Should you open windows in Dubai’s cooler months? Humidity explained

Why this question comes up every winter

As summer ends each year, nights in the UAE cool down, many homes run the AC less and balcony doors stay open longer. In its November climate summary, the National Centre of Meteorology (NCM) describes temperatures falling 4 to 6 °C below October. It also notes that relative humidity rises in the early morning as moist air moves in from the Arabian Gulf, which favours fog and mist, especially along the coast. Its December summary describes the same early-morning pattern. Dubai’s average dew point, a better guide to moisture, falls from about 21 °C in October to about 16 °C in November and 11 °C in December (see Dubai’s humidity month by month).

Fresh air feels good after months of closed windows. But cooler air is not automatically drier air. Whether opening up dries your home or dampens it depends on how much moisture the air holds on each side of the glass, on that day.

Those NCM figures are long-term climate averages, not a forecast. For today’s conditions, check the NCM forecast or a weather app that shows the dew point.

Relative humidity, moisture and dew point, in plain words

Three numbers describe humid air:

• Relative humidity (RH) is the percentage on your weather app or hygrometer. It tells you how close the air is to full, at its current temperature.
• Moisture content is the actual amount of water in the air, in grams per cubic metre (g/m³).
• Dew point is the temperature at which that air would be completely full (100% RH). The higher the dew point, the more water the air holds.

The key fact is this. Cool the same air and its RH goes up, even though no water was added. Warm it and RH goes down. The dew point stays the same either way. That is why the US National Weather Service calls relative humidity misleading and treats dew point as the better measure of how much moisture is in the air.

For the full science, with a calculator, see our guide to humidity, dew point and condensation.

Why comparing indoor and outdoor percentages can mislead you

Your weather app shows 70% outside on a cool evening. Your living room reads 60%. It looks as if opening the window will make the home wetter. Often the opposite is true.

The outdoor air has the higher percentage, but it holds about 2.4 g less water in every cubic metre. Once it comes inside and warms to 25 °C, it would read about 48%. In this case, airing the room out lowers the moisture indoors.

It also works the other way. On a warm, sticky day, 50% outside at 32 °C (dew point about 20 °C) carries far more water than 55% inside at 24 °C (dew point about 14 °C).

The rule: compare dew points. The air with the lower dew point is the drier air.

When opening windows removes moisture, and when it adds it

Opening a window swaps indoor air for outdoor air. It can only carry moisture out if the air coming in is drier.

• Outdoor dew point clearly lower than indoor: opening up removes moisture. The bigger the gap, the faster it works.
• Outdoor dew point higher than indoor: opening up adds moisture. It settles into sofas, curtains, wardrobes and onto the coolest surfaces.
• Within a degree or two of each other: it makes little difference to humidity. Open up for fresh air if you like, but do not expect it to dry the home.

Building science guidance for hot, humid climates treats outdoor air as a moisture load whenever it is more humid than the air indoors. The US EPA’s moisture advice works the same way: use bathroom and kitchen fans, or open a window briefly, to let indoor moisture out, and use air conditioning or a dehumidifier in hot, humid climates.

What changes in the cooler months

Not every UAE home sees the same conditions. An inland villa, a high-floor apartment facing the sea and a ground-floor flat by a lake can have very different air outside the window on the same night. Use the points below as things to check, not fixed rules.

Cooler evenings

As the air cools after sunset, its relative humidity climbs, so night readings of 80% or 90% are common. That alone does not mean the air holds more water than your home. Check the outdoor dew point before you decide.

Foggy and misty mornings

Fog means the outdoor air is full: its dew point equals the air temperature. NCM explains that fog forms with high humidity, light winds and stable conditions, mostly in the early morning. On a 17 °C foggy morning the outdoor dew point is 17 °C. A room at 24 °C and 55% has a dew point of about 14.4 °C. That fog, once it comes in and warms up, would read about 65%. Keep windows closed until the fog lifts, then check again.

Balcony doors and homes near the water

A balcony door moves far more air than a window opened a few centimetres. That is useful when the outdoor air is drier and a problem when it is not. Homes close to the sea, the creek or lakes are more exposed to humid air. If that is your home, see humidity in waterfront homes.

Less AC running

An air conditioner removes water only as a side effect of cooling, while its compressor runs and its coil is cold. When the weather cools, the AC runs less or not at all, so this moisture removal falls just as windows open more. Building scientists flag these mild spring and autumn periods as a hard time to control humidity in humid climates, because outdoor air keeps adding moisture while the AC barely runs. Fan-only mode moves air but does not dry it. Our guide to using a dehumidifier with AC covers how the two work together.

Showers, cooking and indoor laundry

These add moisture inside, whatever the weather. The best fix is to remove it at the source: run the bathroom or kitchen extract fan, and keep the bathroom door closed until the room clears. Wet laundry drying in a closed bedroom adds a lot of water to a small space. Dry it in a room with an extract fan, or by an open window only when the outdoor air is drier.

How to decide in five minutes

1. Measure indoors. Read the temperature and RH on a hygrometer in the room, then find the indoor dew point in the table below.
2. Check outdoors. Look up the outdoor dew point in your weather app. If it only shows temperature and RH, use the calculator in our dew point guide.
3. Compare. Outdoor dew point a couple of degrees or more below indoor: opening up will help. Higher than indoor: keep closed. About the same: it will not change much.
4. Check the air quality. Dry but dusty air is still a reason to wait.
5. Air out, then re-measure. Open up for a short spell, close, let the reading settle and compare dew points before and after.

Indoor dew point at a glance

Should I open the windows? A decision table

Find your situation, check the one thing that decides it, then act.

Two examples (hypothetical)

These two homes are made up to show the method. They are not real homes or measurements.

Example 1, hypothetical: opening up helps. A winter afternoon in an apartment after showers and a load of laundry. Indoors reads 24 °C and 68%: a dew point of 17.7 °C, about 14.8 g of water per m³. Outdoors is 23 °C and 45%: a dew point of 10.4 °C, about 9.2 g/m³. The outdoor air holds roughly 5.6 g less water per m³, and once warmed to 24 °C it would read about 42%. Decision: open windows on opposite sides for a short airing, close up, then re-measure.

Example 2, hypothetical: keep the door shut. A misty early morning in a sea-facing apartment. Indoors reads 24 °C and 58%: a dew point of 15.2 °C, about 12.6 g/m³. Outdoors is 20 °C and 95%: a dew point of 19.2 °C, about 16.4 g/m³. The outdoor air holds roughly 3.8 g more water per m³, and warmed to 24 °C it would read about 74%. Decision: keep the balcony door closed. Run the bathroom and kitchen fans for fresh air and check again once the mist clears.

Calculated with the Magnus formula (Alduchov and Eskridge, 1996), rounded to one decimal place.

How to measure, and how to tell if airing out worked

Use a basic digital hygrometer at about chest height, away from AC vents, windows and the bathroom door. Give it time to settle after you move it, and note the temperature as well as the RH.

Judge the result by the dew point, not the RH alone. Airing out also cools the room, and cooler air reads a higher percentage. For example, a room that goes from 24 °C and 62% to 22 °C and 60% looks almost unchanged. Its dew point has fallen from 16.3 °C to 13.9 °C, and its moisture from about 13.5 to 11.6 g/m³. That airing worked.

Then watch for a few days. Fogging on the inside of the glass in the morning, musty smells or damp-feeling bedding mean moisture is still building up. See condensation on windows to tell inside fogging from outside.

Do not forget dust and air quality

Humidity is not the only thing that comes through an open window. Dubai Municipality’s indoor air quality guidelines point out that ventilation assumes the outdoor air is cleaner than the indoor air, and they list dust among outdoor contaminants. Before a long airing, check Dubai Municipality’s Dubai Air Quality platform for live and forecast PM10 and PM2.5, or your emirate’s official service, and look out for NCM alerts about blowing dust. On dusty days, keep windows closed even when the outdoor air is dry. After a dusty spell, wipe the sills and check your AC filters.

If fine dust is a regular problem indoors, an air purifier with a HEPA filter, sized to the room, removes airborne particles that closing the windows cannot. MedicAir air purifiers, which HawaQ supplies in the UAE, combine a HEPA filter with activated carbon and an enclosed UV-C stage. To see what a purifier does and does not do, read dehumidifier vs air purifier.

Ventilation, cooling and dehumidifying are different jobs

Each one solves a different problem, so it helps to know which one you need.

Your home needs fresh air whatever the season. Do not seal it up or block extract fans, vents or fresh-air grilles to keep humidity out. Dubai Municipality’s guidelines expect a space to have either adequate openable windows or mechanical ventilation. The aim is to choose when you open up, not to stop ventilating.

When you do not need a dehumidifier, and when damp needs investigating

If your rooms stay below 60% through the cooler months, with no fogging inside the glass and no musty smells, airing out on drier days and using extract fans may be all you need. Dubai Municipality guidance is to keep indoor RH below 60%, and below 50% where feasible. The US EPA gives the same upper limit, ideally 30 to 50%.

Investigate further if:

• a room stays above 60% for days, even when the outdoor air is drier
• damp patches or mold keep coming back in one spot
• stains appear after rain, near windows, balcony thresholds or the roof
• AC vents drip, or the AC smells musty when it starts
• only one room is humid while the rest of the home is fine

These point to a leak, a blocked AC condensate drain or extract ventilation that is not working. More air will not fix them. Start with our guide to mold on walls, which explains how to find the moisture source. If you cannot find it, HawaQ’s mould testing maps moisture room by room, checks the AC coils and drains, and samples the air where needed.

Disclosure: HawaQ, the indoor air quality team behind this site, supplies Stadler Form dehumidifiers and MedicAir air purifiers in the UAE. How we recommend.

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