Joseph J. Casby Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph J. Casby Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 9% 3 8% 5% 4 4 4 5% 7% 38%	127330
	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 38%	123019
Transparency	Day	44% 6% 13% 25% 11% 1	130882
	Night	45% 5% 12% 23% 12% 1	127544
Seeing	Day	16% 11% 31% 26% 13% 3	17416
	Night	16% 9% 32% 32% 10% 1	27237

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	8% 13% 6% 20% 6% 6% 5% 3 4 6% 23%	3617
	11th to 20th	Day	5% 12% 7% 13% 7% 6% 9% 8% 8% 4 20%	3620
	21st to end	Day	7% 16% 7% 12% 5% 4 4 3 5% 6% 31%	4464
February	1st to 10th	Day	9% 11% 5% 11% 6% 6% 6% 5% 6% 6% 30%	1800
	11th to 20th	Day	8% 10% 6% 10% 6% 6% 4 4 5% 6% 34%	1754
	21st to end	Day	11% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	1569
March	1st to 10th	Day	12% 8% 3 6% 3 3 3 3 5% 7% 46%	4770
	11th to 20th	Day	10% 9% 3 7% 4 4 4 4 6% 8% 40%	4980
	21st to end	Day	22% 11% 4 9% 5% 4 4 3 5% 6% 26%	5571
April	1st to 10th	Day	17% 11% 4 7% 5% 4 4 3 5% 7% 34%	2400
	11th to 20th	Day	20% 12% 4 8% 5% 4 4 3 4 5% 32%	2374
	21st to end	Day	20% 12% 3 7% 5% 4 5% 4 5% 6% 29%	2580
May	1st to 10th	Day	14% 6% 2 6% 3 3 3 3 4 6% 49%	6177
	11th to 20th	Day	16% 7% 2 5% 3 4 4 3 5% 7% 43%	6319
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 7% 38%	6935
June	1st to 10th	Day	12% 5% 2 4 4 4 5% 4 6% 9% 46%	2800
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	2800
	21st to end	Day	11% 5% 2 6% 6% 5% 6% 4 7% 9% 39%	2800
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 7% 11% 45%	6360
	11th to 20th	Day	9% 3 2 4 3 3 4 3 5% 8% 58%	6360
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 6% 10% 46%	6924
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 42%	4234
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 44%	4171
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 5% 7% 47%	4426
September	1st to 10th	Day	12% 6% 2 5% 5% 4 4 3 5% 8% 46%	2400
	11th to 20th	Day	12% 6% 1 6% 4 4 4 4 6% 8% 44%	2400
	21st to end	Day	13% 6% 3 7% 4 4 3 4 6% 7% 44%	2086
October	1st to 10th	Day	15% 7% 2 5% 4 3 4 3 5% 6% 46%	2000
	11th to 20th	Day	14% 11% 2 6% 5% 4 5% 4 4 7% 38%	2000
	21st to end	Day	17% 13% 3 7% 3 4 4 3 4 6% 37%	2150
November	1st to 10th	Day	12% 13% 4 8% 5% 4 4 3 5% 5% 35%	1689
	11th to 20th	Day	12% 14% 7% 10% 7% 4 4 4 5% 6% 28%	1600
	21st to end	Day	13% 13% 4 9% 6% 3 3 3 5% 7% 34%	1600
December	1st to 10th	Day	8% 11% 5% 16% 12% 6% 5% 4 5% 6% 23%	2610
	11th to 20th	Day	11% 16% 7% 10% 8% 4 5% 3 5% 5% 27%	3008
	21st to end	Day	19% 21% 8% 18% 5% 4 3 2 3 3 14%	3982
January	1st to 10th	Night	7% 12% 6% 16% 9% 5% 4 3 5% 6% 28%	6348
	11th to 20th	Night	7% 12% 5% 12% 8% 7% 9% 9% 7% 5% 20%	6360
	21st to end	Night	9% 17% 7% 11% 5% 5% 4 3 5% 5% 31%	6513
February	1st to 10th	Night	6% 12% 5% 11% 7% 6% 5% 4 5% 6% 33%	2600
	11th to 20th	Night	7% 9% 4 9% 6% 6% 5% 4 5% 8% 38%	2533
	21st to end	Night	12% 11% 3 9% 6% 5% 5% 4 5% 6% 36%	2060
March	1st to 10th	Night	10% 6% 3 7% 4 4 4 3 5% 6% 46%	5125
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 4 7% 8% 39%	4980
	21st to end	Night	24% 12% 4 10% 4 3 3 3 4 4 29%	5358
April	1st to 10th	Night	16% 12% 3 6% 3 4 3 3 6% 6% 37%	2000
	11th to 20th	Night	20% 10% 2 7% 4 4 3 3 5% 5% 35%	1975
	21st to end	Night	18% 10% 3 7% 5% 4 4 3 5% 7% 35%	1817
May	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 5% 50%	3737
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 5% 6% 50%	3596
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	11% 5% 2 4 4 4 4 5% 6% 7% 47%	1600
	11th to 20th	Night	10% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	1600
	21st to end	Night	7% 5% 2 4 4 4 5% 5% 6% 8% 51%	1600
July	1st to 10th	Night	8% 3 1 5% 4 4 4 4 6% 9% 52%	3618
	11th to 20th	Night	6% 3 2 4 4 4 5% 3 6% 9% 54%	3617
	21st to end	Night	8% 4 2 5% 4 4 4 4 6% 7% 51%	3947
August	1st to 10th	Night	12% 6% 3 7% 4 3 4 4 5% 7% 46%	2922
	11th to 20th	Night	14% 5% 2 4 3 3 3 4 5% 8% 50%	2888
	21st to end	Night	9% 6% 2 7% 4 5% 4 4 5% 7% 48%	3475
September	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 5% 6% 53%	2000
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 8% 52%	2000
	21st to end	Night	13% 4 2 4 4 4 3 2 5% 6% 52%	2290
October	1st to 10th	Night	14% 6% 2 6% 4 3 3 3 4 6% 51%	2400
	11th to 20th	Night	17% 9% 3 6% 4 3 3 3 5% 7% 41%	2400
	21st to end	Night	18% 11% 3 7% 3 3 4 2 4 7% 37%	2690
November	1st to 10th	Night	13% 12% 3 6% 4 4 3 3 5% 6% 40%	2653
	11th to 20th	Night	13% 12% 5% 10% 6% 4 4 4 5% 5% 33%	2800
	21st to end	Night	13% 11% 4 9% 6% 4 4 3 4 7% 36%	2800
December	1st to 10th	Night	7% 10% 5% 14% 8% 8% 4 3 5% 7% 29%	4580
	11th to 20th	Night	10% 18% 7% 14% 6% 5% 5% 4 4 5% 23%	5199
	21st to end	Night	21% 16% 9% 16% 5% 3 4 2 4 4 17%	6996

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	64% 4 9% 19% 4	3618
	11th to 20th	Day	61% 5% 7% 17% 10%	3620
	21st to end	Day	55% 6% 16% 17% 7%	4464
February	1st to 10th	Day	52% 6% 14% 17% 9% 1	1800
	11th to 20th	Day	50% 5% 12% 25% 8% 1	1763
	21st to end	Day	51% 7% 13% 20% 10% 1	1569
March	1st to 10th	Day	37% 5% 18% 25% 12% 3	4770
	11th to 20th	Day	41% 4 16% 29% 10% 1	4980
	21st to end	Day	59% 5% 10% 18% 8% 1	5573
April	1st to 10th	Day	50% 5% 12% 21% 12% 1	2400
	11th to 20th	Day	54% 5% 11% 17% 11% 2	2374
	21st to end	Day	54% 6% 12% 19% 9%	2580
May	1st to 10th	Day	35% 5% 12% 29% 16% 2	6216
	11th to 20th	Day	40% 5% 12% 23% 15% 5%	6323
	21st to end	Day	45% 6% 9% 29% 10% 1	6949
June	1st to 10th	Day	34% 8% 14% 29% 13% 1	2800
	11th to 20th	Day	43% 8% 15% 21% 13% 1	2800
	21st to end	Day	39% 9% 15% 25% 10% 2	2800
July	1st to 10th	Day	32% 7% 18% 33% 9%	6291
	11th to 20th	Day	26% 5% 15% 37% 16% 1	6285
	21st to end	Day	32% 8% 15% 29% 16% 1	6728
August	1st to 10th	Day	38% 7% 13% 27% 13% 1	4220
	11th to 20th	Day	36% 6% 13% 34% 10%	4210
	21st to end	Day	36% 6% 17% 28% 13% 1	4425
September	1st to 10th	Day	35% 6% 15% 31% 13%	2400
	11th to 20th	Day	36% 8% 17% 31% 8%	2400
	21st to end	Day	38% 7% 11% 32% 11%	2086
October	1st to 10th	Day	29% 4 8% 35% 23% 1	3260
	11th to 20th	Day	39% 4 12% 26% 17% 1	3240
	21st to end	Day	44% 4 15% 28% 9%	3452
November	1st to 10th	Day	49% 5% 13% 18% 12% 2	1689
	11th to 20th	Day	56% 7% 12% 16% 8% 1	1600
	21st to end	Day	50% 6% 17% 19% 7% 1	1600
December	1st to 10th	Day	61% 6% 15% 13% 5%	2610
	11th to 20th	Day	59% 5% 13% 18% 5%	3005
	21st to end	Day	76% 4 8% 8% 4	3982
January	1st to 10th	Night	58% 4 10% 20% 8%	6348
	11th to 20th	Night	61% 6% 10% 13% 9%	6360
	21st to end	Night	55% 4 11% 22% 7%	6513
February	1st to 10th	Night	51% 6% 12% 21% 9% 1	2600
	11th to 20th	Night	45% 5% 15% 25% 8% 1	2534
	21st to end	Night	49% 5% 12% 21% 11% 1	2060
March	1st to 10th	Night	38% 4 15% 23% 17% 3	5125
	11th to 20th	Night	40% 5% 17% 24% 14% 1	4980
	21st to end	Night	59% 4 10% 17% 10%	5358
April	1st to 10th	Night	48% 5% 9% 23% 15% 1	2000
	11th to 20th	Night	50% 5% 9% 19% 14% 2	1975
	21st to end	Night	48% 5% 9% 21% 14% 2	1817
May	1st to 10th	Night	35% 6% 10% 26% 21% 2	3763
	11th to 20th	Night	34% 5% 12% 23% 20% 7%	3597
	21st to end	Night	44% 6% 11% 26% 13% 1	3950
June	1st to 10th	Night	34% 6% 13% 28% 17% 2	1600
	11th to 20th	Night	35% 8% 14% 23% 18% 2	1600
	21st to end	Night	29% 8% 15% 29% 16% 3	1600
July	1st to 10th	Night	29% 6% 20% 31% 13% 1	3580
	11th to 20th	Night	25% 6% 15% 37% 16% 2	3572
	21st to end	Night	31% 7% 15% 27% 18% 2	3832
August	1st to 10th	Night	37% 6% 12% 28% 14% 3	2913
	11th to 20th	Night	33% 6% 14% 32% 14% 1	2915
	21st to end	Night	35% 7% 16% 28% 12% 2	3475
September	1st to 10th	Night	31% 6% 13% 34% 16% 1	2000
	11th to 20th	Night	30% 6% 18% 30% 16%	2000
	21st to end	Night	33% 5% 14% 31% 15% 1	2290
October	1st to 10th	Night	25% 3 7% 35% 26% 4	3920
	11th to 20th	Night	34% 5% 12% 24% 22% 2	3899
	21st to end	Night	45% 5% 11% 27% 13%	4338
November	1st to 10th	Night	44% 6% 12% 23% 14% 1	2653
	11th to 20th	Night	53% 6% 12% 18% 10% 1	2800
	21st to end	Night	50% 5% 16% 22% 7% 1	2800
December	1st to 10th	Night	55% 6% 17% 18% 4	4580
	11th to 20th	Night	64% 5% 11% 15% 5% 1	5201
	21st to end	Night	72% 4 9% 12% 3	6996

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	13% 13% 35% 27% 11% 2	300
	11th to 20th	Day	17% 14% 41% 22% 5%	300
	21st to end	Day	14% 11% 42% 23% 8% 2	330
February	1st to 10th	Day	14% 17% 44% 20% 4 1	300
	11th to 20th	Day	14% 16% 42% 21% 6% 1	300
	21st to end	Day	18% 14% 41% 25% 2	249
March	1st to 10th	Day	17% 13% 44% 19% 6%	300
	11th to 20th	Day	19% 14% 38% 21% 7% 1	300
	21st to end	Day	28% 14% 27% 22% 7% 3	375
April	1st to 10th	Day	23% 18% 33% 21% 5%	500
	11th to 20th	Day	29% 15% 24% 25% 7% 1	492
	21st to end	Day	27% 14% 29% 21% 8% 1	680
May	1st to 10th	Day	19% 15% 30% 24% 11% 2	703
	11th to 20th	Day	20% 13% 29% 23% 13% 3	725
	21st to end	Day	18% 10% 29% 24% 16% 3	789
June	1st to 10th	Day	11% 9% 27% 33% 17% 3	730
	11th to 20th	Day	15% 12% 34% 27% 11% 2	730
	21st to end	Day	13% 9% 32% 27% 15% 5%	730
July	1st to 10th	Day	9% 3 22% 33% 24% 10%	721
	11th to 20th	Day	9% 5% 29% 29% 21% 7%	718
	21st to end	Day	5% 4 27% 38% 19% 7%	794
August	1st to 10th	Day	11% 5% 23% 34% 21% 6%	726
	11th to 20th	Day	14% 5% 20% 33% 23% 5%	715
	21st to end	Day	10% 7% 24% 34% 20% 5%	658
September	1st to 10th	Day	14% 8% 29% 28% 17% 5%	530
	11th to 20th	Day	13% 5% 28% 30% 21% 3	530
	21st to end	Day	14% 17% 26% 21% 16% 6%	332
October	1st to 10th	Day	15% 15% 28% 27% 11% 4	330
	11th to 20th	Day	20% 20% 36% 18% 7%	330
	21st to end	Day	23% 15% 38% 16% 7% 1	363
November	1st to 10th	Day	20% 17% 33% 20% 9% 1	306
	11th to 20th	Day	19% 19% 40% 18% 3	300
	21st to end	Day	23% 16% 38% 19% 4	300
December	1st to 10th	Day	14% 12% 40% 26% 8% 1	300
	11th to 20th	Day	20% 16% 39% 19% 6%	300
	21st to end	Day	27% 14% 39% 16% 3 1	330
January	1st to 10th	Night	11% 13% 37% 31% 8% 1	1000
	11th to 20th	Night	17% 10% 37% 30% 6% 1	1000
	21st to end	Night	18% 10% 36% 29% 6% 1	1100
February	1st to 10th	Night	13% 12% 40% 28% 7% 1	1000
	11th to 20th	Night	12% 11% 43% 26% 7% 1	979
	21st to end	Night	17% 13% 34% 28% 7%	740
March	1st to 10th	Night	16% 10% 38% 28% 8% 1	695
	11th to 20th	Night	17% 12% 38% 27% 6%	600
	21st to end	Night	22% 12% 29% 25% 11% 1	660
April	1st to 10th	Night	22% 13% 35% 27% 5%	600
	11th to 20th	Night	25% 9% 33% 27% 6%	593
	21st to end	Night	21% 10% 33% 28% 8% 1	600
May	1st to 10th	Night	19% 10% 32% 31% 7% 1	600
	11th to 20th	Night	18% 9% 29% 34% 10% 1	591
	21st to end	Night	14% 8% 25% 38% 14%	650
June	1st to 10th	Night	13% 5% 28% 42% 12% 1	600
	11th to 20th	Night	12% 4 33% 43% 9%	600
	21st to end	Night	10% 4 24% 46% 15% 2	600
July	1st to 10th	Night	5% 3 21% 43% 27% 2	585
	11th to 20th	Night	5% 4 25% 44% 20% 2	582
	21st to end	Night	6% 3 22% 45% 21% 2	657
August	1st to 10th	Night	7% 4 17% 50% 22% 1	597
	11th to 20th	Night	13% 3 19% 36% 27% 3	594
	21st to end	Night	9% 4 25% 38% 23% 2	660
September	1st to 10th	Night	11% 6% 22% 42% 16% 4	600
	11th to 20th	Night	10% 7% 29% 35% 18% 2	600
	21st to end	Night	14% 9% 29% 31% 17% 1	695
October	1st to 10th	Night	17% 8% 28% 33% 13% 1	800
	11th to 20th	Night	21% 11% 33% 28% 6% 1	800
	21st to end	Night	24% 12% 27% 30% 7%	880
November	1st to 10th	Night	19% 10% 33% 30% 7% 1	879
	11th to 20th	Night	20% 12% 35% 26% 7% 1	1000
	21st to end	Night	18% 14% 35% 25% 7%	1000
December	1st to 10th	Night	15% 9% 38% 29% 7% 1	1000
	11th to 20th	Night	19% 9% 39% 27% 6%	1000
	21st to end	Night	24% 11% 34% 25% 5%	1100

Page generated at 2025-10-31 01:02:38. Version ac07

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