It’s harder to exercise at higher altitudes because the air is thinner. Is the same thing also true of hot weather? When it’s 90°F am I getting a lot less oxygen than at 60°F?
60F density 1.22kg/m3
90F density 1.156kg/m3
Pretty much negligible.
It’s harder to exercise in hot weather due to heat stress.
Even if the levels of oxygen differed, your body would try to compensate by changing the rate or depth of breathing, the ratio of breathing in to breathing out, et cetera.
There two different phenomenon at play here.
At higher altitude, as you said the air is thinner - and hence oxygen is less, gets transported less through your lungs and makes it harder to exercise.
At higher temperatures (and likely higher humidity levels), the lungs / skin is not able to effectively thermoregulate your body temperature. So its harder to exercise.
Anyone else also find exercising in cold weather harder? Takes forever to warm up stiff muscles, and the cold air gets into my lungs and makes me cough if I breathe too hard.
Sprinters in Tokyo found the warm weather good for them. People running longer distances were looking for icepacks after the finish. It depends.
Having been a competitive cold weather athlete, the above is fairly normal at first. In time your body gets used to the low ambient temps and you become better at warming up quicker. Also, I found I enjoyed suffered less on training runs in cold temps compared to hot.
That said, I find that as I age dealing with cold temps has become much harder.
That’s what I am trying to figure out. I do a lot of cycling and it’s been hot lately. When it gets up to around 90 I feel like I get worn out faster and don’t have anything left for the last hill. I feel like my sweet spot is in the 60s–warm enough for my system to get warmed up but I don’t feel ragged after an hour. When it’s close to 90 I am breathing much harder when I finish. (I don’t even go out if it’s over 90.)
I feel like growing up in a country with really cold winters is kind of cheating here 
Nowadays I avoid exercising hard enough to have to breathe through my mouth, at least in winter. Otherwise I risk getting a coughing fit that lasts over 30 min and makes me feel like I’m dying.
Don’t think I’ve ever tried exercising when it’s over 30C, that’s very rare in England.
Might you have a touch of asthma? I do. Plus, where I come from (the mid-Atlantic area), winters are exceedingly humid and that doesn’t help either.
I live near Washington DC. We’re having a warm week, today’s high about 95°F (35°C).
That is a ~5% reduction in density, so while it isn’t as big of a difference as going from Los Angeles to Denver (~20% difference) it isn’t negligible when someone is exerting at near 100% VOmax. However, the bigger driver is the moisture content of the air. Cool air carries less moisture even at the same relative humidity, and while the amount of oxygen that is displaced by water vapor is only in the fraction of a percent difference, the absolute increase is almost doubled from 25 °C to 35 °C (77 °F to 95 °F) for the same relative humidity. (Play with the calculator linked above to see.) Humidity is important in maintaining the pulmonary surfactant on the alveolar membrane; too little humidity and the surfactant desiccates and causes inflammation; too much humidity interferes with gas exchange with the air, resulting in a build up of carbon dioxide in the blood.
At high levels of absolute humidity seen at temperatures exceeding 32 °C (90 °F) and above 70% RH, exertion becomes pronounced, and at a wet bulb temperature of 35 °C (95 °F), i.e. relative humidity of 100% at that temperature, it is no longer possible to expel excess moisture from the lungs because the body cannot heat the air above ambient temperature to carry away additional moisture. Of course, at this temperature and relative humidity heat stress is a critical issue as well because the body can no longer cool itself by evaporation (either through respiratory exchange or sweat), but while it is possible to cool someone with icepacks or cold water, it is not possible to improve gas exchange without reducing the humidity content or temperature of the incoming air.
Stranger
I bicycle in hot and cold weather (though won’t go below freezing) and while very hot weather is stifling and exhausting, so is the cold.
A statistic I heard is that there is 8% more wind resistance at 40F than at 80F. The numbers @running_coach gave seem to be at least in the ballpark, though I don’t know if it’s a linear relationship. Anyway, since speed in cycling is mostly a factor of wind resistance when on flat ground, that 8% can still be 2mph when cruising along near 20mph, which is definitely significant.
Also, the colder it is the more effort your body has to expend to maintain core temperature. That’s energy that could be going to pedaling (or running or whatever). Because of the speed cyclists ride, they’re more exposed to wind than runners, so the effect is magnified. Plus cyclists seem to under-dress in cold weather.
Exercise-induced asthma is a thing, and I’ve had the coughing fits after rides in cold weather myself. So I try for steady rhythm instead of intense intervals, which helps. At some point though, the much increased volume of cold air becomes irritating to the lungs, and it seems to loosen mucus which then starts to flow again in warm air, leading to the coughing. It’s just unavoidable sometimes.
to put this in context, your 90F case is consistent with 60F at an altitude of about 900 feet above sea level, which isn’t that high. Draw a meridian through Omaha NE, and pretty much everything east of there (except for the Appalachians) is below that.
Most people notice the altitude in places like Denver, where the elevation is around 5300 feet and the density (at 60F) is around 1.06 kg/m^3.
Yes, that’s one manifestation of the “harder to exercise” thing. At some point you just can’t get enough O2 to maintain your usual peak performance. Go to low enough density, and merely walking with any alacrity becomes challenging.