An instrument that measures the weight of the air above it. The direction and speed of the change matters more than the reading itself for forecasting.
ForecastingWhat is a barometer?
A barometer measures atmospheric pressure — the weight of the entire column of air stacked above the instrument. At sea level that column presses down with a force of about 1013.25 hPa, the value meteorologists call standard pressure. One hectopascal is identical to one millibar, so older Indian and Pakistani instruments marked in millibars read exactly the same numbers as modern ones.
The barometer is the oldest genuinely predictive weather instrument. A thermometer tells you what is happening now. A barometer tells you what the atmosphere is about to do, because pressure changes before the sky does. Evangelista Torricelli built the first one in 1643, and the basic reading technique has not changed since: note the value, note it again three hours later, and pay attention to the difference.
How a barometer works
Mercury barometers are the original design. A glass tube sealed at one end is filled with mercury and inverted into an open reservoir. The air pressing on the reservoir supports a column roughly 760 mm tall at sea level, and the column rises and falls with the pressure. Mercury barometers are accurate and were the standard at official observatories for a century, but they are fragile, heavy, and mercury is now tightly restricted, so they have been largely retired from routine use.
Aneroid barometers — the word means “without liquid” — use a small sealed metal capsule with most of the air pumped out. The capsule flexes as outside pressure changes, and a lever train amplifies that tiny movement into the needle on a dial. This is the dial barometer hanging in a wall bracket. It needs occasional recalibration but survives being carried around, which is why it went into ships, aircraft and field stations.
MEMS sensors are the modern answer. A microelectromechanical pressure chip a few millimetres across sits inside most current smartphones, smartwatches and consumer weather stations. It is the same physics as the aneroid, shrunk onto silicon. Phone barometers drift and are affected by being in a pocket or a sealed car, but they are good enough to log a pressure trend over a few hours.
Reading pressure tendency
The single most useful output of a barometer is tendency: how much the pressure has changed, and in which direction, over the last three hours. Official synoptic reports have carried a coded three-hour tendency for well over a century.
The classic interpretation holds in most of the world:
- Falling pressure means a low-pressure system, a trough or a front is approaching. Faster fall, sooner and rougher the weather.
- Rising pressure means a high or a ridge is building in — clearing skies, lighter wind, and in winter, colder mornings.
- Steady pressure means the situation is not changing much over the next several hours.
A steep, sustained fall over a few hours is the serious signal. It is what a ship’s officer or a coastal fisherman watches for, because it indicates an intensifying low nearby.
Barometers in South Asia
Three regional facts change how a barometer behaves here compared with Europe or North America.
The monsoon is a standing low, not a passing front. Once the monsoon is established, South Asia sits under a persistent trough of low pressure running across the Indo-Gangetic plain. Rain arrives day after day with very little pressure movement. A barometer that reads “rain” all July is not broken; it is describing the season, not tomorrow. The classical falling-barometer rule is simply not diagnostic during the core monsoon months.
The cyclone seasons are when it earns its keep. In the pre-monsoon (roughly April to June) and post-monsoon (roughly October to December) periods, systems over the Bay of Bengal and Arabian Sea intensify rapidly and the pressure gradient becomes sharp. Odisha, West Bengal, coastal Andhra Pradesh, Tamil Nadu, Bangladesh, Gujarat and the Sindh and Balochistan coasts all feel this. The IMD grades these systems by sustained wind speed rather than by pressure — a system becomes a cyclonic storm at 34 knots — but a deep and rapid fall in local pressure is the clearest instrument-based warning an individual can get.
The tropical daily pressure wave is large here. Atmospheric pressure has a natural twice-daily rhythm, with maxima near 10 a.m. and 10 p.m. local time and minima near 4 a.m. and 4 p.m. This semidiurnal tide is strongest in the tropics and weak at high latitudes. In Chennai, Colombo or Karachi it can be big enough to hide a genuine weather-driven trend, which is why afternoon readings should always be compared with the previous afternoon rather than with the morning.
Altitude dominates everything. Pressure drops by roughly 1 hPa for every 8 metres of ascent near the surface. Shimla, Srinagar, Gangtok, Skardu and Leh therefore record station pressures far below sea-level values permanently. Forecast charts get around this by converting every reading to mean sea level pressure, a standardised correction that makes a hill station and a port comparable on the same map.
What a barometer cannot tell you
A single barometer is one point of data in a field that spans thousands of kilometres. It cannot tell you whether the falling pressure is a small thundery low passing forty kilometres away or a cyclone tracking towards your coast. It cannot tell you how much rain will fall. And during the monsoon, as above, it may barely move on the wettest week of the year.
Modern forecasting uses pressure as one input among many — surface observations, radiosondes, satellite imagery, radar and numerical models. Your own barometer is best used as a fast, local confirmation of a trend you have already read about, and as a reason to take a coastal warning seriously when the needle drops hard.
Frequently asked questions
What is a normal barometer reading? Around 1013 hPa at sea level. Anything from roughly 1000 to 1025 hPa is unremarkable at the surface in South Asia; the number matters far less than whether it is rising or falling.
Does falling pressure always mean rain? No. It means an approaching disturbance, which often but not always brings rain. In the settled monsoon, heavy rain regularly falls with pressure almost unchanged.
Is my phone’s barometer accurate? The sensor is reasonably precise but poorly referenced, so the absolute number may be off. It is still useful for tracking change over a few hours, provided the phone is not sealed in a car or pressurised cabin.
Why do weather maps use sea level pressure instead of the actual reading? Because raw station pressure mostly reflects altitude. Correcting every station to sea level removes the terrain signal and leaves the weather signal, so a chart covering both Karachi and Skardu is readable.
Where can I see current pressure for my city? Mausam Online lists surface pressure on every city page, including Chennai, Visakhapatnam, Kolkata, Karachi, Dhaka and Shimla.