Joseph P. Strogen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph P. Strogen Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	12% 7% 3 6% 4 4 4 4 5% 7% 44%	143598
	Night	13% 8% 3 6% 4 4 4 3 5% 6% 46%	138588
Transparency	Day	39% 5% 11% 25% 17% 3	147203
	Night	39% 5% 10% 25% 18% 3	143062
Seeing	Day	18% 13% 29% 25% 13% 3	18063
	Night	18% 12% 31% 27% 11% 1	27794

By Month

Percentage Cloud Cover SE (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	13% 11% 3 9% 4 4 4 3 4 5% 39%	5127
	11th to 20th	Day	8% 10% 3 8% 4 5% 4 3 4 6% 45%	5310
	21st to end	Day	16% 10% 4 7% 3 3 3 3 4 4 41%	5812
February	1st to 10th	Day	14% 11% 4 10% 5% 4 4 3 5% 5% 36%	1890
	11th to 20th	Day	14% 10% 3 7% 6% 4 4 4 4 5% 39%	1900
	21st to end	Day	15% 10% 3 6% 4 4 4 4 5% 6% 40%	1735
March	1st to 10th	Day	15% 10% 3 5% 3 3 3 2 4 5% 47%	5046
	11th to 20th	Day	11% 7% 2 4 3 3 3 3 5% 7% 53%	5090
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 6% 6% 35%	5621
April	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 4 8% 43%	2520
	11th to 20th	Day	17% 9% 2 6% 3 4 5% 3 5% 6% 40%	2494
	21st to end	Day	20% 9% 3 7% 4 3 4 4 6% 6% 34%	2520
May	1st to 10th	Day	16% 7% 3 6% 4 5% 5% 4 6% 7% 37%	7411
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 5% 8% 39%	7722
	21st to end	Day	21% 7% 2 6% 4 4 4 3 5% 7% 34%	9047
June	1st to 10th	Day	9% 6% 2 5% 4 5% 5% 5% 6% 9% 44%	2940
	11th to 20th	Day	9% 4 2 5% 5% 4 5% 4 6% 10% 47%	2940
	21st to end	Day	7% 3 2 4 4 4 5% 4 7% 9% 51%	2940
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 45%	8247
	11th to 20th	Day	6% 4 2 6% 5% 6% 5% 5% 7% 10% 44%	7680
	21st to end	Day	8% 4 2 6% 4 5% 5% 5% 7% 11% 43%	8337
August	1st to 10th	Day	5% 3 1 5% 4 4 5% 4 7% 9% 53%	4364
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 3 5% 9% 46%	4071
	21st to end	Day	7% 5% 2 4 3 4 5% 4 6% 8% 53%	4312
September	1st to 10th	Day	11% 6% 1 4 5% 4 4 3 5% 7% 50%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 6% 50%	2400
	21st to end	Day	8% 4 2 5% 4 4 4 3 5% 7% 53%	2085
October	1st to 10th	Day	12% 4 2 6% 3 2 3 3 4 6% 56%	2000
	11th to 20th	Day	15% 6% 2 4 2 2 2 3 4 4 56%	2000
	21st to end	Day	19% 9% 2 5% 3 2 3 2 4 5% 46%	2200
November	1st to 10th	Day	12% 6% 1 5% 4 3 3 3 4 5% 54%	2024
	11th to 20th	Day	14% 8% 2 5% 4 3 3 2 4 6% 49%	1885
	21st to end	Day	16% 10% 3 6% 2 3 2 4 4 6% 45%	1680
December	1st to 10th	Day	10% 9% 2 6% 3 3 3 3 4 5% 51%	2690
	11th to 20th	Day	11% 11% 4 6% 3 3 3 2 4 5% 47%	3088
	21st to end	Day	19% 20% 5% 9% 3 4 4 3 3 4 26%	4070
January	1st to 10th	Night	13% 11% 5% 11% 5% 5% 3 2 3 4 37%	7779
	11th to 20th	Night	9% 8% 3 11% 5% 5% 4 3 3 4 45%	7680
	21st to end	Night	15% 11% 4 7% 5% 4 3 2 4 4 40%	8402
February	1st to 10th	Night	14% 11% 3 8% 4 3 3 3 5% 6% 38%	2730
	11th to 20th	Night	16% 10% 3 7% 4 4 3 3 4 5% 41%	2605
	21st to end	Night	14% 10% 3 6% 5% 4 4 2 4 6% 42%	2076
March	1st to 10th	Night	17% 12% 2 5% 4 3 3 3 4 4 44%	5040
	11th to 20th	Night	12% 8% 2 4 3 3 3 3 5% 7% 50%	5090
	21st to end	Night	14% 9% 3 7% 5% 5% 4 4 5% 7% 38%	5549
April	1st to 10th	Night	11% 7% 3 5% 4 3 4 2 5% 7% 50%	2100
	11th to 20th	Night	14% 9% 2 5% 3 3 3 3 4 7% 47%	2075
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 7% 39%	2100
May	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 5% 7% 40%	5458
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 6% 8% 45%	5151
	21st to end	Night	20% 9% 3 6% 4 5% 4 3 5% 7% 34%	5139
June	1st to 10th	Night	8% 5% 2 5% 3 4 5% 4 7% 8% 49%	1680
	11th to 20th	Night	7% 5% 1 4 3 4 4 5% 7% 11% 51%	1680
	21st to end	Night	5% 3 1 3 2 4 3 4 6% 10% 59%	1680
July	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 6% 9% 49%	4723
	11th to 20th	Night	5% 3 2 5% 4 4 4 5% 6% 10% 52%	5307
	21st to end	Night	5% 4 2 5% 4 4 4 4 7% 11% 50%	5770
August	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 8% 62%	3016
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 6% 51%	3069
	21st to end	Night	5% 3 2 3 2 3 4 3 5% 9% 62%	3586
September	1st to 10th	Night	12% 5% 2 5% 3 2 3 2 4 7% 55%	2000
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 5% 8% 53%	2000
	21st to end	Night	10% 5% 1 5% 3 3 3 3 5% 6% 56%	2289
October	1st to 10th	Night	7% 4 1 4 2 3 3 3 4 7% 61%	2400
	11th to 20th	Night	15% 5% 1 4 3 4 3 2 4 5% 53%	2400
	21st to end	Night	19% 6% 2 4 3 3 2 3 4 4 51%	2640
November	1st to 10th	Night	12% 6% 2 4 2 3 3 2 4 5% 57%	2492
	11th to 20th	Night	13% 7% 2 6% 4 3 3 2 4 5% 52%	2733
	21st to end	Night	16% 10% 3 5% 4 3 2 2 4 4 47%	2940
December	1st to 10th	Night	10% 9% 3 7% 4 3 3 3 4 5% 48%	4720
	11th to 20th	Night	16% 9% 4 7% 3 3 4 3 3 5% 44%	5339
	21st to end	Night	23% 17% 5% 8% 4 4 3 3 4 4 26%	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	48% 3 10% 24% 13% 1	5128
	11th to 20th	Day	41% 2 8% 27% 16% 5%	5310
	21st to end	Day	47% 3 8% 18% 16% 8%	5812
February	1st to 10th	Day	51% 4 10% 22% 12% 1	1890
	11th to 20th	Day	48% 3 11% 19% 15% 4	1909
	21st to end	Day	45% 4 6% 23% 16% 5%	1735
March	1st to 10th	Day	40% 3 7% 22% 19% 8%	5046
	11th to 20th	Day	32% 3 8% 25% 23% 9%	5090
	21st to end	Day	47% 6% 12% 26% 8% 1	5623
April	1st to 10th	Day	41% 6% 10% 23% 18% 2	2520
	11th to 20th	Day	44% 5% 12% 21% 16% 3	2494
	21st to end	Day	50% 5% 11% 22% 11% 1	2520
May	1st to 10th	Day	44% 7% 12% 23% 15%	7447
	11th to 20th	Day	43% 8% 10% 22% 17% 2	7726
	21st to end	Day	49% 6% 8% 22% 13%	9061
June	1st to 10th	Day	35% 8% 14% 25% 16% 2	2940
	11th to 20th	Day	32% 7% 12% 28% 17% 3	2940
	21st to end	Day	27% 7% 16% 29% 16% 4	2940
July	1st to 10th	Day	35% 7% 13% 24% 19% 2	8178
	11th to 20th	Day	33% 8% 16% 26% 14% 3	7611
	21st to end	Day	33% 7% 20% 31% 8% 1	8155
August	1st to 10th	Day	26% 6% 18% 28% 20% 1	4350
	11th to 20th	Day	36% 6% 13% 30% 15% 1	4110
	21st to end	Day	28% 6% 13% 34% 14% 4	4311
September	1st to 10th	Day	33% 7% 11% 25% 18% 6%	2400
	11th to 20th	Day	34% 5% 10% 25% 24% 2	2400
	21st to end	Day	31% 6% 10% 25% 24% 4	2085
October	1st to 10th	Day	19% 3 5% 19% 41% 13%	3260
	11th to 20th	Day	22% 3 4 24% 35% 11%	3240
	21st to end	Day	33% 3 7% 28% 26% 3	3538
November	1st to 10th	Day	34% 4 10% 26% 22% 5%	2024
	11th to 20th	Day	38% 3 7% 25% 22% 5%	1885
	21st to end	Day	42% 4 8% 28% 16% 1	1680
December	1st to 10th	Day	37% 4 9% 35% 14% 1	2690
	11th to 20th	Day	41% 4 8% 28% 19% 1	3085
	21st to end	Day	64% 4 10% 15% 8% 1	4070
January	1st to 10th	Night	53% 2 10% 23% 10% 1	7780
	11th to 20th	Night	45% 3 6% 25% 18% 4	7680
	21st to end	Night	49% 3 9% 20% 16% 3	8402
February	1st to 10th	Night	47% 4 11% 22% 14% 2	2730
	11th to 20th	Night	45% 4 10% 22% 16% 3	2606
	21st to end	Night	44% 3 8% 23% 17% 5%	2076
March	1st to 10th	Night	45% 4 6% 20% 21% 4	5040
	11th to 20th	Night	34% 4 8% 23% 22% 8%	5090
	21st to end	Night	45% 6% 13% 23% 12% 1	5549
April	1st to 10th	Night	35% 5% 12% 25% 20% 4	2100
	11th to 20th	Night	38% 5% 11% 24% 17% 5%	2075
	21st to end	Night	45% 5% 10% 25% 14% 1	2100
May	1st to 10th	Night	43% 6% 13% 24% 13% 1	5487
	11th to 20th	Night	36% 7% 11% 24% 18% 4	5152
	21st to end	Night	49% 7% 9% 21% 12% 1	5149
June	1st to 10th	Night	31% 8% 15% 26% 18% 3	1680
	11th to 20th	Night	26% 6% 14% 28% 21% 5%	1680
	21st to end	Night	20% 6% 18% 31% 19% 6%	1680
July	1st to 10th	Night	31% 6% 14% 25% 22% 2	4685
	11th to 20th	Night	27% 8% 15% 31% 18% 2	5256
	21st to end	Night	27% 6% 18% 39% 9%	5641
August	1st to 10th	Night	19% 6% 19% 31% 23% 1	3006
	11th to 20th	Night	33% 6% 14% 29% 16% 2	3096
	21st to end	Night	20% 5% 15% 38% 17% 4	3586
September	1st to 10th	Night	31% 5% 9% 26% 22% 7%	2000
	11th to 20th	Night	31% 5% 10% 24% 26% 4	2000
	21st to end	Night	29% 4 10% 25% 30% 4	2289
October	1st to 10th	Night	15% 3 5% 20% 45% 12%	3920
	11th to 20th	Night	24% 3 6% 21% 30% 16%	3899
	21st to end	Night	32% 3 6% 30% 26% 3	4252
November	1st to 10th	Night	31% 3 10% 28% 21% 6%	2492
	11th to 20th	Night	36% 3 7% 25% 23% 5%	2733
	21st to end	Night	42% 4 8% 26% 17% 2	2940
December	1st to 10th	Night	39% 4 8% 33% 15% 1	4720
	11th to 20th	Night	45% 3 9% 24% 18% 1	5341
	21st to end	Night	63% 4 11% 15% 6% 1	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	24% 25% 37% 13% 1	330
	11th to 20th	Day	32% 23% 38% 7% 1	330
	21st to end	Day	23% 26% 39% 10% 1	363
February	1st to 10th	Day	25% 26% 35% 12% 2	330
	11th to 20th	Day	30% 27% 31% 9% 2	330
	21st to end	Day	36% 19% 35% 10% 1	273
March	1st to 10th	Day	27% 22% 39% 10% 1	330
	11th to 20th	Day	35% 16% 34% 12% 1 1	330
	21st to end	Day	32% 27% 32% 8% 1	376
April	1st to 10th	Day	26% 19% 35% 16% 4 1	540
	11th to 20th	Day	30% 20% 30% 16% 4	532
	21st to end	Day	26% 17% 35% 18% 5%	540
May	1st to 10th	Day	21% 17% 36% 21% 5%	675
	11th to 20th	Day	21% 16% 28% 24% 11% 1	775
	21st to end	Day	16% 10% 36% 22% 11% 4	844
June	1st to 10th	Day	11% 7% 27% 31% 19% 4	780
	11th to 20th	Day	10% 5% 27% 39% 17% 2	780
	21st to end	Day	9% 7% 31% 37% 13% 3	780
July	1st to 10th	Day	12% 3 19% 32% 27% 7%	771
	11th to 20th	Day	5% 4 24% 38% 24% 5%	768
	21st to end	Day	9% 3 22% 34% 27% 5%	849
August	1st to 10th	Day	9% 3 21% 39% 23% 4	776
	11th to 20th	Day	10% 4 21% 35% 26% 5%	648
	21st to end	Day	11% 3 17% 32% 28% 9%	583
September	1st to 10th	Day	13% 4 16% 37% 25% 5%	530
	11th to 20th	Day	7% 4 20% 38% 22% 9%	530
	21st to end	Day	10% 11% 25% 34% 17% 4	334
October	1st to 10th	Day	15% 9% 33% 27% 13% 3	330
	11th to 20th	Day	21% 18% 34% 20% 6% 1	330
	21st to end	Day	31% 20% 31% 16% 2	363
November	1st to 10th	Day	18% 16% 42% 18% 5% 1	330
	11th to 20th	Day	24% 18% 35% 16% 6% 1	330
	21st to end	Day	33% 16% 30% 17% 4	330
December	1st to 10th	Day	21% 22% 35% 15% 4 2	330
	11th to 20th	Day	22% 23% 38% 14% 4	330
	21st to end	Day	28% 26% 34% 11% 1	363
January	1st to 10th	Night	22% 18% 38% 20% 3	1050
	11th to 20th	Night	22% 23% 35% 16% 4	1050
	21st to end	Night	20% 21% 37% 21% 1	1155
February	1st to 10th	Night	24% 16% 40% 17% 3	1050
	11th to 20th	Night	25% 18% 37% 18% 1	971
	21st to end	Night	26% 20% 35% 17% 2	692
March	1st to 10th	Night	23% 19% 36% 20% 2	641
	11th to 20th	Night	26% 16% 34% 21% 3	630
	21st to end	Night	27% 16% 36% 18% 3	693
April	1st to 10th	Night	17% 19% 39% 21% 4	630
	11th to 20th	Night	22% 14% 37% 22% 5%	623
	21st to end	Night	22% 13% 37% 22% 6%	630
May	1st to 10th	Night	17% 9% 36% 31% 6%	630
	11th to 20th	Night	18% 9% 29% 29% 14% 1	621
	21st to end	Night	16% 8% 28% 30% 17% 2	683
June	1st to 10th	Night	10% 5% 22% 39% 23% 2	630
	11th to 20th	Night	8% 3 28% 40% 20% 1	630
	21st to end	Night	5% 2 29% 46% 17% 2	630
July	1st to 10th	Night	11% 2 13% 38% 30% 5%	615
	11th to 20th	Night	6% 1 18% 44% 28% 3	612
	21st to end	Night	8% 2 16% 41% 32% 3	690
August	1st to 10th	Night	6% 2 20% 42% 27% 2	627
	11th to 20th	Night	8% 2 16% 44% 28% 2	606
	21st to end	Night	7% 1 15% 40% 31% 5%	660
September	1st to 10th	Night	14% 3 15% 41% 25% 3	600
	11th to 20th	Night	11% 2 18% 35% 28% 5%	600
	21st to end	Night	10% 5% 25% 37% 20% 3	697
October	1st to 10th	Night	10% 6% 28% 32% 21% 3	800
	11th to 20th	Night	18% 12% 34% 29% 8%	800
	21st to end	Night	24% 12% 34% 24% 6%	880
November	1st to 10th	Night	15% 14% 37% 26% 8%	820
	11th to 20th	Night	20% 16% 34% 24% 6%	843
	21st to end	Night	25% 15% 32% 21% 5%	1050
December	1st to 10th	Night	22% 17% 36% 20% 5%	1050
	11th to 20th	Night	24% 15% 36% 21% 4	1050
	21st to end	Night	25% 19% 35% 19% 2	1155

Page generated at 2026-08-12 02:37:14. Version ac07

Copyright © 2026 Attilla Danko.