What Is Millibar (mbar)?
A millibar is a unit used to measure pressure. You will often see it in weather charts and forecasts to show how heavy the air is at a place. When the number in millibars is high, the air pressure is high. When the number is low, the air pressure is low.
The millibar is not an official SI unit, but it is still very common in meteorology, which is the study of weather.
Definition
A millibar is a unit of pressure equal to one thousandth of a bar.
- 1 bar is 100,000 pascals (Pa).
- 1 millibar (mbar) is 0.001 bar.
- So 1 millibar = 100 pascals (Pa).
Pressure tells us how much force is pushing on an area. For example, air in the atmosphere pushes on the ground and on our bodies. The millibar gives us a simple way to write this air pressure with friendly numbers.
Standard sea level air pressure is about 1,013.25 millibars. This is close to what you feel on a normal day at the beach.
History / Origin
The millibar comes from the unit called the bar. The word bar comes from a Greek word that means weight or pressure. The bar was created in the early 1900s as part of the cgs system of units, which was an older system based on centimeters, grams, and seconds.
Scientists wanted a pressure unit that made weather numbers easy to read. Using pascals alone gives big numbers, like 101,325 Pa for normal air pressure. If you use millibars instead, the number becomes 1,013.25, which is shorter and easier to handle. So meteorologists started using millibar for weather maps and reports.
Later, the SI system made the pascal (Pa) the main unit for pressure. Even so, the millibar stayed popular in weather science because it matched old data and tools. In many places, the hectopascal (hPa) is now used instead of the millibar, but they have the same value.
Symbol & Abbreviation
The usual symbol for the millibar is:
- mbar or sometimes mb
Key points:
- Always write the unit symbol in lowercase: mbar.
- The name of the unit is written as millibar.
- In modern weather reports, you may see hPa instead of mbar. The number is the same, only the name of the unit changes.
Current Use Around the World
The millibar is still widely used, especially in weather work and related fields.
- Weather forecasting: Weather maps often show lines of equal pressure called isobars. These lines are usually labeled in millibars or hectopascals. Meteorologists watch how millibar values change to track storms and high or low pressure areas.
- Aviation: Pilots use air pressure to set their altimeters, which show how high the aircraft is. Many aviation reports use hectopascals, which are numerically the same as millibars. In some countries, people still speak about pressure in millibars out of habit.
- Ocean and climate science: Some ocean and climate studies use millibars to describe pressure at the sea surface or in the deep ocean.
- General science and engineering: In lab work and industry, the pascal, kilopascal (kPa), and bar are more common. Millibar is used less often outside weather and climate topics.
Many countries now prefer the hectopascal (hPa). Since 1 hPa = 1 mbar, older millibar data and modern hPa data can be compared directly with no change in the number.
Example Conversions
Here are some useful conversion facts:
- 1 mbar = 100 Pa (pascals)
- 1 mbar = 0.1 kPa (kilopascal)
- 1 mbar = 0.001 bar
- 1 mbar ≈ 0.0145 psi (pounds per square inch)
- 1 mbar ≈ 0.750 torr
- 1 mbar = 1 hPa (hectopascal)
Worked examples:
- Convert 1,013 mbar to pascals
1,013 mbar × 100 Pa per mbar = 101,300 Pa.
This is very close to standard sea level pressure. - Convert 1,000 mbar to bar
1,000 mbar × 0.001 bar per mbar = 1 bar. - Convert 1,000 mbar to psi
1,000 mbar × 0.0145 psi per mbar ≈ 14.5 psi. - Convert 980 mbar to kPa
980 mbar × 0.1 kPa per mbar = 98 kPa. - Convert 750 mbar to hPa
Because 1 mbar = 1 hPa, 750 mbar = 750 hPa.
Related Units
Units related to the millibar include:
- Pascal (Pa): The SI unit for pressure. 1 mbar = 100 Pa.
- Hectopascal (hPa): Very common in weather. 1 hPa = 1 mbar.
- Bar: Larger unit. 1 bar = 1,000 mbar = 100,000 Pa.
- Kilopascal (kPa): Used in many sciences and in engineering. 1 kPa = 10 mbar.
- Standard atmosphere (atm): Based on average sea level pressure. 1 atm ≈ 1,013.25 mbar.
- Torr: Often used in vacuum physics. 1 mbar ≈ 0.75 torr.
- Pounds per square inch (psi): Common in the United States for tire pressure and tools. 1 mbar ≈ 0.0145 psi.
FAQs
What is a millibar used for?
A millibar is mainly used to measure air pressure in weather and climate studies. It helps show if the air pressure is high or low over an area.
Is millibar an SI unit?
No. The millibar is not an official SI unit. The SI unit for pressure is the pascal (Pa). However, many scientists still use millibar in meteorology because it is convenient and well known.
What is the difference between millibar and hectopascal?
There is no difference in value. 1 millibar equals 1 hectopascal. The numbers are the same. The only difference is the name and symbol: mbar or mb compared to hPa.
What is normal air pressure in millibars?
Average sea level air pressure is about 1,013 millibars. This is often written more exactly as 1,013.25 millibars, and it is close to 1 atmosphere of pressure.
How many pascals are in a millibar?
There are 100 pascals in 1 millibar. To change millibars to pascals, you multiply the number of millibars by 100.
Why do weather maps use millibars or hPa instead of pascals?
Because the numbers are easier to read. Writing 1,013 mbar or 1,013 hPa is simpler than writing 101,300 Pa for the same air pressure. This makes weather maps clearer.
Do pilots still use millibars?
Pilots often use hectopascals, which are equal to millibars, especially outside the United States. In some places, people still say millibars in speech, even when the written unit is hPa.
How do I quickly estimate millibars from psi?
A quick rule is: 1,000 mbar is about 14.5 psi. So if you know the psi, you can multiply by about 69 to get millibars, because 1 psi ≈ 69 mbar.
