The iPhone has a remarkable number of weather app options, and you'll find a ton that suit different needs depending on what you want. That said, one shines brighter than the rest, and Yahoo Weather is one of the best full-featured weather apps in the App Store.
Large photos that make the app easy to read and use.
Detailed weather forecast right from the main screen.
Radar, satellite, heat maps, and more if you're interesting in digging further.
Simple and easy to use so you don't waste time just checking the weather.
Where It Excels
Yahoo Weather's biggest achievement is the fact that it's fast, quickly shows you only the information that really matters (today's weather) and then still manages to provide you with additional information if you want it. It doesn't hurt that the app is also beautiful to look at and easy to use. When you first open it up, you're shown today's forecast. If you want more information, including a five-day forecast, detailed satellite information, wind, and more, you just scroll down. Yahoo Weather taps into Flickr and pulls public domain pictures that were taken during the weather conditions on the day you're looking at it, which makes the app feel a lot more dynamic than most weather apps.
Where It Falls Short
Yahoo Weather's main purpose is simplicity, so it only provides information that matters to the most amount of people. You're not going to get insanely detailed wind reports or personalized weather forecasts based on your exact location. Still, for most people, Yahoo Weather does everything you need in a weather app, and it's free.
There are a ton of excellent weather apps on the iPhone that suit different needs. Want a full doppler radar? Sure, that's doable. Prefer something minimal? You have options. Want one that only notifies you about bad weather? Yep, that exists too. All of these are worth checking out to see if they fit your specific needs.
If Yahoo Weather isn't your thing, the four big free apps, Accuweather, The Weather Channel, WeatherBug, and Weather Underground are fantastic alternatives. All have the same basic feature set as Yahoo Weather, but offer slightly different experiences in terms of interface and coverage. The fact is, different apps might have more accurate forecasts than others for your area, so it's worth playing around with a few to see which works best for you.
Today Weather (99¢) may very well be one of the best weather apps around, but even at only 99¢ it's tough to justify paying for a weather app when free alternatives exist. That said, if you like the features of Accuweather, Weather Underground, or the Weather Channel, but hate the ads, then Today Weather is worth a look.
Perfect Weather ($2.99) is another fantastic weather app that features a ton of information if you're willing to pay for it. Perfect Weather's big strength is that it loads up fast, gives you a five day forecast, and then provides with all the additional details you could possibly need at a glance. This includes a full satellite image, hour-by-hour temperature, and plenty more. It's all fast too, so you can get in and out of the app quickly.
Dark Sky ($3.99) offers an entirely different kind of experience for iPhone. Instead of worrying about forecasting everything, it concentrates on hyperlocal, small time increments. Essentially, it'll let you know if it's going to rain (or snow) within the next hour or so. If you're the type to live more in the moment, Dark Sky is one of the best at short-term forecasts (and alerts).
If you're more a fan of minimalist weather apps, you have a few solid options. Blue is a great little app that shows you the forecast for the next day or so in a handy color coded index. Sun is actually a free webapp designed specifically for the iPhone and uses gestures for control. Weather Neue (free) shows simplified current weather and a four-day forecast in a lovely package. Finally, Partly Cloudy might be the prettiest of the bunch by using the visualization of a clock to show you the weather forecast in a surprisingly intuitive way.