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Barometric Pressure
What it is and what it does.
Why Measure Barometric Pressure?
Barometric pressure is one of the most important parameters in weather observation, as the movement of
pressure fronts indicates the movement of weather fronts. Weather stations almost always include a barometer,
be they manned or unmanned. Barometers are also used in data buoys & ships at sea.
Hydrological & Ground water applications need information on barometric surface pressure to take into
account the effect of the hydrostatic pressure of air in different area. Barometric absolute pressure influences
other physical and industrial processes as well. For example, the wavelength of light in a laser interferometer
system changes with the refractive index of air, which is actually a function of air pressure.
Engine performance is also affected by air intake pressure. Aircraft altitude can be calculated from
atmospheric air pressure with aircraft altimeter adjusted according to air pressure readings reported by airports.
THE ACCURACY OF GPS IS AFFECTED BY ATMOSPHERIC PRESSURE. System accuracy may be
enhanced by barometric pressure information at the GPS receiver antenna level.
Barometric Pressure Measurement.
Barometric pressure is defined as atmospheric pressure i.e. the force exerted on a surface of unit area caused by
weight of the air column above, normally between 950 – 1050 hPa at sea Level. It indicates the presence &
movement of weather patterns & affects many physical measurements.
For climatological and meteorological purposes, standard sea-level pressure is said to be 76.0
cm or 29.92 inches or 1013.2 millibars. Scientists often use the kilopascal (kPa) as their
preferred unit for measuring pressure. 1 kilopascal is equal to 10 millibars. Another unit of force
sometimes used by scientists to measure atmospheric pressure is the newton. One millibar equals
100 newtons per square meter (N/m2).
By; Robert Peters for West Toronto Bassmasters
1/19
Barometric Pressure
What it is and what it does.
Atmospheric Pressure
Force exerted by the weight of the air
Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air
above that surface. In the diagram below, the pressure at point "X" increases as the weight of the air above it
increases. The same can be said about decreasing pressure, where the pressure at point "X" decreases if the
weight of the air above it also decreases.
Thinking in terms of air molecules, if the number of air molecules above a surface increases, there are more
molecules to exert a force on that surface and consequently, the pressure increases. The opposite is also true,
where a reduction in the number of air molecules above a surface will result in a decrease in pressure.
Atmospheric pressure is measured with an instrument called a "barometer", which is why atmospheric pressure
is also referred to as barometric pressure.
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.
By; Robert Peters for West Toronto Bassmasters
2/19
Barometric Pressure
What it is and what it does.
Feeling Pressured?
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.
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.
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?
Atmospheric pressure is the pressure caused by the weight of air above any area in the Earth's atmosphere.
Standard atmospheric pressure (atm) is discussed in the next section.
Air masses are affected by the general atmospheric pressure within the mass, creating areas of high and low
pressure.
As elevation increases, fewer air molecules are above. Therefore, atmospheric pressure decreases with
increasing height. The following relationship is a first-order approximation:
,
By; Robert Peters for West Toronto Bassmasters
3/19
Barometric Pressure
What it is and what it does.
where P is the pressure in pascals and h the height in metres. This shows that the pressure at an altitude of
31 km is about 10(5-2) Pa = 1000 Pa, or 1% of that at sea level1.
A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would
weigh approximately 14.7 lbf. A 1 m2 column of air would weigh about 100 kilonewtons.
Standard atmospheric pressure
Standard atmospheric pressure or "the standard atmosphere" (1 atm) is defined as 101.325 kilopascals (kPa).
(see also Standard temperature and pressure)
This can also be stated as:
•
29 117/127 inches of mercury
29.92 inHg
•
760 millimetres of mercury (mmHg)
•
760 torrs (Torr)
•
1013.25 millibars (mbar, also mb)
•
14.7 psia or 0 psig (pounds-force per square inch, absolute or gauge) (lbf/in²)
•
1 6517/196133 technical atmospheres (at)
1.03322745 at
This "standard pressure" is a purely arbitrary representative value for pressure at sea level, and real atmospheric
pressures vary from place to place and moment to moment everywhere in the world.
In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the
"standard" means the equivalent quantity of air at standard temperature and pressure. However, this standard
atmosphere is defined slightly differently: temperature = 68°F (20°C), air density = 0.075 lb/cu ft, altitude = sea
level, and relative humidity = 0%. In the air conditioning industry, the standard is often temperature = 32°F
(0°C) instead. For natural gas, the petroleum industry uses a standard temperature of 60°F (15.6°C).
Intuitive feeling for atmospheric pressure based on height of water
Atmospheric pressure is often measured with a mercury barometer, and a height of
approximately 30 inches of mercury is often used to teach, make visible, and illustrate
(and measure) atmospheric pressure. However, since mercury is not a substance that
humans commonly come in contact with, water often provides a more intuitive way to
conceptualize the amount of pressure in one atmosphere.
1 atmosphere (14.7 PSI) is the amount of pressure that can lift water approximately 33.90
feet (approx. 10.333 meters). Thus when dumping water on someone from a height of
approximately 10 meters (or, equivalently, when someone enters a pool from a 10-meter
platform as is commonly found in olympic swimming pools), the potential energy approximately equivalent to
water at a pressure of one atmosphere. At this height of water, the person and water collide at slightly more than
By; Robert Peters for West Toronto Bassmasters
4/19
Barometric Pressure
What it is and what it does.
50 km/h (slightly more than 30 mi/h), as potential energy is converted to kinetic energy. Thus, many swimmers
wear ear plugs to avoid barotrauma from water pressure transiently (surge at impact) in the one atmosphere
range, where there is little or no time to equalize the ears as can be done when scuba diving.
In terms of city water pressure, one atmosphere is approximately one half to one quarter the pressure of typical
city water mains (i.e. water pressure is around 2 to 4 atmospheres).
How Air Pressure Signals Changes in the Weather
Before Hurricanes could be spotted by satellites from space, people would keep a wary eye on their barometers
during hurricane season. If the air pressure dropped, that was usually a good time to board up windows and
head further inland!
As hurricanes pass over coastal areas, air pressure can drop significantly. At sea level air pressure is normally
around 1013.25mb (29.92 inches of mercury). Extremely strong hurricanes are accompanied by air pressure
drops of between 30 and 70mb. The greater the pressure differences between a low-pressure area and a highpressure area, the stronger the winds! Wind is the natural result of having a low-pressure area next to a higherpressure area since the air molecules in the higher-pressure zone will migrate to the "more spacious"
surroundings of the low-pressure area
How Does Weather Affect Air Pressure?
By Dr. Louis Walter of NASA's Earth Science Enterprise
The most fundamental thing you have to understand is that heavier gases weigh more than lighter gases. Now
that's pretty straightforward - but what does it mean? Well, different chemical elements, as you know, have
different atomic weights. Those which form gases (like nitrogen, oxygen, etc.) often combine two atoms at a
time to form a gaseous molecule - like N2 (two nitrogen’s) or O2 (two oxygen’s).
Now the ATOMIC weight of nitrogen (N) is 14 and of oxygen (O) it's 16. The molecules N2 and O2 have
MOLECULAR weights of 28 and 32, respectively. Doesn't take a rocket scientist to see that a gallon of oxygen
weighs more than a gallon of nitrogen. Matter of fact, it turns out that - at room temperature and normal (sea
level) atmospheric pressure, 28 grams of nitrogen occupies a volume of 22.4 litres and 32 grams of oxygen
occupies the same volume!
In other words, under STP (standard temperature and pressure) the weight of 22.4 litres of a gas in grams equals
the molecular weight of the gas.
Now for some fun with numbers. Air is ABOUT 80% nitrogen and 20% oxygen. How much does a litre of air
weigh? Well.....
If 22.4 litres of nitrogen weighs 28 grams; 0.8 litres weighs (0.8/22.4) X 28 grams = 1 gram (almost exactly)
and 0.2 litres of oxygen weighs (02. /22.4) X 32 = 0.286 grams, so...
A litre of air weighs about 1.286 grams.
Now, let me change the subject just a bit. How much does gaseous water weigh? Not LIQUID water - I mean
steam or vapour. Let's figure it out. The chemical formula for water is H2O. One oxygen atom (atomic weight
16) and two hydrogen atoms (atomic weight 1). The total weight of the molecule is 18. Now how much does
22.4 litres (of the gas) weigh? 18 grams. One litre weighs 18/22.4 grams or 0.8 grams.
By; Robert Peters for West Toronto Bassmasters
5/19
Barometric Pressure
What it is and what it does.
Now we're getting close. You can see that air normally weighs 1,286 grams per litre but, if we substitute water
for some of the air, the mixture becomes lighter. So, if there's water (otherwise known as humidity) in the air,
the air mixture becomes LIGHTER - and it doesn't push down so hard on the mercury and the barometer's
lower.
By; Robert Peters for West Toronto Bassmasters
6/19
Barometric Pressure
What it is and what it does.
By; Robert Peters for West Toronto Bassmasters
7/19
Barometric Pressure
What it is and what it does.
By; Robert Peters for West Toronto Bassmasters
8/19
Barometric Pressure
What it is and what it does.
Barometric Pressure
Rising
Water surface
By; Robert Peters for West Toronto Bassmasters
Shore
9/19
Fish movement to equalize internal pressure
Barometric Pressure
What it is and what it does.
Bottom
Barometric
Pressure
Falling
shore
Surface
Fish movement to equalise internal pressure
Bottom
When the pressure rises the internal pressure (float bladder) of the fish is squeezed causing
unwanted pain, so the fish move to equalize the pressure, it moves closer to shore or to shallower
water.
It is just the opposite when the pressure is dropping the fish moves deeper to equalize the pressure
and easy the pain. So in closing if the barometric pressure is raising fish from DEEP to
SHALLOW, & when falling fish from SHALLOW to DEEP.
Presentation Material:
The barometer is an instrument used to indicate air pressure. It can be a handy tool for anglers to
help determine periods of fish activity. It is not a cure-all for fishless days, but it is another
scientific approach, which helps guarantee fewer of them.
Barometric pressure is relative weight of the atmosphere bearing on the earth’s surface. The
pressure is measured in a column of mercury at sea level at 15 degrees C. Standard Barometric
pressure is 101 kPa (kilopascals) or 29.9 inches. This reading is adjusted in each geographical area
for altitude and temperature.
By; Robert Peters for West Toronto Bassmasters
10/19
Barometric Pressure
What it is and what it does.
Low pressure could then be considered as a reading of less than 101 kPa and a high pressure above
101 kPa.
How does this foretell good or bad fishing? Fish usually feed best when the barometer is in the
moderate range between readings of 100 and 103 kPa (29.6 to 30.4 in.). Often these days are days
of haze and humidity and are excellent fishing periods.
The exception to this is when the barometer begins to drop for a short period during the approach of
a cold front. At this time, a period of intense feeding occurs prior to the arrival of stormy weather.
In periods of high pressure, during reading in excess of 103 kPa (30.4 in.) there is very little fish
activity, except, possibly in the very deep waters. These are the very bright cool days of beautiful
weather associated with a high-pressure area.
Remember, besides fishing during a favourable barometric reading, you should be on a good
fishing structure with an ideal water temperature. If all three “add up,” prepare for plenty of fish.
action.
If the wind is south to southwest and it is overcast with just the right water conditions and you’re
on a good structure, great! You are probably in for a fine day of fishing.
But what if the conditions are generally opposite? There is a strong north wind with a cold front
passing through, its too bright, and you don’t know the fishing spots. Don’t go home because you
can’t line up the right conditions. Make the best of it, continue fishing. Who knows? You may get
lucky!
Referenced, from Bass Digest. Date of print,
Weather plays a most important part in a fisherman’s life. If he has a fishless day, the reason for his lake of
success will invariably concern the weather. It could be too bright, too cloudy, too calm, too windy, too dry, and
too humid . . . and surprisingly enough, his excuse might be right.
To be able to compensate for weather change or an articular condition could be the difference of having or not
having fish.
Signs that could indicate a weather change other than clouds are as follows:
 Rapidly falling barometer – rain
 Higher than usual nighttime temperatures – rain
 Large rings around the sun or moon prior to an overcast sky – rain
 Clouds moving in different directions at different altitudes – rain
 Wind shift to the N/E or N – rain
 Rapidly rising barometer – clearing
 Wind shift to the W or N/W – clearing & cooler
 Wind shift from the N or N/W – warmer.
By; Robert Peters for West Toronto Bassmasters
11/19
Barometric Pressure
What it is and what it does.
Cold Fronts: This is one that here we see often.
How many of you know what a cold front is. It is the leading edge of a mass of cold air advancing into an area
of warm air.
As the cold front approaches and passes through the confrontation between the cold and warm moist air often
causes a great deal of turbulence accompanied by lightning, thunder, rain or hail. The line on the weather map
that distinguishes this cold front is the location of the lowest pressure.
After the front passes the pressure again begins to rise slowly until it reaches a peak a day or two later. At this
time we are in the midst of a high pressure system, brining with it bright, clear invigorating days, just beautiful
weather. However from a fisherman’s standpoint it is usually a “dead loss”.
Cold fronts and good fishing just do not mix. Prior to the arrival of a cold front, the weather may be warm and
humid but not bright. These are the hazy, lazy days with high humidity and a diffused light. During this period,
particularly in the mornings and evenings, fishing will be good, both along the lake structures and in the rivers.
The answer is light. Without strong light, the fish are more active and the fishing is generally favourable. Make
use of the period prior to the storm, as fishing activity will increase. When the storm breaks, head for cover, the
fish will. The crash & rumble of thunder “SCARES” the fish into hugging the bottom and into a mood of
inactivity. The storm will probably last the rest of the day and into the night. The morning will dawn clear crisp
and bright. This is the signal. The fish will head for deep holes and shady areas of the rivers and into the deeper
parts of the lake below the level of intense light penetration. Fish in other than these areas and the day will be a
practically fishless.
This is what the cold front does. It drives the fish to cover and into a sulking mood. This sullen mood is said to
be caused by the high pressure. With the possible exception of early morning and late evenings, it ruins the
fishing. Think back how many times has this happened to you? This semi-fishless spell will last for two or
three days, or until the skies cloud over or become hazy. The whole process will repeat it itself. Fishing will
improve until the next cold front comes through. Cold fronts cannot be avoided, so go fishing anyway. But fish
the shady areas or below the penetration of the suns rays which generally are from 9 to 11 m (30 to 35 feet)
below the water surface.
Water Temperature:
Knowing water temperature is not the only reason for an angler’s success, but it can be extremely important in
his search for all game fish.
All fish have a preferred temperature zone – an area or layer of water at which they are most comfortable and at
which their body functions work best this is their COMFORT ZONE. Find fish in their comfort zone on a good
fishing structure and you usually find them active, hungry and full of fight.
See Chart Below for details:
By; Robert Peters for West Toronto Bassmasters
12/19
Barometric Pressure
What it is and what it does.
Thermocline:
The thermocline is relatively unimportant in shallow lakes. The authors however feel that the complete angler
should be aware of this phenomenon that occurs in deep water lakes, in the event that he seeks the lake trout or
Salmon.
In the summer and fall the waters of large deep lakes form or settle into three distinct different temperatures.
The following sketch illustrates these layers as the Epilimnion (top warm water layer), the Hypolimnium (the
bottom cold water layer) and the Thermocline (a highly oxygenated, cool water layer sandwiched between the
other two layers).
By; Robert Peters for West Toronto Bassmasters
13/19
Barometric Pressure
What it is and what it does.
The thermocline is a layer or strata of water where the temperature undergoes a rapid change over a short
distance. In our illustration the “quick change”, 16 C to 9 C (60- 48 F) takes place in 4.6 m (15 Ft) between the
18 –23 m (60 – 75 ft) depths. This is the thermocline layer.
The location, depth and thickness of the thermocline are dependent upon such things as the size and depth of the
lake, the water clarity, and probably the most important, the wind. It should be noted that because of these
factors, the thermocline may, and often does vary day to day. To find the thermocline use a thermometer and
take temperature readings at 3 m intervals until you see a rapid temperature change in a short distance. If by
chance the thermocline is penetrated by a bottom structure such as a reef or sand bar. A ledge, or drop-off, the
fishing should be concentrated at this area. It might well be one of the lakes hot spots.
Light Penetration:
Color visibility is also dependent upon the light intensity of the sun’s rays, which enter the water. From the
accompanying drawing we note that in early morning when the sun’s rays are at a low angle, lees than 30* to
the horizon, there is a minimum of light penetration. At this time, the first and only color to show in the surface
water is BLUE. As the day progresses and illumination improves, GREEN also begins to penetrate the water
while blue rays penetrate deeper. Progressively, as the sun reaches it’s zenith, YELLOW, ORANGE and RED
By; Robert Peters for West Toronto Bassmasters
14/19
Barometric Pressure
What it is and what it does.
will appear. As late afternoon, then evening approaches the colors begin to reverse order to their morning
appearance.
Fish or man cannot see ultra-violet rays. However, ultra-violet rays convert florescent color pigments into light
rays. Hence florescent red lures, in good light conditions, appear as red lures but are more visible in deeper
water than ordinary red lures.
What about shades, the whites, blacks and metallic finishes? White is an excellent gather of light rays and
shows light in a diffused manner. Black is the absence of color. When practically all light is gone, black is
recommended for maximum contrast. The metallic nickel & silver finishes reflect light like a mirror. Gold &
Brass finishes reflect the yellow rays.
Remember that color penetration as indicated in the chart, is the situation at mid-day and in clear water.
Naturally underwater illumination is lessened when the sun is at a low level, when there is heavy cloud cover, or
when water is in a stained or turbid condition.
By; Robert Peters for West Toronto Bassmasters
15/19
Color Presentation:
Barometric Pressure
What it is and what it does.
In order to best select a lure color, it is desirable to know the water depth at which certain colors are effective.
Light is a composite of the light spectrum colors. When light penetrates water its colors are filtered out in
varying degrees. The accompanying drawing illustrates the greatest depth at which each color in the spectrum is
visible to the fish in a clear water lake. Below these depths fish see colors as grey and black.
Spectrum fishing:
Spectrum fishing is the science of color and light penetration through the water, applied to lure color selection.
It is a known fact, through aquarium experimentation that fish have distinct color preferences; they favour the
warm colors, red, orange and yellow. From these experiments, it can be assumed that fish in natural
surroundings will react to color, as did the laboratory fish.
Fish color preference:
In good light, red is the color that excites a fish most. Orange & Yellow are the next colors fish seem to prefer.
By; Robert Peters for West Toronto Bassmasters
16/19
Barometric Pressure
What it is and what it does.
Remember these are the color preference in bright light. In reduced light situations, blue & green are most apt to
excite fish.
Fish, whose habitat is in deep water, are accustomed to dim light and therefore favour the reflecting shades of
white & silver.
I hope the information I have gathered for you will help to improve your fishing and you catch -ability.
By; Robert Peters for West Toronto Bassmasters
17/19
Barometric Pressure
What it is and what it does.
By; Robert Peters for West Toronto Bassmasters
18/19
Barometric Pressure
What it is and what it does.
By; Robert Peters for West Toronto Bassmasters
19/19