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.

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.

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.