Heat and Humidity - Colorado Climate Blog (original) (raw)

Frog in a rain gauge with condensation. Photo credit: Chris Johns.

Stop me if you have heard this one before: “Last summer I visited [insert east coast state here]. It was 98 degrees and 98% humidity. It was BRUTAL…” What if I told you that everyone who has ever made this claim… was WRONG?! Nobody has ever experienced 98 degrees Fahrenheit with 100% (or near 100%) humidity under natural, outdoor conditions. Why have so many people made or heard such claims, and what are we not understanding about humidity?

Relative Humidity

When humidity is reported as a percentage (from 0% to 100%), it refers to relative humidity. This value indicates the amount of water vapor currently in the air compared to the maximum amount of water vapor the air could hold. In simpler terms, it’s how “full” the air is with water vapor.

So, what does it mean for air to be saturated (100% relative humidity)? It’s a common misconception that 100% relative humidity means the air is entirely composed of water vapor. This is incorrect; if that were the case, there would be no oxygen, and we wouldn’t be able to breathe. Instead, 100% relative humidity signifies that the air has reached its saturation vapor pressure. This is the point of equilibrium where, given a sufficient source of liquid water, the rate at which water evaporates into the air is equal to the rate at which water vapor condenses out of the air.

The crucial point, and often a source of confusion, is that this “magical” saturation vapor pressure is not constant; it depends on the air’s temperature. Warmer air can hold more water vapor before becoming saturated, while cooler air reaches saturation with less water vapor. Saturation vapor pressure has an exponential relationship with temperature. This means that as air temperature increases, the amount of water vapor the air can hold before becoming saturated increases dramatically – not just linearly, but exponentially. Consequently, warmer air can hold significantly more water vapor than colder air.

Clausius-Clapeyron

Consider these examples: At 32 degrees Fahrenheit (0 degrees Celsius), the saturation vapor pressure is approximately 0.61 hectopascals. However, at a much warmer 95 degrees Fahrenheit (35 degrees Celsius), the saturation vapor pressure jumps to about 56.27 hectopascals. This represents nearly a 10-fold increase in the air’s capacity to hold water vapor between cool air and hot air. Another way to visualize this difference is by looking at the actual concentration of water vapor in saturated air. Saturated air at 32 degrees Fahrenheit contains only about 0.5% water vapor by volume, whereas saturated air at 95 degrees Fahrenheit is composed of approximately 5% water vapor. This fundamental principle is described by the Clausius-Clapeyron relationship, a key concept in thermodynamics. The graphic below (as depicted in the helpful article by Roland Stull from the University of British Columbia) visually demonstrates how saturation vapor pressure increases with temperature, highlighting this exponential relationship.

The graphic above shows the saturation vapor pressure (kilopascals) as a function of temperature (Celsuis).

Why Saturated Air at 98°F is not possible

We’ve covered that relative humidity is the percentage of water vapor in the air compared to its saturation point, and that this saturation point is highly temperature-dependent. Let’s talk about the interplay between heat and relative humidity on a warm summer day.

In humid climes, summer mornings might see temperatures of 70-80 degrees Fahrenheit with near 100% humidity. As the sun warms the land and temperatures rise, the air’s capacity to hold moisture (saturation vapor pressure) increases significantly. If the actual amount of moisture in the air remains constant, relative humidity must decrease.

Consider this example from a National Weather Service forecast for Atlanta, Georgia: The top panel shows temperature (red) rising throughout the day, while the dew point (green: a measure of actual moisture content we’ll discuss more later) remains relatively steady. As a result, the relative humidity, shown in the middle panel, drops from 85% overnight to 46% during the day. The air did not lose moisture; it simply gained the capacity to hold much more moisture due to increased temperature, effectively halving the relative humidity.

Therefore, while someone might experience 98 degrees and 98% humidity on the same day, these conditions would not occur simultaneously.

The panels above are National Weather Service forecast plots. These plots show 48hr forecasts for temperature (red), heat index (yellow), and dewpoint (green) (top panel). Surface winds and wind gusts (second panel). Relative humdity (green), precipitation potential (yellow), and cloud cover (blue) (third panel). Rainfall forecast (fourth panel). Thunder probability (bottom panel).

We’ve established that even on a muggy day, rising daytime temperatures with constant atmospheric water vapor lead to a drop in relative humidity. But if 100% humidity is simply an equilibrium where evaporation rate equals condensation rate, why don’t we consistently reach this point at high temperatures? Two primary factors come into play: energy limitations and atmospheric stability.

Energy Limitations: Even with abundant surface moisture (like over a large body of water or very wet soil), achieving 100% humidity at high temperatures (e.g., 98 degrees Fahrenheit) is extremely energy-intensive. It requires massive amounts of energy both to heat the air to 98 degrees and to evaporate enough water to saturate it at that temperature, particularly for the latter. The sun’s energy in a single day simply isn’t enough to achieve both simultaneously, even in the middle of summer, and as the sun sets, temperatures cool.

Atmospheric Stability: Have you ever heard the term “heat rises?” It is true to a point. The atmosphere stratifies by density, with colder, denser air sinking and warmer, less dense air rising. However as air rises, it also expands and cools. For air to continue rising, the air above it must be sufficiently colder (approximately 6 degrees Fahrenheit per 1000 feet of ascent). If not, the air will not rise, or it will sink back down. Meteorologists use “stability” to describe this tendency. Air that is more than 6 degrees Fahrenheit warmer per 1000 feet of elevation gain than the air aloft is considered “unstable” and will rise. Otherwise, it won’t.

Here’s the crucial caveat: moisture. When saturated air rises, it cools, but the water vapor condenses into liquid. This phase change from liquid to vapor releases a large amount of latent heat into the atmosphere, allowing saturated air to continue rising while remaining relatively warm. This is why warm, moist conditions are ideal for thunderstorm formation: the air rises, and water vapor condenses out as rain.

An airmass that is both 98 degrees Fahrenheit and 98% humidity at the surface would almost certainly be incredibly unstable. The only condition that would keep this air at the surface is an otherworldly “dome” of hot air aloft to prevent it from rising. Such an airmass would more likely be violently propelled skyward, leading to extremely severe thunderstorms.

Dew Point

Relative humidity, being dependent on both moisture and temperature, can be confusing. A more intuitive measure of atmospheric moisture is dew point, which quantifies “absolute humidity.” Dew point is the temperature to which air must be cooled to become saturated, given its current moisture content. For example, a 50°F dew point means the air would be saturated if it cooled to 50°F. Higher dew points indicate more moisture in the air.

Here’s a general guide to comfort levels based on dew point:

Humidity in Colorado

Colorado is known for relatively pleasant summer weather in no small part due to the lack of humidity. While we do experience days with temperatures over 90°F across much of the state, even 100**+°F at low elevations during heat waves, it is typically a dry heat. This is because Colorado is a landlocked state at high elevation. More humid air from grazes us from the east or southeast from time-to-time, but high dewpoints at high elevations quickly lead to thunderstorms and a reintroduction of drier air. Dewpoints over about 60°**F are uncommon, though they do occur, especially on the eastern plains. Two general rules of thumb for Colorado are dewpoints decrease (much like temperature) at higher elevations, and dewpoints are usually higher east of the Continental Divide than west of the Continental Divide.

Below is a graphic showing 10:15 AM July 15th, 2025 conditions from the Colorado Agricultural Meteorological Network (CoAgMET): The red numbers next to each station show current temperatures, and the green numbers show current dewpoints. Dewpoints from summer 2024 for the five stations in black boxes have been plotted in the figure below: Stations on the eastern plains (Holly and Avondale in this example) more routinely measure dewpoints in the 50s and 60s than stations at higher elevations or west of the Continental Divide (such as Gunnison or Orchard Mesa).

Colorado Agricultural Meteorological (CoAgMET) Network observed temperatures (red), dewpoints (green), and wind speed/direction (black barbs) for 10:05 AM MST on July 15th, 2025.

Colorado Agricultural Meteorological (CoAgMET) Network observed dewpoints at Holly, Avondale, Salida, Gunnison, and Orchard Mesa Stations (locations shown in black boxes above) for May-August, 2024.

Conclusions

In summary, relative humidity is a confusing indicator that creates communication challenges. It is often misunderstood, leading to inaccurate assessments of summer weather such as “Last summer I visited [insert east coast state here]. It was 98 degrees and 98% humidity. It was BRUTAL…” It is preferable to use a measure of absolute humidity to describe muggy conditions. For example: “Last summer I visited [insert east coast state here]. The temperature was 98°F in the afternoon and the dewpoint was a sticky 73°F. It was BRUTAL!” Thankfully, in Colorado, you are unlikely to encounter such conditions.

If you enjoyed this post and would like to learn more about heatwaves, I recommend checking out our blog on Colorado heat from last summer here. I hope to publish more on heat in the coming months, examining concepts like heat index, wet bulb globe temperatures, and changes to heatwave probabilities in a warming climate.

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By Peter Goble

Peter Goble works at the Colorado Climate Center as the Assistant State Climatologist of Colorado. He received his B.S. in meteorology from the University of Northern Colorado and his M.S. in atmospheric science from Colorado State University. He specializes in climate variability and drought. Recent research projects include investigating the sources of error in western Colorado water supply forecasts, determining areas of Colorado most suitable for expansion of the wine grape industry, understanding the impact of climate change on extreme precipitation patterns in Colorado, and increasing both observational and modeled soil moisture monitoring efforts in Colorado. Peter is the Colorado Community Collaborative Rain, Hail, and Snow Network Coordinator, a community science organization focused on improving precipitation observations around the world.