I exclusively hike in long sleeves long pants. 100% of the time regardless of heat. But some smart choices can make this comfortable –
1. If you’re wearing pants, they MUST be vented. a single vent per leg is ok like the RR Ecomesh https://www.railriders.com/men-eco-mesh-pant-with-insect-shield-p-837.html These are the best overall pants for relatively-cold to relatively-hot weather (like 30-85* approximately).
If the nights wont get below the 50’s and daytime highs are >high 80’s I switch to the RR Bone Flats pants – the greatest hot weather hiking pants ever (after you treat them with permethrin) https://www.railriders.com/men-bone-flats-pants-p-992.html The dual leg vents on the BF pants makes them as cool as shorts IME. They are loose and billow-y, so each step pumps some air through them. Also if there is any wind the double vents on each leg let wind blow through them freely similar to shorts. Choose the lightest color. They show dirt quickly and the mesh wont hold up to thorny brambles very well, but they are Awesome for hot weather. I really cant say enough good things about the bone flats pants in hot hot weather.
Without mesh vents even the lightest weight pants are miserable IME.
Similarly, I hike in a 50/50 Merino/Poly 120gsm shirt from Icebreaker. The entire back panel has the knit holes in it and is quite airy. The 1/4 zip helps too. It isnt as cool as a UL 100% poly shirt, but poly smells so quickly I cant stand it on a multi day hike. Again, choose a very light color like off white or light gray.
Last, the sun umbrella is GOD in exposed hot weather (from Zpacks, GG, SMD, and others). I have done multiple tests and depending on the intensity of the sun and surrounding vegitation, the temp difference under the umbrella vs full sun is 15-35*. For example, 2 weeks ago I was hiking in the lockwood valley of southern california – full desert essentially. Shade temp under my umbrella was ~95*. After 20 mins in the sun my thermometer said 130* and I believe it.
If you hold the umbrella in a perfect line between you and the sun, I can shade 70-90% of my body (including most of my legs). The difference of the umbrella is incredible. Yes most people consider holding the umbrella annoying, but I consider being SOAKED in sweat, sticky, and miserable the worse option. Also since my sweat load is reduced by >50% from the umbrella, I can carry less water and be comfortable.
So if you’re going long sleeve & pants –
1. Light colored pants with 2 or 4 vents is mandatory for comfort
2. A light colored LS shirt is critical. 100% poly or Poly-Merino blend is recommended. Knit holes in the fabric help a lot. A 1/4 zip helps a lot.
3. Portable shade from an umbrella makes your life 15-40* cooler and pays for itself via less water requirement and increased overall comfort. The only time I miss my 2nd pole is doing steep descent, but in that case I usually put away the umbrella and suffer in the heat for a little while to save my knees