Meteorology 101: Understanding Indexes

Your Vantage Pro2 or Vantage Vue gives you more than just raw weather data. While raw data is very useful, our systems offer several indexes that can increase your understanding of what that data means if you happen to be, you know, a human being. They can tell you what you want to know in terms of your comfort in the current conditions.

WIND CHILL: This index tells you not just what the air temperature is on a cool day, but what it feels like it is. On a cold day, humans create a thin layer of warm air close to the skin. A brisk wind can whisk away this layer, leaving us feeling colder. How much colder is expressed by the wind chill index. We’ve all experienced wind chill. When we hit the ski slopes in the morning, the air temperature was 18°F/-8°C and the wind was calm. Lovely! But about 2 in the afternoon, the wind picked up to a brisk 20 mph. Even with the air temperature warmer at 20°F/-6°C, it now feels like 4°F/-16°C. As wind increases, so do the dangers associated with cold. Should that wind rise to say, 30 mph and air temp drops to 5°F/-15°C, the wind chill will be -19°F/-28°C, making frostbite likely in 30 minutes or less.

Where you can see your wind chill index: On the Vantage Vue or Vantage Pro2 console, pressing 2ND and WIND(CHILL) shows you the wind chill in the field that shares dew point and heat index. On the Vantage Vue console, you'll also see the minimum wind chill recorded for the day and the time it occurred in the Weather Center. In the WeatherLink app, scroll down to see it in the "Feels Like" section. On WeatherLink.com, you'll see it on your bulletin dashboard with other temperature readings (including outside temp and dew point.)

HEAT INDEX: We’ve all heard the grumbles of people enduring a heatwave in a location with high humidity: “It’s not the heat, it’s the humidity!” What they are referring to is the fact that a person sitting in the shade on a 100°F/38°C day in Phoenix might be quite comfortable (especially if she has an ice-laden lemonade in her hand), while a person sitting in the shade on a 100°F/38°C degree day in New Orleans is facing imminent heat stroke, lemonade or not. What could have made the difference? If the humidity in New Orleans is 60% (that’s below the 72.5% year-round average in New Orleans), the heat index will be 129°F/54°C. Her counterpart with the lemonade in Phoenix in the humidity of, let’s say of 30%, is going to feel like it’s just 102°F/39°C (a perfectly balmy day in Phoenix!). The person in Phoenix will perspire and the perspiration will evaporate, making her feel much cooler. But for the folks on that hot day in New Orleans, perspiration will not evaporate quickly into the already sodden air. The body can be overwhelmed in its struggle to stay cool and heat stroke, a dangerous condition that requires emergency treatment, is a real threat.

Where you can you see your heat index: On the Vantage Vue or Vantage Pro2 console, pressing 2ND and TEMP(HEAT) shows you the heat index same field that shares dew point and wind chill. On the Vantage Vue console, you'll also see the maximum heat index recorded for the day and the time it occurred in in the Weather Center. In the WeatherLink app, scroll down to see it in the "Feels Like" section. On WeatherLink.com, you'll see it on your bulletin dashboard with other temperature readings (including outside temp and dew point.)

THW INDEX: This index adds a little more information to the heat index by adding wind. (The heat index assumes winds are calm.) While we usually think of wind as cooling, if it is hot enough (over 93°F/34°C), wind will make you feel even hotter.

Where you can you see your THW Index: In the WeatherLink app, scroll down to see it in the "Feels Like" section. On WeatherLink.com, you'll see it on your bulletin dashboard in it's own tile, if you have selected it from the parameters list.

THSW INDEX: This is the granddaddy of all the indexes! If you know the THSW, you know how it will feel to a person out there in the current conditions. It stands for Temperature Humidity Sun Wind: all the players! Like the heat index, it uses temperature and humidity. Like the wind chill and THW indexes, it also includes the effect of wind. And then it adds further clarity by including the heating effects of solar radiation. (That is why you need a Vantage Pro2 Plus, or an added-on solar radiation sensor to see this index.) It is calculated only when air temperature is at or above 68°F/20°C.

Where you can you see your THSW Index: On a Vantage Pro2 console (listening to a sensor suite with a solar radiation sensor), press 2ND and TEMP(HEAT), then TEMP(HEAT) again to see the THSW index in the same field that shares dew point and wind chill. In the WeatherLink app, scroll down to see it in the "Feels Like" section. (It will appear instead of THW.) On WeatherLink.com, you'll see it on your bulletin dashboard in it's own tile, if you have selected it from the parameters list.

(Now if only we could come up with a Weather Enthusiast Index that gives us an indication of how interesting today’s combination of weather data will feel to the average weather buff. How fun would that number be?)