Humidity, Dew Point and Fog
Learn why dew point tells you more than relative humidity, and how a clear, calm night turns the Oldman River valley into a bowl of fog.
In this lesson
- What water vapour is, and why warm air can carry far more of it than cold air
- Why relative humidity swings all day even when the moisture in the air hardly changes
- Why dew point is the number forecasters actually watch
- How dew, frost and the different kinds of fog form
- Why fog loves the river valley and coulees around Lethbridge
The invisible water all around you
Every breath you take contains water. Not droplets, but water vapour: water in its gas form, scattered among the nitrogen and oxygen molecules. Water vapour is completely invisible. The white “steam” above a kettle is not vapour at all; it is a cloud of tiny liquid droplets that formed when the vapour hit cooler air.
The amount of vapour the air can hold before it starts turning back into droplets depends almost entirely on temperature. Warm air can carry a lot; cold air can carry very little. A rough rule of thumb is that the capacity roughly doubles for every 10 to 11 degrees of warming.
Scientists will point out that air doesn’t really “hold” water like a sponge. What actually happens is that at higher temperatures, water molecules move fast enough to stay as gas. But the sponge picture works well for everyday thinking: a warm sponge is big, a cold sponge is small, and when the sponge is full, water has to come out.
Air at −20°C can carry only about one-twentieth of the water vapour that air at +20°C can. That is why a January cold snap feels so dry, even when it’s snowing.
Relative humidity: the number that misleads
Relative humidity (RH) is the figure you see on most weather apps. It tells you how full the sponge is, as a percentage of what it could hold at the current temperature. At 100 percent the air is saturated; at 50 percent it holds half of what it could.
The trouble is that the size of the sponge changes all day as the temperature changes. On a typical summer day in Lethbridge, the actual amount of moisture in the air may barely change from dawn to dusk, yet RH might read near 80 percent at sunrise and below 30 percent by mid-afternoon. Nothing about the moisture changed. The air simply warmed up, the sponge got bigger, and the percentage dropped.
Winter shows the same effect in reverse, indoors. Take outdoor air at −20°C with a perfectly ordinary 80 percent RH, bring it inside and heat it to 21°C. You haven’t removed a drop of water, but the sponge is now enormous, so indoor RH falls to the single digits. Hello, static shocks, cracked knuckles and dry sinuses.
Relative humidity tells you how close the air is to saturation right now. It does not tell you how much moisture is actually there. For that, look at the dew point.
Dew point: the honest number
The dew point is the temperature the air would have to cool to (without adding or removing any moisture) for it to become saturated. Cool the air to its dew point and water starts to condense: dew on the grass, fog in the valley, beads on the outside of a cold glass of iced tea.
Because it depends only on how much vapour is present, the dew point stays fairly steady through the day while RH bounces around. That makes it the better measure of how humid the air really feels and how much moisture is available for clouds and storms.
How dew point feels
These are general guides rather than exact cut-offs, and people differ:
- Below about 10°C: dry and comfortable. This is home territory for southern Alberta most of the year.
- Mid-teens: you start to notice the humidity, especially in the sun.
- Around 20°C and up: sticky and muggy, the kind of air common in Ontario or the American Midwest in summer, and a rare visitor here.
The spread tells the story
The gap between the temperature and the dew point is called the dew point spread (or dew point depression). A big spread means dry air with room to spare. As the spread shrinks toward zero, the air is close to saturation and fog, low cloud or dew become likely.
Dew and frost: the ground gets there first
On a clear night the ground radiates heat out to space, and the grass, car roofs and deck boards cool faster than the air a couple of metres up. When a surface drops below the dew point, vapour condenses right onto it as dew. That is why you can find a soaking-wet lawn on a morning when the official temperature never got close to the dew point.
If the surface is below freezing, the vapour skips the liquid stage and turns straight into ice crystals: frost. Strictly speaking, the temperature at which this happens is called the frost point, which is a little higher than the dew point in below-freezing air. Frost can coat your windshield even when the station reads a degree or two above zero, because the glass facing the open sky is colder than the air.
Hoar frost is the delicate, feathery crystal growth that builds directly from vapour on still, cold nights. Rime is different: it forms when tiny supercooled fog droplets freeze on contact with trees and fences, building up on the side facing the breeze. Both turn the coulees into a postcard.
How fog forms
Fog is simply a cloud touching the ground. It forms whenever air near the surface reaches its dew point, either by cooling down or by having moisture added. Meteorologists name fog by how that happens:
- Radiation fog: clear skies, light winds and a long night let the ground and the air above it cool to the dew point. This is the classic early-morning valley fog that burns off by late morning.
- Advection fog: moist air drifts over a colder surface, such as snow cover, and is chilled from below. It can hang around all day.
- Upslope fog: moist air is pushed up a gradual rise in the land and cools as it rises. The prairie climbs steadily westward toward the Rockies, so an easterly wind does exactly this in southern Alberta.
- Steam fog: very cold air passes over open, relatively warm water. The water evaporates into the cold air, which immediately condenses it. It looks like the river is smoking.
- Freezing fog and ice fog: freezing fog is made of supercooled liquid droplets below 0°C that leave rime on everything. Ice fog, made of ice crystals, needs bitter cold, typically below about −30°C, and is most common near towns where vehicles and chimneys add vapour.
Our air is usually dry. With dew points often well below the temperature, fog is less common here than in many places, and when it does form it tends to have a clear cause. Watch for three set-ups: valley fog on calm autumn mornings after rain, when the Oldman valley fills while West Lethbridge and the downtown side sit in sunshine; upslope fog and low cloud on a raw easterly wind, which can blanket the whole region for a day or more; and steam fog rising off open water on a frigid winter morning. At the other extreme, a chinook can drop the relative humidity into the teens, drying out lawns, skin and wood fences in a hurry.
Fog can be patchy, so you may drive from bright sun into near-zero visibility in seconds, especially dropping into the river valley. Slow down, use low-beam headlights (high beams reflect back at you), make sure your tail lights are on, and leave much more following distance. In freezing fog, watch for black ice on bridges and overpasses.
Try it yourself
Check the temperature and dew point on the live station this evening and again first thing tomorrow. How much did the spread shrink overnight? If the sky was clear and the wind light, did you find dew or frost on the lawn or the car, even though the air temperature stayed above the dew point? Then compare the relative humidity at breakfast with mid-afternoon and notice how much it moves while the dew point barely budges.
West Lethbridge now (4:08 am): 0°C, wind calm, humidity 94%. Live station ›
Key terms
- Water vapour
- Water in its invisible gas form, mixed in with the rest of the air.
- Saturation
- The point where air holds all the water vapour it can at its current temperature; any more and it condenses.
- Relative humidity
- How close the air is to saturation, as a percentage. It changes with temperature even when the moisture doesn’t.
- Dew point
- The temperature the air must cool to for saturation. A direct measure of how much moisture is really there.
- Dew point spread
- The gap between the temperature and the dew point. Near zero means fog, dew or low cloud are likely.
- Frost point
- The below-freezing temperature at which vapour turns straight into ice crystals on a surface.
- Radiation fog
- Fog formed when the ground and nearby air cool overnight under clear skies and light winds.
- Upslope fog
- Fog formed when moist air is pushed up rising ground and cools to its dew point.
Check your understanding
1. On a sunny summer day the relative humidity drops from 80 percent at dawn to 30 percent in the afternoon. What most likely happened?
Relative humidity depends on temperature. As the air warms, the same amount of moisture becomes a smaller percentage of what the air could hold.
2. Which number is the better guide to how much moisture is actually in the air?
The dew point depends only on the amount of water vapour present, so it stays steady while relative humidity swings with temperature.
3. The temperature is 4°C and the dew point is 3°C on a clear, calm evening. What should you expect overnight?
A spread of only one degree means the air is nearly saturated. Clear skies and calm wind will let it cool to the dew point quickly.
4. Why does fog often fill the Oldman River valley while West Lethbridge stays clear?
On calm nights the coldest air flows into low ground like water into a bowl, so the valley cools to saturation before the uplands.
5. Why does a heated house in January feel so dry?
Bitterly cold air can carry very little water. Warming it indoors greatly increases its capacity, so the relative humidity can fall to single digits.
Weather School is for learning and enjoyment. It is not official guidance and may contain errors. For watches and warnings, always use Environment Canada (WeatherCAN app or weather.gc.ca).