Note: | Vantage Pro2 measures Heat Index only when the air temperature is above 57° F |
| (14° C), because it’s insignificant at lower temperatures. (Below 57°, Heat Index = |
| the air temperature.) The Heat Index is not calculated above 135° F (52° C). |
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|
| Temperature/Humidity/Sun/Wind (THSW) Index |
| The THSW Index uses humidity and temperature like the Head Index, but |
| also includes the heating effects of sunshine and the cooling effects of |
| wind (like wind chill) to calculate an apparent temperature of what it |
| “feels” like out in the sun. The THSW Index requires a solar radiation sen- |
| sor. |
| Humidity |
| Humidity itself simply refers to the amount of water vapor in the air. How- |
| ever, the amount of water vapor that the air can contain varies with air tem- |
| perature and pressure. Relative humidity takes into account these factors |
| and offers a humidity reading which reflects the amount of water vapor in |
| the air as a percentage of the amount the air is capable of holding. Relative |
| humidity, therefore, is not actually a measure of the amount of water vapor |
| in the air, but a ratio of the air’s water vapor content to its capacity. When |
| we use the term humidity in the manual and on the screen, we mean rela- |
| tive humidity. |
| It is important to realize that relative humidity changes with temperature, |
| pressure, and water vapor content. A parcel of air with a capacity for 10 g |
| of water vapor which contains 4 g of water vapor, the relative humidity |
| would be 40%. Adding 2 g more water vapor (for a total of 6 g) would |
| change the humidity to 60%. If that same parcel of air is then warmed so |
| that it has a capacity for 20 g of water vapor, the relative humidity drops to |
| 30% even though water vapor content does not change. |
| Relative humidity is an important factor in determining the amount of |
| evaporation from plants and wet surfaces since warm air with low humid- |
| ity has a large capacity to absorb extra water vapor. |
| Dew Point |
| Dew point is the temperature to which air must be cooled for saturation |
| (100% relative humidity) to occur, providing there is no change in water |
| vapor content. The dew point is an important measurement used to predict |
| the formation of dew, frost, and fog. If dew point and temperature are close |
| together in the late afternoon when the air begins to turn colder, fog is |
| likely during the night. Dew point is also a good indicator of the air’s |
| actual water vapor content, unlike relative humidity, which takes the air’s |
| temperature into account. High dew point indicates high water vapor con- |
| tent; low dew point indicates low water vapor content. In addition a high |
| dew point indicates a better chance of rain, severe thunderstorms, and tor- |
| nados. |
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