Atmospheric Pressure

The Earth’s atmosphere is held in place by Gravity. That means that air has weight. Cold air is heavy and dense, while warm air is lighter and less dense. As the air at the surface is heated, it becomes lighter and rises, creating an area of relative low pressure. As the air at the surface cools, it becomes heavier and denser, creating an area of relative high pressure. The air under higher pressure is forced toward an area of low pressure. A instrument to measure atmospheric pressure would be very useful in weather forecasting.

The Mercury Barometer

Around 1643, Evangelista Torricelli, a student of Galileo turned a tube of mercury upside down in a bowl of mercury and realized that he was measuring atmospheric pressure.

By Kilom691 - The New Student's Reference Work, Public Domain,

The difference between the level in the bowl and the level in the tube measured atmospheric pressure.

The unit of measure used to measure atmospheric pressure is just a little bit confusing. Inches/millimeters of mercury was likely used by Torricelli to measure the difference between the level of mercury in the bowl and the level of the mercury in the tube.

The international system of units(SI) is used to measure atmospheric pressure can be expressed in either Pascals (Pa) or bars (bar).

        • Standard Atmospheric Pressure  =  1.01325 bar (bars)

        • 1.01325 bar (bars) = 1013.25 mb (millibars)

        • Standard Atmospheric Pressure  =  101325 Pa (Pascals)

        • 101325 Pa (Pascals) = 101.325 kPa (kilopascals)

        • 101325 Pa (Pascals) = 1013.25 hPa (hectopascals)

The numbers are the same. It’s the decimal that determines what you call it and also confuses. Let’s confuse things a little more. While Canada uses hectopascals (hPa), the U.S. is using inches of mercury (inhg).

        • Standard Atmospheric Pressure  =  29.9213 inhg (inches of mercury)

The Aneroid Barometer

The fact that the Mercury Barometer did not travel well led to the invention of the Aneroid Barometer, the instrument that we are all more familiar with today. The sealed cell (box) is compressible to accommodate an increase in pressure, but also contains a spring, which allows the cell to decompress with a decrease in pressure. Through a series of linkages, the compression and decompression of the cell is transmitted to a needle that moves on a calibrated dial.

The sketch

What it looks like on the Wall

High and Low Pressure

What causes atmospheric pressure to increase or decrease locally? 

The only thing that I remember from grade 8 is Land Breezes and Sea Breezes. What I didn’t fully appreciate at the time was that the air temperature over the water changes very little from day to night, while the air temperature over the land can be “higher than, in the day” and “less than, at night”.

At the end of a warm sunny day (evening), the area of relatively low pressure is over the land, while the area of relatively high pressure is over the water. The result is a cool Sea (or lake) Breeze (off the water).

At the end of a clear cool night (morning), the area of relatively low pressure is over the water, while the area of relatively high pressure is over the land. The result is a warm Land Breeze (off the land).

Horizontal air movement (wind) at the surface  is caused when the area of warmer, lighter and less dense air (area of low pressure) rises and the heavier, cooler and more dense air (area of high pressure) moves in to fill the void.

Atmospheric Circulation

The solar heating of the troposphere is uneven for many reasons, but the more significant reasons include the angle of the earth’s surface to the sun’s radiation and  earth’s rotation. The heat absorbed by the earth’s surface is most significant at the equator and least significant at the poles. The earth’s rotation means there is daytime heating and nighttime cooling. The troposphere is continuously compensating for the resulting temperature/pressure differentials. What we experience at the surface is wind.

The Tropics

As the troposphere heats up at the equator, the air becomes less dense (lighter) and rises. The lighter air is replaced by heavier, denser air moving in from the north and the south. The resulting circulation in the atmosphere is called the “Hadley Cell”.

The resulting surface winds are typically northerly (north of the equator) and southerly, south of the equator. These winds are deflected to the west by the Coriolis effect.


The Coriolis Effect


Anything (not attached to the planet) that is moving north in the northern hemisphere will appear to be deflected to the east as a result of the earth’s rotation. In other words, while a object (that is not attached) is heading north, the surface of the planet is heading west, leaving the object somewhere to the east of directly north.

The earth’s rotation deflects (Coriolis effect) the Hadley cell’s upper winds to the East and surface winds to West. These surface winds are commonly referred to as the “Trade Winds” and the “Northeast Trade Winds” in the Northern Hemisphere. They are also known as the tropical easterlies and are the prevailing winds in the tropical regions.

The Mid-latitudes

As we move north (or south) of the “Hadley Cell“, we find a another circulating cell called the “Ferrel Cell“. The main distinction is that the “Ferrel Cell” circulates in the opposite direction. Generally speaking, the “Hadley Cell” is positioned between Equator and the 30th parrallel, while the “Ferrel Cell” sits between the 30th parrallel and the 60th parrallel (mid-latitudes).

The resulting surface winds in the mid-latitudes (northern hemisphere) are typically southerly, but the earth’s rotation deflects (Coriolis effect) the Ferrel cell upper winds to the west and surface winds to the east. These surface winds are commonly referred to as the “Prevailing Westerlies” in both hemispheres.


Air flows from an area of high pressure to an area of low pressure. The greater the pressure differential, the faster the flow. This air flow is called wind. From the perspective of a mariner, it is important to remember that the wind is the flowing of air over the earth’s surface. The air moving at the surface (the wind we experience) is slowed by friction with the surface. Irregular land surfaces provide much more friction than flat water.

A screen shot from showing surface winds

The above screen shot from shows an area of low pressure sitting over Moncton, New Brunswick, Canada and an area of high pressure sitting off shore in the Atlantic.

Wind forecasts are for surface winds.

As the southeast winds approach the Cape Breton coast, they are forecast to be 30-40 kts. Because of minimal friction, wind speeds on the water are always higher than they are on adjacent land areas.

In the Bras d’or Lakes area (inland Cape Breton), the forecast winds have slowed to 15-20 kts. This is due primarily to the increased friction of the land surface.

As the winds come off the north coast they are forecast to be 35-40 kts. After blowing over the Bras d’or Lakes, the winds continue up and over the Cape Breton highlands which has elevations between 300 and 500 meters. The winds then blast down from the highlands and onto the waters of the Gulf of St. Lawrence.

The winds offshore are always higher that on nearby land. The wind is the air flowing over the earth’s surface and it’s direction and speed are significantly influenced by friction.

A screen shot from showing surface winds

In this screen shot of the Great Lakes from, you can compare wind velocities on land with those on the surfaces of the Lakes. The colour coded wind speed scale in knots is located in the lower right corner of the screen.

Weather Maps

The traditional surface analysis maps were drawn by hand. The more common surface weather map these days is computer generated. The surface analysis map shown below contains a significant amount of information about weather conditions at the surface, including atmospheric pressure, weather fronts, frontal movements, wind direction, wind strength and much more.

Low Pressure Area

The picture above shows a simplified drawing of a low pressure area. The lines (isobars) are lines of equal pressure. They are displayed very much the same as how depth contour lines are displayed on a nautical chart. Each line is labeled with the pressure  expressed in millibars, with the center of the low pressure area showing a pressure of 993 mb (millibars). Note that on the traditional map, the first 2 digits of the pressure are dropped on the isobars, while the 4 digits are used for the pressure at the center of the lows and highs.

The closeness of the isobars indicates the pressure gradient or the rate at which the pressure changes. The closer the isobars, the more pronounced change in the pressure and is also an indicator of wind strength. The steeper the pressure gradient, the stronger the wind.

High Pressure Areas

The only real difference with how a high pressure area is displayed is the direction of the pressure gradient.

The Coriolis Effect

Since high pressure is attracted to low pressure, we would expect the surface winds to be moving more or less at right angles to the isobars toward the center of low pressure. But because of the coriolis effect, the winds are deflected to the right in the northern hemisphere. All of the winds near the low pressure are being deflected to the right.

Besides being responsible for the wind’s deflection to the right in the northern hemisphere, the coriolis effect is also causing a rotation of the earth’s surface relative to the surface air. For each rotation of the planet, surface rotates from zero at the equator to 1 full rotation at the poles. The rotation of the surface causes the winds around a low pressure to spiral inward counterclockwise in the northern Hemisphere and clockwise in the southern hemisphere.

The Coriolis Effect is relative to the wind speed. Because surface friction is greater over land than water, the wind speed and consequently the Coriolis Effect is less over land than it is over water. Generally, the wind crosses the isobars at roughly 300 over land, while closer to 150 over water.

Buys Ballot's Law

Where is the low pressure? Buys Ballot, a dutch meteorologist came up with this back in 1857.

      • stand with your face to the wind

      • hold your right hand straight out from your right side

      • move your extended arm backwards 15(if over open water), or

      • move your extended arm backwards 30(if over open land)

At this point, you should be be pointing at the low pressure area. In the southern hemisphere, use your left hand.

The Beaufort Scale

Some time ago, Admiral Francis Beaufort developed a scale of wind velocities based on observed wind conditions. In the days before wind instruments, this scale served as a reference for sailors.

Wind Shifts

Wind shift refer to a change in wind direction over a period of time. 

If the wind shift is progressively clockwise, it is referred as a veering wind. In the northern hemisphere, a veering wind suggests a passage of a front (warm or cold).

If the wind shift is progressively counterclockwise, it is referred as a backing wind. In the northern hemisphere, a backing wind suggests a passage of a low pressure area to the south.

Local Winds

Regular winds that are influenced by the local geographic conditions are referred to a local winds. Significant local winds often get names. In Canada, most people are familiar with the Chinook winds, warm winds that blow down from the Rocky Mountains in Alberta.

The Wreckhouse winds in Western Newfoundland might be of concern, if sailing near the southwest corner of Newfoundland.