Weather apps, especially on the iPhone or iPad, are usually great looking and informative when it comes to the conditions and forecast. However, for some kinds of data, it requires just a little extra bit of extra knowledge to understand what the app is trying to tell you. This applies in the cases of atmospheric pressure and sunrise/sunset times.
1. Atmospheric Pressure
There are two numbers of interest for atmospheric pressure. Most of the time, you'll see the pressure adjusted to sea level. That's the number you'll see in your local TV weather forecasts and in many of the Weather apps you might have on your iPhone.
[Note: not all the screen shots below were taken at the same time of day, so there will be minor variations in the actual numbers.]
For example, here's what one might see in Apple's iOS weather app. Note that normal sea level atmospheric pressure is 29.92 inches of Mercury (Hg) or 1013 millibars, so anything above or below that number is above or below normal pressure.
The reason your local atmospheric pressure is corrected to sea level, which is likely higher than your actual local pressure, is explained by the National Weather Service.
The sea level pressure is the atmospheric pressure at sea level at a given location. When observed at a reporting station that is not at sea level (nearly all stations), it is a correction of the station pressure to sea level ... Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.
That sea level number is also important to pilots so they can set their altimeter correctly.
Most of the weather apps you'll use show the sea level corrected pressure, but if you want to know your local pressure, called the station pressure, you can either use a weather app that reports that number or an app that accesses the barometer right in your iPhone. (Starting with the iPhone 6, Apple added a real barometer.) Also see this article by our Kelly Guimont on crowdsourcing local pressures.
The Weather Pro app shows both pressures. The top is the sea level pressure and the bottom is the local pressure.
A nifty iOS app simply called Barometer also shows both the sea level corrected pressure and the local pressure in units of your choice, all with a great GUI.
Yahoo Weather, on the other hand, shows the local (station) atmospheric pressure to the right of the windmills.
Over the years, I have noticed that developers exhibit varying levels of expertise in both the sources they use and the interpretation of what they get from the source. If you're ever in doubt about your local sea level pressure, an aviation source is generally a reliable way to go. I often use an app called Aero Weather. Parts of it look like this:
2. Sunrise and Sunset
The reported times of sunrise and sunset are usually looked up from tables or sources that compute it. The calculation for a specific location can be complex and is dependent on the location's latitude. Also, atmospheric refraction comes into play and can change the time by several minutes. In Denver, mountains change the time of sunset by a minute or so.
If you are significantly east or west of the location where the sunrise and sunset times are reported from, the time may also be off by several minutes.
Some apps try to be helpful in terms of twilight times. For example, the Weather Underground app shows a term called "first light" and "last light." But these terms don't have a specific technical meaning, and it's not clear in the app how the developer defined them. [UPDATE: the developer contacted me and pointed to the app help which defines First and Last Light as Civil Twilight. We're still exploring the different times compared to other apps.]
The terms that are technically defined are Civil, Nautical and Astronomical sunrise and sunset. They are defined by the U.S. Naval Observatory as follows (excerpted here).
Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished....
Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible....
Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening, scattered light from the Sun is less than that from starlight and other natural sources....
In some locations, civil twilight is used as a standard for when you're driving and must turn your car lights on and off.
A truly great app, one that I've reviewed, that provides all these times is called Nav Clock. The developer (Dale Rask) has an expert understanding of not only this data, but all the data he presents in his Nav Clock app, and I've never found any occasion to question his numbers. Here's the horizontal mode:
Into the Sunset
Armed with this with this technical background, you'll now be in a very good position to download the right apps and assess what you see in some of these weather apps that present atmospheric pressure, sunrise and sunset times.