Weather Terms

Posted in About Weather.

There are lots of sources of information for weather terms online and Weather Shop has selected a few which we think may be of interest. Please read below and click for extra information and a link to the original source.

Cloud Base

The cloud base is often calculated by measuring the Temperature and Humidity. This way is hugely simplified compared to how professional meteorologists would do it, however the basic principle is the same.

A professional meteorologist would use a chart called a Tephigram which will give them the various temperature and humidity readings at various altitude. The altitude is measured as a pressure rather than a height: as height varies according to the environmental temperature.

If you imagine a ball of air in front of you, under normal circumstances it will exhibit the same properties as the air around it in terms of temperature and humidity. This is what is being read by your weather station. If you were to lift this ball of air to say a different altitude, the ball will now expand due to the lower pressure. As pressure, temperature and volume are all related, dropping the pressure and keeping the volume constant will cause the temperature to fall. In dry air the rate at which the temperature drops is relatively liner.

Humidity is the amount of water vapour that can exist in that ball of air for the given conditions. The higher the temperature the more water vapour can exist within that air. Conversely when the air cools the less water vapour can exist. When we measure humidity we are measuring the amount of water vapour in the air.

So as we are lifting the ball of air we are keeping the amount of water in the air fixed. However, as we cool the air we eventually get to a temperature where the the total amount of water vapour that can exist equals the amount of water vapour that we have in the ball of air. When this happens the water vapour will condensate into water droplets.

By taking into consideration the rate at which the temperature falls with height, you can calculate the height at which the cloud base will form.

Professional meteorologists also have at their disposal another piece of kit that can tell them the height of the cloud base and that is a ceilometer. This basically shines an infra-red laser vertically in pulses. The pulses are then timed so that when the are reflected back you can deduce the height of the cloud base. Using this method you can also deduce the height of the cloud. (MJB)

Heat Index

The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.

The difference between Relative and Absolute Pressure

The difference between Relative and Absolute PressureAbsolute air pressure provides the display of the true measured air pressure of the current time and location. Relative air pressure is the one value that is calculated back to sea level from the local absolute air pressure and can thus be taken as a reference for weather condition and weather development for the entire country.

Dew Point

The dew point is the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water. Condensed water is called dew when it forms on a solid surface.

Fog

Reduction in visibility to under 1 km caused by suspension of minute water droplets (water fog) or ice crystals (Ice fog – q.v.). Water fogs are further sub-divided according to the process by which the fog forms, e.g. Radiation fog (caused mainly due to loss of surface heat from the ground at night in conditions of near-calm wind and high relative humidity); Advection fog (caused by movement of humid air over a relatively colder surface); Upslope fog (adiabatic cooling of air having high relative humidity as it climbs over high ground) and Evaporation fog (caused by evaporation into cold air which lies over a relatively warm water surface). (If the visibility is below 200M but greater than ~50M then it is usually referred to as ‘Thick Fog’ (& colloquially in the UK as ‘motoring fog’) & if below 50M then ‘Dense Fog’. However, there are different criteria for climatological stations, and other services will have different rules – treat this note as a guide only.)

Fog Point

(strictly fog-point temperature) The air temperature (as measured in a standard thermometer screen) at which fog is expected/does form. Its calculation (before an event) is usually based on empirical work which employs either the surface air temperature/dew point at some time earlier in the day, or by construction on a thermodynamic diagram. The fog point is lower than the air-mass dew point, because as air cools through the evening and night, moisture is condensed out on contact with the chilled land surface, and this lowers the dew point from afternoon values.

Freezing Fog

As for the definition of fog (above), but the droplets are super-cooled (i.e. temperature below zero), and strictly, the fog should be depositing rime-ice. However, in METAR/TAF coding, as long as the air temperature is below zero degC, then fog is coded as ‘freezing’ irrespective of whether rime is observed.

Basic Barometer Forecast Rules

  • Look at the change in 3 hours.
  • If the pressure is descending, there is a low pressure coming.
  • If it’s ascending (or rising), the low is passing or a high pressure is coming.
  • When the pressure is changing rapidly (> 6 hPa/3 hours), it’s windy (or potentially windy).
  • More detailed: Sinking (falling) slowly (0.5 – 3 hPa in 3h): low is weak, dying or moving slowly. You might get some rain but typically no high winds.
  • Sinking (falling) moderately (3-6 hPa/3h): rapid movement or deepening low. Moderate winds and rain in warm front. The low is passing you fast so day after tomorrow will typically be fine.
  • Sinking (falling) 6-12 hPa/3h: Storm.
  • Rise is connected to gradually drier weather.

If the centre of a low pressure system drops 24mb within 24 hours this is storm is considered to be a ‘bomb’ : mostly due to the explosive nature of its development. These ‘bombs’ are also short lived, lasting approximately 48 hours. The most notable features of ‘bombs’ are very heavy rain and strong winds. The great storm (‘Hurricane’) of 1987 is an example of one of these ‘bombs’.

“Thunder heard!”: rumbles, crackles and bangs – why the difference?

The noise we hear when a thunderstorm is in progress is caused by the near-instantaneous expansion of air (due to intense heating) along the path of the lightning discharge. The type of noise we hear (rumble, crackle or bang) depends upon the distance of the observer from the lightning path and the path ‘structure’, e.g. simple stroke, branched pattern etc. A lengthy drawn-out rumble/crackle of thunder is due to the slow speed of sound (330 m/s) through air – the lightning ‘flash’ of course travels at the speed of light (several orders of magnitude faster) – effectively our eyes see the discharge as soon as it happens; the sound wave takes much longer to reach our ears, unless the lightning strike is very close by – when we would experience a violent ‘bang’ . . and hopefully nothing worse!

When lighting undergoes much ‘branching’, both within and outside the cloud, although the whole flash would take place over a small fraction of a second, the sound waves from each part of the path (and the branches), take different periods of time to reach an observer: the sound will be heard as either a continuous crackling (relatively close-by storm), or low-intensity ‘rumble’ (distant storm), or a mix of the two, again depending upon the character of the discharge path.

The reason why near thunder ‘cracks’ and distant thunder ‘rumbles’ is this: sharp cracks are composed of sound waves with high frequency (or short wavelengths); these are rapidly damped owing to irregularities in wind & temperature. The lower frequency (longer wavelength) portion of the sound wave is not absorbed anything like as much, and therefore it is these we hear from distant storms.

Thunder, under normal atmospheric conditions, has an absolute maximum ‘travel’ of about 20 km, and more usually 8 to 10km. In mountainous areas and under conditions of ‘anomalous’ low-level refraction, then greater distances may be achieved.

Facts About Rainbows Random Facts about Rainbows

  • A Rainbow is light refracted through millions of droplets of water.
  • The angle of light refraction to create a Rainbow is 42 degrees to the eye of the person watching
  • Sir Isaac Newton discovered the seven distinct colors of the visible spectrum
  • Rainbows are God’s promise – Genesis 9
  • Rainbows in your environment create good Feng Shui
  • Everyone loves a Rainbow!