WMO turns Sixty

23rd March is World Meteorological Day.

On this day each year meteorologists around the world celebrate a chosen theme together to commemorate the anniversary of the founding of the World Meteorological Organization (WMO) in 1950.

The theme this year is “Sixty Years Service for Your Safety and Well-Being”

Historic beginnings and a NZ connection:

Back in the mid 1800s severe storms caused an increase in shipwrecks in the Atlantic.  The weather was a problem and people needed to be warned about it.  An international meeting was convened in Brussels in 1853, instigated by Lt. Matthew F. Maury of the US Navy.  Subsequently, the British Government commissioned a former New Zealand Governor, Robert FitzRoy, as first ever “Meteorological Statist” to gather records on the weather and its behaviour.

Robert FitzRoy. as Second Governor for New Zealand , 1843-1845.

FitzRoy did more than gather records.  He started a network of observations and produced weather maps, and published his book on how to forecast the weather by watching the trends on a barometer: The Weather Book: a Manual of Practical Meteorology. (London: Longman, Green, Longman, Roberts, & Green.)  There are still many wall barometers that carry quick forecasts written around the dial – something which FitzRoy documented back in 1863.

Another conference was held in Vienna in September 1873 attended by twenty governments and called the First International Meteorological Congress.   An ad-hoc Permanent Committee was formed, presided over by C.H.D. Buys Ballot (Netherlands), and they formed The International Meteorological Organization (IMO) which was to be the basis for international collaboration for many decades.

WMO’s founding documents were signed on 23rd March 1950.  WMO became a specialized agency of the United Nations (UN) in 1951.   In 1953, the UN ratified WMO to replace the IMO.

Advances in Sixty Years

As WMO turns sixty, this is a good opportunity to reflect back on how the study of weather and climate has evolved over the past six decades.

The science of weather forecasting follows the four main elements of all science: 1) gathering data, 2) analysing these data for patterns and trends, and to gain physical insights into processes, 3) constructing mathematical models of the known physics, and implementing computer programs that extrapolate the patterns and trends into the future, and then 4) researching the results and reporting expectations to the public and various users in a manner best suited to help in decision making.

The first step is to gather data. Since meteorology does not recognize political boundaries, war is no impediment to cooperation amongst members of the WMO, and the planet’s observational network has extended to cover the entire globe.  The Global Observing System now includes about 10,000 land stations; 1,000 upper air stations; more than 1,000 ships; 1,200 drifting buoys; 200 moored buoys; 3,000 ARGO profiling floats; and 3,000 commercial aircraft.  This is supplemented by six geostationary satellites, five polar orbiting satellite,  five operational environmental satellites and about another 50 research satellites.  These facilities are all owned and operated by WMO members, each undertaking certain responsibilities in an agreed global scheme so that all can benefit.

Global Observing System, click to enlarge. NMS=National Meteorological Service

WMO’s role has evolved over the past 60 years, along with advances in technology with satellites and computing.  WMO has set up six Regional Associations around the world and several programmes with the aim of securing and enabling the same reliable weather information everywhere and for everyone:  It is now an exceptional scientific and technological institution with a primary focus on the safety and well being of all.  WMO-sponsored research is flourishing and a full summary of this is available in the official publication for World Met Day and a special message from the WMO Secretary-General, Michel Jarraud.  We have a small number of hard copies of the official World Meteorological Day (WMD) information pack (click for pdf) which we can post out with our compliments.  Email your postal details to bob dot mcdavitt at metservice dot com.

There are now 189 countries and territories in WMO, following the recent incorporation of the Democratic Republic of Timor-Leste.   New Zealand’s Permanent Representative to WMO is MetService’s General Manager of Science Research and Development,   Dr. Neil Gordon.

In the past sixty years MetService has also made great strides in weather forecasting for the safety and well-being of New Zealanders.  By extending our rain radar network, and providing our forecasters with state-of-the-art training and tools we have, in the past year, been able to start a Severe Thunderstorm Warning Service that covers most New Zealanders.  MetService is also working with neighbouring countries in the South Pacific in a regional initiative to supply web based tools and training to help in weather warning services, thereby enhancing the safety of people in the South Pacific.

Looking back at its performance over the past sixty years, WMO has been fortunate that its founders chose to set it up on a solid base laid out by the IMO, allowing for the free and unrestricted international exchange of weather data and products.  In some ways WMO is like a big club, and yet in other ways it is a unique example of how nations can work together, united by the challenges thrown at us by weather and climate.

Please join with the meteorological community around the world in a round of applause in celebrating World Meteorological Day 2010.

Frizzy February Weather

As the cicada season moves out and the mushroom season moves in, people have been commenting on the high humidity we had in February.  This is no great surprise: the sea-surface temperature of the seas around New Zealand reach their annual peak in late February/early March, and the sea is the source for most of the water vapour in our air

When relative humidity (RH) is high, human hair absorbs moisture from the air and creeps or expands.  Curly air frizzes and straight hair goes limp.  From dry to saturated, hair length can change by three percent.  In 1783, Swiss physicist and geologist, Horace Bénédict de Saussure used this principle to build the first humidity meter or hygrometer. These instruments, measuring relative humidity, became very popular. They were often in the form of a  Swiss or German weather house in which a lass dressed in light clothes would come out in dry weather, and a lad dressed in wet-weather clothes would come out in damp weather.  Because hair responds to relative humidity, these hygrometers helped to popularise RH as the main  measure of humidity.

Relative humidity measures how close the air is to being saturated with water vapour, where 100% means fully saturated.  However, the word saturation here can be misleading. It is a misconception is say that air “holds” or dissolves water vapour or that saturation is reached when the air can “hold” no more water vapour.  There is always plenty of space between the air molecules for more water vapour molecules, and the saturation point, in fact, refers to a sustainable equilibrium.

Think in terms of energy. Temperature measures the kinetic energy of a material, or how fast its molecules are vibrating.   In a pool of liquid water any molecules that are vibrating fast enough (as determined by the water temperature) will break free or evaporate and become water vapour, taking with them that extra kinetic energy  — so when sweat evaporates from skin, or saliva evaporates from a panting dog,  it helps cool things down.

Here’s an experiment to show evaporative cooling:

  • Lick the back of your hand.
  • Blow gently across your hand. You should already feel a cooling sensation.
  • Now, use a towel to dry that hand and then use the opposite hand to feel the temperature of the skin on the dried hand. It should feel cooler to the touch!

Water vapour molecules that are moving slow enough (as determined by air temperature) will plunge back into water liquid, transferring energy to the molecules near where they hit and joining the liquid — this is called condensation, and it’s a process that helps warm solids and liquids.

Here’s an experiment to show condensation and its warming impact:

  • Breathe onto a pair of spectacles
  • The moisture in your breath will raise the RH of the air touching the glass to 100%, and the glass will fog up with water droplets, and warm a bit
  • Leave the spectacles alone for a while and the fog will evaporate, and the spectacles will cool a bit

When the air is “saturated” and RH is 100%, the rate of evaporation equals the rate of condensation, so the amount of water vapour in the air maintains equilibrium.    This amount varies exponentially with the temperature.

“Muggy weather” is a term we use for a hot day when the humidity in the air becomes uncomfortable.  On a hot day we sweat and thus keep our cool by evaporative cooling, but if the amount of humidity is high enough to disrupt this cooling then we feel hot and sticky, tired out.  Cold air (as at dawn in winter) can have 100% RH and be foggy but since it isn’t warm enough to raise a sweat it will not feel muggy at all.  So RH is not a measure of  mugginess.

Dew point is the temperature you need to cool grass, or a glass, down to get dew or beads of moisture from the air.  It is the temperature at which condensation occurs. It is a good measure of the amount of water vapour in the air and thus a good measure of “mugginess”.

Steve Horstmeyer of WKRC TV (1)  has compiled the following table linking dew point to human perception of humidity:

Dew Point                               Perception

Less than -30ºC                     prolonged exposure will crack the skin

Less than 10ºC                      A bit dry for some

10-12ºC                                   very comfortable

13-16ºC                                   comfortable

16-18ºC                                   OK for most, but all perceive humidity

18-21ºC                                   somewhat uncomfortable for most people

21-24ºC                                   very humid, uncomfortable

24-26ºC                                   extremely uncomfortable, oppressive

Higher than 26ºC                   severely high.  Highest measured has been 35C

Dew point depends on the region that the air reaching us comes from, and thus mainly changes only when the wind changes.

The following graphs of temperature, relative humidity, and dew point during February 2010 around the country show what made late summer so frizzy.  It was a month when anticyclones lingered around southern Tasman Sea and central New Zealand, causing sunny warmer and drier than normal conditions. Between these high-pressure systems some passing troughs brought brief periods of rain, most notably the active front over Southern Alps on 13 February which caused the abandonment of the mountain run and kayak leg in this year’s Coast to Coast.

Kerikeri, February 2010: Relative humidity on top, air temperature and dew point (ºC) below, with daily time stamps at 1pm NZDT (0000UTC).

The above trace is taken at Kerikeri and shows the warmest day to be 29ºC on 14 February.  After that there was an extended period of northerly winds which raised the dew point to 20ºC.  A southerly change on Friday 19 February helped to drop the humidity for a few days and then a further period of northerly winds raised it back to 20ºC on 24 and 25 February.  Note the “flat-lining” of the RH trace at 100% on 24 February.

Auckland, February 2010: Relative humidity on top, air temperature and dew point (ºC) below, with daily time stamps at 1pm NZDT (0000UTC).

In Auckland the trace is similar to that at Kerikeri, but the warmest days were only near 27ºC, and there was nearly a week of “twenty degree nights” and dew points above 18 ºC from 14 to 19 February and again on 25 February.

Wellington, February 2010: Relative humidity on top, air temperature and dew point (ºC) below, with daily time stamps at 1pm NZDT (0000UTC).

In Wellington, February’s humidity was not as extreme as in Auckland or Northland.  The warmest day was 27ºC on 6 February, and the warmest nights were 18ºC on 13 and 24 February.

Christchurch, February 2010, Relative humidity on top, air temperature and dew point (ºC) below, with daily time stamps at 1pm NZDT (0000UTC).

In Christchurch, the relative humidity was regularly “flat-lining” at 100% in the last half of the month.  On 23 February the wind came from the northwest, the temperature shot up to nearly 31ºC, and the dew point dropped to around 3ºC, producing a very low RH of 17 per cent.

Dew point readings from selected stations are given in the coded observations page on metservice.com (in the 4 group: a number such as 40117 means the dew point is 11.7ºC).  Relative humidity readings are shown on our National page (see below).  MetService’s “feels like” temperature applies to people in the shade and takes into account wind chill when the air temperature is 10ºC, or, when it is above 14ºC, calculates the apparent temperature that accounts for any high humidity, as seen below.  For air temperatures between 10ºand 14ºC there is a roll over from wind chill to apparent temperature.

Example of humidity and apparent "feels like" readings at metservice.com

References

  1. Horstmeyer, Steve (2006-08-15). “Relative Humidity….Relative to What? The Dew Point Temperature…a better approach”. Steve Horstmeyer, Meteorologist, WKRC TV, Cincinnati, Ohio, USA.