5.2 (HUMIDITY) WEATHER EARTH SCIENCE

Transcription

1 EARTH SCIENCE 5.2 (HUMIDITY) WEATHER Text: The water vapor content of the air is called humidity. The amount of water vapor air can hold depends on its temperature. Warm air can hold far more moisture than cool air. The image above shows the relationship between air temperature and the ability of the atmosphere to hold water vapor. The higher the temperature, the more water vapor the air can hold. Although we do not usually feel humidity as readily as we feel temperature, the air may seem clammy or sticky on very humid days. In humid weather, sweat does not evaporate as readily. This makes it more difficult for you to keep a comfortable body temperature. Meteorologists usually express humidity as relative humidity. This is a comparison of the actual water vapor content of air with the maximum amount of water vapor air can hold at a given temperature. For example, if the relative humidity is 50%, the air is holding half as much moisture as it could hold at the present temperature. At 100% relative humidity, the air is holding as much moisture as it can at that temperature, and the air is said to be saturated. Although relative humidity does not tell the amount of water vapor that a given volume of air holds, most people find relative humidity more useful than humidity expressed in other ways. The dew-point temperature may be used as an indication of absolute humidity. The image above shows that as air is cooled, its ability to hold moisture decreases. If moisture is not taken out of the air, it can be cooled until the air is saturated. The temperature at which this occurs is known as the dew-point temperature. Sometimes, 9 th GRADE 5

2 EARTH SCIENCE 5.2 (HUMIDITY) WEATHER this temperature is just called the dew point. It is called the dew point because when the outside temperature falls below the dew point, dew (or frost) can form on exposed surfaces. After reading, synthesize the information from the text by answering the questions below. According to the text, what is humidity: According to the text, what is relative humidity: According to the text (and image), what is the relationship between humidity and temperature: According to the text, what is the dew point: 9 th GRADE 6

3 EARTH SCIENCE 5.2 (HUMIDITY) WEATHER Practice: Challenge: How is the dew point related to humidity? 9 th GRADE 7

4 As we learned in our last lesson, relative humidity is not likely to remain the same if the air temperature changes. Consider a parcel of air that is cooling. If there is no evaporation or condensation, the absolute humidity (amount of moisture) in this body of air will remain constant. The ability of air to hold moisture decreases with decreasing temperature. As the air becomes cooler, it is closer to saturation, and the relative humidity increases. This frequently occurs at night when temperatures decrease, leading to the formation of dew or frost. The figure below shows typical changes in air temperature, dew point, and relative humidity over a period of one day. The thick orange line indicates changes in air temperature. The changes in temperature are the result of overnight cooling and daytime heating by insolation. Over this period there is little change in the dew point, indicating that the absolute humidity is relatively constant, as shown by the dotted line. However, changes in relative humidity depend on air temperature and dew point. At night, the thin line indicates that as the air temperature cools to the dew point, the relative humidity increases to 100 percent. This situation changes when the sun warms the air in the morning. Relative humidity drops as the air is warmed and its ability to hold water vapor increases. 2

5 One of the reasons meteorologists measure air temperature and determine the dew point is to predict precipitation. When the air temperature and dew point are close, the chance of precipitation increases. On this date, what time is precipitation most likely: Rising relative humidity indicates that the air is coming closer to saturation in the late afternoon. Again, this is a strong indicator of precipitation. Based on this graph, what could you say is the relationship between relative humidity and probability of precipitation: 3

6 Quick Check: Practice: 1.) 2.) 3.) 4

7 4.) 5.) 6.) 5

8 On the above graph, indicate with arrows where the chance of precipitation is both most likely and least likely. 7.) Base your answers to the next (3) questions on the graph below: 6

9 8.) 9.) 10.) 7

10 11.) 8

11 12.) What time would the probability of precipitation be both highest and lowest, respectively? A) 12:00 AM (Midnight) and 5:00 PM B) 10:00 AM and 10:00 PM C) 5:00 PM and 5:00 AM D) 12:00 PM (Noon) and 10:00 AM xp 9