DENSITY OF AIR - Density is affected not only by temperature and pressure, but also by the amount of water vapor in air. Thus, the standard values are only an approximation. The Ideal Gas Law may be used to calculate density. Once again, the result is only an approximation that is most accurate at low temperature and pressure values. The density of air is the mass per unit volume of atmospheric gases. The density of air or how light it is depends on the temperature and pressure of the air.

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Density of Air

What Is the Density of Air at STP?

How the Density of Air Works

by Anne Marie Helmenstine, Ph.D.

What is the density of air at STP?

In order to answer the question, you need to understand what density is and how STP is defined.

Key Takeaways: Density of Air at STP

·  The value for the density of air at STP (Standard Temperature and Pressure) depends on the definition of STP. The definition of the temperature and pressure isn't actually standard, so the value depends on who you consult.

·  The ISA or International Standard Atmosphere states the density of air is 1.225 kg/m3 at sea level and at 15°C.

·  The IUPAC uses an air density of 1.2754 kg/m3 at 0 °C and 100 kPa for dry air.

·  Density is affected not only by temperature and pressure, but also by the amount of water vapor in air. Thus, the standard values are only an approximation.

·  The Ideal Gas Law may be used to calculate density. Once again, the result is only an approximation that is most accurate at low temperature and pressure values. 

The density of air is the mass per unit volume of atmospheric gases. It is denoted by the Greek letter rho, ρ.

The density of air or how light it is depends on the temperature and pressure of the air. Typically the value given for the density of air is at STP or standard temperature and pressure.

STP is one atmosphere of pressure at 0° C. Since this would be a freezing temperature at sea level, most of the time dry air is less dense than the cited value.

However, air typically contains a lot of water vapor, which would make it more dense than the cited value.

Density of Air Values

The density of dry air is 1.29 grams per liter (0.07967 pounds per cubic foot) at 32° Fahrenheit (0° Celsius) at average sea level barometric pressure (29.92 inchs of mercury or 760 millimeters).

·  At sea level and at 15°C , the density of air is 1.225 kg/m³. This is the value of the ISA or International Standard Atmosphere. In other units, this is 1225.0 g/m³, 0.0023769 slug/(cu ft), or 0.0765 lb/(cu ft).

·  The IUPAC standard of temperature and pressure (0 °C and 100 kPa), uses a dry air density of 1.2754 kg/m³.

·  At 20 °C and 101.325 kPa, the density of dry air is 1.2041 kg/m³.

·  At 70 °F and 14.696 psi, the density of dry air is 0.074887 lbm/ft³.

Affect of Altitude on Density

The density of air decreases as you gain altitude. For example, air is less dense in Denver than in Miami.

The density of air decreases as you increase temperature, providing the volume of the gas is allowed to change.

As an example, air would be expected to be less dense on a hot summer day versus a cold winter day, providing other factors remain the same.

Another example of this would be a hot air balloon rising into a cooler atmosphere.

STP Versus NTP

While STP is standard temperature and pressure, not many measured processes occur when it's freezing.

For ordinary temperatures, another common value is NTP, which stands for normal temperature and pressure. NTP is defined as air at 20^oC (293.15 K, 68^oF) and 1 atm (101.325 kN/m², 101.325 kPa) of pressure.

The average density of air at NTP is 1.204 kg/m³ (0.075 pounds per cubic foot).

Calculate the Density of Air

If you need to calculate the density of dry air, you can apply the ideal gas law. This law expresses density as a function of temperature and pressure.

Like all gas laws, it is an approximation where real gases are concerned, but is very good at low (ordinary) pressures and temperatures. Increasing temperature and pressure adds error to the calculation.

The equation is:

ρ = p / RT

where:

· ρ is the air density in kg/m³

· p is the absolute pressure in Pa

· T is the absolute temperature in K

• R is the specific gas constant for dry air in J/(kg·K) or is 287.058 J/(kg·K)

Anne Marie Helmenstine, Ph.D.

·   Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

·   Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

·   ThoughtCo and About Education chemistry expert since 2001.

·   Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

Anne Marie Helmenstine, Ph.D.​

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/density-of-air-at-stp-607546

PRECIPITATION TYPES - Precipitation or hydrometeor is any particle of water that originates in the atmosphere and falls to the ground. Rain is comprised of liquid water droplets, known as raindrops. Snow is millions of tiny ice crystals that collect and form into flakes, which we know as snowflakes. Graupel, (also known as "snow pellets" or "soft hail") will crush and break apart into granules. If a snowflake partially melts, but then refreezes, you get sleet. Hail is 100% ice, falls only during thunderstorms. Freezing rain is similarly to sleet. They freeze when they strike objects on the ground whose surface temperatures are 32°F or colder.

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Precipitation Types

Rain, Snow, Sleet, and Other Types of Precipitation

By Tiffany Means

Precipitation. Some find it an intimidatingly long word, but it simply means any particle of water (be it liquid or solid) that originates in the atmosphere and falls to the ground.

In meteoroloty, an even fancier term that means the same thing is hydrometeor.

There are only so many forms water can take, and because of this, only a limited number of precipitation types. The main types include:

Rain

Rain is comprised of liquid water droplets, known as raindrops.

Rain is unique because it's one of the few precipitation types that can occur during any season. As long as air temperatures are above freezing (32°F), rain will fall.

Snow

While we think of snow and ice as two different things, snow is actually millions of tiny ice crystals that collect and form into flakes, which we know as snowflakes.

In order for snow to fall outside your window, air temperatures at the ground and well above the surface must stay below freezing (32°F).

It can be slightly above freezing at some pockets and still snow as long as they aren't substantially above the freezing mark and stay above it for very long, or else the snowflakes will melt.

Graupel

If supercooled water droplets freeze onto falling snowflakes, you get what's called "graupel."

When this happens, the snow crystal loses it's identifiable six-sided shape and instead becomes a clump of snow and ice.

Graupel, (also known as "snow pellets" or "soft hail") looks white like snow. If pressed between your fingers, it will usually crush and break apart into granules. When it falls, it bounces like sleet does.

Sleet

If a snowflake partially melts, but then refreezes, you get sleet.

In other words, sleet forms when a thin layer of above-freezing air is sandwiched in-between a deep layer of sub-freezing air high up in the atmosphere and another down at low levels.

In such a scenario, the precipitation starts out as snow, falls into a layer of warmer air at mid-levels and partially melts, reenters subfreezing air, and refreezes while falling in it towards the ground.

Sleet is small and round, which is why it is sometimes referred to as "ice pellets." It makes an unmistakable sound when hitting and bouncing off of the ground and your house.

Hail

Often confused with sleet, is hail, which is 100% ice but is not necessarily a wintertime event. It usually falls only during thunderstorms.

Hail is smooth, rounded (although parts of it can be flat or have spikes), and can be anywhere from pea-sized to as large as a baseball.

Although hail is ice, it is more of a threat to damaging property and vegetation than it is to causing slick travel conditions.

Freezing Rain

Freezing rain forms similarly to sleet, except the icecream sandwich is layer of warm air at mid-levels is deep.

Precipitation either starts out as snow or supercooled raindrops, but becomes all rain in the warm layer.

While freezing air may hug the ground, it is such a thin layer that the raindrops don't have enough time to freeze into sleet before reaching the ground.

Instead, they freeze when they strike objects on the ground whose surface temperatures are 32°F or colder.

If you think the "rain" in freezing rain makes this winter weather event somewhat harmless, think again!

Some of the most disastrous winter storms and ice storms are due primarily to freezing rain.

That's because when freezing rain falls, it covers trees, roadways, and everything else on the ground with a smooth, clear coating of ice or "glaze," which can make for hazardous travel.

Ice accumulations can also weigh down tree branches and power lines, causing damage from downed trees and also widespread power outages.

Tiffany Means is a meteorologist, science writer, and avid cloud watcher/photographer.

Experience

Tiffany has been finding beauty skyward and sharing it with others since the age of 5. By twelve, she knew she wanted to pursue weather professionally—thanks in part to the release of the blockbuster film Twister. Since those days, Tiffany has interned with the domestic and international weather departments at CNN, written monthly climate reports for NOAA’s National Centers for Environmental Prediction, and participated in a number of science outreach events (such as the Science Olympiad Competition). She has personally experienced such weather greats as the Blizzard of 1993, and the floods of Hurricane Francis (2004) and Ivan (2004).

Education

Tiffany holds a bachelor’s degree in Atmospheric Science with a concentration in weather forecasting from the University of North Carolina at Asheville.

Tiffany is a proud member of the American Meteorological Society (AMS).

Tiffany Means

"Weather affects us all. We check it on a daily basis, and talk about it with complete strangers...but it is so much more than 5-day forecasts and small talk! Through my enthusiasm for and expertise in the weather sciences, I hope to spark your curiosity about our atmosphere, create an awareness that will keep you weather ready and safe, and strengthen your environmental responsibility to our atmosphere, water, and earth."

Contact Tiffany: Tiffany can be reached at aboutweatherexpert@gmail.com with questions, comments, reprint requests, or suggestions. You can also connect with her via the social links below.

https://www.thoughtco.com/precipitation-types-3444529

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THE PRINTING PRESS - Johannes Gutenberg and His Revolutionary Printing Press - Johannes Gutenberg was a goldsmith and businessman from the mining town of Mainz in southern Germany. Little is known about his early life. Gutenberg had begun experimenting with printing techniques using metal movable type. Using a modified wine press, Gutenberg created his printing press. The ink was rolled over the raised surfaces of movable handset block letters held within a wooden form and the form was then pressed against a sheet of paper. By 1455 Gutenberg had printed several copies of the Bible, consisting of three volumes of text in Latin,

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Gutenberg And The Printing Press

Johannes Gutenberg and His Revolutionary Printing Press

by Mary Bellis

Books have been around for nearly 3,000 years, but until Johannes Gutenberg invented the printing press in the mid-1400s they were rare and hard to produce.

Text and illustrations were done by hand, a very time-consuming process, and only the wealthy and educated could afford them.

But within a few decades of Gutenberg's innovation, printing presses were operating in England, France, Germany, Holland, Spain, and elsewhere.

More presses meant more (and cheaper) books, allowing literacy to flourish across Europe. 

Books Before Gutenberg

Although historians can't pinpoint when the first book was created, the oldest known book in existence was printed in China in 868 A.D.

"The Diamond Sutra," a copy of a sacred Buddhist text, isn't bound like modern books are; it's a 17-foot-long scroll, printed with wooden blocks.

It was commissioned by a man named Wang Jie to honor his parents, according to an inscription on the scroll, though little else is known about who Wang was or why he had the scroll created.

Today, it is in the collection of the British Museum in London.

By 932 A.D., Chinese printers regularly were using carved wooden blocks to print scrolls.

But these wooden blocks wore out quickly, and a new block had to be carved for each character, word, or image that was used.

The next revolution in printing occurred in 1041 when Chinese printers began using movable type, individual characters made of clay that could be chained together to form words and sentences.

Printing Comes to Europe

By the early 1400s, European metalsmiths also had adopted wood-block printing and engraving.

One of those metalsmiths was Johannes Gutenberg, a goldsmith and businessman from the mining town of Mainz in southern Germany.

Born sometime between 1394 and 1400, little is known about his early life.

What is known is that by 1438, Gutenberg had begun experimenting with printing techniques using metal movable type and had secured funding from a wealthy businessman named Andreas Dritzehn.

It is unclear when Gutenberg began publishing using his metal type, but by 1450 he had made sufficient progress to seek additional funds from another investor, Johannes Fust.

Using a modified wine press, Gutenberg created his printing press.

The ink was rolled over the raised surfaces of movable handset block letters held within a wooden form and the form was then pressed against a sheet of paper.

Gutenberg's Bible

By 1452, Gutenberg entered into a business partnership with Fust in order to continue funding his printing experiments.

Gutenberg continued to refine his printing process and by 1455 had printed several copies of the Bible.

Consisting of three volumes of text in Latin, Gutenberg's Bibles had 42 lines of type per page with color illustrations.

But Gutenberg didn't enjoy his innovation for long. Fust sued him for repayment, something Gutenberg was unable to do, and Fust seized the press as collateral.

Fust continued printing the Bibles, eventually publishing about 200 copies, of which only 22 exist today.

Few details are known about Gutenberg's life after the lawsuit.

According to some historians, Gutenberg continued to work with Fust, while other scholars say Fust drove Gutenberg out of business.

All this is certain is that Gutenberg lived until 1468, supported financially by the archbishop of Mainz, Germany.

Gutenberg's final resting place is unknown, although he is believed to have been laid to rest in Mainz.

Mary Bellis

·   Film producer and director

Experience

Mary Bellis was a former writer for ThoughtCo. She covered inventors for ThoughtCo for 18 years. She was a freelance writer, film producer, and director.  Forbes Best of the Web credited Mary for creating the number one online destination for information about inventors and inventions. Her writing has been reprinted and referenced in numerous educational books and articles. One of her films was a documentary on Alexander Graham Bell, the inventor of the telephone. She also worked as a curator specializing in computer-generated art. 

Education

Mary Bellis held a Master of Fine Arts degree in film and animation from the San Francisco Art Institute.

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/johannes-gutenberg-and-the-printing-press-1991865