... to help you better understand the information presented on the home page and the graphs page here at sanpanchoweather.com.
Outside Temperature is measured in mid-town San Pancho, just off Third World Avenue, near the football (soccer) field. The sensor is in a vented and shielded enclosure that minimizes the solar radiation induced temperature error.
Inside Temperature reading is not significant since it measures the temperature inside my home, most of which is outside!
Wind Chill - Unlikely in San Pancho!
Heat Index is an apparent temperature reading which uses temperature and the relative humidity to determine how hot the air actually feels. When humidity is low, the apparent temperature will be lower than the 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 apparent temperature feels higher than the actual air temperature, because perspiration evaporates more slowly.
The Heat Index is calculated only when the air temperature is above 57 F or 14 C, because it's insignificant at lower temperatures. It is also not calculated above 135 F or 52 C (which we wouldn't want to be around for in any case).
Humidity Out is simply 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, 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. This reading, then, is the relative humidity of the air in San Pancho. Relative humidity is an important factor in determining the amount of evaporation from plants and wet surfaces since warm air with low humidity has a large capacity to absorb extra water vapor.
Humidity Inside is the relative humidity in my home, which is largely open to the outdoors. See the definition of Humidity above.
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 and fog (and frost). It 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. A high dew point indicates a better chance of rain, severe thunderstorms, and tornados.
Barometer - 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 high the atmospheric pressure. This in turn means that atmospheric pressure changes with altitude. For example, atmospheric pressure is greater here at sea-level in San Francisco that it is in Mexico's central highlands.
To compensate for this difference and to facilitate comparison between locations with different altitudes, atmospheric pressure is generally adjusted to the equivalent sea-level pressure. This adjusted pressure is know as barometric pressure. Barometric pressure also changes with local weather conditions, making barometric pressure and extremely important and useful weather forecasting tool.
High pressure zones are generally associated with fair weather. Low pressure zones are general associated with poor weather. In general, rising pressure indicates improving weather conditions while falling pressure indicates deteriorating weather conditions.
Solar Radiation - What we call current solar radiation is technically known as Global Solar Radiation. It is a measure of the 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 on the display above gives a measure of the amount of solar radiation hitting the San Pancho weather station's solar radiation sensor at any given time. Solar Radiation is expressed in Watts/sq. meter. The range of this reading is from 0 to 1800 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. UV radiation can be expressed on two scales: MEDs (Minimum Erythemal Dose ... or the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure), or on the UV Index.
I have chosen to display the UV Index, with is an intensity measurement first defined by Environment Canada and since adopted by the World Meteorological Organization. UV Index assigns a number between 0 and 16 to the current UV intensity. The US EPA categorizes the Index values as shown below. The lower the number, the lower the danger of sunburn. The Index Values you see are real-time measurements.
Index Value/Exposure Categories are:
0 - 2 / Low
3 - 4 / Moderate
5 - 6 / High
7 - 9 / Very High
10+ /Extreme
Daily ETo - 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 the stoma or pores on the leaves) with the amount of water vapor returned through transpiration (exhaling of moisture through plant skin) to arrive at a total. Another way to look at it: ET is the opposite of rainfall. It is expressed in the same units of measure, inches or millimeters. The San Francisco weather station uses air temperature, relative humidity, average wind speed, and solar radiation data to estimate ET, which is calculated once an hour, on the hour.
Wind Direction and Speed - See the compass? Right.
Forecast - Located below the wind gauge, this is a short-term outlook generated by the weather station. Don’t use this prediction for important decisions!
Rain - The weather station incorporates a tipping-bucket rain collector that measures 0.01 inch for each tip of the bucket and those measurements are displayed here in real-time. Four separate variables track rain totals: rain storm, daily rain, monthly rain, and yearly rain.
Note the start time for each rain storm as rainfalls regularly occur over one or more days. The rain rate is a calculation based on the interval of time between each bucket tip, as described above.
Comments on the Trend/Graphs Page:
Temperature-Humidity Graph - Tracking these two factors during the day. Note the temperature axis is on the right, the Humidity axis is on the right. The temperature axis will change to accommodate the range of today’s readings.