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 2024-03-18.

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% 3 5% 5% 3 5% 5% 6% 8% 41%	78659
	Night	16% 6% 2 5% 5% 3 4 4 5% 7% 43%	81128
Transparency	Day	41% 7% 12% 23% 15% 3	78365
	Night	40% 6% 12% 23% 16% 3	80941
Seeing	Day	19% 13% 27% 23% 13% 5%	14034
	Night	18% 11% 30% 29% 11% 1	23563

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	15% 6% 2 6% 4 3 4 4 4 5% 46%	2517
	11th to 20th	Day	14% 10% 4 9% 7% 3 4 3 4 6% 36%	2721
	21st to end	Day	19% 8% 2 7% 5% 3 5% 4 5% 6% 34%	3124
February	1st to 10th	Day	14% 9% 3 6% 5% 3 4 4 5% 8% 39%	1710
	11th to 20th	Day	16% 7% 3 4 6% 4 4 5% 5% 7% 41%	1752
	21st to end	Day	19% 11% 4 5% 5% 3 3 3 4 5% 38%	1570
March	1st to 10th	Day	15% 5% 2 5% 5% 3 4 4 4 6% 47%	1976
	11th to 20th	Day	16% 6% 3 5% 5% 3 4 4 5% 8% 42%	2058
	21st to end	Day	19% 7% 3 6% 5% 3 4 4 5% 5% 39%	2256
April	1st to 10th	Day	14% 7% 3 5% 6% 4 5% 5% 4 8% 41%	2160
	11th to 20th	Day	15% 5% 2 4 5% 3 4 5% 4 7% 44%	2134
	21st to end	Day	18% 5% 3 4 5% 3 5% 5% 5% 8% 40%	2160
May	1st to 10th	Day	17% 7% 2 6% 5% 4 4 5% 6% 6% 38%	2298
	11th to 20th	Day	20% 6% 3 5% 5% 4 5% 5% 5% 7% 35%	2478
	21st to end	Day	13% 5% 3 5% 5% 3 4 5% 7% 10% 40%	2725
June	1st to 10th	Day	12% 4 3 6% 4 3 5% 6% 7% 11% 38%	2520
	11th to 20th	Day	13% 5% 3 6% 6% 3 5% 5% 7% 9% 38%	2520
	21st to end	Day	10% 4 2 5% 5% 3 6% 6% 7% 10% 42%	2520
July	1st to 10th	Day	10% 5% 2 5% 6% 4 6% 6% 8% 9% 40%	2520
	11th to 20th	Day	6% 3 2 5% 6% 4 6% 6% 8% 11% 43%	2520
	21st to end	Day	9% 6% 3 7% 6% 3 6% 7% 7% 9% 37%	2605
August	1st to 10th	Day	10% 5% 2 5% 6% 4 7% 6% 7% 8% 39%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 6% 7% 9% 39%	2179
	21st to end	Day	12% 5% 2 5% 5% 3 5% 6% 7% 8% 42%	2376
September	1st to 10th	Day	16% 5% 2 5% 5% 3 4 4 5% 7% 43%	2160
	11th to 20th	Day	13% 5% 2 4 4 2 3 4 5% 8% 50%	2160
	21st to end	Day	18% 7% 3 5% 4 2 4 5% 6% 7% 39%	1923
October	1st to 10th	Day	18% 5% 2 5% 5% 3 4 4 4 7% 43%	1800
	11th to 20th	Day	18% 5% 3 4 3 2 5% 4 5% 6% 45%	1800
	21st to end	Day	17% 6% 2 4 4 3 4 4 5% 6% 45%	1980
November	1st to 10th	Day	13% 5% 2 4 4 2 4 3 4 7% 51%	1673
	11th to 20th	Day	16% 6% 2 4 3 2 4 5% 4 6% 48%	1710
	21st to end	Day	16% 5% 3 5% 4 4 5% 4 4 6% 45%	1525
December	1st to 10th	Day	18% 7% 2 6% 5% 3 4 4 5% 8% 38%	1520
	11th to 20th	Day	19% 7% 2 5% 5% 2 4 4 5% 8% 40%	1910
	21st to end	Day	16% 7% 3 6% 7% 4 5% 5% 6% 6% 36%	2772
January	1st to 10th	Night	15% 6% 3 6% 5% 3 4 4 5% 7% 42%	4429
	11th to 20th	Night	18% 8% 2 5% 4 3 3 3 4 6% 43%	4235
	21st to end	Night	21% 8% 3 6% 5% 2 3 3 4 5% 39%	4532
February	1st to 10th	Night	16% 8% 3 6% 5% 2 4 4 5% 6% 41%	2470
	11th to 20th	Night	17% 7% 3 6% 4 3 4 4 5% 5% 43%	2314
	21st to end	Night	17% 8% 3 5% 5% 3 5% 4 4 6% 40%	1884
March	1st to 10th	Night	15% 5% 2 4 5% 2 4 4 5% 7% 47%	2200
	11th to 20th	Night	16% 7% 3 5% 3 3 5% 4 4 5% 44%	2068
	21st to end	Night	17% 7% 3 5% 5% 3 4 3 4 6% 43%	2098
April	1st to 10th	Night	17% 7% 3 5% 5% 3 4 4 5% 8% 40%	1800
	11th to 20th	Night	15% 5% 2 5% 4 3 4 4 6% 9% 43%	1775
	21st to end	Night	20% 7% 2 4 3 2 3 4 5% 7% 43%	1800
May	1st to 10th	Night	18% 7% 2 6% 5% 3 4 5% 5% 8% 39%	1677
	11th to 20th	Night	18% 7% 2 6% 5% 3 4 4 5% 7% 37%	1415
	21st to end	Night	14% 6% 3 4 5% 3 4 5% 5% 9% 41%	1550
June	1st to 10th	Night	12% 5% 2 5% 4 3 5% 6% 5% 9% 44%	1440
	11th to 20th	Night	13% 5% 2 6% 5% 3 5% 6% 6% 9% 40%	1440
	21st to end	Night	11% 4 2 5% 5% 3 5% 5% 8% 10% 43%	1440
July	1st to 10th	Night	10% 5% 3 7% 7% 4 5% 6% 7% 8% 38%	1440
	11th to 20th	Night	5% 4 2 4 5% 3 6% 7% 7% 10% 46%	1440
	21st to end	Night	8% 5% 2 5% 6% 2 7% 6% 7% 10% 43%	1708
August	1st to 10th	Night	9% 5% 2 5% 5% 3 6% 6% 7% 9% 42%	1611
	11th to 20th	Night	11% 6% 3 5% 6% 4 6% 5% 6% 9% 39%	1702
	21st to end	Night	12% 4 2 5% 4 3 5% 5% 6% 9% 44%	1980
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 5% 4 6% 46%	1800
	11th to 20th	Night	12% 5% 3 4 3 2 3 3 5% 7% 53%	1800
	21st to end	Night	18% 6% 2 5% 5% 3 4 4 5% 6% 42%	2033
October	1st to 10th	Night	20% 6% 2 6% 4 2 3 4 5% 5% 43%	2160
	11th to 20th	Night	17% 5% 2 3 3 3 3 4 5% 6% 49%	2160
	21st to end	Night	18% 5% 2 4 4 2 4 4 5% 5% 48%	2376
November	1st to 10th	Night	11% 5% 2 5% 4 2 4 4 5% 5% 53%	2402
	11th to 20th	Night	16% 6% 2 5% 5% 2 3 4 4 5% 48%	2470
	21st to end	Night	13% 4 2 5% 5% 2 4 4 6% 7% 46%	2650
December	1st to 10th	Night	18% 7% 3 5% 6% 3 5% 4 5% 6% 39%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 3 4 5% 5% 7% 42%	3285
	21st to end	Night	16% 7% 3 5% 5% 3 5% 5% 6% 8% 34%	4884

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	39% 5% 13% 27% 13% 4	2518
	11th to 20th	Day	50% 5% 10% 19% 13% 3	2721
	21st to end	Day	50% 6% 11% 23% 9% 1	3124
February	1st to 10th	Day	44% 5% 14% 25% 11% 2	1710
	11th to 20th	Day	43% 5% 11% 25% 14% 3	1761
	21st to end	Day	48% 4 10% 20% 13% 4	1570
March	1st to 10th	Day	40% 4 13% 26% 15% 2	1976
	11th to 20th	Day	41% 4 12% 20% 17% 5%	2058
	21st to end	Day	48% 5% 9% 19% 17% 3	2256
April	1st to 10th	Day	42% 6% 12% 19% 16% 5%	2160
	11th to 20th	Day	38% 7% 12% 22% 16% 5%	2134
	21st to end	Day	42% 6% 11% 20% 18% 4	2160
May	1st to 10th	Day	45% 7% 10% 20% 15% 3	2298
	11th to 20th	Day	47% 7% 12% 19% 12% 3	2482
	21st to end	Day	39% 9% 15% 22% 12% 2	2725
June	1st to 10th	Day	38% 9% 13% 23% 15% 2	2520
	11th to 20th	Day	41% 9% 13% 22% 12% 3	2520
	21st to end	Day	35% 9% 16% 24% 16% 1	2520
July	1st to 10th	Day	38% 10% 14% 21% 15% 3	2450
	11th to 20th	Day	31% 11% 15% 26% 14% 2	2445
	21st to end	Day	39% 10% 13% 21% 14% 2	2459
August	1st to 10th	Day	39% 10% 13% 23% 14% 1	2314
	11th to 20th	Day	37% 10% 14% 21% 15% 2	2179
	21st to end	Day	37% 8% 14% 23% 14% 4	2376
September	1st to 10th	Day	41% 6% 12% 20% 18% 4	2160
	11th to 20th	Day	34% 5% 11% 26% 21% 4	2160
	21st to end	Day	43% 7% 9% 20% 20% 2	1923
October	1st to 10th	Day	41% 6% 11% 24% 17% 2	1800
	11th to 20th	Day	39% 5% 11% 22% 21% 2	1800
	21st to end	Day	41% 4 11% 25% 17% 2	1980
November	1st to 10th	Day	34% 5% 13% 25% 19% 4	1673
	11th to 20th	Day	37% 5% 11% 24% 20% 3	1710
	21st to end	Day	41% 4 11% 30% 12% 2	1525
December	1st to 10th	Day	45% 5% 15% 22% 11% 1	1520
	11th to 20th	Day	43% 5% 11% 31% 10% 1	1906
	21st to end	Day	47% 7% 14% 21% 11% 1	2772
January	1st to 10th	Night	41% 5% 11% 27% 12% 3	4429
	11th to 20th	Night	43% 4 11% 26% 14% 2	4235
	21st to end	Night	47% 4 8% 29% 11% 2	4532
February	1st to 10th	Night	44% 5% 13% 24% 12% 2	2470
	11th to 20th	Night	44% 5% 10% 20% 19% 3	2316
	21st to end	Night	45% 5% 10% 20% 16% 4	1884
March	1st to 10th	Night	36% 5% 11% 26% 18% 2	2200
	11th to 20th	Night	41% 5% 11% 23% 17% 3	2068
	21st to end	Night	43% 5% 10% 20% 17% 5%	2098
April	1st to 10th	Night	43% 5% 11% 19% 18% 4	1800
	11th to 20th	Night	38% 6% 12% 21% 19% 5%	1775
	21st to end	Night	41% 6% 11% 20% 17% 5%	1800
May	1st to 10th	Night	43% 8% 10% 20% 15% 4	1677
	11th to 20th	Night	46% 7% 11% 15% 19% 2	1416
	21st to end	Night	38% 11% 16% 21% 12% 3	1551
June	1st to 10th	Night	36% 9% 11% 24% 17% 2	1440
	11th to 20th	Night	40% 8% 14% 21% 16% 1	1440
	21st to end	Night	34% 8% 14% 26% 16% 2	1440
July	1st to 10th	Night	42% 10% 14% 18% 12% 3	1400
	11th to 20th	Night	31% 10% 17% 23% 17% 2	1392
	21st to end	Night	34% 9% 17% 21% 18% 1	1613
August	1st to 10th	Night	35% 10% 16% 24% 15% 1	1600
	11th to 20th	Night	40% 9% 14% 19% 15% 2	1702
	21st to end	Night	37% 8% 14% 22% 16% 3	1980
September	1st to 10th	Night	39% 7% 11% 20% 18% 5%	1800
	11th to 20th	Night	32% 4 11% 24% 25% 4	1800
	21st to end	Night	42% 5% 9% 20% 20% 3	2033
October	1st to 10th	Night	42% 6% 11% 22% 17% 2	2160
	11th to 20th	Night	36% 5% 12% 27% 18% 2	2160
	21st to end	Night	37% 5% 9% 27% 19% 2	2376
November	1st to 10th	Night	32% 5% 12% 26% 20% 5%	2402
	11th to 20th	Night	39% 4 9% 23% 19% 5%	2470
	21st to end	Night	36% 5% 13% 27% 17% 2	2650
December	1st to 10th	Night	46% 5% 11% 23% 13% 1	2660
	11th to 20th	Night	41% 6% 13% 21% 16% 3	3288
	21st to end	Night	43% 9% 15% 22% 10% 1	4884

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	26% 27% 29% 14% 3 1	180
	11th to 20th	Day	28% 28% 29% 14% 1	238
	21st to end	Day	20% 29% 34% 12% 5%	297
February	1st to 10th	Day	24% 21% 39% 11% 4 1	270
	11th to 20th	Day	26% 27% 38% 8% 1	359
	21st to end	Day	30% 20% 34% 14% 3	382
March	1st to 10th	Day	20% 24% 32% 20% 4	460
	11th to 20th	Day	22% 22% 35% 14% 5% 1	449
	21st to end	Day	23% 20% 33% 17% 5% 2	462
April	1st to 10th	Day	19% 21% 36% 17% 6% 1	420
	11th to 20th	Day	17% 20% 30% 19% 11% 3	412
	21st to end	Day	25% 15% 32% 19% 7% 1	420
May	1st to 10th	Day	24% 15% 27% 20% 11% 2	423
	11th to 20th	Day	22% 12% 29% 24% 11% 2	447
	21st to end	Day	20% 6% 21% 27% 22% 6%	485
June	1st to 10th	Day	14% 7% 31% 25% 16% 6%	450
	11th to 20th	Day	16% 9% 25% 28% 15% 5%	450
	21st to end	Day	12% 6% 26% 33% 16% 7%	450
July	1st to 10th	Day	13% 4 16% 29% 26% 13%	445
	11th to 20th	Day	5% 3 17% 37% 28% 10%	444
	21st to end	Day	12% 3 23% 32% 22% 7%	488
August	1st to 10th	Day	11% 4 19% 32% 26% 9%	446
	11th to 20th	Day	11% 3 17% 33% 24% 12%	439
	21st to end	Day	11% 5% 20% 32% 22% 12%	495
September	1st to 10th	Day	20% 6% 18% 24% 21% 12%	450
	11th to 20th	Day	15% 3 23% 26% 23% 9%	450
	21st to end	Day	23% 10% 22% 25% 16% 5%	450
October	1st to 10th	Day	22% 7% 22% 27% 15% 8%	450
	11th to 20th	Day	18% 12% 27% 26% 15% 2	450
	21st to end	Day	19% 13% 34% 23% 9% 2	495
November	1st to 10th	Day	18% 10% 30% 27% 13% 3	365
	11th to 20th	Day	17% 19% 32% 21% 10%	370
	21st to end	Day	18% 22% 35% 19% 6% 1	185
December	1st to 10th	Day	24% 23% 31% 18% 3 2	180
	11th to 20th	Day	27% 22% 31% 15% 6%	180
	21st to end	Day	22% 19% 32% 19% 7%	198
January	1st to 10th	Night	18% 16% 43% 19% 3	760
	11th to 20th	Night	23% 16% 33% 25% 3	760
	21st to end	Night	17% 18% 42% 22% 2	836
February	1st to 10th	Night	20% 18% 37% 23% 2	760
	11th to 20th	Night	22% 22% 39% 16% 1	743
	21st to end	Night	21% 21% 38% 18% 3	628
March	1st to 10th	Night	18% 19% 37% 22% 4	760
	11th to 20th	Night	20% 19% 36% 20% 5% 1	752
	21st to end	Night	21% 13% 39% 22% 5% 1	792
April	1st to 10th	Night	21% 17% 37% 20% 4	720
	11th to 20th	Night	18% 13% 34% 28% 8%	711
	21st to end	Night	23% 12% 32% 26% 8%	720
May	1st to 10th	Night	21% 9% 30% 31% 8% 1	597
	11th to 20th	Night	21% 7% 31% 33% 8%	354
	21st to end	Night	16% 5% 19% 40% 17% 2	389
June	1st to 10th	Night	13% 3 25% 40% 18% 1	360
	11th to 20th	Night	14% 4 22% 43% 17% 1	360
	21st to end	Night	10% 2 19% 53% 16%	360
July	1st to 10th	Night	12% 2 16% 39% 29% 2	350
	11th to 20th	Night	6% 3 13% 45% 30% 3	348
	21st to end	Night	9% 2 16% 46% 25% 2	528
August	1st to 10th	Night	11% 1 14% 44% 28% 2	537
	11th to 20th	Night	13% 4 15% 37% 29% 3	638
	21st to end	Night	13% 3 15% 38% 26% 4	792
September	1st to 10th	Night	18% 3 18% 34% 23% 4	720
	11th to 20th	Night	13% 4 14% 37% 27% 5%	720
	21st to end	Night	20% 4 20% 34% 20% 2	720
October	1st to 10th	Night	21% 5% 22% 29% 19% 4	720
	11th to 20th	Night	18% 8% 28% 32% 13% 1	720
	21st to end	Night	21% 10% 32% 25% 12%	792
November	1st to 10th	Night	14% 9% 34% 31% 11% 1	740
	11th to 20th	Night	19% 15% 30% 28% 7% 1	760
	21st to end	Night	15% 16% 34% 28% 7%	760
December	1st to 10th	Night	22% 16% 34% 21% 6% 1	760
	11th to 20th	Night	19% 13% 41% 23% 5%	760
	21st to end	Night	19% 15% 37% 26% 2	836

Page generated at 2024-03-15 14:01:45. Version ac07

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