Weather for NearSys Station, February 2018

The first half of the month was warmer than usual and the last have was colder than normal. February saw no precip until after mid-month. But it was cold enough that most precipitation fell as snow. The afternoons were warm enough that most snow melted within the day.

All Sky Photometer for NearSys Station, 27 February 2018

The All Sky Photometer began collecting data at 5:11 AM MST. The day started out clear and became partly cloudy mid-afternoon. Because of the low sun elevation of winter, the sky only got three times brighter by mid-afternoon.

The afternoon clouds were patchy.

Visibility for NearSys Station, 24 February 2018

The UAVSonde indicated that the visibility was only on the order of 30 miles at 4:30 PM MST. These images show the storms NearSys Station was experiencing this afternoon. Graupel fell shortly after this flught. Therefore, I suspect the precip visible in these images is also graupel.

The thermal imager indicated a temperature difference of 10 degrees F between the ground and the sky. If we assume a dry adiabatic lapse rate of 5.4 degrees per thousand feet, the cloud base was at 1,800 feet. That means the UAV reached 22% of the way to the cloud base. A moist adiabatic lapse rate of 3 degrees per thousand feet, then the cloud base was at 3,300 feet and the UAV climbed 12% of the way to the cloud base.

Based on the images, it appears that 1,800 feet is a more accurate altitude of the cloud base.

Looking east. Nearby precip is visible in the southeast. 

Looking West.

Looking south.

Looking North.

Thermal and Visible Imaging Test of a Snowstorm

Snowstorms passed through the area this afternoon and I wondered they appeared in thermal infrared. You can see in the images below that a snowstorm several miles away does not appear in thermal infrared. Therefore, I conclude that the air beneath a cloud is at the same temperature as falling snow. In actuallity, graupel or soft hail fell shortly after these images were taken. So it is probably more graupel rather than snow in these images.

Visible image of a snowstorm several miles away

The same storm at nearly the same time. The falling snow is indistinguishable from the air below the clouds.

All Sky Photometer for NearSys Station, 19 February 2018

The All Sky Photometer began collecting data at 7:09 AM MST. The data shows the sky was very cloudy twice today.

Visible and Thermal Images of a Cumulonimbus

A storm cloud approached NearSys Station today. I was curious about it's temperature variations and so took several visible and thermal images to compare. Here are two of the best pairs.

Based on color, the foreground cloud is warmer than the background one.

Bluer regions indicate colder patches of clouds. I think the warmer clouds are those that appear slightly brighter. Slightly darker regions in visible images may be higher altitude portions of the cloud.

Sunspot Observation at NearSys Station, 18 February 2018

The sunspot group that was visible last week is gone. Since it appeared at the sun's meridian (from Earth's perspective) eight days ago and the group has rotated at least 90 degrees, the sun must rotation once on its axis in less than 4X8 or 32 days.

No visible sunspots here.

Visibility for NearSys Station, 18 February 2018

The UAVSonde flight at 8:30 AM MST indicated the visibility is at least 30 miles. Clouds appearing over the Boise Mountains may indicate the visibility is at least 50 miles.

Looking south

Looking north

Looking east

Looking west

Sky and Ground Thermal Observation for NearSys Station, 15 February 2018

A thermal measurement at 6:15 PM MST indicated a ground temperature of 14 *F And a cloud temperature of -2 *F. At a difference of 16 degrees and assuming a dry adiabatic lapse rate of 5.4 degrees per 1,000 feet, the cloud base is 3,300 feet above the ground.

The stratocumulus above NearSys Station were producing small amounts of virga earlier.

Sunspot Observation at NearSys Station, 13 February 2018

Before that sunspot has a chance to rotate out of view, I got a second picture.

Changes in the Estimated Altitude of Stratus Clouds as Snow Approaches

Meteorologists predicted snow for the lower Treasure Valley on Sunday afternoon ( February 11th). So I started making measurements of ground and sky temperatures using my Seek Reveal thermal imager. Then assuming a dry adiabatic lapse rate of 5.4 degrees per thousand feet, I calculated the base height of the clouds. It was readily apparent their altitude was lowering as the day progressed. The data below seems to confirm that qualitative observation.

The snow didn't start until sometime after 9:15 PM MST and was finished by 4:30 AM MST the next morning. Therefore, there is no data during the time of snow at NearSys Station. The snow we did get was light and amounted to only 1/4 inches.

Ground and Air Temperatures

Estimated Altitude Corresponding to the Temperature Differences

Visibility for NearSys Station, 11 February 2018

The UAVSonde made a visibility assessment at 8:00 AM MST, 11 February 2018. After reviewing the images, it was determined the visibility is in excess of 50 miles.

Looking north

Looking east, the Boise Mountain range is clearly visible

Looking south

Looking west

Sunspot Observation at NearSys Station, 10 February 2018

SpaceWeather.com reported the appearance of a large sunspot group crossing the sun. This is at a time when the sun should be entering a quieter time as it approaches solar minimum.

The SunSpotter telescope showed five sunspots in the group now crossing the sun's center.

By counting pixels and comparing them to the sun's known diameter, the largest sunspot was determined to have an umbral diameter of 18,300 miles and a penumbral diameter of 30,500 miles. That means the largest sunspot is nearly four times wider than Earth's diameter.

All Sky Photometer for NearSys Station, 10 February 2018

Photometer data was collected starting later than usual, at 7:52 AM MST rather than prior to 6:00 AM. It was a clear day with few if any clouds. However, since data collection began after dawn, the sky only brighten by a factor of two over the day.

This is a good reason to standardize the photometer readings, it would permit different days to be compared to each other.

Sky and Ground Thermal Observation for NearSys Station, 8 February 2018

A thermal observation at 4:00 PM MST indicated the ground had a temperature of 33 *F and the altostratus over NearSys Station a temperature of -2 *F. Assuming a dry adiabatic lapse rate of 5.4 *F per thousand feet, The cloud base had an altitude of 6,500 feet.

The altostratus above NearSys Station in visible light

The altostratus above NearSys Station in long-wave infrared 

Visibility for NearSys Station. 4 February 2018

The UAVSonde was launched at launched later than usual because of high winds. Visibility appears to be barely 50 miles.

Looking north

Looking east, the Boise Mountain range is barely visible, it at all.

Looking south

Looking West into the setting sun

All Sky Photometer for NearSys Station, 4 February 2018

NearSys Station started out cloudy, but cleared up for lunch time. Then it became cloudy again. The spike in blue is puzzling and needs to be watched in future measurements.