Ryan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ryan Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	15% 6% 2 6% 4 4 4 4 6% 8% 40%	126366
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	122687
Transparency	Day	40% 7% 12% 23% 16% 3	129954
	Night	40% 6% 11% 23% 17% 3	127168
Seeing	Day	19% 13% 27% 23% 13% 4	16072
	Night	18% 11% 30% 29% 11% 1	25405

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% 6% 2 6% 4 4 4 3 5% 6% 47%	3597
	11th to 20th	Day	13% 8% 3 10% 5% 4 4 2 4 5% 41%	3899
	21st to end	Day	16% 8% 3 7% 4 4 4 3 5% 6% 40%	4466
February	1st to 10th	Day	14% 10% 3 6% 4 4 4 3 5% 8% 38%	1800
	11th to 20th	Day	17% 8% 2 5% 5% 4 4 3 6% 6% 40%	1847
	21st to end	Day	18% 11% 3 6% 4 4 3 3 4 5% 39%	1650
March	1st to 10th	Day	19% 5% 2 4 3 4 4 3 4 6% 46%	4663
	11th to 20th	Day	11% 6% 3 6% 4 4 4 3 6% 7% 45%	4980
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 5% 42%	5682
April	1st to 10th	Day	15% 8% 2 6% 5% 5% 5% 4 5% 7% 39%	2400
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 4 5% 8% 43%	2374
	21st to end	Day	17% 5% 2 5% 4 4 5% 4 5% 8% 41%	2400
May	1st to 10th	Day	25% 7% 2 6% 4 4 4 4 6% 8% 30%	5767
	11th to 20th	Day	25% 7% 3 7% 4 4 4 3 6% 7% 31%	6318
	21st to end	Day	17% 5% 2 6% 4 4 5% 4 7% 9% 37%	6934
June	1st to 10th	Day	12% 5% 2 7% 4 4 5% 5% 7% 10% 39%	2800
	11th to 20th	Day	13% 5% 2 6% 6% 4 5% 4 7% 9% 38%	2800
	21st to end	Day	10% 4 2 6% 4 5% 5% 5% 7% 10% 43%	2800
July	1st to 10th	Day	7% 5% 2 6% 4 5% 6% 4 8% 10% 41%	6360
	11th to 20th	Day	10% 7% 3 7% 6% 6% 7% 6% 8% 11% 30%	6360
	21st to end	Day	9% 6% 2 7% 5% 4 5% 5% 7% 10% 39%	6566
August	1st to 10th	Day	19% 5% 2 6% 4 5% 5% 5% 6% 8% 34%	4234
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 7% 9% 40%	3945
	21st to end	Day	11% 5% 2 5% 4 3 4 4 6% 7% 50%	4312
September	1st to 10th	Day	15% 5% 2 6% 4 4 4 3 5% 6% 45%	2400
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 7% 49%	2376
	21st to end	Day	20% 8% 2 6% 4 4 4 3 5% 7% 37%	2028
October	1st to 10th	Day	18% 6% 2 6% 4 3 4 3 5% 7% 44%	1900
	11th to 20th	Day	17% 5% 2 4 3 4 4 3 5% 6% 47%	1900
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 6% 46%	2090
November	1st to 10th	Day	13% 5% 2 4 4 3 4 3 4 7% 51%	1763
	11th to 20th	Day	15% 6% 1 5% 3 3 4 5% 4 6% 48%	1800
	21st to end	Day	15% 6% 2 5% 4 5% 4 3 4 6% 45%	1605
December	1st to 10th	Day	13% 6% 3 7% 4 4 4 4 5% 6% 44%	2600
	11th to 20th	Day	20% 9% 2 5% 3 3 3 3 5% 7% 40%	2990
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 6% 37%	3960
January	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	6349
	11th to 20th	Night	14% 8% 2 5% 3 3 3 3 4 6% 49%	6057
	21st to end	Night	16% 8% 3 6% 4 3 3 3 4 5% 45%	6490
February	1st to 10th	Night	16% 9% 3 7% 5% 3 4 3 5% 6% 40%	2600
	11th to 20th	Night	18% 8% 3 6% 4 3 4 3 5% 5% 42%	2439
	21st to end	Night	15% 9% 3 6% 4 5% 4 3 4 6% 40%	1980
March	1st to 10th	Night	17% 8% 2 5% 4 3 3 2 4 6% 45%	5173
	11th to 20th	Night	11% 7% 3 5% 3 4 3 3 4 6% 51%	4980
	21st to end	Night	14% 7% 3 6% 5% 4 4 3 5% 6% 45%	5248
April	1st to 10th	Night	18% 8% 2 7% 4 4 4 3 6% 7% 39%	2000
	11th to 20th	Night	15% 5% 2 6% 4 4 3 3 7% 9% 42%	1975
	21st to end	Night	18% 7% 2 4 3 3 3 3 5% 7% 44%	2000
May	1st to 10th	Night	25% 7% 2 6% 4 4 5% 4 5% 8% 29%	4157
	11th to 20th	Night	24% 10% 3 7% 4 4 3 3 6% 7% 31%	3575
	21st to end	Night	20% 6% 2 5% 4 3 4 4 6% 8% 38%	3920
June	1st to 10th	Night	12% 6% 1 6% 4 4 4 5% 6% 9% 44%	1600
	11th to 20th	Night	13% 5% 2 7% 4 5% 5% 4 6% 9% 40%	1600
	21st to end	Night	11% 4 1 6% 4 4 5% 4 8% 10% 43%	1600
July	1st to 10th	Night	8% 6% 3 7% 5% 6% 6% 5% 7% 9% 39%	3598
	11th to 20th	Night	11% 8% 3 9% 5% 6% 6% 5% 8% 10% 30%	3597
	21st to end	Night	10% 7% 2 6% 4 4 5% 4 6% 9% 42%	4280
August	1st to 10th	Night	19% 6% 2 6% 4 4 5% 4 6% 9% 35%	2915
	11th to 20th	Night	12% 7% 3 7% 4 5% 5% 4 6% 8% 40%	3069
	21st to end	Night	12% 5% 2 6% 3 4 4 3 6% 8% 48%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 4 6% 48%	2000
	11th to 20th	Night	12% 6% 2 5% 3 4 3 3 5% 7% 52%	1981
	21st to end	Night	20% 7% 2 5% 4 4 4 3 5% 6% 40%	2147
October	1st to 10th	Night	20% 6% 2 6% 4 3 4 3 4 5% 43%	2280
	11th to 20th	Night	16% 5% 2 4 3 4 3 3 5% 6% 50%	2280
	21st to end	Night	17% 5% 2 4 3 2 4 3 5% 6% 49%	2508
November	1st to 10th	Night	11% 5% 1 5% 4 3 4 4 5% 5% 54%	2532
	11th to 20th	Night	15% 7% 2 5% 5% 3 3 3 5% 5% 47%	2600
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 6% 7% 46%	2790
December	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 47%	4580
	11th to 20th	Night	22% 6% 2 4 4 4 3 3 5% 7% 40%	5205
	21st to end	Night	18% 8% 3 6% 4 4 5% 4 6% 6% 36%	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	36% 4 10% 25% 21% 3	3598
	11th to 20th	Day	46% 4 8% 20% 16% 6%	3899
	21st to end	Day	44% 6% 11% 24% 14% 2	4466
February	1st to 10th	Day	44% 5% 13% 25% 11% 2	1800
	11th to 20th	Day	44% 5% 11% 24% 14% 3	1856
	21st to end	Day	47% 4 10% 21% 13% 4	1650
March	1st to 10th	Day	39% 5% 10% 24% 17% 6%	4663
	11th to 20th	Day	37% 5% 11% 27% 15% 6%	4980
	21st to end	Day	44% 5% 8% 18% 19% 5%	5682
April	1st to 10th	Day	43% 6% 11% 19% 16% 5%	2400
	11th to 20th	Day	39% 7% 12% 21% 16% 5%	2374
	21st to end	Day	41% 6% 11% 21% 18% 4	2400
May	1st to 10th	Day	50% 7% 12% 18% 10% 2	5799
	11th to 20th	Day	51% 7% 10% 21% 10% 1	6322
	21st to end	Day	41% 8% 12% 23% 15% 1	6949
June	1st to 10th	Day	38% 9% 13% 22% 16% 2	2800
	11th to 20th	Day	40% 9% 13% 23% 12% 3	2800
	21st to end	Day	34% 9% 15% 23% 17% 1	2800
July	1st to 10th	Day	35% 11% 15% 20% 16% 3	6290
	11th to 20th	Day	43% 12% 16% 20% 8% 1	6285
	21st to end	Day	37% 9% 11% 22% 21% 1	6378
August	1st to 10th	Day	46% 9% 13% 23% 9%	4221
	11th to 20th	Day	36% 9% 12% 28% 14% 1	3984
	21st to end	Day	32% 6% 12% 28% 19% 2	4312
September	1st to 10th	Day	39% 6% 11% 20% 20% 4	2400
	11th to 20th	Day	34% 5% 10% 26% 21% 4	2376
	21st to end	Day	45% 6% 9% 19% 19% 2	2028
October	1st to 10th	Day	37% 5% 8% 18% 28% 5%	3160
	11th to 20th	Day	26% 5% 11% 25% 23% 9%	3140
	21st to end	Day	25% 3 12% 29% 28% 4	3428
November	1st to 10th	Day	33% 5% 13% 24% 20% 4	1763
	11th to 20th	Day	36% 5% 11% 25% 20% 3	1800
	21st to end	Day	40% 4 12% 30% 12% 2	1605
December	1st to 10th	Day	40% 5% 16% 25% 11% 3	2600
	11th to 20th	Day	45% 4 13% 29% 9% 1	2986
	21st to end	Day	44% 7% 15% 19% 14% 1	3960
January	1st to 10th	Night	37% 4 10% 26% 19% 4	6349
	11th to 20th	Night	37% 4 11% 24% 18% 6%	6057
	21st to end	Night	41% 4 9% 27% 18% 2	6490
February	1st to 10th	Night	44% 5% 13% 24% 12% 1	2600
	11th to 20th	Night	44% 4 10% 21% 19% 3	2441
	21st to end	Night	45% 5% 10% 20% 16% 4	1980
March	1st to 10th	Night	40% 5% 8% 26% 12% 9%	5173
	11th to 20th	Night	33% 6% 12% 29% 14% 6%	4980
	21st to end	Night	41% 5% 8% 19% 25% 3	5248
April	1st to 10th	Night	45% 5% 11% 19% 17% 4	2000
	11th to 20th	Night	39% 6% 12% 20% 19% 4	1975
	21st to end	Night	39% 6% 11% 21% 19% 5%	2000
May	1st to 10th	Night	51% 8% 10% 16% 11% 3	4187
	11th to 20th	Night	51% 7% 10% 19% 11% 1	3576
	21st to end	Night	43% 8% 11% 23% 12% 3	3927
June	1st to 10th	Night	37% 9% 11% 24% 18% 2	1600
	11th to 20th	Night	40% 8% 14% 23% 15% 1	1600
	21st to end	Night	35% 8% 14% 25% 17% 2	1600
July	1st to 10th	Night	40% 10% 15% 17% 14% 3	3560
	11th to 20th	Night	46% 11% 17% 17% 9% 1	3552
	21st to end	Night	36% 8% 13% 23% 20%	4154
August	1st to 10th	Night	45% 8% 17% 20% 9% 1	2904
	11th to 20th	Night	40% 7% 11% 26% 14% 1	3096
	21st to end	Night	34% 7% 11% 30% 17% 2	3586
September	1st to 10th	Night	37% 7% 10% 21% 21% 4	2000
	11th to 20th	Night	34% 4 10% 24% 24% 4	1981
	21st to end	Night	44% 5% 9% 20% 19% 3	2147
October	1st to 10th	Night	43% 6% 9% 19% 21% 2	3800
	11th to 20th	Night	24% 5% 11% 26% 28% 6%	3779
	21st to end	Night	25% 4 11% 32% 24% 4	4120
November	1st to 10th	Night	32% 5% 12% 25% 22% 5%	2532
	11th to 20th	Night	39% 4 9% 24% 19% 5%	2600
	21st to end	Night	37% 5% 13% 26% 17% 2	2790
December	1st to 10th	Night	37% 5% 12% 30% 14% 2	4580
	11th to 20th	Night	44% 5% 13% 21% 14% 3	5208
	21st to end	Night	45% 7% 12% 18% 17% 1	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	29% 27% 28% 13% 3 1	200
	11th to 20th	Day	29% 28% 29% 13% 1	265
	21st to end	Day	21% 29% 34% 12% 5%	330
February	1st to 10th	Day	27% 21% 38% 10% 3 1	300
	11th to 20th	Day	30% 26% 35% 7% 1	394
	21st to end	Day	28% 21% 35% 14% 3	414
March	1st to 10th	Day	21% 24% 32% 19% 4	500
	11th to 20th	Day	22% 24% 35% 14% 5% 1	500
	21st to end	Day	23% 20% 34% 16% 4 2	550
April	1st to 10th	Day	22% 22% 34% 16% 6% 1	500
	11th to 20th	Day	20% 21% 29% 18% 9% 3	492
	21st to end	Day	24% 16% 32% 20% 8% 1	500
May	1st to 10th	Day	26% 15% 27% 21% 10% 2	503
	11th to 20th	Day	22% 13% 29% 23% 11% 3	527
	21st to end	Day	20% 7% 22% 26% 20% 5%	573
June	1st to 10th	Day	15% 8% 31% 24% 15% 7%	530
	11th to 20th	Day	17% 9% 25% 30% 15% 5%	530
	21st to end	Day	12% 7% 26% 33% 15% 7%	530
July	1st to 10th	Day	12% 5% 17% 31% 23% 12%	525
	11th to 20th	Day	6% 3 19% 36% 26% 9%	524
	21st to end	Day	11% 5% 24% 33% 21% 6%	576
August	1st to 10th	Day	13% 4 20% 32% 23% 8%	526
	11th to 20th	Day	12% 3 19% 32% 22% 11%	519
	21st to end	Day	10% 5% 22% 31% 22% 10%	583
September	1st to 10th	Day	18% 8% 19% 25% 20% 10%	530
	11th to 20th	Day	15% 4 22% 26% 24% 8%	522
	21st to end	Day	26% 10% 22% 23% 14% 4	490
October	1st to 10th	Day	22% 8% 23% 26% 14% 8%	490
	11th to 20th	Day	17% 13% 27% 26% 14% 3	490
	21st to end	Day	18% 13% 33% 24% 10% 2	539
November	1st to 10th	Day	17% 10% 31% 27% 12% 3	395
	11th to 20th	Day	16% 20% 33% 22% 10%	400
	21st to end	Day	21% 22% 33% 18% 5%	205
December	1st to 10th	Day	23% 23% 33% 17% 3 2	200
	11th to 20th	Day	28% 22% 31% 15% 6%	200
	21st to end	Day	23% 20% 32% 18% 7%	220
January	1st to 10th	Night	19% 17% 42% 18% 3	800
	11th to 20th	Night	23% 16% 33% 25% 3	800
	21st to end	Night	17% 18% 42% 21% 2	880
February	1st to 10th	Night	20% 18% 37% 22% 2	800
	11th to 20th	Night	24% 21% 38% 16% 1	783
	21st to end	Night	21% 21% 38% 18% 3	660
March	1st to 10th	Night	19% 20% 37% 21% 4	800
	11th to 20th	Night	20% 19% 36% 20% 5% 1	800
	21st to end	Night	21% 15% 39% 21% 4	880
April	1st to 10th	Night	23% 17% 37% 20% 4	800
	11th to 20th	Night	19% 13% 34% 27% 7%	791
	21st to end	Night	21% 12% 33% 27% 7%	800
May	1st to 10th	Night	21% 9% 30% 32% 8%	662
	11th to 20th	Night	20% 9% 29% 34% 8%	394
	21st to end	Night	17% 5% 20% 40% 16% 2	433
June	1st to 10th	Night	13% 3 25% 41% 18% 1	400
	11th to 20th	Night	15% 4 22% 43% 16% 1	400
	21st to end	Night	11% 2 20% 50% 17%	400
July	1st to 10th	Night	12% 3 16% 40% 27% 2	390
	11th to 20th	Night	7% 3 13% 45% 30% 3	388
	21st to end	Night	9% 2 16% 46% 25% 2	588
August	1st to 10th	Night	12% 2 13% 43% 27% 2	597
	11th to 20th	Night	13% 4 15% 37% 29% 2	710
	21st to end	Night	13% 3 16% 39% 25% 4	880
September	1st to 10th	Night	16% 3 20% 35% 23% 4	800
	11th to 20th	Night	14% 4 14% 37% 26% 5%	793
	21st to end	Night	23% 4 19% 33% 19% 2	760
October	1st to 10th	Night	21% 6% 22% 29% 19% 4	760
	11th to 20th	Night	17% 9% 28% 32% 13% 1	760
	21st to end	Night	20% 10% 32% 26% 13%	836
November	1st to 10th	Night	14% 9% 34% 32% 11% 1	780
	11th to 20th	Night	19% 15% 31% 28% 7% 1	800
	21st to end	Night	16% 16% 34% 27% 7%	800
December	1st to 10th	Night	22% 16% 35% 21% 6% 1	800
	11th to 20th	Night	20% 13% 40% 23% 5%	800
	21st to end	Night	20% 15% 36% 26% 3	880

Page generated at 2025-09-16 01:49:16. Version ac07

Copyright © 2025 Attilla Danko.