I forget this every year. I am running a Half Marathon on Turkey weekend. It will be 40 degree at the start and probably close to 40 degrees when I finish 90 minutes later.
What should my clothing strategy be?
40 is a funny temp. So is it 40 and cold and dry and windy or 40 and humid and cloudy? I’d probably go shorts and then either a long sleeve or a short sleeve depending on wind. As for hat I would just skip it. Maybe some gloves. Ideally I would want to be quite cold at the beginning of the race and probably thinking I’m under dressed.
Shorts and running hat for sure. If its windy I’d probably go with arm warmers and maybe gloves with a singlet. Less wind just singlet and gloves.
If I’m wearing shorts I’m probably bringing a light scully to boot
EDIT; I don’t really have a lot of hair so take that into account
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Shorts and light long sleeve, or even short sleeve if the sun is shining. No way on a hat you’ll be carrying it. Headband at most but still will end up carrying it. Especially if you can run a half in 90 minutes.
I’d wear a singlet, shorts, and light-weight throw-away gloves if it was sunny and no wind. Once it heats up enough, toss the gloves at an aid station. If you are generally cold, wear a cheap cotton long sleeve shirt on top. Once you warm up, toss that too.
I do generally overheat a ton though while running and have gone shirtless in 30-35 degrees before. Just need gloves!
Shorts and a T-Shirt.
Go the the dollar store and get a beanie, gloves and long socks. Cut out the toes of the long socks and use as arm arm warmers. All of the dollar store items can be tossed mid-race if need be.
I ran a 15k race in Chicago a week or so ago. It was 35ish at the start and maybe 40 when I finished. I wore tights, a fleece beanie, gloves, a base layer on top. If I had to do over, I would have worn shorts, a tshirt with arm warmers running hat and maybe gloves, depending on wind. Definitely not tights, a base layer, and a beanie. I finished the race holding the beanie and my gloves.
Don’t forget fuel. You’ll need 13 gels of various flavors. 4 power bars, 2 bananas, a peanut butter sandwich and 3 bottles of gatorade. So, whatever you wear - make sure it will hold all that. 
There are some awesome replies here with ingenious ways of dealing with the cold…
You guys do know that you can get run specific gear for almost any temp? You don’t have to cut up socks and throw away gloves.
For me, tights would be a bit too warm for 40 deg’s racing. On a training run, I might wear them. I would go with shorts.
For a top, easy, long sleeve tech shirt.
I don’t know what you guys are talking about carrying hats? I wear running hats in 100deg+ weather. Are you wearing running caps? My head never gets too hot and the primary purpose of the hat is to collect sweat not keep you warm.
Beanie would be too much imo.
One thing I have noticed is that the Halo II Pullover keeps your head pretty darn warm and would work well in 40 deg weather. They also have the Halo Anti-Freeze that covers the ears…I would probably consider that as well.
I’m a big fan of compression shorts - not the new, fancy, technical stuff, but the older stuff that’s just kind of like tights that only cover your thighs. I find that they really work well in a large range of temperatures.
Sometimes I’ll have a throw-away sweater taht i’ll wear at the start and for the first 1/2 mile or so, but if you do that every race it starts to add up.
I’d go with shorts and a long-sleeve jersey, unless it’s going to be warmer by the end, in which case I might use a short-sleeve jersey. I also might use a headband I have that covers my ears (a Pearl Izumi cycling item), which I would then pull down around my neck, probably within the first mile.
I forget this every year. I am running a Half Marathon on Turkey weekend. It will be 40 degree at the start and probably close to 40 degrees when I finish 90 minutes later.
My rule of thumb, which works quite well for me, and a bunch of other people who now use it is to dress as what would be approporate for the temp + 20*. In your instance, I typically wear shorts and a long sleeve t-shirt in 60* weather, so I wear running shorts and a long sleeve shirt to run @ 40*. I’d also be pretty comfortable wearing pants @ 60*, so tights would also be OK.
Where it’s really helpful is to look at 20* or 30* real temp, would you wear a coat or jacket at 40* or 50*?
I agree with several of the above posts: unless windy and chilly, I’d be in shorts and a light, long-sleeve technical shirt or heavier short-sleeve technical. Possibly an earband.
For a dollar, you can get some inexpensive, basic, cotton gloves at WalMart. Throw 'em away at an aid station once you’re warm. Also, inexpensive tube socks with the toes cut off make great disposable arm warmers.
Staying warm pre-race is pretty easy – garbage bag with holes cut for head and arms.
I just did a half that started just below 40 degrees, sunny, and a slight cool breeze. I wore a singlet, shorts, a headband, armwarmers, and gloves. The gloves came off after a couple miles, but otheriase I felt like I dressed perfectly. Of course there were people wearing full tights and jackets and others racing with just a top and shorts.
FWIW- 1/2 I ran a couple of weeks ago started around 40 but got a little higher. I wore a singlet with shorts and arm warmers and a bball style hat. I was perfect (if not a little cool for the first couple of miles) and my running partner wore a long sleeve base layer and gloves and was covered in salt by the end. We finished about 5 minutes behind where you plan
Good luck!
I wore a very lightweight longsleeve running shirt in NYC with the DeSoto wind vest. The wind vest is really light and can even fold up and fit in your pocket. I would just unzip it when I was in the sun and the wind wasn’t bad and zip it up little by little as conditions changed…worked really well at regulating temperature when conditions were constantly changing.
Thanks folks. Got it.
Shorts, long sleeve tech tee, gloves, baseball hat and PLASTIC BAG. Should be good.
But, it is a very low key race. Most likely can wear sweats until very near the start.
Mike
I think what also might matter is it an “A” race or just another race. If it’s an “A” race go one less on top and bottom than you would on a normal training run at the temp. Another piece of advice is to do a warmup run prior to the start with warmer layers and then if possible peel those right before the gun goes off.
It’s always easier to bring extra clothes and then make a decision as you get out of your car race morning as well. Another option is to test out the temp on one of your taper runs prior to the event and see what works.
What nobody has mentioned is what pace do you plan on running and what is your goal.
If I’m doing a half marathon in 40 degree weather but plan to tear up the course sub 1:30, then I’m going minimal with shorts, singlet, gloves and beanie or cap.
If I"m running the race with my wife or sister and we are going to be chatting the entire way at the back of the pack and coming in after 2 hours, then I’m probably dressing a little warmer with a long sleeve shirt and maybe even tights.
Running 6:45s will heat me up good, but running 9s or 10s will keep my HR so low that I’m not sweating or overheating with the extra clothes.
Good luck