Sheffield Weather

This page shows some recent weather measurements made at approximately 70 metres above mean sea level just to the north of Sheffield in the UK.

Note that, in all of the graphs on this page, the right-most edge corresponds to the time at which this page was downloaded (7:33) and, if everything is working correctly, each graph will show measurements, taken at 15-minute intervals over the past 24 hours, from which the code for this page will attempt to make weather predictions.

Please note that there will be a slight delay while the data are analysed, plotted (using JPgraph) and then transmitted, so the graphs will appear sometime after the text.

Temperature

The two graphs below show the air temperature as well as the temperature of a horizontal surface exposed to the sky. This surface is black, so will have a temperature similar to a road surface. Both temperature sensors are located about 1 metre above the gutter of a single-storey building.

When there is a lot of cloud cover, day or night, the air and surface temperatures will be somewhat similer, so the two curves will be closely spaced, if not superimposed. (As the red [surface temperature] points are plotted after the blue [air temperature] points, the blue points may not be visible if the curves are closely similar.) A rise in the surface temperature relative to the air temperature during the day, or a fall in the surface temperature relative to the air temperature at night, should be interpreted as a break in the cloud cover. A sudden and simultaneous dip in both the air and the surface temperatures, especially if the surface temperature dips below the air temperature during the day, is likely to indicate a rain shower. In fact, if the surface temperature is below the air temperature at any time during daylight hours, assume that rain is falling at that time.

The Maxim DS18B20 temperature sensors used to provide the temperature measurements have a measurement uncertainty of 0.5C (and a resolution of only 0.06C) over the expected temperature range.

The average rate at which the air temperature has changed over the past hour is 0.15 degrees C per hour.

Over the past 24 hours, the maximum and minimum air temperatures were 21.1C and 13.3C, respectively, and the maximum and minimum surface temperatures were 24.6C and 13.1C, respectively, as will be seen in the graph below.

Atmospheric pressure

The graph below should show atmospheric pressure over the past 24 hours. The pressure sensor is at a height of about 70 metres above mean sea level, but all of the measurements have been corrected to mean sea level for compatibility with other data sources. The Bosch BMP180 pressure sensor used for the pressure measurements has an absolute uncertainty of typically 1mB over the potential measurement range, a relative uncertainty of 0.12mB, and a resolution of 0.01mB. The fine relative uncertainty and resolution mean that the rate of change of pressure can be determined much more accurately than the absolute uncertainty (over the full range of pressure) suggests. If the pressure curve appears "noisy" this probably means that a gusty wind is blowing.

Standard atmospheric pressure is considered to be 1013.25mB, but the pressure can vary over the range ~950mB to ~1050mB.

If the atmospheric pressure is high, expect fine weather in summer, or cold mainly dry weather in winter; however, if the pressure is low, and particularly if it is falling, expect wet and/or windy weather - especially if the pressure is falling faster than "slowly", at which point alarm bells should begin to ring as a storm is very likely to be approaching. The violence of such a storm, and how close it will pass, will depend on the extent, and rate, of the pressure drop.

Current atmospheric pressure

The current pressure (1005.8mB, 754.4mmHg or 29.7inHg) is a little below average, so the weather should be considered to be 'changeable'. (You may be interested to know that a pressure of 1005.8mB indicates that water would boil at a temperature of 99.6C - calculated using the Antoine Equation.)

Current rate of change of atmospheric pressure

The mean slope of the pressure curve over the past 3 hours is 0.08mB/h, which means that the pressure is (in weather-forecasting terminology) 'rising slowly'.

Weather prediction (if any)

The weather is likely to become colder but clear.

Humidity

The graph below shows the output of a DHT11 humidity sensor having a measurement uncertainty of 5% over a Relative Humidity range of 20% to 90%.

The most-recent temperature indicated at the location of the humidity sensor is 182C and the relative humidity is reported to be 73%. These suggest a dew point of 132.7C. As the air temperature (13.7C) is close to the dew point (13C) mist is possible, especially in low-lying areas.

Sunshine

The sensor used to detect the sun is a simple photo-transistor whose output is sampled at one-second intervals by the PIC 16F627 controller that collects the sensor data. If the brightness is above an arbitrary level (corresponding to a bright overcast sky, i.e., thin cloud) this is recorded as such by the controller. The sunshine data shown on the chart, below, are simply the percentage of the one-second samples, that correspond to bright sunlight, recorded over the previous 15 minutes.