Kilimanjaro can get pretty cold, despite its location near the equator. In high camps you may see temps in the 20s or even a little chillier. The summit can see zero or less. So you need to be prepared for that.
Monkey Man Jacket
I think you're close to having what you need, but you might want to beef up your cold-weather kit. If I were heading to Kili, I'd start with a layer of mid-weight long underwear, such as REI's Midweight MTS crew top and bottom ($30 each; www.rei.com). Next, a layer of good fleece, around 200 or 300 weight, such as Mountain Hardwear's Monkey Man Jacket ($115; www.mountainhardwear.com). And a good waterproof-breathable shelltop and bottom, which you indicate you already have. Starting from scratch, I'd probably go with L.L. Bean's Mountain Guide Gore-Tex XCR Jacket ($279; www.llbean.com), then Marmot's PreCip Pants ($70; www.marmot.com), although the pants might end up being "sacrificial" given the sometimes rough conditions on Kilimanjaro. But they'll work fine.
I am inclined to suggest you pack along a light down-like piece. It doesn't have to be terribly exoticPatagonia's Micro Puff Pullover ($145; www.patagonia.com) would be fine. But it'll add an excellent layer in case it's both cold and windy when you summit. It's also a useful ski-layering piece for when you're back home.
Of course, a good selection of warm gloves and hats also is essential.
Still, cold aside, the main thing will be your reaction to altitude. Don't rush the trip and take time to acclimate. Good luck!
Want to know what American mountain tamer Ed Viesturs plans next? Get yourself a copy of Outside's August issue, on newsstands now.
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