Hat
Most of the time you probably won’t need a hat, but they are useful in certain situations.
Figures vary from about 7% to 75%, but whichever it is, a lot of body heat is lost through your head. For cold weather, a good, simple solution to this is to carry a lightweight, waterproof, silk, 
woollen or wool mix cap/hat
For the sun, choose a hat that is light and soft instead of a stiff one so it packs well. It should have a wide brim which will protect your face, ears, and neck.
Make sure you have a chin strap, if not, make one out of a shoelace to stop your hat blowing away (as it inevitably will).
Baseball caps provide some protection from the sun and cold and are very portable, but they are not ideal as they leave the back of your neck exposed in the sun.
My hat of choice is the Tilley Airflo Hat (shown right). This is such a good hat, everyone should buy one.
It blocks 98% of UVA and UVB rays, has a UPF 50 rating and mesh in the crown for ventilation when it gets really hot. The brim is wide and stiff which keeps both the sun and the rain off your face and neck.
It floats in water, repels rain and mildew, and has a secret velcro pocket where you can store id’s and spare cash, keys, or credit cards.
It has a cord that wraps under your chin and another at the back of your head for when it is windy. It is machine washable, and is guaranteed for life by Tilley against shrinkage or wear.
Jacket/coat
A thin coat or jacket on top of multiple layers is better than a bulky, thick one. Down jackets are best if it is really cold.
Rain jacket
In moderate climates a rain jacket will be better in the rain than an umbrella as it will keep both your hands free.
A rain jacket will also make a good wind-breaker or light jacket for cooler weather. One with a hood is even better.
In hot climates, an umbrella may work out better in the rain (an extra jacket may be too hot) and has the added advantage of being able to be used as a parasol or sunshade. A rain jacket can also be used as a ground cloth for sitting on the (usually, wet) ground.
Try to find a cover for your backpack too. You can buy these custom-made but a simple black plastic bag will do just as well.
Poncho
A waterproof poncho can be used as a rain coat, a backpack cover, a ground sheet or picnic blanket, and even a tarpaulin for sleeping under in an emergency. You can also spread it over your mattress to protect against bedbugs. They can be bought very cheaply in most cities in the world.
I use an Ultra-Sil Tarp-Poncho.
Rain trousers
In wet climates these can be useful (essential when mountaineering). They keeping your trousers dry and can keep you warm when it is cold.
Not really recommended in hot climates, but can be used as trousers if worn alone. And you never know when you may need extra protection on chilly nights or something to wear when on a boat or when rafting.
Gloves
Not necessary in hot countries, and unless they are thin synthetics, they can be bulky to carry around with you, especially if you never use them. And anyway, they can be bought easily and cheaply if you need them in cold climates.
Try to find fast-drying ones, such as pile and polypropylene, if you decide to buy some.
