Moisture in your everyday life

ATMS 100

Moisture in Your Everyday Life

The questions on the next several pages are related to real life experiences that you have had or may encounter at some time in your life. Answer as many questions as you can as completely as possible in the time provided. You should work with at least one other person. Your group will be given a specific question number to start with. At the end of the allotted time each group will discuss their answers.

  1. You live in Kansas and it is January. Your clothes dryer just broke and you have to hang your clothes out to dry on the clothesline. The weather conditions indicate a temperature of 40 °F and a dewpoint temperature of 20 °F. Will the clothes dry? Explain.

Yes, the clothes will dry. Clothes dry when water evaporates from them. As long as the air is not saturated, water can be evaporated into it. Once the air is saturated it contains as much water as it possibly can (for that temperature) and you cannot add any more vapor into the air. The clothes will dry because the dewpoint and air temperature are far apart, indicating that the air is far from being saturated.

  1. Explain why does it take longer for vegetables to cook at higher elevations. (Hint: consider the elevation and the temperature at which water will boil)

At the surface, the boiling point of water is 100 °C (212 °F). As water boils, bubbles of vapor rise to the top of the liquid and escape. The saturation vapor pressure exerted by the bubbles must equal the pressure of the atmosphere, otherwise the bubbles would collapse. So, boiling will only occur when the saturation vapor pressure of the escaping bubbles is equal to the total atmospheric pressure.

As you move to higher elevations, air pressure decreases. This means that you will be able to get water to boil at lower temperatures (92 °C). Recall that SVP is related to temperature. Lower temperatures produce lower SVP. Once water boils, the temperature remains constant. If you continue to heat the water, the energy is used to convert the liquid water to water vapor.

Because the vegetables are boiling at a lower temperature, they will have to boil longer to cook thoroughly.

  1. Would you expect water in a glass to evaporate more quickly on a windy, warm, dry summer day or on a calm, cold, dry winter day? Explain.

The water would evaporate more quickly on the summer day. The windy conditions will enhance evaporation (as opposed to the calm winter day conditions.) Both days are “dry”, however they are very different temperatures. The air has a larger capacity for water vapor at higher temperatures and so more water will evaporate into the air on a warmer day than a colder day.

  1. If you take a hot shower in the bathroom, the mirror will fog up. Explain why this happens. Explain why aiming a stream of air from a hair dryer at it will make the mirror clear again.

With the hot shower on, you are increasing the dewpoint in the bathroom as water evaporates from the stream of water from the shower into the air. When the amount of water vapor in the air is the maximum amount possible, then the air is saturated (the dewpoint temperature = air temperature). At this point, adding any more vapor into the air will cause condensation to occur. Water droplets begin to condense on the mirror (they are easy for us to see but they are condensing on all the surfaces in the bathroom). If you blow air over the condensed water, evaporation will start to occur. If the air is warm, the saturation vapor pressure of the air near the mirror will increase, thereby decreasing the relative humidity and causing evaporation to occur.

  1. Two people with long thick hair live in different parts of the country; one in Arizona and one in North Carolina. In the summer if they both wash their hair and leave it to air-dry. One will have dry air within an hour while the other persons hair not dry all day. Whose hair will remain damp and why?

The person living in North Carolina will have damp hair. Hair dries as water evaporates from it. When the air is close to saturation it is more difficult to evaporate water into the air. When the air is further from being saturated water will evaporate easily into it. In North Carolina in the summer, the relative humidity is typically high because there is ample moisture in the air from the Gulf and the Atlantic Ocean. In Arizona, the air is dry as there is not plentiful moisture source available.

  1. Explain what is meant by the phrase “It’s not the heat, it’s the humidity.”

This phrase is often used in conjunction with hot summer temperatures. When air temperature is high, but there is little moisture in the air (low humidity), our bodies are able to cool easily by evaporating perspiration. When temperatures are high and humidity is high, it is more difficult to cool our bodies and so it feels oppressively hot.

  1. During hot summer days in California, many people seek refuge in the shade where it is much cooler than in the sun. Explain why this method of keeping cool does not work in Illinois in mid-summer.

In California, humidity is typically very low because there is no significant moisture source (ocean is too cold to effectively evaporate large quantities of water vapor). When it is hot in the sun, you can move into the shade and immediately feel a significant temperature difference. During mid-summer in Illinois the humidity is high and so it feels a lot hotter than it really is (see #6). When you move into the shade, you lose the direct sunlight but the humidity does not change under the tree and so it still feels relatively hot.

  1. From a scientific perspective, what time of day would be the best time to water your lawn?

For the most effective watering you want to have the least amount of evaporation. This would occur when the relative humidity is high. When RH is high, it is difficult to evaporate much more water vapor into the air. The RH is typically highest in the early morning hours and so this would be the most effective time to water the lawn.

  1. Why can you see your breath on a cold morning? Does the air temperature have to be below freezing for this to occur?

When the air temperature is cold, the amount of water vapor needed to saturate the air is also low (saturation vapor pressure is low). When we exhale, we add moisture into the atmosphere. Adding moisture raises the dewpoint temperature. If the air is already close to saturation, the moisture we add to the atmosphere may bring the air to saturation and eventually condensation may occur; hence we see our breath as a fog. The air temperature does not have to be below freezing for this to occur, just close to saturation.

  1. Explain why icebergs are often surrounded by fog.

The icebergs are frozen and very cold and so they have a chilling effect on the surrounding air. (Air cools by conduction of heat from air to iceberg; small amount of ice may melt and evaporate into the atmosphere.) If the air is cooled to the dewpoint temperature, a fog will form (condensation takes place).

  1. During a summer visit to New Orleans, you stay in an air-conditioned motel. One afternoon, you put on your sunglasses, step outside, and within no time your glasses are “fogged up.” Explain what has apparently caused this.

Inside the motel, the air temperature is cool, and it is relatively dry (air conditioning tends to dry out the air). Your sunglasses therefore are also cool. Cooler temperatures do not require as much water vapor to saturate the air (lower saturation vapor pressure). When you step outside, you encounter warmer, more humid air. There is plenty of moisture available outside (high humidity or high dew point). Since your glasses are still cold, the temperature of your glasses and the dew point temperature are probably close together, near or at saturation. Water vapor condenses on the glasses creating a fog.

  1. On a winter night, the air temperature cooled to the dew point and fog formed. Before the formation of fog, the dewpoint remained almost constant. After the fog formed, the dew point began to decrease. Explain why.

Fog is composed of liquid water drops. When fog forms, water changes from a vapor to a liquid. Dewpoint temperature is a measure of the amount of water vapor in the atmosphere. If the amount of water vapor in the atmosphere decreases, then the dewpoint will decrease also.

  1. Is humid air or dry air more dense?

Humid air is less dense than dry air. If the air is completely dry then there is no water vapor in it. Approximately 99% of the atmosphere is Nitrogen (N2) and Oxygen (O2), which have molecular weights of 28 and 32, respectively. This means that the molecular weight of dry air would be approximately 29.

If the air is humid then there is water vapor in it. This means that some of the dry air must be replaced with water vapor. Water vapor has a molecular weight of 18 (2 Hydrogen = 2; 1 Oxygen = 16). Therefore, you are replacing some of the 29 molecular weight air with 18 molecular weight air, thereby making the air “weigh” less. Density is a mass (or weight) per unit volume and therefore humid air is lighter and less dense than dry air.

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