Yes, this can be used outside in the sun. However, don't use it outside in the sun if the surface of your vehicle is super hot.
A quick test to tell if it's okay to apply is that you should be able to comfortably hold your hand against your paint. If you can't do this, then it is way too hot outside and you need to wait for it to cool down a bit by either applying early in the morning or later in the evening.
And just a reminder to only use 1 spray per 2 x 2' section. This product is unlike any on the market as it doesn't contain any unnecessary fillers such as coloring or fragrances, so very little is needed.