Temperature, Frost, Humidity and Clouds
Temperature
Paterson's proximity to the coast affects its temperatures by reducing extremes. The area does not receive the intensity of hot days in the summer and cold nights in the winter compared to the Upper Hunter Valley. It is, however, far enough inland to receive some very hot days (>35°C) in most summers and some frosts in most winters.
Extremes however do occur, on 4 July 1984 the maximum temperature at Tocal reached only 7.0°C at 0900 and rose to 17.6°C at 1500 hours. There were widespread snowfalls throughout all the southern states and into Queensland.
The following graph shows the average monthly temperatures at Tocal. They indicate a fairly mild climate with few extremes on average.
Soil temperature
For root crops like potatoes and for seed germination of crops like maize, the temperature of the soil is more important than that of the air. Potatoes need a soil temperature of at least 8°C, no more than 28°C with an ideal temperature of about 18°C, which is the minimum temperature for germination of sorghums and millets.
The soil type used for this measurement is a podsolic type and is not representative of the alluvials used for cropping in the district. It is also above the height of the alluvial soils which would suggest readings would be higher than those occurring on the flats. So when applying these readings, adjustments for the site would need to be made.
Soil temperature graphs follow the air temperature measurements, without the daily fluctuations common with air temperatures. The graph below shows soil temperatures measured at 10 cm and 20 cm at 9.00 am. The temperatures rise during the day, with the 10cm readings averaging 3-4°C higher at 3pm and the 20cm readings showing a smaller rise of 1-2°C at 3pm
View base soil temperature data.
Frost
Severe frosts are uncommon at Tocal. When they occur, they are associated with stationary high pressure systems and still, dry air in mid winter.
The frost figures show that, as expected, July experiences the most frosts, with August the next most frost-prone month.
District experience shows that frost incidence is extremely variable. The variation is dependent on topography and local air movements. For example, the Tocal weather station site rarely receives a noticeable frost as it is on a hill above the river valley. On many occasions the river and creek flats can receive a white frost while no frost is recorded at the weather station. Land with a higher elevation than the weather station would receive even fewer frosts.
The ridges and mountain range tops on the edge of the Paterson Valley are basically frost free, which allowed the growing of early tomatoes early this century. This industry was superseded by glasshouse tomatoes - an industry that no longer exists.
Humidity
Humidity is the actual amount of water vapour in the air and is measured by a wet bulb thermometer. If the wet bulb thermometer shows a temperature of 15°C it means that condensation will take place in the form of clouds or fog if the air temperature (as measured by a dry bulb thermometer) falls to 15°C. Relative humidity is therefore a measurement of how much water vapour the air is actually holding compared to how much it could hold at that time. This is why the average relative humidity for Tocal is much the same for January as July. The air actually holds much less moisture in July but it is about 73 per cent saturated for that temperature.
The graph below shows that humidity, while not fluctuating greatly throughout the year, remains relatively high, which is typical for coastal districts. High humidity is often associated with discomfort in humans and animals when combined with high temperatures. Tocal is far enough from the coastal breezes to experience this discomfort, especially in the summer months.
Clouds
Total cloud cover is measured in oktas, which is a scale of 1–8. At Tocal measurements are taken at 9 am and 3 pm. The graph below shows the average monthly cloud cover at 9 am Tocal.
Clouds come in many different forms. We naturally think of clouds as bearing at least the chance of rain, and to show this relationship, we have included the graph below which illustrates that rain and cloud cover are inextricably linked.
