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Here are some weather terms which will help to understand the data provided by our weather station.

Air Pressure

The force exerted on a surface by the weight of the air above it.

Apparent Temperature

A measure of human discomfort due to combined heat and humidity. It measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities. The apparent temperature is less than the actual air temperature when the humidity is relatively low and that the apparent temperature indicates the reduced stress and increased comfort associated with the higher rate of evaporative cooling of the skin.

Apparent temperatures greater than 80F (27C) are likely to produce some discomfort. Values in excess of 105F (41C) may be dangerous and even life-threatening, with severe heat exhaustion or heat stroke possible if the exposure is prolonged or physical activity is high. The degree of stress may vary with age, health, and body characteristics.

The apparent temperature does not consider the effects of air movement (wind speed) or exposure to sunshine on the degree of discomfort or stress.


An instrument used for measuring air pressure. The two most common types are the mercury barometer and the aneroid barometer.

Barometric Pressure

The actual pressure value indicated by a pressure sensor.

Barometric Tendency

The amount and direction of change in barometer readings over a three-hour period.

Degree Day

It gauges the amount of heating or cooling needed for a building using 65F as a baseline. To compute heating/cooling degree-days, take the average temperature for a day and subtract the reference temperature of 65F. If the difference is positive, it is called a Cooling Degree Days. If the difference is negative, it is called a Heating Degree Days. The magnitude of the difference is the number of days.

For example, if your average temperature is 50F for a day in September, the difference of the average temperature for that day and the reference temperature of 65F would yield a minus 15. Therefore, you know that you are going to have Heating Degree Days that day. Since the magnitude of the difference is 15, you know that you are going to have 15 Heating Degree Days. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs.

Dew Point (Dew-Point Temperature)

A measure of atmospheric moisture. The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater amount of water vapor in the air mass.

Electromagnetic Radiation

The energy produced by an oscillating electrical (and magnetic) field, transmitted by photons.


A process by which liquid changes into a gas or vapor.


Combination of evaporation from free water surfaces and transpiration of water from plant surfaces to the atmosphere.

Heat Index

The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. To find the Heat Index (HI), look at the Heat Index (HI) Chart below. As an example, if the air temperature is 90F (found at the left side of the table) and the Relative Humidity (RH) is 70% (found at the top of the table), the Heat Index (HI)--or how hot it actually feels--is 106F. This is at the intersection of the row 90F and the 70% column.

This index was devised for shady, light wind conditions. Exposure to full sunshine can increase Heat Index (HI) values by up to 15F. Also strong winds, particularly with very hot, dry air, can be extremely dangerous. Any value Heat Index (HI) greater than 105F is in the Danger Category. When the Heat Index is between 105-115F for 3 hours or more, a Heat Advisory will be issued by the local National Weather Service Forecast Office.

Category Classification Heat Index/Apparent Temperature (F) General Affect on People in High Risk Groups
I Extremely Hot 130F or Higher Heat/Sunstroke HIGHLY LIKELY with continued exposure
II Very Hot 105F - 130F Sunstroke, heat cramps, or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity
III Hot 90F - 105F Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity
IV Very Warm 80F - 90F Fatigue POSSIBLE with prolonged exposure and/or physical activity

See Apparent Temperature.


Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.


Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red).

Relative Humidity

A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

Solar Radiation

The electromagnetic radiation emitted by the sun.


Water discharged into the atmosphere from plant surfaces.

Ultraviolet Radiation

The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.

UV (Ultraviolet) Index

This index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays. It was designed by the National Weather Service and the Environmental Protection Agency (EPA). Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities.

Currently, the computation of the UV Index does not include the effects of variable surface reflection (e.g., sand, water, or snow), atmospheric pollutants or haze. By following the few simple precautions in the table below, you can greatly reduce your risk of sun related injuries (blistering sunburns, as well as longer-term problems like skin cancer and cataracts).

UV Index Value Exposure Category Time to Burn Actions to take at noon
0-2 Minimal 60 minutes Apply SPF sunscreen
3-4 Low 45 minutes Apply SPF sunscreen, wear a hat.
5-6 Moderate 30 minutes Apply SPF 15, wear a hat
7-9 High 15-24 minutes Apply SPF 15 to 30 sunscreen, wear a hat and sunglasses. Limit midday exposure
10+ Very High 10 minutes Apply SPF 30 sunscreen. Wear a hat, sunglasses, and protective clothing.
Time to burn and actions apply to people with a Type II, fair skin that sometimes tans and usually burns. People with lighter skin need be more cautious. People with darker skin may be able to tolerate more exposure. But even dark skin can burn.

When the Index is High or Very High, try to minimize your outdoor activities between the peak hours of 10 AM and 4 PM when the sun is most intense. When the Index is 10 or higher, stay indoors if possible, otherwise be sure to take all the other necessary precautions.

Wind Chill

The wind chill is the effect of the wind on people and animals. The wind chill temperature is based on the rate of heat loss from exposed skin caused by wind and cold and is to give you an approximation of how cold the air feels on your body.

As the wind increases, it removes heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder. If the temperature is 0F and the wind is blowing at 15 mph, the wind chill temperature is -19F. At this level, exposed skin can freeze in just a few minutes.

The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool. The inanimate object will not cool below the actual air temperature. For example, if the temperature outside is -5F and the wind chill temperature is -31F, then your car's radiator temperature will be no lower than the air temperature of -5F.


See many more weather terms in the National Weather Service's JetStream Online Weather School Glossary



Sandy Bay Yacht Club, P.O. Box 37, 5 T Wharf, Rockport, MA 01966 USA
(978)546-9433 Clubhouse, (978)546-6240 Office

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