Barometric pressure, or atmospheric pressure, is a measure of the amount of force with which the atmosphere pushes down on a given area. Low barometric pressure often indicates that stormy weather is on the way. Barometers operate by measuring how much liquid, usually mercury, the atmosphere displaces, or how much it compresses a spring with an electronic sensor. The simplest way to know your barometric pressure is to look it up, but you can also build your own barometer or purchase one. Finding barometric pressure at sea level is the same as finding it anywhere else -- the only difference is that pressure drops as altitude increases.
Look up your location's barometric pressure online. Using the weather website given as a resource, or any similar website, input your Postcode or the name of the nearest town or city. You should see an up-to-date barometric pressure reading.
Purchase a barometer. The quality and price of these instruments can vary widely. A non-fluid-based barometer, which measures the force on a small spring, may be the cheapest option. Buying online is probably best, though you can also find barometers at some large retailers such as Home Depot.
Build your own barometer. Tape a ruler to the inside of a glass or beaker with straight sides, so that the numbers are visible.
Tape about a foot of clear plastic tubing to the ruler in the glass, making sure that the bottom of the tube is a bit above the bottom of the glass.
Fill the glass halfway with water. Add several drops of food colouring so it's easier to see.
Use the tube like a straw, but don't drink the water. Pull it up to around two-thirds the height of the tube, and hold it in place by covering the tube with your tongue.
Using a piece of wet clay, chewing gum, or something similar, seal the tube without letter the water fall back down.
Note the height of the water in the tube. As the air pressure changes, the atmosphere will press more or less on the water in the beaker, forcing the column of water higher or lower.
Observe your barometer over the course of a few weeks, recording your readings every day. The device will take about a day to acclimate to changes in pressure. The results using your homemade instrument will not be as precise as those obtained with commercial barometers, but it provides an excellent demonstration of the principles involved.