Dew, Frost, Condensation, and Radiation

Ghost Rock, Alamo Hueco Mountains

Ghost Rock with Three Faces, New Mexico

A common experience is waking up in the morning when the grass and plants close to the ground are covered with dew and the tent fly or tarp is covered with condensation. What causes this? Why is there more dew on the grass than on the rocks? Why does dew or frost sometimes form on a car roof or window when there is none on the ground? How can frost appear on the grass when the air temperature is above freezing? Why is dew more prevalent on clear nights when there’s more moisture in the air on cloudy nights? Why does condensation form on the top of my thick sleeping bag and nowhere else? Why does condensation form on my tarp even though its very well ventilated?

The answer to all these questions lies in heat transfer, especially radiation heat transfer. We begin the discussion at night where, in the absence of fire, all the radiation heat transfer is in the infrared region. All objects emit infrared radiation. The rate is proportional to absolute temperature raised to the fourth power times the emissivity of the material. In visible light, dark, especially black, objects have a high emissivity. Lighter colored objects (white, shiny metal) have a low emissivity and emit and absorb little radiation.

Objects also absorb radiation. The fraction of the incident radiation absorbed is termed the absorptivity. At any constant wavelength of light the absorptivity and emissivity of any object are the same. Shiny objects neither emit nor absorb much radiation, they reflect it.

The situation becomes interesting when we switch wavelengths to the longer wavelength infrared (sometimes called heat rays). In the visible bands our eyes are good indicators of emissivity/absorptivity, but we are blind in the infrared. Aluminized materials “space blankets” have low absorptivity (and emissivity) in all wavelengths of interest, but this is not true of most colors. Most materials of interest, with the exception of dry air and aluminized surfaces, are “black” in the infrared wavelengths. Tables of infrared absorptivity are available (e.g., if more precise information is desired.

The switch in properties between the visible and infrared wavelengths has very important implications for how our environment behaves. We’ll begin with the sky. In the absence of clouds, pollution, and dust, the atmosphere is clear, allowing most visible radiation to pass. Dry air is also clear to infrared radiation, but water vapor is “black” in the infrared. Carbon dioxide is also “black” to infrared radiation, leading to global warming. Infrared radiation is emitted constantly by all objects (tent fly, outer jacket, leaves, soil, rocks). If the lost energy is not replaced, the temperature of the objects will drop.

On a clear, dry, night, the sky is largely clear with respect to infrared radiation and most of the emitted radiation passes out to space. The dry air itself, has a low emissivity and emits only a small amount of infrared radiation. The result is that the objects all tend to lose energy and cool. As they cool, they are warmed by the other heat transfer mechanisms: conduction, convection, and condensation. Conduction works best when the object is connected to a large thermal reservoir (the ground) through a pathway with high thermal conductivity. This is the case for large rocks and rock outcrops. Conduction of heat from the earth limits their cooling.

Convection heat transfer works best when the wind blows. On a windy night the rate of convection heat transfer is high and most objects stay close to air temperature. On calm nights convection is less important and objects can cool, by net loss of radiation, well below the air temperature.

Cooling is greatest on calm nights for objects isolated from the earth (no conduction heat transfer pathway from the ground heat source, little convection heat transfer from the air). The leaves of plants, especially plants close to the ground surface, are most susceptible to cooling. Tarps, tent flies, the roofs of houses and cars, and the tops of well insulated (thick) sleeping bags when sleeping out usually cool below air temperature on clear, calm nights. Yes, we said that right, objects tend to cool to below ambient air temperature on clear nights.

For a high school science project I had my daughter construct a small quilt of different outdoor related fabrics. Each night we were camping she would hang out the quilt and then read the temperature of each fabric in the morning before then sun rose substantially above the horizon. In the figures below the temperature difference between the fabric and the air was recorded. Negative numbers (all of them) indicate that the fabric was colder than the air. There is a lot of scatter but two observations are that the portion with aluminized fabric (space blanket) on the bottom sewed to nylon fabric on top was typically colder than other fabrics presumably because the space blanket portion was towards the ground and thus this piece of the quilt was not warmed as much from ground sourced infrared radiation and the mesh (taken from an old tent door) was closer to air temperature. The more a fabric cools below air temperature the greater likelihood that it will reach the dew point and have condensation.

Fabric cools below air temperature

Figure 1. Difference between air and fabric temperature in the morning for different fabric types.

When objects cool, air touching the objects also cools, raising the relative humidity, and leading to condensation or frost. All of the objects susceptible to cooling also tend to receive higher amounts of dew, condensation, and frost. Many nights the minimum air temperature is near the dew point. When temperatures begin to drop below the dew point condensation releases large amounts of energy that tends to prevent the temperature from dropping much further.

On cloudy and/or humid nights much of the infrared radiation emitted by the earth is reflected back to the earth and the water vapor in the atmosphere also emits infrared radiation depending upon the water vapor content and the atmospheric temperature. In this situation incoming and outgoing radiation are closer to balancing and objects have less of a tendency to cool below atmospheric temperature. In the figure below the results are shown as a function of the extent that clouds and/or trees blocked the clear night sky. When the sky is cloudy, or when the tent/tarp is set under a tree, there is less tendency for the fabric to cool below the air temperature and thus less of a tendency for condensation to form on the fabric. This is shown in Figure 2 where we see that the fabrics in the quilt stayed closer to ambient temperature when the night was cloudy and/or with tree cover above.

Cloud and forest cover reduce condensation

Figure 2. Difference between air and fabric temperature in the morning for different cloud and tree cover.

What can you do to minimize condensation (e.g., in a single wall tent)?  Figure 3 shows how much moisture air can hold, the key to avoiding condensation is to keep the air at less than 100% relative humidity. Anywhere to the left of the dark blue (100% relative humidity) is an invitation for condensation to form on the tent wall or anywhere else of concern.

Assume the air under a tarp has a water vapor content of 0.01 kg/cubic meter at 50% relative humidity and a temperature of 22.5 degrees. What will cause this air to become super saturated with water vapor, potentially leading to condensation on the tarp? Well, if temperature drops during the night and additionally the temperature of the tarp is below the air temperature (typically the case on clear, calm, nights with low amounts of moisture in the air), then when the tarp temperature gets below about 12 degrees Celsius, condensation becomes likely. Additionally moisture may be added from other sources such as your breathing, evaporation from wet clothes, and evaporation from the ground surface. If temperature remains constant, doubling the water content to 0.02 kg/cubic meter would lead to saturation and potential condensation.

Figure 3. Amount of moisture in air at different temperature and relative humidity of 25, 50 75 and 100%.

So . . . . both temperature and water vapor additions are important. What can be done to control them?

Keeping the fabric warm is important. Net loss of infrared radiation (IR) at night causes fabric temperature to drop below the air temperature and leads to condensation. Clouds, wind, and staying beneath trees all help with keeping the fabric from dropping below ambient temperature. Clouds and tree branches send IR radiation back toward the fabric, lessening the radiation imbalance. Wind increases convection heat transfer thereby keeping the fabric near the air temperature. The lesser amounts of condensation on windy nights have more to do with the wind keeping the fabric near air temperature rather than ventilating moisture, although both are important.

Keeping the air dry is also important. Useful strategies include the use a ground cloth to reduce moisture flux from the ground (for tarps and tent vestibules), limiting the amount of wet clothing in the tent, and (usually) increasing ventilation.

How do the different types of tent work?
Double wall tents work by allowing condensation to form on the outer waterproof layer where it runs down to the ground without contacting the people and gear inside.. The inner layer is permeable to vapor and, because it is insulated from the outside air and night time sky by the outer layer - it remains warmer. Low water content and high temperature reduce the possibility of condensation. Variants on the double wall design such as two impermeable (coated) layers can also work as long as the moisture can get out through a vent (e.g., the Stephenson Warmlite tents). The most important factor is the temperature of the inner layer is kept sufficiently high to prevent condensation.

Tarps work by having high rates of ventilation which keeps the moisture content of the air under the tarp nearly the same as in the ambient air. Good ventilation and blocking ground moisture with a ground cloth help, The pervasive common wisdom (which is wrong) is that with great ventilation one should never see condensation on a tarp. Unfortunately the infrared radiation imbalance which occurs on clear, calm nights causes the tarp fabric temperature to drop below the air temperature - leading to condensation even in the absence of additional moisture sources such as human breathing and sweat. Ventilation will not always prevent condensation.

Single wall tents with waterproof/breathable fabrics allow moisture to escape through the fabric but are not thermally insulated from the outside as is the case with double wall tents. Condensation can occur when the breathing rate of the fabric is too low and/or when the fabric temperature drops below air temperature. Single wall tents of non breathable fabric are condensation prone and ventilation is usually important for minimizing condensation. For non-breathable tarps and single wall shelters the current best option is to make the shelter of aluminized sil-nylon on the top. The lower infrared emissivity keeps the material warmer at night, thereby minimizing condensation. To my knowledge none of the cottage manufacturers incorporates this concept yet but the material is currently available from Seattle Fabrics and aluminized fabrics do reduce (but not eliminate) condensation, especially on the calm, clear nights where condensation is so prevalent.

An ideal tent would have a waterproof/breathable lightweight and strong fabric that would keep in warmth while letting vapor escape. A future avenue for improvement would be to manage the infrared energy by adding a low IR emissivity coating to the outside of the fabric. This would lower the loss of IR during the night thereby increasing the temperature of the fabric and lowering the degree of condensation. An (ugly) non breathable example would be to use an aluminized silnylon fabric with the aluminized side pointing to the outside. A better option might be to utilize some of the IR emissivity controlling coatings developed by the US military when they become available for civilian use. Adding a low emissivity coating to the outside of a single wall tent or tarp fabric should significantly lower the frequency and amounts of condensation.

Condensation inside fabrics is a related issue that depends less on radiation. The human body sweats continuously, more when overheated. As the water vapor from sweat moves through a clothing system it will condense inside the fabric if the dew point is reached inside the clothing. When the water vapor condenses it wets the fabric and moves by wicking (capillary movement) rather than vapor diffusion. The capillary movement can be towards the outside air (good) and also towards the body (bad). The two major changes to the warmth of the clothing are a) wet fabric tends to have greater thermal conductivity and b) evaporation/condensation cells may set up inside the fabric causing even more heat loss. In freezing conditions the water may freeze inside the clothing system causing even greater problems. In the future I plan on performing calculations to illustrate these issues. In extreme cold the problem can be prevented by donning vapor barrier clothing or sleeping bag liners. These should be non breathable coated material. This shuts off the supply of water from the skin into the fabric. In less extreme environments a useful approach is to dress in layers to prevent sweating and allow the compromised layers to be dried in the sun or wind.

Return to Window Outdoors: A Guide to Understanding the Outdoor World