Measurements

 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 page 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 30kph, 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).