forget we're actually feeling air pressure all the time!

Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.

How much pressure are you under? Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!

Why doesn't all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.

Run for Cover! Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.

Control the weather! Use a special JavaScript barometer designed to demonstrate how air pressure and weather conditions are linked. You will also find instructions on how to build your own barometer!

What Happens if Air Pressure Changes?

Why do my ears pop? If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit.

As you climb higher, air temperature decreases. Typically, air temperatures decrease about 3.6° F per 1,000 feet of elevation.

Do you think a decrease in temperature could be explained in terms of air pressure? How?

 The atoms and molecules that make up the various layers in the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface they exert a force on that surface in what we observe as pressure.

Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable.

Air pressure can be increased (or decreased) one of two ways. 

• First, simply adding molecules to any particular container will increase the pressure. A larger number of molecules in any particular container will increase the number of collisions with the container's boundary which is observed as an increase in pressure.
• A good example of this is adding (or subtracting) air in an automobile tire. By adding air, the number of molecules increase as well a the total of the collisions with the tire's inner boundary. The increased number of collisions forces the tire to expand and pressure increase.
• The second way of increasing (or decreasing) is by the addition (or subtraction) of heat. Adding heat to any particular container can transfer energy to air molecules. The molecules therefore move with increased velocity striking the container's boundary with greater force and is observed as an increase in pressure.

Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.

In the International Space Station, the density of the air is maintained so that it is similar to the density at the earth's surface. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch).

Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, meaning a decrease in air pressure. In fact, while the atmosphere extends more than 15 miles (24 km) up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).

Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we need to convert the station air pressures reading to a value with a common denominator.

The common denominator we use is the sea-level. At observation stations around the world, through a series of calculations, the air pressure reading, regardless of the station elevation, is converted to a value that would be observed if that instrument were located at sea level.

The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars". 

• Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. 
• 
  
• At sea level, standard air pressure in inches of mercury is 29.92.
• Millibars comes from the original term for pressure "bar". Bar is from the Greek "báros" meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.

Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.

• Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.

The Pascal

The scientific unit of pressure is the Pascal (Pa) named after after Blaise Pascal (1623-1662). One pascal equals 0.01 millibar or 0.00001 bar. Meteorology has used the millibar for air pressure since 1929.

When the change to scientific unit occurred in the 1960's many meteorologists preferred to keep using the magnitude they are used to and use a prefix "hecto" (h), meaning 100.

Therefore, 1 hectopascal (hPa) equals 100 Pa which equals 1 millibar. 100,000 Pa equals 1000 hPa which equals 1000 millibars. The end result is although the units we refer to in meteorology may be different, their numerical value remains the same. For example the standard pressure at sea-level is 1013.25 millibars and 1013.25 hPa.

Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.

How are changes in weather related to changes in pressure?

The FALL of the barometer (decreasing pressure)

• In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind.
• In frosty weather, the fall of the barometer denotes thaw.
• If wet weather happens soon after the fall of the barometer, expect but little of it.
• In wet weather if the barometer falls expect much wet.
• In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.
• The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind).

The RISE of the barometer (increasing pressure)

• In winter, the rise of the barometer presages frost.
• In frosty weather, the rise of the barometer presages snow.
• If fair weather happens soon after the rise of the barometer, expect but little of it.
• In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.
• In wet weather, if the mercury rises suddenly very high, fine weather will not last long.
• The barometer rises highest of all for north and east winds; for all other winds it sinks.

The barometer UNSETTLED (unsteady pressure)

• If the motion of the mercury be unsettled, expect unsettled weather.
• If it stands at "MUCH RAIN" and rises to "CHANGEABLE" expect fair weather of short continuance.
• If it stands at "FAIR" and falls to "CHANGEABLE", expect foul weather.
• Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather.

These pressure observations hold true for many other locations as well but not all of them. Storms that occur in England, located near the end of the Gulf Stream, bring large pressure changes. In the United States, the largest pressure changes associated with storms will generally occur in Alaska and northern half of the continental U.S. In the tropics, except for tropical cyclones, there is very little day-to-day pressure change and none of the rules apply.

Why doesn't all that pressure squash me? 

• Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.

Pressure Underwater

The pressure underwater works the same as for atmospheric pressure. The pressure exerted underwater depends on the amount of water overhead. The density of water is about 1 gram per cubic centimeter, which is around 800 times that of air so that the pressure underwater changes much more quickly with depth than the atmospheric pressure changes with height. By a depth of 32.8 feet (around 10 meters), the water above the fish leads to a pressure of 1 atmosphere or 101,000 Pascals (101 kPa). At this depth, however, this is not the total pressure because we excluded the pressure exerted by the atmosphere. The total pressure at this depth is that due to the water, 1 atm, and that due to the atmosphere, 1 atm, for a total pressure of 2 atm, twice that at the surface of the ocean where the pressure felt is dominated by the atmospheric pressure.Water is nearly incompressible (cannot squeeze or make it expand easily [by changing the pressure] and so water's density does not change with depth). This means that for every change of 32.8 feet (or ~10 meters) in depth, the pressure increases by 1 atm (101 kPa).This is remarkable. The rate at which the pressure increases as one descends in the ocean, does not depend on the depth.
At a depth of 1,000 meters then, the pressure a fish feels is around 100 times the pressure that it feels at the surface of the ocean. The pressure at 1,000 meters is huge, it corresponds to a pressure of around 1,470 pounds per square inch squeezing the fish.

REVIEW:

Air Pressure helps forecastersNot only do differences in air pressure help determine wind speed and direction, they help forecast precipitation and clear weather. High pressure indicates sinking air where clouds cannot form. Low pressure indicates rising air, which allows clouds to form, bringing rain or snow.

Air Flows From High Pressure to Low Pressure

Rising and Sinking Air

Since warm air is less dense and creates less air pressure, it will rise; cold air is denser and creates greater air pressure, and so it will sink. When warm air rises, cooler air will often move in to replace it, so wind often moves from areas where it's colder to areas where it's warmer. The greater the difference between the high and low pressure or the shorter the distance between the high and low pressure areas, the faster the wind will blow. Wind also blows faster if there's nothing in its way, so winds are usually stronger over oceans or flat ground. Meteorologists can forecast the speed and direction of wind by measuring air pressure with a barometer.

Discussion Questions:

1.	If you were on a mountain, would the weight of the air above you (air pressure) be greater than or less than it is now?
2.	What do you think causes wind?
3.	If you lived in Kansas and observed a sudden drop in air pressure, what kind of weather would you expect to see soon?
4.	Why do hot air balloons rise?
5.	Air weighs less than water, would you expect the pressure exerted by water to be greater or less than the pressure exerted by the same amount of air?

BALLOON - AIR PRESSURE EXPERIMENT

• take notes with all senses
• give an explanation for the results of the experiment

WATER CYCLE - QUIZ

WATER CYCLE - QUIZ

• take quiz 
• write your name, quiz name, & score on notebook paper
• turn into drawer

AIR PRESSURE: ALL AROUND US

To understand pressure, consider a sponge. 

When left alone, a sponge stretches out to its full size. As you squeeze down on the sponge, what happens? How small can you make it? Can you hide the sponge entirely within your hand? When the sponge is squished up as tightly as you can make it, how does it feel? The sponge should feel hard and tight. Now, as you let go of the sponge and it once again takes its original size and shape, how does it feel? In this state, it should feel soft again.

The atmosphere is similar to a sponge in that it can be compressed, or squished up. The weight of the air above compresses the air below tighter and tighter. The lower we travel down into the atmosphere, the more weight there is above, and so the tighter the air is squished or compacted. The higher we travel into the atmosphere, the less air there is above and so the less weight there is pushing down, and as a result, the less squished up, or the less compact the air.

Half of the air making up our atmosphere is squished down into the first three layers, and 90% of the atmosphere is squished into the first 10 miles (16km). Above this altitude the air is so thin that the pressure is even less than that of the best vacuums on the surface of the Earth.

When your friend squeezes your arm, you feel pressure! That's because molecules collide with each other and things like your arm, the ground, or a tree. They exert a force on those surfaces.

Molecules in Earth’s atmosphere constantly bounce off each other and everything else around them. The force exerted by these air molecules is called air pressure.

Air Pressure: How to measure the air around us.

Air pressure is the force exerted on you by the weight of tiny particles of  air (air molecules). Although air molecules are invisible, they still have  weight and take up space. Since there's a lot of "empty" space between  air molecules, air can be compressed to fit in a smaller volume. Air Pressure: At sea level, the air pressure is about 14.7 pounds per square inch. As your altitude increases (for example, if you climb a mountain), the air pressure decreases.  At an altitude of 10,000 feet, the air pressure is 10 pound per square inch (and there is less oxygen to breathe).

Measurement of air pressure: Millibar: Small units of pressure commonly found on surface weather maps. 1013.25mb   = is the average sea level air pressure. 34 millibars = 1 inch of   mercury. 1 millibar = 1/1000 of a bar 1 bar = 100,000 newtons 1 newton is forced it takec to move an object of 1kg at a rate of  1 meter per second. Isobars: The lines that join points on a map having the same air pressure at a given time. Units of Pressure meteorologists use millibars In aviation and television weather reports, pressure is given in inches of mercury ("Hg), while meteorologists use millibars (mb), the unit of pressure found on weather maps. As an example, consider a "unit area" of 1 square inch. At sea level, the weight of the air above this unit area would (on average) weigh 14.7 pounds! That means pressure applied by this air on the unit area would be 14.7 pounds per square inch. Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars.

Measuring Air Pressure: Barometer: •  Instrument used to measure pressure. A barometer is a device that measures air (barometric) pressure. It measures the weight of the column of air that extends from the instrument to the top of the atmosphere. There are two types of barometers commonly used today, mercury and aneroid (meaning "fluidless"). Earlier water barometers (also known as "storm glasses") date from the 17th century. The mercury barometer was invented by the Italian physicist Evangelista Torricelli (1608 - 1647), a pupil of Galileo, in 1643. Torricelli inverted a glass tube filled with mercury into another container of mercury; the mercury in the tube "weighs" the air in the atmosphere above the tube. The aneroid barometer (using a spring balance instead of a liquid) was invented by the French scientist Lucien Vidie in 1843.  2 types of Barometers used   •  Aneroid Barometer •  Toricelli Mercury Barometer

Aneroid Barometer:

An aneroid barometer is a flexible metal chamber that has been tightly 

sealed after having some air removed. As the higher atmospheric 

pressures pushes the metal chamber the attached needle is moved. When 

the atmosphere has lower pressures it allows the chamber to expand 

which moves the needel in the opposite direction.

Toricelli Mercury Barometer

A glass tube from which the air has been removed is inserted into a dish of mercury. The air pressing down on the mercury in the dish forces some of the mercury up into the glass tube.

The height that the mercury rises in the glass tube is directly related to the
atmospheric pressure. This pressure is usually measured in inches of mercury.
A standard mercury barometer has a glass column about 30 inches long.
A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.

ATMOSPHERE PRESSURE - VIDEO

AIR PRESSURE - ACTIVITY

AIR PRESSURE - ACTIVITY  #2

ATMOSPHERE - HYDROLOGIC/WATER CYCLE

The hydrologic cycle involves the continuous circulation of water in the Earth-Atmosphere system. At its core, the water cycle is the motion of the water from the ground to the atmosphere and back again. Of the many processes involved in the hydrologic cycle, the most important are...

• This change of state of water occurs in the atmosphere and between the earth's surface and atmosphere.
• change of state or phase change = solid > liquid > gas

This basic cycle is seen almost daily around the world in the formation and dissipation of clouds. When a cloud develops it is water vapor becoming a liquid. Conversely, when a cloud dissipates, liquid water changes state back into a gas.

The Ocean's Role

The vast bulk of the water in the water cycle is found in the oceans. The oceans hold 96.5% of the earth's water and due to their size it may take thousands of years for a water molecule to move from the ocean to the atmosphere. This is in spite of an average 45 inches (114 cm) of water that evaporates from the ocean each year. (An additional 1% of salty water is also found in saltwater lakes and saline groundwater.)

The highest rate of evaporation from the oceans occurs in winter for both the Northern and Southern Hemispheres. The location of greatest evaporation is found on the east coasts of continents. (See maps lower right.) This is due primarily to winter storms that move off the east coasts of continents which tend to have strong winds. These winds help carry water vapor away from its source thereby allowing more evaporation to take place.

Maximum global evaporation rates. 

The other factor is the warm ocean currents that move pole-ward along the east coasts of continents. The cold winter-time air masses that move over the water allow for large differences in air and sea temperatures so evaporation is also large. Then, when these differences in air and sea temperatures are combined with strong winds it makes evaporation in these regions very efficient.

Yet, of all evaporation that occurs over the oceans, a little over 90% of the moisture falls directly back into the sea as precipitation. And after spending upwards of a few thousand years in the ocean, a water molecule, on average, will only spend about nine days in the atmosphere before returning to earth. This is a very simple water cycle!

But over land, the water cycle can become quite complicated. The remaining 10% of moisture is transported over land and falls as precipitation from where it can travel a myriad of paths. If the precipitation falls as snow, it can remain frozen for a day or two then melt and flow into a river. Or the snow can become compacted and be locked up in a glacier for centuries.

Some water may infiltrate the soil or percolate into the groundwater. While most groundwater returns to the ocean, some groundwater can bubble up to the surface as a spring and evaporate back into the atmosphere, flow into a river, or even be captured and bottled for human consumption.

But remember, of all the water on the earth only 2.5% is fresh water and nearly all of that fresh water is locked up in glaciers and groundwater. Perhaps surprisingly, the atmosphere only contains about one-thousandths percent of all water on the earth.

The distribution of fresh water as well as an estimation of the time a water molecule remains in various features, can be seen in the table (below). 

Estimate of global FRESH water distribution	Total Water (%)	FRESH Water (%)	Duration
Ice caps, Glaciers, & Permanent Snow	1.74	68.7	> 1000 years
Groundwater	0.76	30.1	~300 years
Soil Moisture	0.001	0.05	~280 days
Ground Ice & Permafrost	0.022	0.86
Lakes	0.007	0.26	1-100 years
Atmosphere	0.001	0.04	9-10 days
Wetlands	0.0008	0.03
Rivers	0.0002	0.006	12-20 days
Humans / Animals / Plants	0.0001	0.003

• evaporation
• transpiration
• condensation
• precipitation
• runoff

Evaporation  (CLICK)
Evaporation is the change of state in a substance from a liquid to a gas. In meteorology, the substance we are concerned about the most is water.

For evaporation to take place, energy is required. The energy can come from any source: the sun, the atmosphere, the earth, or objects on the earth such as humans.

Everyone has experienced evaporation personally. When the body heats up due to the air temperature or through exercise, the body sweats, secreting water onto the skin.

The purpose is to cause the body to use its heat to evaporate the liquid, thereby removing heat and cooling the body. It is the same effect that can be seen when you step out of a shower or swimming pool. The coolness you feel is from the removing of bodily heat to evaporate the water on your skin.

Transpiration

Transpiration is the evaporation of water from plants through stomata. Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues. In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Of the transpired water passing through a plant only 1% is used in the growth process of the plant. The remaining 99% is passed into the atmosphere.

CONDENSATION
Condensation is the process whereby water vapor in the atmosphere is changed into a liquid state. In the atmosphere condensation may appear as clouds or dew. Condensation is the process whereby water appears on the side of an uninsulated cold drink can or bottle.

Condensation is not a matter of one particular temperature but of a difference between two temperatures; the air temperature and the dewpoint temperature. At its basic meaning, the dewpoint is the temperature where dew can form. Actually, it is the temperature that, if the air is cool to that level, the air becomes saturated. Any additional cooling causes water vapor to condense. Foggy conditions often occur when air temperature and dewpoint are equal.

Condensation is the opposite of evaporation. Since water vapor has a higher energy level than that of liquid water, when condensation occurs, the excess energy in the form of heat energy is released. This release of heat aids in the formation of hurricanes.

Precipitation

Precipitation is the result when the tiny condensation particles grow too large, through collision and coalesce, for the rising air to support, and thus fall to the earth. Precipitation can be in the form of rain, hail, snow or sleet.

Precipitation is the primary way we receive fresh water in earth. On average, the world receives about 38½" (980 mm) each year over both the oceans and land masses.

Runoff

Runoff occurs when there is excessive precipitation and the ground is saturated (cannot absorb anymore water). Rivers and lakes are results of runoff. There is some evaporation from runoff into the atmosphere but for the most part water in rivers and lakes return to the oceans.

If runoff water flows into the lake only (with no outlet for water to flow out of the lake), then evaporation is the only means for water to return to the atmosphere. With evaporation only pure water evaporated, and therefore any contaminates and salts are left behind. The result is the lake becomes salty as in the case of the Great Salt Lake in Utah or Dead Sea in Israel.

Evaporation of this runoff into the atmosphere begins the hydrologic cycle over again. Some of the water percolates into the soil and into the ground water only to be drawn into plants again for transpiration to take place.

GROUND WATER POLLUTION CLEAN-UP

REVIEW:  
1. Evaporation is the change of state of water )a liquid) to water vapor (a gas).  On average about 47 inches (120 cm) is evaporated into the atmosphere from the ocean each year.

2. Transpiration is evaporation of liquid water  from plants and trees  into the atmosphere.   Nearly all (99%) of all  water that enters the roots transpires into the atmosphere.

3. Sublimation is the  process where ice and snow (a solid) changes into water vapor (a gas) without moving through the liquid phase.

4. Condensation is the process where water vapor (a gas) changes into water droplets
 (a liquid). This is when we begin to see clouds

5. Transportation is the movement of solid, liquid and gaseous water through the atmosphere. Without this movement, the water evaporated over the ocean would not

 precipitate over land. 

6. Precipitation is water that falls to the earth.  Most precipitation falls as rain but includes snow, sleet, drizzle, and hail. On average, about 39 inches (980 mm) of  rain, snow and sleet fall each year around the world.

7. Deposition is the  reverse of sublimation. Water vapor (a gas)  changes into ice (a 

 solid) without going through the liquid  phase. This is most often seen on clear, cold

 nights when frost forms on the ground

8. Infiltration is the movement of water into the ground from the surface.  Percolation is  movement of water past the soil going deep into the groundwater. 

9. Surface flow is the river, lake, and stream  transport of water to the oceans. 

Groundwater is the flow of water under- ground in aquifers. The water may return to the

 surface in springs or eventually seep into the oceans.

10. Transpiration/Plant uptake  is water  taken from the  groundwater flow and  soil moisture. Only 1% of water the plant  draws up is used by  the plant. The remaining 99% is passed back into the atmosphere.

HYDROLOGIC CYCLE - ACTIVITY  #1

HYDROLOGIC CYCLE - ACTIVITY  #2

HYDROLOGIC CYCLE - ACTIVITY  #3

HYDROLOGIC/WATER CYCLE - ACTIVITY  #4

HYDROLOGIC CYCLE - ACTIVITY  #5

HYDROLOGIC CYCLE - VIDEO

(click) REVIEW THIS GRAPHIC BEFORE TAKING THE QUIZ

HYDROLOGIC CYCLE - QUIZ

• DO NOT USE NOTES
• write your quiz name, score & your name on notebook paper & turn into drawer

thanks to NASA http://www.srh.weather.gov/jetstream/atmos/hydro.htm