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Foothill Ranch, CA Weather

Sensor Definitions

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This page explains some of the weather sensor readings.


Wind Chill

Wind Chill takes into account how the speed of wind affects our perception of the air temperature.  Our bodies warm the surrounding air molecules by transferring heat from our skin.  If there's no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules.  However, wind sweeps that warm air surrounding the body away.  The faster the wind blows, the faster heat is carried away and the colder you feel.  Wind has a warming effect at higher temperatures.


Heat Index

The Heat Index uses temperatures and the relative humidity to determine how hot the air actually "feels."  When humidity is low, the temperature we will feel lower than the actual air temperature, since perspiration evaporates rapidly to cool the body.  However, when humidity is high (i.e., the air is more saturated with water vapor) the temperature will feel higher than the actual air temperature, because perspiration evaporates more slowly.


THSW Index (temperature/ humidity/ sun/ wind)

The THSW Index is similar to the heat index but also includes the heating effects of sunshine and the cooling effects of wind (like wind chill)



Humidity itself simply refers to the amount of water vapor in the air.  However, the amount of water vapor that the air can contain varies with air temperature 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 is a ratio of the air's water vapor content to its capacity.  So, for this site, when you see humidity, that means relative 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 10g of water vapor which contains 4g of water vapor, the relative humidity would be 40%.  Adding 2g more water vapor (total of 6g) would change the humidity to 60%.  If that same parcel of air is then warmed so that it has a capacity for 20g of water vapor, the relative humidity drops to 30% even though water vapor content does not change. 


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 content: low dew point indicates low water vapor content.  High dew point also indicates possible rain, severe thunderstorms, and tornados.


You can also use dew point to predict the minimum overnight temperature.  Provided no new fronts are expected overnight and the afternoon relative humidity is greater than or equal to 50%, the afternoon's dew point indicates an idea of what the minimum temperature to expect overnight, since the air can never get colder than the dew point.


Barometric Pressure

The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth.  This pressure is known as atmospheric pressure.  Generally, the more air above an area, the higher the atmospheric pressure, this means that atmospheric pressure changes with altitude.  Atmospheric pressure is greater at sea-level than on a mountain top.  To compensate for this difference and to compare between different locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea-level pressure.  The adjusted pressure is known as barometric pressure.


Barometric pressure also changes with local weather conditions, making barometric pressure an extremely important and useful weather forecasting tool.  High pressure zones are generally associated with fair weather while low pressure zones usually mean poor weather conditions.  For forecasting purposes, the barometric pressure value is less important than the change in barometric pressure.  Rising pressure indicates improving weather and falling pressure indicates deteriorating weather conditions.


UV Index

UV Index assigns a number between 0 and 16 to the current UV intensity.  The higher the number, the higher the danger of sunburn.  If the index value is between 0-2, you have a Low exposure category.  Between 3-4, your risk is Moderate.  Between 5-6, your risk is High.  Between 7-9, your risk is Very High.  If it's 10+  your risk is Extreme.


UV Index

Index Values

Exposure Category

0 - 2


3 - 4


5 - 6


7 - 9



Very High


Evapotranspiration (ET)

Evapotranspiration (ET) is a measurement of the amount of water vapor returned to the air in a given area.  It combines the amount of water vapor returned through evaporation (from wet vegetation surfaces and leaves) with the amount of water vapor returned through transpiration (exhaling of moisture through plant skin) to arrive at a total.  Essentially, ET is the opposite of rainfall and it's expressed in the same unit as rain.  (Inches)


Solar Radiation

What we call "current solar radiation" is technically known as Global Solar Radiation, a measure of intensity of the sun's radiation reaching a horizontal surface.  This irradiance includes both the direct component from the sun and the reflected component from the rest of the sky.  The solar radiation reading gives a measure of the amount of solar radiation hitting the solar radiation sensor at any given time, expressed in Watts/sq. meter.


UV (Ultra Violet) Radiation

Energy from the sun reaches the earth as visible, infrared, and ultraviolet (UV) rays.  Exposure to UV rays can cause numerous health problems, such as sunburn, skin cancer, skin aging, cataracts, and can suppress the immune system.  The UV sensor can help analyze the changing levels of UV radiation and can advise of situations when levels become too high.



MED (Minimum Erythemal Dose) is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure.  In other words, 1 MED will result in a reddening of the skin.  Because different types burn at different rates, 1 MED for persons with dark skin is different from persons with very light skin.


Both the U.S. Environmental Protection Agency (EPA) and Environment Canada have developed skin type categories correlating characteristics of skin with rates of sunburn. See Table A-1 “EPA Skin Phototypes” and Table A-2 “Environment Canada Skin Types and Reaction to the Sun” for a description of skin types.

EPA Skin Phototypes

Skin Phototype

Skin color

Tanning & Sunburn history

1 - Never tans, always burns

Pale or milky white; alabaster

Develops red sunburn; painful swelling, skin peels

2 - Sometimes tans, usually burns

Very light brown; sometimes freckles

Usually burns, pinkish or red coloring appears; can gradually develop light brown tan

3 - Usually tans, sometimes burns

Light tan; brown, or olive; distinctly pigmented

Rarely burns; shows moderately rapid tanning response

4 - Always tans; rarely burns

Brown, dark brown, or black

Rarely burns; shows very rapid tanning re-sponse

Table A-1


Environment Canada Skin Types and Reaction to the Sun1

Skin Type

Skin Color

History of Tanning & Sunburning



Always burns easily, never tans



Always burns easily, tans minimally


Light Brown

Burns moderately, tans gradually


Moderate Brown

Burns minimally, tans well


Dark Brown

Burns rarely, tans profusely



Never burns, deep pigmentation

Table A-2

 1Developed by T. B. Fitzpatrick of the Harvard Medical School

  UV Dose (MEDs)

UV Dose and Sunburn - Use this plot to estimate the MED dose leading to sunburn. A person with Type II (Environment Canada) skin type might choose 0.75 MED as the maximum for the day; in contrast, a person with Type V (Environment Canada) Skin Type might consider 2.5 MEDs a reasonable dose for the day. NOTE: the Vantage Pro assumes a Fitzpatrick (Environment Canada) Skin Type of II.