The RealFeel Temperature is an index from Boston.com's weather partner Accuweather that describes what the temperature really feels like. It is a composite of the effects of temperature, wind, humidity, sunshine intensity, cloudiness, precipitation and elevation on the human body -- everything that affects how warm or cold a person feels.

Temperature by itself gives only part of the picture. Other measures, like the wind chill or heat index, include temperature and only one additional element like wind speed or humidity and many are designed to measure effects on an inanimate object or an unclothed person. None of them tell what it really feels like to an appropriately dressed person. RealFeel Temperature includes everything that affects how warm or cold a person feels.

For example, on a steamy July day without a breath of wind, with the noon sun beating down, the official temperature (which is measured in the shade) may be 92 degrees. But when you stand outside in the scorching sun, it may feel like 118 degrees. In contrast, when the official temperature is 92 degrees with a nice breeze blowing and low humidity, with bright sunshine but late in the day so the sun intensity is not as strong, it may feel like 86 degrees.

The Tanning Index is a forecast of the amount of ultraviolet expected to reach the Earth's surface when the sun is highest in the sky. The higher the index, the faster UV radiation causes damage to the skin and eyes. The index depends on the elevation of the sun in the sky, the cloud cover, and amount of ozone. It predicts UV levels on a 0-10+ scale.

Be sure to take extra precautions against exposure to the sun when the UV Index predicts exposure levels of moderate to above (5-10+).

"Heating degree days" is a measure of the amount of energy needed to heat a house to an inside temperature of 65 degrees fahrenheit over a 24-hour period. It is calculated by taking the difference between 65 degrees and the average outside temperature for the day.

For example, if the average temperature for a day was 40 degrees, the heating degree days would be 25. If the average daily temperature is above 65 degrees, heating degree days are 0.

The UV Index represents the amount of skin-damaging ultraviolet radiation reaching the earth's surface. It is influenced by factors such as time of day, season, cloud cover, ozone, altitude, and latitude. The index is reported on a scale of 1 to 11+, with 1-2 rated as low, 3-5 as moderate, 6-7 as high, 8-10 as very high, and 11+ as extreme. The US Environmental Protection Agency recommends sunglasses and sunscreen in all but the very lowest index category.

In Boston, values typically range from 1 to 2 (low) in the winter to 6-7 (high) in the summer. The sunniest summer days, however, can easily be in the very high range.