So we have to look elsewhere for the problem. The photo lab could be the culprit, but because you said the graininess seemed evident only on certain types of pictures, I tend to discount that. Bad developing would render most or all of the photos grainy. You drop an important clue when you say the problem-prone photos were taken of sky and clouds, or on glaciers. My guess is that the camera was under-exposing the film. The resultant "thin" negative will tend to render a grainier looking print than one with proper exposure.
How to solve this problem? Two ways. One, the Rebel has a control that lets you skew the exposure to either the over- or under- side. You can adjust that control to over-expose by one F-stop, and that should do it. Alternatively, most cameras such as the Rebel allow you to "lock" the exposure on one subject, then hold that exposure setting when focusing on another. Figure out how to do that on the Rebel (check the manual it may be as easy as slightly depressing the shutter release), then, when next confronting a very bright scene, set your exposure with a reading off your bare hand. Then aim the camera at the snowy/sunny scene and click away. This should actually be a bit more accurate than arbitrarily over-exposing everything.
Of course, keep in mind that during the summer you probably can get by just fine with 100-speed film. The lower the number, the more light the film needs - but you also get finer grain and better sharpness.