Scottish summers can be all things, but as Billy Connolly said "There's no such thing as bad weather, only the wrong clothes".
It is entirely possible to experience all weathers in the Highlands during the summer months, I have been soaked, snowed on, battered and baked, all in the same day. The good news is that you shouldn't have to take everything with you on the hill. Careful selection of equipment appropriate to your needs on the day means you need not stagger around under the weight of an unwieldy rucksack. Here follows a list of kit and clothing typical to summer mountaineering in the UK.
For a more detailed look at the lightweight philosophy here is an article from the archive:
This is typically what I'd be wearing:
Thermal base layer
Wool, synthetic or a mixture. Cotton as a last resort as it tends not to wick away sweat and takes a long time to dry.
Walking or mountaineering socks, synthetic or wool mixture, I like socks made by Smartwool, calf or knee length.
Lightweight walking trousers, trekking style, stretchy or soft shell. The latter will be a bit more abrasion resistant for scrambling and climbing. 'Technical' mountain trousers may well have articulated knees and a diamond crotch, meaning that when you do the splits or take a high step the waist band doesn't slip down and the ankles don't ride up to half mast. Some people like shorts, but I would say over trousers are then essential if it gets wet and windy.
Light weight fleece, power-stretch top, something with a hood is very handy for a quick warm up.
Endless discussion and debate is possible here, you could quite feasably wear, trekking boots, B1s, approach shoes or mountain trainers. It very much depends on what you're used to and what you intend doing in them. Often an actual conversation is worthwhile here.
A bit of personal choice here, they can be handy in very boggy terrain, but I tend to find them quite warm, you can shed a lot of excess heat through the skin of lower legs and forearms.
Map & compass*
The Silva Expedition 4 compass is recommended, it has all the features you will need including a variety of roamer scales. Have a map of relevant areas in a small waterproof bag. There are various mountain maps available for any given area of the UK, my article here gives a basic explanation.
* Note that I have included this in what I'm wearing. I highly recommend having your map and compass made small and kept in a jacket or trouser pocket. This is where it will be useful, as opposed to inside a rucksack where it will probably stay! You can trim maps, take the card backing off or print them from online software so that you don't have a huge folded wad of paper bursting out of a pocket or a sail flapping wildly around your neck!
Hat & gloves
Even in summer hat and gloves are essential, even if they never come out of your bag they could be considered emergency equipment. Definitely a concession to summer temperatures can be made, there's no need for winter or ski gloves, wind proof fleece is fine for most things.
This will need to be in the region of 25 - 40 litres capacity. Simple, clean designs are good for climbing, for hillwalking and multi day expedtitions a bag which has compartments and pockets make it easier to organise and access things. Rather than using a big rucksack liner I prefer to have stuff in individual dry-bags, with one big liner snow or rain will get inside every time you need to get something. Here is a review of the Osprey Mutant 28, which although not that big is very clever so you can do a lot with it.
The bag should be packed with gear least likely to be used nearer the bottom. Having said that, for the climbers especially, is a good idea to have the heavy kit near the lumbar area of your sack. If it's high up in the bag you'll very top heavy.
1st aid kit
This doesn't need to be massive: some tape, plasters, Compeed blister kit, a large wound dressing, a tick remover, energy gel and some painkillers. Anything serious enough to need more than this and you're going to have rescue help coming to you in short order.
Emergency group shelter
An essential piece of safety equipment, inexpensive, small, packable and a potential life saver. A bit like a tent fly without the poles, it provides an instant habitat allowing you to get out of the weather.
The one I use is from Lifesystems. It is vacuum packed, foil backed plastic.
Even in summer this is pretty much essential. Down or synthetics are very good but synthetic is more effective than down when damp. This garment is handy for chilly conditions, but combined with a survival blanket and a group shelter could be a lifesaver during an unplanned night out.
A beanie type that fits under a helmet if mountaineering, or perhaps in sunny weather a hat with a peak which shelters you from the glare of the sun but also driving rain.
Waterproof jacket and trousers
Goretex, Paramo or similar. Lightweight waterproofs are great in that they take up little space in your rucksack, but bear in mind they tear easily in contact with rock, so think about what sort of activity you are likely to be doing and equip accordingly.
Of course summer can be wet and cold, it can even snow on Scottish mountain tops! Powerstretch or fleece gloves work quite well, of course they'll get wet but they are easily wrung out and will still give some protection from the chill when damp. Gloves with leather reinforcements are good for climbing and handling ropes.
Think carbohydrates like bread, pasta, rice etc, plus protein rich food like lean meat, cheese, tuna, nuts, humous, quorn. Sugary foods like cake, flapjack and perhaps energy gels are useful for a short sharp pick-me-up. Bear in mind that the greater the refined sugar content the more likely to experience a 'sugar crash' shortly after consumption. I would suggest dried fruit as an alternative.
Scotland is endowed with a lot of water! This is fine, but I do avoid drinking from streams near livestock or human habitation. Despite this you'll want to have a good supply if your going to spend much of the day on the tops or along ridge lines because most of the drainage channels and collecting features will be below you. On a hot day I could easily go through two litres which is heavy but essential to stay hydrated. A great bit of kit is a water bladder with a tube as it means you can drink sips on the go, the net result of this I have found to be less fatigue and muscle soreness. I would recommend a set up that features a bite valve with an on/off so you don't inadvertently leak water all over yourself!
(LED) head torch essential. I used to take spare batteries, but have you ever tried fiddling with a battery compartment in the dark, whilst it's blowing a hoolie all around? Way, way easier to just take a spare torch, for example the Petzl E-lite, which is tiny and weighs nothing.
Good for bright days when glare can be uncomfortable.
Even at the modest altitudes of Scotland's mountains UV is pronounced, so it's important to be protected, particularly if you are fair skinned – it is possible to get sunburned on a cloudy day!
CE rated mountaineering helmet that fits you whilst wearing a hat.
Very much personal preference here and they can take a bit of getting used to. Poles turn you into a quadruped, they take some of the load off your legs and with practise can make getting up and down hills a fair bit easier.
Some may say essential. Nowadays people use their phone as a camera and GPS etc, all of which uses up a lot of battery so it's very important to keep the phone close to your body for warmth. If the battery gets cold the chemical reactions will be sluggish, resulting in loss of power. Additionally it needs to be waterproof, either in a 'tough case' or a plastic wallet or bag.
There are a multitude of types available so if purchasing, particularly for the first time, it's important to get specific advice. Check out the link below for a good article from UKClimbing on choosing shoes.
One that will fit over multiple layers of clothing, is easy to adjust with gloves and with plenty of gear loops. I find the 'closed system' fastening easier to deal with, as opposed to the kind of buckle that you have to manually thread.
When you come on a course, all climbing equipment will be supplied. If you're keen and have bought a harness it may be worth buying a few useful bits and pieces to go with it, for example:
Belay plate and screw gate karabiner
Necessary for belaying your instructor, your friends and also abseiling.
120cm sling and screw gate karabiner
Handy for transitioning to abseil mode and various other uses.
Two spare screw gate karabiners
For attaching yourself to anchors.
A camera is great to take out so that you can relive all the glory when you're old and your knees don't work, plus in this day and age a basic outdoor camera is way cheaper to replace than a dropped or broken iPhone!
It's good to have either a weather proof model or a good protective case!