Modern weather reporting aboard sailboats is pretty extensive. We have come a long way from relying upon superstitions such as "Red sky at night, sailor's delight, red sky in the morning, sailor's warning." This superstition originates with the fact that dust and small particles can become trapped in the air of high pressure systems which scatters blue light and makes red light more visible. So a red sunset portends high pressure from the west which usually relates to dry pleasant weather. If a red sky is in the East the high pressure has already passed you and a cold front is approaching bringing low pressure and rain.
Today's cruisers have access to weather faxes over radios, satellite communications, cell phone, cell data, or even wifi depending on their location from shore. Today you can even have your own small weather station on your boat. These conveniences make weather reporting and weather routing seem almost mundane. Compared to crossing oceans 200 or even 50 years ago we have it made. One of the things all the former devices rely on though is electricity. It's true electricity is also nearly ubiquitous on cruising boats today, but things always go wrong. An analog back up for something as dangerous as a storm at sea seems like a fairly good investment. Especially given the price tag is less than $150.
The modern, if you can call it modern, aneroid barometer is an excellent weather predictor. It requires no electricity, little space, and almost no maintenance. This little piece of kit will quietly keep track of all the pressure systems moving along your tiny piece of earth. Whether your sailing in perfect conditions or riding the storm out. It will even work if you start to sink because you weren't paying attention to it. Be advised aneroid barometers may be adversely affected while surrounded by millions of gallons of salt water.
An aneroid barometer consists of a chamber enclosing a vacuum which is deformed based on the pressure around it. This chamber is connected to a pointer which indicates the pressure on a dial. Most barometers will also have a manually adjustable dial. This is used to note where the pressure is when you take a reading. You can check the barometer reading, set the indicator and when you re-check the barometer at a later time you can record the change. The change in pressure is what you will use to deduce the approaching weather conditions.
Barometers will read in millibars or inches, and likely both. This refers to height of mercury and harks back to mercury barometers. Aneroid barometers don't use mercury, but the unit of measure is still employed. The standard pressure at sea level is 1013 millibars or 29.92 inches. You can think of the barometric, or atmospheric, pressure exactly like water pressure in the ocean. The further down you descend the more pressure you feel, because you have more water above you working with gravity to create more pressure. The higher you are in the air the less pressure there is working on you. Since sailboats are supposed to stay at sea level, the biggest factor affecting changing pressure is weather. A change of 1mbar/.03 inches an hour is fairly stable and the winds will not be too strong. If you notice more than 2mbar or .06 inches, this should be an indicator of increasing winds approaching. The faster the rate of drop the stronger the approaching winds. Hurricanes are low pressure systems. The lowest pressure hurricane recorded was in 2005 with a pressure of 892mbar. If you see 892 on your barometer calmly call the manufacturer for a repair or grab the VHF and hail the Coast Guard, whichever better suits the outside conditions. Generally speaking an increase in pressure and temp mean fair weather. A decrease in pressure and temp indicate storms. Below is a general cheat sheet.
Hopefully this little bit of info will help you read and avoid any bad weather while you're out on the water. If all else fails, the $150 investment may just save your $100,000 home on the water.
Fair winds and steady pressures.