For four days this April, I had the opportunity to see the inner machinations of the Bermuda Weather Service at work. Seeing behind the curtain was everything that I hoped it would be and more!
For the most part the weather was benign, but this worked mostly to my benefit as the meteorologists on duty would have more time to dedicate to my interest. The knowledge gained from this experience has been invaluable for my current endeavors.
Launching the balloon:
The weather balloon is a large helium filled balloon that essentially has a small weather station and GPS attached to it. The balloon rises into the atmosphere and through the use of radio signals, sends information on temperature, dew point, wind speed and direction, pressure, and location all back to a computer dedicated to receiving this information. This computer then takes this data and transforms it into a graph – a transect of the atmosphere – which is sent around the world for use in global computer models such as the Global Forecasting System used in the United States. Additionally, it can be used for short term local forecasting and a copy of the graph is printed for the forecaster on duty.
The graph is called a Skew-T graph where all the variables sampled by the weather balloon, and transmitted to the computer are combined into this one graph. From this graph, the forecaster on duty can determine the atmospheric set up for the day and it can be especially useful in forecasting thunderstorms and rain as it reveals atmospheric instability and moisture.
The balloon launch typically occurs around 8:15am – weather stations around the world launch their balloons at roughly the same time so that it can be used in the 1200UTC computer models and synoptic charts. It requires air traffic clearance for safety reasons; typically, there isn’t much air traffic to contend with and the balloon isn’t delayed too much. In about an hour, it reaches its maximum height which is typically less than 100hPa -meteorologists measure altitude in units of pressure rather than units of distance as pressure decreases with altitude and is more meteorologically relevant. However, we know that 850hPa is about 5,000ft, 700hPa is about 10,000ft, 500hPa is about 17,500ft, 300hPa is about 30,000ft, and 100hPa is over 50,000ft.
Balloon launches typically occur once a day, however in times of approaching significant storm systems or failed launches, a second or even third balloon may be launched later in the day. The data from multiple balloon launches is used in the same way.
Making an Observation:
The Bermuda Weather Service typically makes a meteorological observation once an hour at 5 minutes to the top of the hour. These observations primarily serve as guidance for any air traffic that may be incoming. What is in an observation? The observer on duty will take a reading of the current temperature, humidity, pressure, 10-minute sustained wind, any gusts 10kts greater than the sustained wind, and then make a note of the sky conditions with a comment on cloud altitudes.
The on-duty observer typically takes these reading from an automated weather system setup on the airport’s two runways. They will use the observation that is taken from the runway that is in use at the time. However, at 00, 06, 12, and 18UTC, the observer must also make a note of the rainfall recorded over 6 hours, 12 hours, and 24 hours.
There are four barometers that are used to make sure that they are still accurate – the atmospheric pressure, next to wind speed, is one of the most important observations at the airport as aircraft use it to determine their altitude. There are the two barometers on the runways, two analogue ones in the office and a further digital barometer in the office.
Wind speed is observed to World Meteorological Organization (WMO) standards; 10-minute sustained wind with gusts 10kts or greater than that figure. If anemometers on the runways fail for any reason, there are back up hand-held anemometers for use. If winds get above 20kts, a marine small craft warning must be issued; 25kts, air traffic has to be warned.Wind direction is also important as it determines which runway is in use and, in severe wind storms, the operation of both the airport and the causeway.
Temperatures at these specified times are measured from the Stevenson’s screen, a box designed to keep thermometers in the shade and allow airflow around them. There are four thermometers in here; the maximum thermometer, the minimum thermometer, a normal thermometer, and a wet-bulb thermometer. The max and min thermometers differ from the normal thermometer in that they have a piece that rises with the liquid in the tube, but doesn’t fall, or falls with the liquid but doesn’t rise respectively. The wet-bulb thermometer is just that; a thermometer with the bulb kept moistened by a wick that is dipped in water – this ‘wet-bulb temperature’ helps determine humidity in association with the ‘dry-bulb temperature’ measured from the normal thermometer.
What is in a Forecast?:
The forecaster on duty will use a variety of global forecast models and their own climatological knowledge of Bermuda to make a forecast. They must provide not only a public and marine synopsis, but also a forecast for the airport called a TAF, or Terminal Area Forecast.
The tools used for forecasting include the Skew-T graph, prior observations, the global forecast models (GFS, UKMET, ECMWF), radar data, satellite data, and the input of the Bermuda Weather Service staff during their briefing every morning. For the time I was at the station, the weather was fairly benign, however when there is a lot of short term forecast disagreement, or when there is a significant weather event that is too small scale for the models to pick-up on accurately, the briefing can become much more important.
The forecaster produces a forecast at 5:30 and 11:30 am and pm each day, they include a five day forecast for the public and for the marine area which extends 25 nautical miles from Bermuda’s coastline, and any appropriate watches or warnings. If a significant weather event develops that isn’t addressed by the forecast, an intermediate forecast may be posted with any associated watches or warnings. Tropical Cyclone watches and warnings are issued as advised by the National Hurricane Center out of Miami, FL. The Bermuda Weather Service communicated all watches and warnings with the Bermuda Government Emergency Measures Organization.
For the sake of consistency and credibility, the forecaster keeps the forecasts from changing too significantly from forecast to forecast even across a change of forecaster – each forecaster has their own equally correct way to do things!
In addition to creating the five day forecasts, the TAF, and the weather briefing, a forecaster at the Bermuda Weather Service also puts together the synoptic charts using a fancy program. They use satellite, and model analysis overlays to determine the position of different fronts and centers of high and low pressure and draw them onto the map.
In all, my experience at the Bermuda Weather Service was a good one, an educational one, and one that I would gladly repeat!