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Thinner robes? Seriously the broad brimmed hat is great for sunny weather or wet. If the robes are heavy wool they'll be almost water proof and plenty warm, thinner wool for cool spring and autumn weather and linsey-woolsey, a, usually, 50-50 blend of wool and the flax fibre used to make linen for the summer months in cooler climates, cotton, linen or silk according to local availability where it gets really hot in the summer. The Bedouin of the Sahara traditionally wear pale colored linen robes and head scarves to help them survive the desert heat. By sticking to a clothing pattern and varying only the materials wizards won't find themselves unexpectedly encumbered by a change in the weather.

Lots of desert people wear long robes; if the sun is punishing enough, you'll want to cover up even if you are quite dark skinned. Loose fitting airy robes help keep you cool in those sorts of conditions, and by way of a bonus stop you getting too cold too quick when the sun sets, as in deserts the nights are often cold.

I won't try and ramble on here; images are the only really interesting things, and a) I'm finding it hard to locate really nice images worth showing you (because I'm a bit rubbish at image searching) and b) really, the photos will show a bunch of people in robes in interesting places. You don't really need to see them to know that yes, robes can and do work in very hot desert areas, and have been used for that exact purpose for well over a thousand years.

"Freedom of movement for casting" is ill defined, but constricting clothing is absolutely not what you want in very hot weather, so that's ok.

If your wizards want robes, then they can have robes. No problem

Tuareg man. I think he's wearing cotton; certainly it appears to be quite a light fabric. More info on Tuareg clothing, plus links.

(as an unrelated footnote, Tuaregs are islamic but it is the men who wear the veils)

Bedouin Sheikh. Note the black robes; when the sun is hot enough, the colour of your clothing doesn't really matter that much. White won't save you. Traditional bedouin robes were woolen, too.

And who could forget the classic strange bearded, robed guy with magical powers who lived in a desert?

(it is little bit pointless for Jedi to wear those sort of robes if they live in non-desert environments, but somewhere along the way Lucas et al confused practical clothing with a uniform)

For use outside of desert climates, robes become increasingly less attractive as the weather gets wetter. A nice woolen cloak does an excellent job of keeping the rain off, but once you get inside you'll want to take it off. You will find some robes and robe-like garments in warm, wet locations however:

(Source)

Note the off-the-shoulder motif; not so good for stopping the sun, but not as stifling in hot, high-humidity conditions. You may also note how this style is often associated with people with strange and mystical powers...

The Gandalfesque wizard is a European wizard.

Robes make sense if you are in England. If you want a wizard from a hot climate, take a look at how people in that climate dressed historically when they wanted to put on a show and impress. These Javanese performers are not wizards (I don't think) but their outfits would be great for a hot climate wizard: lots of skin, lots of ornaments, creepy masks.

You can find wizards from anywhere. Googling around found Tahitians, Khmer, Indians; use search term sorcerer and you do better with African and pre-contact Americas. There is no shortage of amazing stuff for you to copy for your fiction.

The idea that a wizard wears a long robe is, surprisingly, a pretty old one. It's not a modern fantasy stereotype.

At least since the time of a Roman toga there was an implicit connection between long loose clothing and the lack of necessity to fight or work for a living.

In the 10th century the whole Europe started adopting Byzantine fashions, and we see kings in loose long robes on the miniatures, while fighting men and peasants wear shorter tunics. The Church had also adopted Byzantine fashions, so both Catholic and Orthodox priests and monks even now wear clothes that didn't change much from 10th century.

Free townspeople of high standing and scholars wore long robes in 15th century too.

In 17-19th century all the manners of loose silk robes of Eastern origin were worn as a part of domestic clothing by gentlemen of leisure, philosophers and writers. This banyan is an ancestor of a modern dressing gown and a bathrobe.

So we can conclude that the idea that an older man who doesn't need to fight or engage in other physical activity, but rather spends his time reading, writing, studying, teaching and discussing with other similar gentlemen, will wear long loose clothes, is entrenched enough in European culture that it doesn't need a reality check.

As for the materials, there are several ways to approach it. If we position medieval and not modern approach to sweat and body odor, you can continue wearing wool in a lot of environments. Ancient Greek and Roman clothes were mainly wool - and it gets pretty hot in Mediterranean. As other answers write, Bedouin clothes were traditionally wool.

If you are not afraid of your own sweat, wool is pretty comfortable when hot - maybe because it's a material more or less similar to human hair, being made from animal hair :)

Other materials like silk and cotton are already covered in other answers.

About the only environment you don't won't to wear wool in is warm and hot tropical. Buddhist monks still manage to wear long loose clothes there, even though it's rather a wraparound thingie and not a tailored robe.

UPD: as far as carrying the gear is concerned, in a medieval-ish world a wizard shouldn't carry all his gear with books and potions on his person. Just like a knight wouldn't always wear his armor and weapons. A knight would have a packhorse and a squire. A high-ranking wizard will have a mule and an apprentice to take care of that for him.

Using popular cultural as our guide, we only need to look at Disney for the wide range of sartorial options available to the modern magic user.

The traditional robe and pointy hats are suitable for everyday wear and official events too

But, wizards are nothing if not adaptable and comfortable in any manner of dress. Here we have Merlin strutting in the latest Bermuda fashions -- ensemble by Brooks Brothers of East Hollywood.

Even a wizard as notable as Gandalf with rock the silk robe, appropriate for a night on the town or hot Arabian nights

You could look at the fashions of South American, African, SE Asian, and Pacific islander witch doctors, medicine men, shamans, sangomas, and whatever other names have been given to people claiming to use supernatural powers. Some of those are barely more than a loincloth. (With other paraphernalia added, often distinguishing their occupation: headdresses, bracelets, beads, staffs, etc. etc.)

Comfort isn't always the most important thing, looking like you are what you are is also very important.

Think of medieval royalty and rich people, their outfits didn't really change summer or winter, as the point of the clothes was to be RICH, and to let every single person know they are RICH

Is this a summer of winter outfit for for the queen, short answer both