Read...
In the last section, our discussion of pressure was focused on the pressure at sea level. But, most of the United States (and the rest of the world's land masses) aren't at sea level, so why make that distinction? Well, in order to analyze the horizontal patterns of surface air pressure that govern weather, meteorologists require a "level playing field," and that's why they're interested in "sea-level pressure."
To see what I mean, consider this: if you brought a barometer with you on a trip to Denver, Colorado (elevation about one mile above sea level), it would regularly measure a pressure of about 850 millibars. That's much lower than the typical range of sea-level pressures we talked about in the previous section. The pressure on your barometer would be so low because pressure decreases with increasing height everywhere in the atmosphere. The reason why that's the case is fairly intuitive: the higher the altitude, the less air (and weight) there is above you in an atmospheric column. In fact, pressure at the top of the troposphere is typically less than 300 millibars (less than 30 percent of the pressure at sea level).
So, the fact that pressure decreases with increasing height explains why the surface pressure at high elevation locations (like Denver) is much lower than at sea level. For meteorologists, surface pressure's dependence on elevation presents a bit of a problem. To see what I mean, check out the map of long-term average surface pressure (called station pressure) across the United States below.
The map of long-term average station pressure across the United States shows very low pressures in the Rocky Mountains (less than 780 millibars in some areas) because of the high elevations in the region.
Credit: Earth System Research Laboratory
The first thing you might notice on the map is the area of very low pressures in the Rocky Mountains (less than 780 millibars in some areas). Is there some kind of monster low-pressure system permanently parked in the Rockies? Of course not! The station pressures are always low there because of the high elevations in the Rockies. The dramatic variation in station pressure based on elevation makes it virtually impossible for meteorologists to use station pressure to track centers of high and low pressure. Regardless of the strength and position of various high- and low-pressure systems, the map of station pressure would always look something like the one above (lowest pressures in the highest-elevation regions). So, in order to level the playing field, meteorologists adjust station pressure to sea level.
By adjusting to sea level, meteorologists are essentially pretending that high-elevation locations (like Denver) are located at sea level, and as such, they adjust all barometer readings to what they would be if they were located at sea level. To do so, meteorologists "correct" the station pressure to sea level by estimating the weight of an imaginary column of air that extends from station to sea level. I'm skipping the details, but the bottom line is that this estimated weight of the imaginary air column gets converted into a pressure adjustment that is added to the observed station pressure (this schematic may help you visualize the adjustment process). While the estimating process isn't perfect (especially for very high elevation locations), the end result is a sea-level pressure value that can be used to plot useful weather maps, which help meteorologists track high- and low-pressure systems more effectively.
Thus, the contour maps of pressure that meteorologists most commonly work with (and that we'll most commonly work with) are maps of sea-level isobars (remember that "isobar" is the name of a contour of equal pressure). Maps of sea-level isobars help weather forecasters quickly spot areas of low and high pressure, which can help them identify areas of potentially stormy weather. For example, check out the analysis of sea-level pressure from 12Z on October 30, 2017 below, and note the strong low-pressure system centered just north of New York state (marked by the "L") in the Canadian Province of Quebec).
The analysis of sea-level pressure from 12Z on October 30, 2017 shows a strong low-pressure system located over the Northeast United States and Southeast Canada.
Credit: University of Illinois
If you remember how to interpret contour maps, you should be able to estimate the pressure at the center of this strong low. The contour interval on this map is four millibars and the innermost labeled closed isobar around the low is 984 millibars. There's one unlabeled closed isobar inside the 984-millibar isobar, which represents 980 millibars. The center of the low is located inside that isobar, so its lowest pressure must have been less than 980 millibars, but greater than 976 millibars (otherwise there would have been a 976-millibar isobar drawn).
To place this sea-level pressure in perspective, check out the barograph showing the range of sea-level pressures again. A sea-level pressure less than 980 millibars represents a pretty strong low, so we might expect some pretty "active" (stormy) weather in the Northeast U.S. around the low-pressure system depicted above. Indeed, that was the case! Check out the 1145Z infrared satellite image from October 30, and note the fairly bright white shading in the region, indicative of an organized area of cold cloud tops. This storm brought drenching rains to the Northeast, along with damaging wind gusts (the National Weather Service office in Boston compiled this list of strongest wind gusts, including several reports of gusts greater than 75 miles per hour). More than a million people lost power in New England from this storm.
A sample station model with sea-level pressure and the three-hour pressure tendency highlighted.
Credit: David Babb
So, where do the pressure observations come from that are used to make maps of sea-level isobars like the one above? Station pressure is routinely measured at surface weather stations (along with temperature, dew point, wind, etc.) and is reported in the station model after being adjusted to sea level. Back when we first covered the station model, we didn't discuss the pressure information that it contains (highlighted in the image on the right), but now it's time. In particular, we're going to focus on the three digits in the upper-right corner (the pressure tendency information is not always reported, so we're going to ignore it in this course). The three digits in the upper-right-hand corner of the station model represent the last three digits of the station's sea-level pressure, expressed to the nearest tenth of a millibar. Thus, to decode the pressure reading, you must first add a decimal in front of the right-most digit. Then you need to place either a "9" or a "10" in front of the three digits.
How do you decide whether a "9" or a "10" should go in front of the three digits? This is where knowing the typical range of sea-level pressures is helpful. Remember that nearly all values of sea-level pressure are between 950 millibars and 1050 millibars (unless you're dealing with an intense hurricane, or an extremely strong Arctic high in winter). So, in the example on the right, we must need a "10" in front of the "046" to give 1004.6 millibars. Placing a "9" in front would have given 904.6 millibars, which wouldn't make sense (unless an extremely intense hurricane was right near the station).
Ultimately, if the three digits you see on the station model are less than "500," you'll typically place a "10" in front of them, while if the three digits are greater than "500," you'll typically place a "9" in front of them. In most cases, you want to choose whichever will give you a sea-level pressure between 950 mb and 1050 mb. Some exceptions to this rule exist (intense hurricane or very strong Arctic highs in the winter), but in the scheme of things, the exceptions are rare. To get some practice with decoding sea-level pressure from station models, check out the Key Skill and Quiz Yourself sections below. After you've finished with those, up next we'll start to examine the forces that control the wind so that we can use patterns of isobars to diagnose how the wind will blow.
Key Skill...
A key skill in this section is decoding pressure on a station model. Experiment with the station model tool and observe how different pressures are coded. For example, type in pressures of 999.6 mb, 986.2 mb, and 1028.9 mb and see how they appear on the station model. Practice decoding some random 3-digit coded pressures (decode "953", "069", and "395", for example) and check your answers with the tool by typing your answer into the "Current Conditions" panel and see if the station model displays the 3-digit code that you started with.
Think you have a good handle on decoding pressure from a station model now? Here are a few more examples for you to try. If you don't get these right on the first try, you may need to spend more time exploring with the interactive station model...
Example #1:
You see a station model with "957" in the upper-right corner. What is the sea-level pressure at this station?
Click for answer...
Answer: 995.7 millibars. We arrive at this conclusion by placing a 9 in front. If we had put a 10 in front, we'd have had 1095.7 millibars, which would be much higher than any sea-level pressure ever measured on Earth.
Example #2:
You see a station model with "234" in the upper-right corner. What is the sea-level pressure at this station?
Click for answer...
Answer: 1023.4 millibars (most likely). We arrived at this conclusion by placing a 10 in front. If we had put a 9 in front, we'd have 923.4 millibars, which is really only possible in a hurricane.
Example #3:
You see a station model with "701" in the upper-right corner. What is the sea-level pressure at this station?
Click for answer...
Answer: 970.1 millibars (most likely). We arrived at this conclusion by placing a 9 in front. If we had put a 10 in front, we'd have 1070.1 millibars, which would be near the highest sea-level pressure ever recorded on Earth (an extremely rare situation).
FAQs
The three digits in the upper-right-hand corner of the station model represent the last three digits of the station's sea-level pressure, expressed to the nearest tenth of a millibar. Thus, to decode the pressure reading, you must first add a decimal in front of the right-most digit.
How do you read a pressure trend? ›
The pressure trend has two components, a number and symbol, to indicate how much (in tenths of millibars) in the past 3 hours and the trend in the change of the pressure during the same period. In above case, the pressure was falling after steady or slightly rising and becoming 3 mb LOWER than it was three hours ago.
What are the rules for pressure readings on a station model? ›
Air Pressure: when coding air pressure on a station model, use the following rule:
- if the air pressure on the station model is 500 or more, place a 9 in front of this number. ...
- if the air pressure number on the station model is less than 500 add a 10 in front of the number.
The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 psi.
What are the three types of atmospheric pressure? ›
Pressure is classified into three types, they are:
- Absolute pressure.
- Gauge pressure.
- Differential pressure.
- Sealed pressure or vacuum pressure.
Simply put, barometric pressure is the measurement of air pressure in the atmosphere, specifically the measurement of the weight exerted by air molecules at a given point on Earth.
What is the difference between pressure and barometric pressure? ›
Atmospheric Pressure refers to the pressure exerted by the atmosphere. Barometric Pressure is a term used to describe the pressure measured by a barometer.
What does a barometric pressure of 29.9 mean? ›
Normal is 29.9; range ~29.6 - 30.2 inches Hg (752-767 mm Hg)… at SEA LEVEL! If you are in Merrill, for example, at 1300 ft. altitude, this range changes. Rarely (at sea level) do readings exceed 30.4 inches Hg (773 mm Hg)…
What is the ideal pressure reading? ›
As a general guide: ideal blood pressure is considered to be between 90/60mmHg and 120/80mmHg. high blood pressure is considered to be 140/90mmHg or higher.
What is a good pressure reading number? ›
A normal blood pressure level is less than 120/80 mmHg. No matter your age, you can take steps each day to keep your blood pressure in a healthy range.
998: Pressure, to the nearest tenth of a millibar. Add either a 10 or 9 in front based on which would bring the value closer to 1000. The pressure here is 999.8 millibars (mb).
What are the 5 unit of pressure? ›
There are several other units of pressure such as pounds per square inch and bar, unit of atmospheric pressure is atm, centimetres of water, millimetres of mercury or inches of mercury (used as a unit of blood pressure), torr, MSW and FSW.
What are the two types of pressure? ›
There are two basic pressure types - absolute and gauge - distinguished by what pressure they are compared to, which is called the reference pressure.
What are the 5 most common units of pressure? ›
Answer and Explanation: The SI unit of pressure is Pascal (Pa) or Newton per meter squared. Some other common units of pressure are bar (bar), standard atmosphere (atm), torr (torr) or millimeters of mercury (mmHg), inches of mercury (inHg), and pounds per square inch (psi).
What are the 3 main factors that influence air pressure? ›
1)The 3 main factors that affect barometric (air) pressure are:
- Temperature.
- Altitude or Elevation.
- Moisture ow water vapour.
Pressure is defined as the physical force exerted on an object. The force applied is perpendicular to the surface of objects per unit area. The basic formula for pressure is F/A (Force per unit area). Unit of pressure is Pascals (Pa). Types of Pressures are Absolute, Atmospheric, Differential, and Gauge Pressure.
What are the different type of pressure? ›
Types of pressure: Absolute pressure, gauge pressure, differential pressure.
What affects barometric pressure? ›
Barometric pressure increases as altitude decreases, with air molecules in the upper layers compressing the layers below them. Barometric pressure fluctuates based on elevation levels, wind patterns, and temperatures.
What is an example of barometric pressure? ›
For weather example: during a rainy day, the barometric pressure is lower than it would be on a sunny day. The barometric pressure also varies based on altitude. The higher you are, the smaller the barometric pressure, which makes sense because when you move to a higher altitude, there is less air on top of you.
What causes high barometric pressure? ›
Areas of high and low pressure are caused by ascending and descending air. As air warms it ascends, leading to low pressure at the surface. As air cools it descends, leading to high pressure at the surface.
The air pressure at sea level at a temperature of 59°F (15°C) is equal to one atmosphere (Atm), and this is the baseline reading for determining relative pressure. Atmospheric pressure is also known as barometric pressure because it is measured using a barometer.
How is weather pressure measured? ›
Atmospheric pressure is measured by a barometer that employs a silicon capacitive pressure sensor having excellent repeatability and long term stability characteristics.
Why is water not used as a barometric? ›
Water is highly inconvenient to take a tube of height 10.4 m for a barometer. The vapor pressure of water is high so its vapors in the vacuum space will make the reading inaccurate.
Is 30.33 barometric pressure high? ›
A barometer reading of 30 inches (Hg) is considered normal. Strong high pressure could register as high as 30.70 inches, whereas low pressure associated with a hurricane can dip below 27.30 inches (Hurricane Andrew had a measured surface pressure of 27.23 just before its landfall in Miami Dade County). According to Dr.
Is 29.83 low barometric pressure? ›
A barometric reading below 29.80 inHg is generally considered low, and low pressure is associated with warm air and rainstorms.
What is a comfortable barometric pressure? ›
People are most comfortable with barometric pressure of 30 inches of mercury (inHg). When it rises to 30.3 inHg or higher, or drops to 29.7 or lower, the risk of heart attack increases. A barometric reading over 30.20 inHg is generally considered high, and high pressure is associated with clear skies and calm weather.
What is H and L in weather forecast? ›
High and Low Pressure Systems
Areas of low pressure can be stormy. Air pressure is indicated on weather maps with a blue "H" at each center of high pressure and a red "L" at each center of low pressure.
What numbers are high and low air pressure? ›
Understanding Pressure Readings
- High Pressure: above 30.2 inHg.
- Normal Range: 29.8 inHg - 30.2 inHg.
- Low Pressure: below 29.8 inHg.
Low-pressure areas are places where the atmosphere is relatively thin. Winds blow inward toward these areas. This causes air to rise, producing clouds and condensation. Low-pressure areas tend to be well-organized storms.
What is a high pressure in weather? ›
High pressure often means dry weather with sunshine. Low pressure often means clouds and precipitation. High pressure is associated with sinking air. Air pressure is higher because it is pushing DOWN on the ground. When air sinks from high in the atmosphere to the lower levels it warms up and dries out.
High pressure systems are generally associated with warm and dry conditions. Low pressure systems take the air at the surface and force it up high into the atmosphere. This process causes air to cool and condense, which is how we get clouds. Thunderstorms and winter storms are associated with low pressure systems.
What is used to measure pressure? ›
Instruments used to measure and display pressure mechanically are called pressure gauges, vacuum gauges or compound gauges (vacuum & pressure). The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.
How does a pressure sensor measure pressure? ›
Pressure transducers have a sensing element of constant area and respond to force applied to this area by fluid pressure. The force applied will deflect the diaphragm inside the pressure transducer. The deflection of the internal diaphragm is measured and converted into an electrical output.
What is the normal range for pressure? ›
Normal blood pressure for most adults is defined as a systolic pressure of less than 120 and a diastolic pressure of less than 80. Elevated blood pressure is defined as a systolic pressure between 120 and 129 with a diastolic pressure of less than 80.
How many pressure is normal? ›
If you have normal blood pressure, your blood pressure is less than 120/80.
What are the standard pressure values? ›
Standard Pressure is 1 Atm, 101.3kPa or 760 mmHg or torr.
STP is the "standard" conditions often used for measuring gas density and volume. At STP, 1 mole of any gas occupies 22.4L.
What is a high pressure read? ›
Elevated blood pressure is when readings consistently range from 120-129 systolic and less than 80 mm Hg diastolic. People with elevated blood pressure are likely to develop high blood pressure unless steps are taken to control the condition.
What is the normal pressure by age? ›
Table 5.1: Estimated Normal Blood Pressure for Age
| Age | Normal Systolic Range | Normal Diastolic Range |
|---|
| Adolescent (14–18 years) | 90–120 mm Hg | 50–80 mm Hg |
| Adult (19–40 years) | 95–135 mm Hg | 60–80 mm Hg |
| Adult (41–60 years) | 110–145 mm Hg | 70–90 mm Hg |
| Older adult (61 and older) | 95–145 mm Hg | 70–90 mm Hg |
4 more rowsNormal pressure is 120/80 or lower. Your blood pressure is considered high (stage 1) if it reads 130/80. Stage 2 high blood pressure is 140/90 or higher. If you get a blood pressure reading of 180/110 or higher more than once, seek medical treatment right away.
What does 1012 mb pressure mean? ›
Values around 1013 mb and higher indicate an influence from a high pressure system. Strong highs will have values well above 1013 mb such as 1030 millibars. Low pressure tends to bring in cloudier, windier and stormy weather while high pressure tends to bring in less clouds, lighter wind and fair weather.
1030 mb (30.42 inches of mercury): Strong High Pressure System. 1013 mb (29.92 inches of mercury): Average Sea Level Pressure. 1000 mb (29.54 inches of mercury): Typical Low Pressure System.
Is 1000 hPa high or low pressure? ›
Pressure is usually around 1000hPa, and at sea level, it rarely gets lower than 950hPa or higher than 1050 hPa. So high pressure gives fine, dry weather – warm in summer (remember how glorious July was!) but with cold nights in winter. But on the other hand, low pressure brings clouds, rain and strong winds.
What is 14.7 psi in atmospheric pressure? ›
Normal atmospheric pressure is 14.7 psi, which means that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds. Normal atmospheric pressure is defined as 1 atmosphere. 1 atm = 14.6956 psi = 760 torr.
What does an atmospheric pressure of 760 MMHG mean? ›
Standard sea-level pressure, by definition, equals 760 mm (29.92 inches) of mercury, 14.70 pounds per square inch, 1,013.25 × 103 dynes per square centimetre, 1,013.25 millibars, one standard atmosphere, or 101.325 kilopascals.
How do you write 1 atmospheric pressure? ›
A pressure of 1 atm can also be stated as: ≡ 101325 pascals (Pa) ≡ 1.01325 bar. ≈ 1.033 kgf/cm.
How do you explain atmospheric pressure? ›
The atmospheric pressure at a point is defined as the force acting normally on a unit area around that point, due to the total height of the air column of the atmosphere above it. That pressure is known as atmospheric pressure, also known as air pressure.
What are 5 units of pressure? ›
There are several other units of pressure such as pounds per square inch and bar, unit of atmospheric pressure is atm, centimetres of water, millimetres of mercury or inches of mercury (used as a unit of blood pressure), torr, MSW and FSW.
Is 40 psi too much air? ›
Specifically, the level of 40 psi can be suitable for passenger cars or sports cars. But this is too high for small cars with a recommendation below 35 psi, while 40 psi is too low for large trucks. The recommended level for the tires of famous sports cars and passenger cars is between 32 -40 psi.
What does a 30 barometric pressure mean? ›
A barometer reading of 30 inches (Hg) is considered normal. Strong high pressure could register as high as 30.70 inches, whereas low pressure associated with a hurricane can dip below 27.30 inches (Hurricane Andrew had a measured surface pressure of 27.23 just before its landfall in Miami Dade County).
How do you read 760 mmHg? ›
And we also know that one mmHg is also equal to 1 torr, which is 1 / 760 of atmospheric pressure (atm) that is 1 atm = 760 mmHg. Moreover, the unit of a millimetre of mercury is considered obsolete and the S.I unit Pascal (Pa; 1 atm = 1,01,325 Pa) ought to be used.
The new guidelines categorise blood pressure as normal (<120/80 mm Hg), pre-hypertension (120/80 to 139/89), stage 1 hypertension (140/90 to 159/99), and stage 2 hypertension (≥160/100 or higher).
What does it mean 15 20 mmHg? ›
mmHg stands for millimeters of mercury and it is a measurement of pressure. In addition to being used on compression socks, it is also used for blood pressure readings. Common levels for compression socks include; 8-15mmHg, 15-20 mmHg, 20-30 mmHg and 30-40 mmHg.
How do u calculate pressure? ›
Pressure and force are related, which means you can calculate one if you know the other by using the physics equation: pressure = force/area. This pressure can either be documented in pounds per square inch, psi, or newtons per square meter (N/m2).
Is 1 bar the same as 1 atm? ›
Definition and conversion
and 1 bar is approximately equal to: 0.986923 atm.
What is the difference between air pressure and atmospheric pressure? ›
Air pressure is the pressure exerted by the air around us while Atmospheric pressure is the pressure exerted by the atmosphere on the earth. Air pressure is measured by tore gauge while atmospheric pressure is measured using mercury barometer.
What causes atmospheric pressure? ›
Air pressure is caused by the weight of the air molecules above. Even tiny air molecules have some weight, and the huge numbers of air molecules that make up the layers of our atmosphere collectively have a great deal of weight, which presses down on whatever is below.
What is the importance of atmospheric pressure? ›
The atmosphere that surrounds Earth has weight and pushes down on anything below it. The weight of air above a given area on Earth's surface is called atmospheric pressure. It is an important factor influencing Earth's weather and climate.
