My personal sling psychrometer obtained in the mid 1970s.
Before electronic hygrometers became widely available, one of the primary instruments used to measure atmospheric humidity was the sling psychrometer.
James Hutton was the first to discover that a thermometer moistened by water will show a cooler temperature than a dry thermometer when exposed to the wind. Psychrometric equations were later developed and refined during the 19th Century.
The sling psychometer consists of two thermometers mounted side by side on a frame. One thermometer is kept dry and measures the ambient air temperature; this is known as the dry-bulb thermometer. The other thermometer has its bulb wrapped in a cotton wick that is saturated with distilled water. This is the wet-bulb thermometer.
The frame is attached to a handle with a swivel joint, allowing the thermometers to be rapidly swung through the air. As air passes over the wet wick, evaporation occurs, causing the temperature of the wet-bulb thermometer to decrease. The amount of cooling depends on the moisture content of the air. Drier air promotes more evaporation and produces a larger temperature difference between the wet-bulb and dry-bulb readings, while humid air results in less cooling.
After the instrument has been spun for a sufficient period, the wet-bulb and dry-bulb temperatures are quickly recorded. Using psychrometric tables or charts, these readings can then be used to determine the relative humidity, absolute humidity, or dew point temperature.
Although modern electronic sensors have largely replaced sling psychrometers, the instrument remains an important teaching tool and a reliable reference standard for humidity measurements.
Before electronic and digital sunshine sensors became available, daily sunshine duration was measured using the Campbell–Stokes sunshine recorder.
The first sunshine recorder was invented by John Francis Campbell in 1853 and later refined by George Gabriel Stokes in 1879. Campbell’s original design used a water-filled glass sphere that focused the Sun’s rays onto a wooden surface. The instrument was carefully aligned for the local latitude and the ecliptic so that the time scale corresponded with solar time. The water-filled sphere was later replaced with a solid glass sphere, but the repeated scorch marks on the wooden surface made the original design impractical.
To improve the instrument, a specially designed photosensitive card was introduced. The glass sphere concentrated the Sun’s rays onto the card, burning a hole when the sunlight was sufficiently intense and leaving only a scorch mark under weaker conditions. As the Sun moved across the sky, the burn or scorch trace progressed across the card, creating a record of the day’s sunshine. At the end of the day, the card was removed and examined. The length of the burn trace was measured to determine the duration of bright sunshine, while the appearance of the marks provided a rough indication of solar intensity.
Although the Campbell–Stokes recorder was a significant advance for its time and served as the international standard for many decades, it had several limitations. The Sun’s rays at sunrise and sunset were often too weak to burn the card, causing sunshine during those periods to go unrecorded. Intermittent sunshine produced broken or faint burn traces that required subjective interpretation, leading to inconsistencies between observers. In addition, rain could damage the recording card, while frost, ice, snow, or condensation on the glass sphere could reduce its ability to focus sunlight accurately, resulting in measurement errors.
Overall, the year’s temperatures were near normal. The annual mean of 50.4 F was 0.1 F above normal. The annual precipitation of 42.71 inches was 1.18 inches above normal.
2025 Weather Data
January
January was the coldest month of 2025 and with a monthly mean of 23.7.7 F, which was 3.5 F below normal, with a high of 44.8 F and a low for the month of -8.7 F. Precipitation was 1.46 inches below normal with 1.46 inches of liquid precipitation. There were 9.3 inches of snow for the month, with a maximum daily total of 2.4 inches on January 4.
High wind speed of 37 mph on January 29.
February
February was a near-normal month for temperature, with a mean of 29.4 F, which was 0.3 F above normal. The high was 53.8 F, and the low was 7.9 F. Precipitation was 0.42 inches above normal at 2.88 inches. There were 5.4 inches of snow for the month, with a maximum daily total of 1.6 inches on February 16.
High wind speed of 40 mph on February 16.
March
March temperatures were 6.5 F above normal with a mean of 43.5 F. The high was 74.8 F, and the low was 14.1 F. Precipitation was 1.17 inches below normal with 2.21 inches of liquid precipitation. There was no measurable snow accumulation. There was 0.3″ of snow accumulation.
The highest recorded wind speed was 46 mph.
April
Temperatures were 2.4 F above normal in April with a mean temperature of 51.7 F. The high was 85.0 F, and the low was 19.4 F. Rainfall was 0.57 inches above normal at 4.08 inches.
There was a trace of snow in April.
High wind speed of 49 mph on April 29.
May
Temperatures were 1.3 F below normal with a mean temperature of 58.5 F. The high was 78.7 F, and the low was 36.9°F. Precipitation was 8.25 inches, 4.61 inches above normal. May was the rainiest month of 2025.
The high wind speed was 34 mph.
June
Temperatures for the month were 2.1 F above normal with a mean of 70.2 F. The high for the month was 95 F and the low was 36 F. Precipitation was 1.72 inches above normal, with 5.81 inches of rainfall.
The high wind speed was 35 mph.
July
July was the warmest month, and temperatures were 2.3 F above normal with a mean of 74.5 F. The high for the month was 93.7 F, and the low was 52.2 F. Rainfall was 1.51 inches above normal, with 5.30 inches of precipitation.
The high wind speed was 25 mph.
August
August temperatures were 3.1 F below normal with a mean of 67.3 F. The high was 90.8 F, and the low was 39.4 F. Precipitation was 2.32 inches below normal at 1.84 inches.
The high wind speed was 23 mph.
September
Temperatures were 0.7 F above normal with a mean of 63.9 F. The high was 88.5 F, and the low was 36.5 F. There were 3.46 inches of rainfall, which was 0.49 inches below normal.
High wind speed was 28 mph.
October
October was near normal with temperatures at 0.3 F below normal, with a mean of 51.6 F. The high was 81.6 F and the low was 27.2 F. Rainfall was 3.26 inches, which was 0.20 inches below normal.
The high wind speed was 32 mph.
November
Temperatures in November were near-normal at 0.6 F below normal with a mean of 40.6 F. The high was 67.9 F and the low was 23.4 F. Precipitation was 2.24 inches, 0.75 inches below normal. The first snowfall of the season was on November 11.
The high wind speed was 39 mph.
December
December temperatures were 3.8 F below normal, with a mean temperature of 28.4 F. The high for the month was 54.3 F deg. F, and the low was 1.1 F. Precipitation with 1.92 inches of liquid precipitation recorded, which was 1.26 inches below normal. There were 7.8 inches of snowfall.
July was 2.3 F above normal for temperature. The high for the month was 93.7 deg. F recorded on July 30. The low for the month was 52.2 deg F, recorded on July 22. There were 6 days at or above 90 deg F. There were 15 heating degree days and 311 cooling degree days.
July was an above-normal month for precipitation, with 5.30 inches of rainfall recorded, which was 1.51 inches above normal. The maximum rainfall in a single day was 1.79 inches recorded on July 26. There were 9 days of rain >.01 in, 9 >.10 in, and 1 >1 in.
Overall, the year’s temperatures were above normal. The annual mean of 52.8 F was 2.6 F above normal. The annual precipitation of 43.74 inches was 2.21 inches above normal.
2024 Weather Data
January
January was the coldest month of 2024 but was warmer than normal with a monthly mean of 31.3 F, which was 4.1 F above normal, with a high of 62.6 F and a low for the month of 4.8 F. Precipitation was 1.93 inches above normal with 4.85 inches of liquid precipitation. There were 8.3 inches of snow for the month, with a maximum daily total of 5.5 inches on January 7.
High wind speed for the month and year was 47 mph on January 14.
February
February was an above-normal month for temperature, with a mean of 36.1 F, which was 7.0 F above normal. The high was 61.4 F and the low was 4.8 F. Precipitation was 0.72 inches below normal at 1.74 inches. There were 6.3 inches of snow for the month, with a maximum daily total of 4.0 inches on February 17.
High wind speed of 39 mph on February 28.
March
March temperatures were 7 F above normal with a mean of 44 F. The high was 74.2 F and the low was 18.5 F. Precipitation was 0.02 inches below normal with 3.36 inches of liquid precipitation. There was no measurable snow accumulation.
High wind speed of 45 mph on March 11.
April
Temperatures were 2.9 F above normal in April with a mean temperature of 63.0 F. The high was 84 F and the low was 28.0 F. Rainfall was 2.41 inches above normal at 5.92 inches. The last day of temperatures below freezing was April 22.
The high wind speed for the month was 38 mph.
May
Temperatures were 2.6 F above normal with a mean temperature of 62.4 F. The high was 85.4 F and the low of 40.1 F. Precipitation was 5.40 inches, 1.76 inches above normal.
The high wind speed was 34 mph.
June
Temperatures for the month were 1.9 F above normal with a mean of 70 F. The high for the month was 93.6 F and the low was 43.9 F. Precipitation was 1.64 inches below normal with 2.45 inches of rainfall.
The high wind speed was 27 mph.
July
July was the warmest month and temperatures were 1.9 F above normal with a mean of 74 F. The high for the month was 94.7 F and the low was 49.5 F. Rainfall was 1.49 inches below normal with 2.30 inches of precipitation.
The high wind speed was 29 mph.
August
August temperatures were 0.7 F below normal with a mean of 69.7 F. The high was 92.9 F and the low was 45.4 F. Precipitation was 5.20 inches above normal at 9.36 inches. August was the rainiest month of 2024.
The high wind speed was 28 mph.
September
Temperatures were normal with a mean of 63.5 F. The high was 83.9 F and the low was 42.4 F. There were 1.94 inches of rainfall, which was 2.01 inches below normal.
High wind speed was 25 mph.
October
October was mild with temperatures at 1.4 F above normal with a mean of 53.3 F. The high was 80.4 F and the low was 29.7 F. The first freeze was on October 15. Rainfall was 0.95 inches, which was 2.51 inches below normal.
The high wind speed was 25 mph.
November
Temperatures in November were mild at 3.1 F above normal with a mean of 44.3 F. The high was 72.2 F and the low was 20.9 F. Precipitation was 2.55 inches, 0.44 inches below normal. The first snowfall of the season was on November 22.
The high wind speed was 30 mph.
December
December temperatures were near normal, with a mean temperature of 32.2 F. The high for the month was 61.8 F deg. F, and the low was 4.1 F. Precipitation with 2.92 inches of liquid precipitation recorded, which was 0.26 inches below normal. There were 2 inches of snowfall.
On October 10 I gave my Davis weather station an overhaul. The dew point readings seemed to be off and I had ordered and received an overhaul kit from Davis Instruments. The instrument package has been in use since December 2015.
I removed the radiation shield, cleaned the accumulated mildew from the plates, and replaced the temperature/humidity sensor. I also installed a new battery and solar panel. I replaced the rain sensor with a more accurate one. The spikes are to deter birds from clogging the rain collector.
I will work on the wind vane and anemometer in the Spring. They were replaced about 4 years ago.
Two other sensors are also part of the instrument suite. A UV sensor measures the UV index and a solar radiation sensor provides readings of Watts per square meter.
The other part of my weather station is the air quality sensor. Its operation remains nominal. It measures 1, 2.5, and 10-micrometer particulates in units of micrograms per cubic meter.
July was 1.8 F above normal for temperature. The high for the month was 94.7 deg. F recorded on July 15. The low for the month was 49.5 deg F, recorded on July 2. There were 7 days at or above 90 deg F. There were 24 heating degree days and 302 cooling degree days.
July was a below-normal month for precipitation with 2.30 inches of rainfall recorded, which was 1.49 inches below normal. The maximum rainfall in a single day was 1.28 inches recorded on July 22. There were 6 days of rain >.01 in, 3 >.10 in, and 1 >1 in.
Stormstown Dispatch 