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Forecaster's Discussion. Submit Storm Report. Text Bulletins. Climate Plots. Fire Weather. Air Quality. Follow us on Twitter. It could also be very humid but cool or cold low dew point and you would feel comfortable. The temperature to which air must be cooled at a constant pressure to become saturated. In western Europe, this period may exist from the first week in July to mid- August and is often the period of the greatest frequency of thunder.
Named for Sirius, the Dog Star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months. DROUGHT Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.
The temperature recorded is considered identical to air temperaure. One of the two therometers that make up a psychrometer. DUSK The period of waning light from the time of sunset to dark. See twilight and dawn. See lunar eclipse or solar eclipse.
This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process. Days and nights are most nearly equal in duration.
In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September It is the opposite physical process of condensation. EYE The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles.
In general, when the eye begins to shrink in size, the storm is intensifying. More commonly used in areas that observe the English system of measurement. Created in by Gabriel Daniel Fahrenheit , a German physicist, who also invented the alcohol and mercury thermometers. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.
FLOOD High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist. Also called a prediction. The temperature at which a liquid solidifies under any given set of conditions.
Pure water under atmospheric pressure freezes at 0 degrees Celsius or 32 degrees Fahrenheit. It is the opposite of fusion. In oceanography, the freezing point of water is depressed with increasing salinity. FROST The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing. For marine interests, it can be categorized as a moderate gale 28 to 33 knots , a fresh gale 34 to 40 knots , a strong gale 41 to 47 knots , or a whole gale 48 to 55 knots.
In , the World Meteorological Organization defined the categories as near gale 28 to 33 knots , gale 34 to 40 knots , strong gale 41 to 47 knots , and storm 48 to 55 knots. HAIL Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel.
Individual lumps are called hailstones. HEAT A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.
This is not the actual air temperature. For more information, please see our Heat Stress index page. Heat Stress is more of the condition that occurs when you have a high Heat Index so Heat Index is the more common term, but either is acceptable. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.
The signs of heat stroke include when an individual's body temperature is greater than degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness.
Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun. Please see our Heat Stroke Symptoms page for more information. It could last from several days to several weeks. The Weather Channel uses the following criteria for a heat wave: a minimum of ten states must have 90 degree plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.
It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity,and specific humidity. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are example of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation.
Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spray. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects. See the psychrometer as an example.
See What is a Hygrometer for more detail. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold. ICE The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail, for example. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust".
Primarily occurs during a thaw in the late winter or early spring. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems. It usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes more or less rapidly as it flows. See glaze. One inch of mercury is equivalent to See barometric pressure. First divised in by Evangelista Torricelli , an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.
A first frost normally precedes this warm spell. KNOT A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations.
A knot is equivalent to 1. LIGHTNING A sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air.
For an example, see ball lightning. The moon does not have any light of its own, instead, it reflects the sun's light. During a lunar eclipse, the moon is in the earth's shadow. It will often look dim and sometimes copper or orange in color. It uses a long glass tube, open at one end and closed at the other.
After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends.
The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern.
Measurement is taken in inches of mercury. Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. First used by Evangelista Torricelli , an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few.
A scientist who studies the atmosphere and atmospheric phenomena. Also referred to as the temperate region. For sites that report two observations over a hour period, MSC uses the following procedure to determine:. A time that best suits the observer for regular observations should be determined and maintained every day. A cloud forms in the atmosphere as a result of condensation of water vapour.
Condensation nuclei, such as smoke or dust particles, form a surface upon which water vapour can condense.
Normals for some elements are derived from less than 30 years of record but can still be considered useful. The minimum number of years used are indicated by a "code" defined as:. For example, a day with a mean temperature of Cooling degree-days are used primarily to estimate the air-conditioning requirements of buildings.
All cooperative climate stations are representative of their area and have trained and reliable volunteer observers who have instruments and associated equipment installed on their property. CCN observation data undergoes basic automatic quality checks.
Depending on individual climate observing programs and climate elements, weather observations at a climate station are recorded at three basic intervals:. Clocks are moved ahead one hour on the second Sunday in March, and back one hour on the first Sunday of November. Degree-days for a given day represent the number of Celsius degrees that the mean temperature is above or below a given base temperature.
Saturation occurs when the air is holding the maximum water vapour possible at that temperature and atmospheric pressure. The direction of the maximum gust true or geographic, not magnetic from which the wind blows.
Expressed in tens of degrees 10's deg , 9 means 90 degrees true or an east wind, and 36 means degrees true or a wind blowing from the geographic North Pole. Drizzle drops are too small to cause appreciable ripples on the surface of still water.
The drops appear almost to float in the air, thus making visible even slight movements of air. The dry bulb temperature is the value taken from a thermometer when the bulb of the thermometer is dry. It reflects the ambient temperature independent on the moisture content of the air. The elevation in metres m refers to the elevation of the observing location above mean sea level.
The elevation of each site is given to the nearest metre and is generally the height of ground on which the instruments are exposed. Prior to April 1, , the elevation at principal stations located at airports was generally the established by the elevation of the aerodrome. For principal stations not located at airports the elevation was established by the elevation of the barometer cistern. The physical process by which a liquid, such as water, is transformed into a gaseous state, such as water vapour.
It is the opposite physical process of condensation. Evaporation refers to the calculated lake evaporation occurring from a small natural open water-body having negligible heat storage and very little heat transfer at its bottom and sides.
It represents the water loss from ponds and small reservoirs but not from lakes that have large heat storage capacities. Lake evaporation is calculated using the observed daily values of pan evaporative water loss, the mean temperatures of the water in the pan and of the nearby air, and the total wind run over the pan. A visible aggregate of minute water droplets suspended in the air at or near the surface of the earth, reducing horizontal visibility to less than 1 km.
It is created when the temperature and the dew point of the air have become the same, or nearly the same. Drizzle, the drops of which freeze on impact with the ground or with objects at or near the ground.
This obstruction to vision consists mainly of super-cooled droplets that usually deposit rime or glaze on objects or surfaces with below-freezing temperatures. The definition of freezing fog is the same as for fog , except that it occurs when the temperature is in the range of It may be reported at temperatures colder than Alternately, frost or hoar frost describes a deposition of ice crystals on objects by direct sublimation of water vapour from the air.
Number of frost free days is calculated based on the last occurrence of frost in spring and the first occurrence of frost in autumn. This is an especially important parameter for agriculture, because the variability in the number of frost free days is crucial for many agricultural activities such as planting and harvesting, but the impact of a strong frost can become an economic problem that affects the prices.
Gusts are sudden, rapid and brief changes in the wind speed. They are characterized by more or less continual fluctuations between the high peak and low lull speed. The extreme gust speed is the instantaneous peak wind observed from the anemometer dials, abstracted from a continuous chart recording, or from a data logger.
Precipitation of small balls or pieces of ice with a diameter ranging from 5 to 50 mm or more. Hail is generally observed during heavy thunderstorms. Heating degree-days are used primarily to estimate the heating requirements of buildings. Humidex is an index to indicate how hot or humid the weather feels to the average person. It is derived by combining temperature and humidity values into one number to reflect the perceived temperature.
For example, a humidex of 40 means that the sensation of heat when the temperature is 30 degrees and the air is humid feels more or less the same as when the temperature is 40 degrees and the air is dry. Hourly Humidex values are only displayed when the air temperature is 20C or greater and the humidex value is at least 1 degree greater than the air temperature.
The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail for example. Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust. They are mainly visible when they glitter in sunshine and occur only at very low temperatures and stable air masses.
A type of fog composed of suspended particles of ice or ice crystals 20 to microns resulting from the freezing of tiny supercooled water droplets. Precipitation of transparent or translucent pellets of ice, which are spherical or irregular shaped, having a diameter of 5 mm or less.
They are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the partial melting of pellets.
Ice pellets usually bounce when hitting hard ground and make a sound on impact. They can fall as continuous precipitation or in showers. Latitude co-ordinates for climate stations are generally for the instrument site; however prior to April 1, at principal stations airports the locations given were normally that of the official airport locations. The accuracy of these locations depended on the quality of the reference maps available at the time. The latitude of each site is given to the nearest second or to the nearest 0.
All locations in Canada are north of the equator. The Local Standard Time LST is used for observation purposes and is that of the standard time zone in which the station is located, whether or not "daylight saving time" is adopted for other purposes. In Canada, Local Standard Time is commonly used for archiving surface weather observations. Longitude co-ordinates for climate stations are generally for the instrument site; however prior to April 1, at principal stations airports the locations given were normally that of the official airport locations.
The longitude of each site is given to the nearest second or to the nearest 0. The accuracy of these locations depends on the quality of the reference maps available at the time. Negative values of longitude denote degrees west of the Greenwich Meridian. All locations in Canada have negative values of longitude. See climatological day. The atmospheric pressure at mean sea level usually determined from the observed station pressure.
Mean sea level pressure is computed from the station pressure and reported so that the barometric pressures at stations of different elevations can be compared at a common level for analysis purposes.
These tables give the average number of days per month or year on which a specific meteorological event occurs. For example:. Observations of atmospheric phenomenon including the occurrence of weather and obstructions to vision have been taken at many hourly reporting stations. Otherwise it will appear with a modifier of moderate or heavy. At some stations observations are made by automatic aviation weather reporting systems.
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