Even though Yosemite is in sunny California, the weather can be cold year round. In fact, at the highest points in the park, you can find patches of snow throughout the summer. This means that if you are backpacking or at risk of spending the night inthe back country, bring warm clothes as hypothermia can be an issue anytime of the year.
Of course, it can also be very hot. The day I hiked Half Dome, the valley floor temperature was 100 degrees!!
Temperatures in April can range between 32 (freezing) and 66 (relatively pleasant), but the weather can go from gorgeous to dangerous in a matter of an hour - very unpredictable, still, in April.
That being said, I don't want to discourage you from going because it's a beautiful time of the year, waterfalls are flowing strongly as the snow is well into melting (although High Country areas such as Glacier Point and Tuolumne will likely not open until the end of May), and it's a more quiet time than later in the summer when the majority of tourists are in the valley ("a little more civilized by then").
Just come prepared for the vagaries of early Spring in the High Sierras!
All of the hotels and lodges in the valley, as well as Wawona, are open - for high-end luxury you've got the Ahwahnee & the Wawona Hotel; for mid-price there's Yosemite Lodge at the Falls; and for less expensive take a look at Curry Village. You can get unheated tents at Curry Village for around $60, or heated tents for around $120 - both include breakfast. See: http://www.yosemitepark.com/default.aspx for more information.
Hope this helps - I love Yosemite at any time of the year, as long as I've brought along the right clothes! :-)
Fondest memory: Camping in November with our two boys (13 & 8 at the time), sitting by the campfire of an evening, listening to the haunting strains of "Twinkle, Twinkle, Little Star" played by our older son on his flute; then hearing the other campers spontaneously applaud as the last note floated away.
Favorite thing: The Yosemite valley is just like a postcard, wonderfull picture spots, landscapes at every corner. We went in the spring and that's a good time to go, not too hot so that you still can hike. In the late summer and fall the waterfalls and lakes (mirror lake) may be dry, although still beautiful, you miss a great part of this fairy tale place.
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