Low pressure means that the air is lighter than its surrounding masses. This light air rises causing the water vapour in the air to condense, thus forming clouds. Hope this answers the question.

Dear Sakiba Nabi,

Thanks for clearing my doubt.

Dear Tahmidul Azom Sany

Please kindly consider the explanation provided by the UCAR Center for Science Education:

https://scied.ucar.edu/learning-zone/how-weather-works/highs-and-lows-air-pressure

This is due to relationship between altitude and temperature. High temperature makes air light and hence high pressure.

Thanks Janusz Pudykiewicz and Kiprotich Kiptum. I know this is very basic knowledge but I was confused about some assumptions that were made. Now I'm cleared.

Dear Tahmidul Azom Sany this is certainly a technical question of fundamental interest and importance. In addition to the expert answers which have already been provided, please also read the Wikipedia article on "Cloud chamber" at https://en.wikipedia.org/wiki/Cloud_chamber.

The cloud chamber (https://www.nuledo.com/en/) is a device in which the path of radioactive particles can be visualized. It's also based in part on the principles outlined in your question.

There are a number of instructive YouTube videos available which show cloud chamber experiments, such as this one:

Cloud Chamber

https://www.youtube.com/watch?v=e3fi6uyyrEs

Dear Tahmidul Azom Sany and Readers,

Please consult also the following entry about cloud physics:

https://en.wikipedia.org/wiki/Cloud_physics

“This process occurs when one or more of three possible lifting agents—cyclonic/frontal, convective, or orographic—causes air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling.”

The link between large scale dynamics of a geophysical fluid and condensation process is nicely explained in the above Wikipedia article and the information presented is a good starting point for the further studies.

Dear Tahmidul Azom Sany , barometers measure the air pressure acting on them, that pressure changes as you have experimentally observed (high pressures for "fair-weather" and low pressures for cloudy/rainy days. Atmospheric pressure is the weight of the atmosphere over an area. As most of us have seen in pictures -and astronauts and recently some people in commercial rockets- the atmosphere thickness is more or less uniform. So what makes the difference in pressure? The answer is the composition of it. When you have a sunny day, all the atmosphere column avobe your area (the barometer area) is mainly air:

https://en.wikipedia.org/wiki/Atmosphere_of_Earth

"By volume, dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere."

When the day is cloudy or rainy we have the same atmosphere column over our barometer, but this time the water proportion is higher, the water displaces some N2 and O2 from the column.

Now a bit of maths and chem. Area is a constant in your experiment, so we only must worry about weight of the atmosphere column (column´s heigth is also constant)

High pressure (sunny day):

Atmospher composition: mainly N2 (78%), and O2 (21%), we know the molecular weight of each gas (N2=14, O2=32), and therefore we can get the weight of each gas multiplying by its weight fraction (N2=14x78, O2=32x21)and hence the weight of the atmosphere column by adding the weights of each gas).

Low pressure (cloudy day):

Now we have H2O in a higher %, so the fraction of N2 and O2 will be lower, because some % of dry air is replaced by H2O, but water molecular weight is 18, that is a bit more than N2, but quite less than O2, so the weight of the atmosphere column would be lighter when it contains water (clouds), the more lighter (less pressure) it will be as the more water it contains.

Hope this helps.

Dear Manuel Gómez and Readers,

In order to answer the original question properly we should go back to the roots of the problem which is in the Norwegian Cyclone Model and its numerous extensions.

Please kindly consult the following elementary information:

https://www.inscc.utah.edu/~steenburgh/classes/5110/lecture_notes/norwegian_cyclone_model.pdf

https://journals.ametsoc.org/view/journals/bams/aop/bamsD200078/bamsD200078.xml

https://wasatchweatherweenies.blogspot.com/2011/04/updating-norwegian-cyclone-model.html

https://en.wikipedia.org/wiki/Low-pressure_area

I hope that the information from the sites above can provide a satisfactory answer to the question posed by Tahmidul Azom Sany

The further clarification is provided in the outstanding paper by Max Margules; this paper was recently translated by one of the members of the RG community and it is available at:

Article The last paper "On the theory of storm" (Zur Sturmtheorie) p...

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. https://www.americangeosciences.org/education/k5geosource/content/weather/why-is-the-weather-different-in-high-and-low-pressure-areas

High in the atmosphere, air pressure decreases. ... A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses, forming clouds and often precipitation. https://scied.ucar.edu/learning-zone/how-weather-works/highs-and-lows-air-pressure

Kindly read on this paper.

https://www.ametsoc.org/ams/index.cfm/education-careers/education-program/k-12-teachers/project-atmosphere/training-opportunities/project-atmosphere-peer-led-training/project-atmosphere-peer-training-resources/clouds-activity/

Low pressure is caused by a hot air mass being displaced by moving cold air. This hot air is less dense and rises, causing a drop in pressure. Hot air has a greater capacity to hold moisture, so when it rises and comes into contact with cold air it cools and the moisture particles join together to form clouds or even precipitate. This happens because as the air cools its volume decreases and the water particles begin to coalesce. Intuitively you can see in winter that on the window glass the water condenses in winter as the warm air in your room cools down. You will also see that before a big storm, there is a lot of heat and humidity in the air, which abruptly drops its temperature before it rains.

Low barometric pressures only brings rain clouds, if there is moisture upstream AND if the dew point is correct--see the end of https://www.ecoseeds.com/cool.html -- A lot of times in the Middle East and North Africa, instead of rain clouds--what you get instead with low pressure--is dust storms, which can dramatically decrease the chance of rain clouds from forming.

However, by planting back the native vegetation in barren desert areas, you can do two things--you cut the amount of atmospheric dust and also correct the dew point to allow rain clouds to form. The vegetation in the mountains above Salalah, Oman is a good example. The other thing you can get with low pressure and the dew point is off, is a massive Heat Index, which will make certain places uninhabitable by people without air conditioning in the future.

A wet-bulb temperature of 35 °C, or around 95 °F, is pretty much the absolute limit of human tolerance, says Zach Schlader, a physiologist at Indiana University Bloomington. Above that, your body won't be able to lose heat to the environment efficiently enough to maintain its core temperature.

Dear Craig Carlton Dremann

For the explanation of the mechanism causing a dust storm please kindly consider the following

Article The Formation and Behaviour of a large Saharan Haboob

Yes, dust storms probably form by different weather conditions in different desert areas, and my studies have been mainly focused on the Arabian Peninsula and Pakistan.

The Sahara could have very different weather conditions, than Arabia, to get the dust storms started. But the basics are the same for what happens at the earth's surface--in both cases--grazing of domesticated animals or plowing for dry-farmed fields, has stripped off the native plant cover, which then allows the dust to get airborne.

Once airborne, that atmospheric dust even at a very low level of only 40 micrograms per cubic meter, can stop a Category-5 cyclone like GONU that you can see at https://www.ecoseeds.com/GONU3.html

Dear Craig Carlton Dremann

The mechanisms responsible for the dust storm events in the region mentioned in your comment are discussed extensively in the following paper

Article Haboob dust storms of the southern Arabian Peninsula

As you can see, the Authors mention the synoptic scale shamaal winds as well as Haboob dust storms. Despite of some slight differences between the Saharan and Arabian Peninsula Haboob cases, we still can identify a single common mechanism that is related to a cold density current from convective clouds.

locations to the south and east of a low center (a region known as the "warm sector"), moist weather. Because winds flow counterclockwise around a low in the Northern Hemisphere, winds in the warm sector are generally from the south, which results in milder air being fed into the system. Showery cloudiness, precipitation and thunderstorms also occur here, but they are specifically at the boundary of a warm sector and the leading edge of the cold front.

Locations behind or to the west of a low center will see cold, dry weather. This is because the counterclockwise flow of winds around the low are from a northerly direction, suggesting colder temperatures. It's also typical to see conditions clearing here as the colder, denser air is more stable.

As the air leaves the high-pressure area, the remaining air sinks slowly downward to take its place. That makes clouds and precipitation scarce because clouds depend on rising air for condensation. High-pressure areas usually are areas of fair, settled weather. This causes air to rise, producing clouds and condensation.

A rapid drop in atmospheric pressure means that a low-pressure system is arriving. Low pressure means that there isn't enough force, or pressure, to push clouds or storms away. Low-pressure systems are associated with cloudy, rainy, or windy weather. A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses, forming clouds and often precipitation. Increasing high pressure corresponds with clear, sunny weather. Decreasing pressure corresponds with cloudy, rainy weather. Falling pressure typically indicates increasing clouds, possibly bringing precipitation, while rising pressure means clearing skies with improving weather conditions. In a depression air is rising and blows in an anticlockwise direction around the low. As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. 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. Warm air is lifted and the water vapor within it condenses, forming heavy clouds and often, precipitation. Therefore, low-pressure systems are generally associated with cloudy and rainy weather because as warm air rises, it cools in the atmosphere, creating condensation.