Pulvis Stellarum Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pulvis Stellarum Observatory clear sky chart from 2005-11-06 to 2026-01-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	17% 7% 2 6% 4 4 4 4 5% 7% 40%	127040
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	124654
Transparency	Day	42% 7% 13% 21% 15% 3	130586
	Night	43% 6% 12% 21% 16% 3	129195
Seeing	Day	19% 16% 33% 22% 8% 2	16590
	Night	20% 14% 33% 25% 7% 1	25595

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	10% 7% 3 6% 4 4 4 3 6% 6% 47%	3677
	11th to 20th	Day	14% 8% 2 6% 4 3 4 3 4 5% 48%	3618
	21st to end	Day	18% 8% 3 6% 4 4 3 3 4 4 43%	3960
February	1st to 10th	Day	18% 8% 2 5% 3 5% 5% 3 6% 6% 38%	1916
	11th to 20th	Day	14% 7% 2 5% 5% 4 5% 4 5% 7% 42%	1948
	21st to end	Day	20% 7% 3 6% 4 4 4 4 4 6% 40%	1650
March	1st to 10th	Day	20% 11% 3 6% 3 3 3 2 4 6% 39%	4476
	11th to 20th	Day	15% 9% 2 5% 4 4 4 3 5% 6% 43%	4767
	21st to end	Day	22% 9% 3 7% 4 4 4 4 5% 5% 33%	5780
April	1st to 10th	Day	19% 8% 3 6% 4 3 3 3 6% 7% 38%	2400
	11th to 20th	Day	18% 7% 2 5% 3 3 5% 3 6% 7% 40%	2374
	21st to end	Day	20% 7% 2 6% 4 4 4 3 5% 6% 39%	2595
May	1st to 10th	Day	29% 9% 3 6% 4 3 4 3 5% 7% 28%	5886
	11th to 20th	Day	22% 8% 3 7% 5% 5% 4 3 5% 6% 34%	5895
	21st to end	Day	20% 6% 2 6% 4 4 4 5% 6% 8% 36%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 7% 8% 35%	2800
	11th to 20th	Day	15% 6% 2 5% 5% 5% 5% 4 7% 8% 37%	2800
	21st to end	Day	14% 6% 2 6% 4 5% 5% 4 6% 9% 39%	2800
July	1st to 10th	Day	15% 7% 3 7% 5% 5% 6% 4 6% 10% 33%	6360
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 10% 35%	6360
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 41%	6926
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 9% 32%	4244
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	4061
	21st to end	Day	7% 4 2 5% 4 5% 6% 5% 7% 8% 45%	4312
September	1st to 10th	Day	15% 5% 3 6% 4 4 4 3 5% 6% 45%	2395
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 52%	2200
	21st to end	Day	19% 6% 2 6% 4 4 4 3 4 5% 43%	2200
October	1st to 10th	Day	22% 6% 2 4 3 3 3 2 4 5% 45%	2200
	11th to 20th	Day	16% 6% 2 5% 3 3 3 3 5% 7% 47%	1974
	21st to end	Day	19% 5% 2 5% 4 4 4 3 5% 4 45%	1980
November	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 5% 5% 47%	1851
	11th to 20th	Day	16% 6% 2 4 4 3 4 3 5% 6% 48%	1890
	21st to end	Day	18% 6% 2 6% 4 3 4 3 4 5% 45%	1890
December	1st to 10th	Day	14% 8% 3 5% 3 3 3 3 5% 6% 48%	2792
	11th to 20th	Day	19% 6% 2 5% 4 2 3 2 3 5% 50%	3066
	21st to end	Day	19% 10% 3 7% 4 4 3 2 4 5% 39%	4048
January	1st to 10th	Night	14% 5% 2 4 3 3 4 3 4 6% 52%	6489
	11th to 20th	Night	15% 9% 3 7% 4 4 3 3 4 4 43%	6400
	21st to end	Night	17% 9% 3 7% 4 5% 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 3 5% 5% 4 4 3 4 5% 41%	2479
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	2338
	21st to end	Night	17% 9% 2 6% 5% 4 4 3 5% 4 41%	1980
March	1st to 10th	Night	23% 9% 2 4 3 4 4 3 4 6% 39%	5364
	11th to 20th	Night	17% 10% 3 6% 4 4 3 3 4 6% 41%	5191
	21st to end	Night	24% 10% 2 6% 4 4 4 3 5% 5% 34%	5149
April	1st to 10th	Night	19% 9% 3 6% 4 5% 4 3 5% 6% 37%	2000
	11th to 20th	Night	19% 8% 2 6% 4 3 4 3 5% 6% 39%	1975
	21st to end	Night	20% 8% 3 7% 4 4 3 3 4 5% 41%	1805
May	1st to 10th	Night	29% 8% 3 7% 5% 3 4 3 4 6% 27%	4054
	11th to 20th	Night	21% 9% 3 8% 5% 5% 4 3 5% 7% 29%	4033
	21st to end	Night	21% 7% 3 7% 5% 4 4 4 6% 7% 32%	3934
June	1st to 10th	Night	17% 7% 2 7% 5% 4 4 4 6% 7% 37%	1600
	11th to 20th	Night	18% 7% 2 6% 5% 4 4 3 6% 8% 38%	1600
	21st to end	Night	15% 8% 2 5% 4 3 4 4 6% 9% 40%	1600
July	1st to 10th	Night	16% 8% 3 7% 4 4 5% 4 6% 9% 34%	3600
	11th to 20th	Night	14% 9% 2 7% 5% 4 5% 5% 6% 9% 34%	3600
	21st to end	Night	14% 7% 3 6% 5% 4 4 3 5% 7% 41%	3926
August	1st to 10th	Night	20% 7% 2 7% 5% 4 6% 4 7% 8% 30%	2873
	11th to 20th	Night	9% 7% 3 7% 6% 6% 7% 6% 7% 8% 36%	2969
	21st to end	Night	8% 4 2 5% 3 4 3 3 5% 8% 56%	3586
September	1st to 10th	Night	13% 6% 3 7% 4 4 4 4 6% 6% 44%	2005
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 7% 51%	2200
	21st to end	Night	19% 8% 3 5% 4 3 4 3 4 5% 41%	2200
October	1st to 10th	Night	19% 6% 2 6% 3 3 4 3 4 6% 44%	2200
	11th to 20th	Night	16% 4 2 4 4 4 4 3 5% 7% 48%	2421
	21st to end	Night	20% 6% 2 5% 4 4 3 2 5% 6% 42%	2860
November	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 4 6% 49%	2667
	11th to 20th	Night	17% 7% 2 5% 3 3 4 3 4 5% 47%	2730
	21st to end	Night	15% 6% 2 5% 3 3 4 3 5% 7% 46%	2730
December	1st to 10th	Night	16% 9% 2 5% 4 3 3 2 4 4 49%	4602
	11th to 20th	Night	22% 8% 2 5% 3 3 3 2 4 4 43%	5348
	21st to end	Night	16% 10% 4 7% 4 3 3 2 4 6% 42%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	36% 5% 8% 26% 21% 4	3678
	11th to 20th	Day	39% 6% 10% 21% 17% 8%	3648
	21st to end	Day	45% 5% 9% 21% 19% 2	3960
February	1st to 10th	Day	45% 6% 15% 18% 14% 3	1916
	11th to 20th	Day	42% 6% 11% 23% 15% 4	1957
	21st to end	Day	45% 6% 12% 19% 15% 3	1650
March	1st to 10th	Day	45% 5% 12% 14% 12% 11%	4479
	11th to 20th	Day	38% 7% 15% 27% 11% 2	4767
	21st to end	Day	52% 5% 8% 14% 16% 6%	5780
April	1st to 10th	Day	43% 7% 12% 18% 16% 3	2400
	11th to 20th	Day	43% 7% 9% 20% 18% 3	2374
	21st to end	Day	44% 8% 10% 19% 15% 4	2595
May	1st to 10th	Day	55% 8% 16% 12% 7% 1	5897
	11th to 20th	Day	49% 8% 15% 16% 11% 2	5899
	21st to end	Day	45% 9% 13% 20% 13% 1	6949
June	1st to 10th	Day	43% 9% 15% 20% 12% 1	2800
	11th to 20th	Day	41% 9% 15% 21% 13% 1	2800
	21st to end	Day	40% 10% 15% 21% 13% 1	2800
July	1st to 10th	Day	46% 9% 13% 19% 12% 1	6290
	11th to 20th	Day	41% 8% 17% 24% 9% 1	6285
	21st to end	Day	40% 6% 12% 24% 16% 1	6728
August	1st to 10th	Day	48% 8% 16% 19% 8%	4230
	11th to 20th	Day	40% 8% 15% 27% 9% 1	4100
	21st to end	Day	31% 8% 16% 25% 18% 2	4312
September	1st to 10th	Day	38% 8% 11% 22% 19% 1	2395
	11th to 20th	Day	32% 6% 12% 25% 23% 3	2200
	21st to end	Day	45% 6% 9% 20% 20% 2	2200
October	1st to 10th	Day	37% 4 10% 23% 20% 5%	3590
	11th to 20th	Day	30% 4 9% 25% 28% 4	3190
	21st to end	Day	31% 4 14% 29% 19% 3	3180
November	1st to 10th	Day	39% 6% 13% 20% 20% 2	1851
	11th to 20th	Day	37% 5% 11% 21% 21% 4	1890
	21st to end	Day	42% 5% 10% 27% 13% 3	1890
December	1st to 10th	Day	39% 4 13% 27% 16% 1	2792
	11th to 20th	Day	40% 3 14% 18% 19% 6%	3066
	21st to end	Day	48% 6% 8% 20% 14% 4	4048
January	1st to 10th	Night	34% 4 10% 31% 19% 3	6489
	11th to 20th	Night	44% 4 11% 20% 15% 6%	6445
	21st to end	Night	48% 6% 10% 18% 15% 5%	6996
February	1st to 10th	Night	46% 5% 12% 22% 13% 2	2479
	11th to 20th	Night	44% 5% 10% 19% 17% 4	2340
	21st to end	Night	46% 5% 9% 21% 15% 4	1980
March	1st to 10th	Night	47% 4 10% 16% 14% 7%	5374
	11th to 20th	Night	45% 7% 16% 19% 11% 2	5192
	21st to end	Night	51% 5% 6% 11% 21% 6%	5149
April	1st to 10th	Night	47% 5% 10% 19% 15% 4	2000
	11th to 20th	Night	45% 6% 11% 17% 19% 2	1975
	21st to end	Night	46% 5% 10% 19% 15% 4	1805
May	1st to 10th	Night	57% 9% 15% 11% 8% 1	4060
	11th to 20th	Night	52% 9% 13% 15% 8% 3	4034
	21st to end	Night	51% 10% 13% 17% 9% 1	3935
June	1st to 10th	Night	45% 8% 12% 19% 14% 1	1600
	11th to 20th	Night	44% 7% 13% 23% 13% 1	1600
	21st to end	Night	41% 8% 15% 24% 12% 1	1600
July	1st to 10th	Night	49% 7% 11% 20% 12% 1	3560
	11th to 20th	Night	48% 8% 14% 21% 8%	3552
	21st to end	Night	42% 6% 13% 24% 14% 1	3809
August	1st to 10th	Night	51% 10% 13% 17% 8% 1	2863
	11th to 20th	Night	40% 11% 15% 26% 8%	2996
	21st to end	Night	27% 6% 17% 28% 21% 1	3586
September	1st to 10th	Night	40% 7% 10% 24% 17% 1	2005
	11th to 20th	Night	34% 6% 12% 23% 24% 2	2200
	21st to end	Night	47% 7% 8% 17% 18% 3	2200
October	1st to 10th	Night	39% 5% 11% 24% 17% 3	3590
	11th to 20th	Night	26% 4 9% 26% 32% 4	3944
	21st to end	Night	33% 4 12% 28% 21% 3	4610
November	1st to 10th	Night	37% 6% 13% 22% 19% 3	2667
	11th to 20th	Night	40% 5% 9% 23% 19% 4	2730
	21st to end	Night	38% 5% 14% 27% 13% 2	2730
December	1st to 10th	Night	41% 4 12% 27% 16% 1	4602
	11th to 20th	Night	46% 4 12% 16% 19% 2	5348
	21st to end	Night	45% 5% 10% 20% 18% 3	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 25% 34% 17% 3	220
	11th to 20th	Day	20% 29% 38% 9% 3	212
	21st to end	Day	19% 26% 38% 12% 4	220
February	1st to 10th	Day	24% 23% 36% 15% 1	441
	11th to 20th	Day	26% 25% 38% 9% 1	496
	21st to end	Day	22% 23% 37% 14% 3	414
March	1st to 10th	Day	25% 22% 33% 17% 2	500
	11th to 20th	Day	21% 23% 37% 16% 2 1	500
	21st to end	Day	26% 20% 37% 12% 5% 1	550
April	1st to 10th	Day	25% 24% 32% 14% 5%	500
	11th to 20th	Day	24% 22% 37% 13% 3	492
	21st to end	Day	24% 18% 35% 17% 4 2	500
May	1st to 10th	Day	29% 18% 35% 15% 4	503
	11th to 20th	Day	20% 17% 41% 16% 5% 1	527
	21st to end	Day	17% 11% 35% 28% 7% 2	573
June	1st to 10th	Day	17% 11% 37% 25% 9% 2	530
	11th to 20th	Day	15% 13% 37% 27% 8% 1	530
	21st to end	Day	15% 11% 35% 29% 9% 1	530
July	1st to 10th	Day	15% 7% 27% 35% 14% 2	525
	11th to 20th	Day	12% 7% 27% 37% 16% 2	524
	21st to end	Day	13% 7% 35% 34% 10% 1	576
August	1st to 10th	Day	11% 8% 32% 34% 11% 3	526
	11th to 20th	Day	15% 6% 26% 35% 15% 4	519
	21st to end	Day	10% 6% 35% 29% 14% 7%	583
September	1st to 10th	Day	17% 9% 28% 26% 15% 4	530
	11th to 20th	Day	14% 9% 26% 27% 17% 7%	530
	21st to end	Day	22% 8% 32% 25% 10% 3	530
October	1st to 10th	Day	22% 12% 26% 22% 12% 5%	530
	11th to 20th	Day	18% 17% 31% 21% 11% 2	477
	21st to end	Day	22% 17% 30% 21% 9% 2	462
November	1st to 10th	Day	15% 18% 36% 22% 8% 2	425
	11th to 20th	Day	20% 24% 33% 18% 4	430
	21st to end	Day	23% 21% 32% 18% 6% 1	430
December	1st to 10th	Day	21% 21% 36% 17% 5%	293
	11th to 20th	Day	27% 27% 31% 11% 3	220
	21st to end	Day	21% 21% 37% 13% 5% 2	242
January	1st to 10th	Night	20% 22% 40% 16% 3	840
	11th to 20th	Night	27% 22% 33% 15% 2	825
	21st to end	Night	18% 25% 35% 19% 2	880
February	1st to 10th	Night	22% 20% 38% 20% 1	800
	11th to 20th	Night	24% 22% 40% 12% 1	783
	21st to end	Night	23% 21% 36% 18% 2	660
March	1st to 10th	Night	23% 20% 35% 19% 4	800
	11th to 20th	Night	21% 16% 39% 21% 3 1	800
	21st to end	Night	25% 15% 38% 18% 4	880
April	1st to 10th	Night	23% 18% 35% 21% 3	800
	11th to 20th	Night	20% 16% 39% 21% 4	791
	21st to end	Night	23% 15% 36% 21% 5%	605
May	1st to 10th	Night	26% 11% 32% 25% 6%	600
	11th to 20th	Night	21% 12% 40% 22% 4 1	592
	21st to end	Night	22% 5% 27% 37% 8% 1	441
June	1st to 10th	Night	19% 7% 28% 36% 9% 1	400
	11th to 20th	Night	19% 6% 29% 38% 9%	400
	21st to end	Night	18% 6% 26% 38% 12%	400
July	1st to 10th	Night	16% 5% 26% 38% 14% 1	390
	11th to 20th	Night	15% 4 23% 42% 14% 2	388
	21st to end	Night	14% 2 26% 44% 13%	438
August	1st to 10th	Night	13% 4 27% 43% 12% 1	568
	11th to 20th	Night	12% 3 26% 40% 18% 1	650
	21st to end	Night	14% 4 23% 38% 19% 3	880
September	1st to 10th	Night	15% 5% 29% 34% 16% 2	800
	11th to 20th	Night	16% 5% 26% 32% 18% 4	800
	21st to end	Night	24% 7% 26% 29% 14% 2	800
October	1st to 10th	Night	22% 10% 28% 25% 13% 2	800
	11th to 20th	Night	18% 12% 33% 24% 11% 2	800
	21st to end	Night	21% 14% 37% 21% 7%	880
November	1st to 10th	Night	18% 14% 34% 24% 10% 1	820
	11th to 20th	Night	20% 20% 34% 19% 6%	840
	21st to end	Night	19% 21% 32% 23% 5% 1	840
December	1st to 10th	Night	24% 19% 33% 20% 4 1	840
	11th to 20th	Night	23% 22% 36% 16% 3	840
	21st to end	Night	23% 20% 36% 17% 3	924

Page generated at 2026-01-14 01:14:31. Version ac07

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