The weather station is maintained and managed by the Energy Studies Programme at the Department of Physics for the purposes of obtaining meteorological data specific to the University of Otago campus. Measurement and analysis of climatic conditions is essential for the assessment of the viability of renewable energy generation options such as solar panels and wind turbines, and to assess the thermal performance of buildings. Hence the data collected by the Energy Studies Weather Station is useful for various research projects being carried out onsite currently, and those that may be carried out sometime in the future, as well as for teaching laboratories and the interest of the general public.
- Latitude: 45 degrees 52 minutes south
- Longitude: 170 degrees 31 minutes east
- Pressure transducer - approximately 40 m above mean sea level
- Wind instruments - approximately 47 m above mean sea level
- Other instruments - approximately 45 m above mean sea level
Dunedin is located in the south-east of the South Island of New Zealand, built on the coast around Otago Harbour, adjacent to the Otago Peninsula. The New Zealand Metservice maintains two weather stations in Dunedin, one located in Musselburgh in the south of the city, and one at Momona Airport, inland from the city. Dunedin North, where the Energy Studies Weather Station is situated, often experiences slightly different conditions compared to these sites because of the influence of the surrounding hills.
Northward panoramic view over North Dunedin from the weather station site.
The Energy Studies Weather Station measures temperature and relative humidity using a Vaisala HMP45A Temperature and Humidity Probe (pictured below). Temperature measurement is based on the changing electrical resistance of a platinum sensor. Humidity measurement is based on the electrical capacitance of a thin polymer film. The sensors are contained in an enclosure that allows air to freely pass through while sheltering it from direct sunlight and high winds that might affect the reading.
Measurements from these sensors are taken every twenty seconds and averaged to give a single reading for each sensor every five minutes.
The amount of rainfall is measured with a ‘tipping-bucket’ rain gauge (pictured bottom of page). A funnel collects rain and drains it out a small hole. The trickle of rain fills a small cup beneath, which tips and drains when it receives a certain volume of water, moving an identical cup under the stream. These two cups tip back and forth, each movement closing a circuit and indicating 0.1 mm of rain has fallen.
Interpretation - Temperature
Although air temperature is intuitively simple to understand, there are a number of factors to bear in mind. The influence of solar radiation may make the perceived temperature much greater than the actual ambient air temperature. Energy from the sun is transmitted as electromagnetic radiation at a wide range of wavelengths at various intensities. Although the atmosphere absorbs some sunlight resulting in its direct heating, most of it reaches the Earth’s surface. A person outside on a clear day will absorb this solar energy directly, and hence may mistake the sun’s radiative heat for an elevated air temperature. (See the solar energy section for more information.)
The influence of wind flow - the movement of air increases the effect of convection - a mechanism of heat transfer. Wind flow over a person’s body will increase convective heat loss (provided the air temperature is cooler than their surface temperature) and this increased heat loss can be mistaken for a lower air temperature. The air temperature required to create the same heat loss in still air compared to a real situation with moving air is known as ‘wind-chill factor’. For instance, if the air temperature is 10°C and the wind speed is 30 kph, the wind chill factor is approximately 1°C.
Interpretation - Humidity
Humidity is a measurement of the amount of water vapour in the air. Readings on the display are given as ‘relative humidity’ (RH). This gives humidity as a percentage of the maximum amount of water vapour the air can carry at its current temperature. Air’s water vapour carrying capacity increases with increasing temperature, and decreases with decreasing temperature. This is why on the display graph temperature and RH mirror each other’s trends. If the air temperature drops, its carrying capacity is reduced. So while the actual amount of moisture in the air may have remained constant, it is a larger percentage of the maximum. Hence an increase in RH is observed.
Things to look for:
- Temperature decreasing overnight, and sharply increasing in the morning when the sun rises.
- Sharp decreases in temperature coinciding with sharp decreases in pressure and changes in wind-direction – this indicates a cold frontal system has arrived.
- 100% relative humidity – if the air is still, this indicates the formation of fog.
- Sub-zero temperatures and still air will normally cause a frost
Interpretation – Rain
Five minute total rainfall readings are given on the graph. It should be easy to observe if it has been raining steadily (similar sized bars, evenly distributed) or if it has been raining in violent squalls (sharp spikes).
Wind speed is measured with a Vector A101M Pulse Output Anemometer (pictured below). It consists of a wind-driven rotor that creates an electric pulse every complete revolution. The data-logger counts the pulses and calculates wind speed every 20 seconds. The highest of these is the maximum gust, and the average of all of them over five minutes is the average speed. Wind direction is measured by a Vector W200P/L windvane. The position of the vane determines the resistance across a potentiometer. The data-logger calculates the average wind bearing based on these resistance readings.
Air pressure is measured with a Vaisala PTA 427 Barometric Pressure Transducer. It utilises the varying capacitance of a sliver of silicon crystal caused by it flexing due to pressure fluctuations.
Interpretation - Wind
Wind speed is given in kilometres per hour. Interpretation of wind speed is conventionally done using the Beaufort scale, which equates ranges of wind speed with observable effects.
The scale on the chart goes up to 120 km/hr, which equates to a Beaufort force number of 11 (a violent storm). Note that the weather station is considerably elevated, meaning it is likely to get marginally stronger winds than at ground level.
Wind direction is given in degrees of arc, as with a normal compass. The bearing indicated is the direction the wind is coming from. Note that because the scale is circular and 0° and 360° both equate to north, points are likely to occur at the top and bottom of the chart when the wind is coming from approximately that direction. Also note that when the conditions are quite still, wind direction points will be scattered randomly as there will be no strong directional influence on the windvane.
Interpretation - Pressure
Mean sea level atmospheric pressure is 1013 hPa. Pressure readings below this indicate a low pressure air-mass is in the region, and readings above indicate a high pressure air-mass. Low pressure is associated with wet, cold weather and high pressure with fine, warm weather. Rapid change in the pressure reading, along with rapid changes in temperature and wind direction indicates a frontal system is moving in. Note that pressure also decreases with increasing altitude, and elevated as it is, the pressure transducer will read approximately 5 hPa lower than it would at sea-level.
Our sun emits a very broad range of wavelengths of radiation. The distribution of the intensities of these wavelengths reaching the Earth’s atmosphere is given by the upper line in the graph below. As this radiation penetrates the atmosphere, scattering processes and absorption by ozone, water vapour and oxygen molecules modify the intensities of the different wavelengths, resulting in a distribution given by the lower line below. This is known as ‘attenuation’. Note that the exact effect of attenuation will depend on local conditions. The total solar power reaching the earth’s surface (represented by the area under the lower line) is known as incoming solar radiation or ‘global’ radiation.
Graph of the amount of power the sun radiates at different wavelengths (upper line) and that which reaches the Earth's surface (lower line)
The solar spectrum is divided into three ranges:
Ultraviolet. This range is subdivided in UVC, B and A (given from shortest to longest wavelengths). UVC is wholly absorbed by atmospheric ozone, and UVB partly so. Ozone depletion causes an increase in the amount of UVB reaching the Earth’s surface, causing increased danger of sunburn and skin cancer. UVA is not absorbed by ozone, but is less dangerous.
Visible. Also known as PAR, which stands for ‘photo-synthetically active radiation’, this range corresponds to the wavelengths that the human eye and plant life are sensitive to.
Infrared. This is long wavelength radiation, undetectable to the human eye but can be felt as transmitted heat.
Global radiation is measured using a Li-Cor LI200X Pyranometer (pictured right). It has a silicon photodiode that uses the photo-voltaic effect (a photon-atom collision that results in the ejection of an electron) to produce a current. This is changed to a voltage using a shunt resistor and measured directly by the data-logger. Special scaling factors and calibration are used to estimate true global irradiation as the sensor only responds to the shorter wavelengths (i.e. not long wavelength infrared) and does so in a non-linear fashion. Readings are given in Watts per square meter (W/m2), or in other words, joules of energy received by a metre-square area in a second. Like all the other sensors, it is mounted horizontally.
PAR is measured using a Li-Cor LI190SB Quantum Sensor. Like the other solar sensors, the quantum sensor uses a silicon photodiode, but with special filters to give enhanced response in the visible range (400 – 700nm). The current produced is used to estimate the number of photons incident on a metre squared every second, hence the measurement unit micromoles per second per square metre (µmol/m2s). Photon counting is often used for agricultural and photovoltaic power system research as both are dependent on the number of photon-molecule interactions rather than the total incident thermal power.
UVA and UVB are measured with Skye High-Output Sensors, models SKU 420 and SKU 430 respectively. Both use a photo-voltaic sensor in combination with special filters to detect the wavelengths they are designed for.
Notes/Things to look for
- These sensors record radiation coming from all directions, including solar beam or ‘direct’ radiation, sky or ‘diffuse’ radiation and reflected radiation.
- A perfectly cloudless day should result in a smooth bell shaped curves for all solar sensors. However, you may notice small ‘tails’ at dawn and dusk – this is where the sun is behind the surrounding hills and direct radiation is blocked, but diffuse radiation is still present.
- If there are clouds surrounding (but not obscuring) the sun, a noticable lensing effect can occur. This is commonly seen in the sensor readings as sharp peaks either side of troughs on days with a largely smooth bell-curve profiles: a small cloud moves close to the sun causing lensing and a sharp peak, then obscures it causing a trough. A second peak is caused as it moves away from the sun.
- Because the maximum height in the sky the sun reaches each day changes through the year, so will maximum possible readings of the sensors (discounting incidences of lensing). The maximum possible maximum will occur at summer solstice on December 21st and the minimum possible maximum at winter solstice on June 21st.
- The scale for PAR on the chart has been set such that the approximate radiative power of PAR (in W/m2) can be read off the scale for ‘Global’ on the left-hand side.
- The fact that UV can still burn even with cloud cover can be observed on the weather station charts: cloud diminishes the amount of UV, but it does not eliminate it.