Rainwater Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rainwater Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	12% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	125815
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 46%	123987
Transparency	Day	38% 6% 11% 21% 18% 6%	129531
	Night	37% 5% 10% 21% 20% 8%	128346
Seeing	Day	16% 8% 23% 25% 20% 9%	16368
	Night	16% 8% 24% 29% 20% 4	26025

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% 10% 3 8% 3 3 2 2 4 5% 44%	3921
	11th to 20th	Day	14% 6% 3 7% 3 3 3 3 4 6% 50%	4080
	21st to end	Day	20% 12% 3 5% 3 3 2 2 3 5% 42%	4728
February	1st to 10th	Day	17% 8% 2 6% 5% 3 4 4 5% 7% 40%	2000
	11th to 20th	Day	22% 10% 2 6% 3 3 4 3 4 4 39%	1948
	21st to end	Day	19% 9% 3 7% 4 4 3 2 4 5% 40%	1650
March	1st to 10th	Day	21% 9% 3 8% 5% 4 4 4 5% 6% 33%	4480
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 3 5% 7% 44%	4930
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 5% 6% 39%	5865
April	1st to 10th	Day	13% 5% 2 6% 5% 4 4 4 6% 6% 45%	2400
	11th to 20th	Day	15% 6% 2 6% 4 4 3 4 5% 6% 46%	2374
	21st to end	Day	10% 5% 2 7% 5% 5% 4 3 5% 7% 47%	2400
May	1st to 10th	Day	16% 7% 3 7% 5% 4 5% 4 6% 8% 38%	5405
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 5% 7% 8% 35%	5627
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 6% 9% 39%	6433
June	1st to 10th	Day	9% 5% 3 7% 6% 6% 6% 5% 7% 9% 38%	2723
	11th to 20th	Day	5% 5% 2 6% 6% 5% 6% 6% 7% 9% 43%	2800
	21st to end	Day	6% 4 2 6% 7% 6% 5% 5% 8% 9% 41%	2800
July	1st to 10th	Day	6% 5% 2 7% 7% 7% 7% 6% 9% 10% 34%	6360
	11th to 20th	Day	7% 4 2 6% 6% 6% 8% 7% 10% 12% 33%	5910
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 6% 9% 11% 30%	6087
August	1st to 10th	Day	4 3 1 4 3 4 4 4 6% 6% 61%	3905
	11th to 20th	Day	5% 4 2 7% 5% 6% 5% 5% 7% 10% 45%	3849
	21st to end	Day	8% 4 2 7% 4 5% 5% 5% 6% 8% 47%	4312
September	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 7% 5% 5% 4 4 4 7% 45%	2301
	21st to end	Day	11% 4 2 5% 3 4 4 4 5% 7% 51%	2200
October	1st to 10th	Day	7% 4 1 3 4 4 4 3 5% 6% 59%	2160
	11th to 20th	Day	10% 4 1 4 3 3 4 3 4 6% 57%	1980
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	2090
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 3 3 6% 55%	1829
	11th to 20th	Day	13% 6% 2 4 3 3 3 2 4 5% 54%	1800
	21st to end	Day	14% 7% 4 6% 4 3 3 3 4 6% 46%	1800
December	1st to 10th	Day	17% 8% 3 5% 3 4 3 3 5% 6% 43%	2930
	11th to 20th	Day	16% 10% 4 7% 4 4 4 2 4 5% 41%	3356
	21st to end	Day	13% 8% 2 6% 4 4 3 3 4 6% 47%	3982
January	1st to 10th	Night	18% 9% 3 7% 3 3 3 2 4 4 46%	6026
	11th to 20th	Night	15% 9% 5% 6% 3 2 2 2 4 4 48%	5900
	21st to end	Night	23% 9% 2 5% 3 2 3 3 3 4 42%	6234
February	1st to 10th	Night	18% 9% 3 6% 4 3 3 3 4 6% 41%	2400
	11th to 20th	Night	21% 8% 2 6% 4 3 3 3 3 4 42%	2338
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	19% 11% 3 6% 4 4 4 3 4 5% 36%	5373
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 5% 7% 50%	5072
	21st to end	Night	17% 7% 3 6% 4 4 3 3 4 6% 43%	5062
April	1st to 10th	Night	13% 7% 2 5% 4 3 4 4 5% 6% 48%	2000
	11th to 20th	Night	13% 6% 1 5% 3 4 4 3 5% 7% 50%	1975
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 7% 50%	2000
May	1st to 10th	Night	15% 6% 2 6% 4 3 4 4 6% 8% 41%	4490
	11th to 20th	Night	18% 7% 2 7% 5% 5% 4 4 6% 7% 35%	4264
	21st to end	Night	9% 5% 3 7% 5% 5% 5% 5% 7% 10% 39%	4421
June	1st to 10th	Night	5% 5% 2 6% 5% 5% 6% 5% 5% 9% 46%	1671
	11th to 20th	Night	4 3 2 4 4 4 4 5% 7% 10% 52%	1600
	21st to end	Night	5% 4 2 4 5% 4 5% 5% 7% 11% 50%	1600
July	1st to 10th	Night	4 6% 2 7% 5% 5% 6% 6% 10% 13% 38%	3618
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 5% 9% 15% 41%	4047
	21st to end	Night	6% 6% 3 8% 5% 6% 6% 5% 9% 12% 34%	4755
August	1st to 10th	Night	3 1 1 3 2 3 3 3 7% 8% 66%	3244
	11th to 20th	Night	3 2 1 5% 3 4 5% 4 7% 13% 53%	3200
	21st to end	Night	5% 4 2 6% 4 4 5% 5% 7% 8% 51%	3586
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 6% 7% 52%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 48%	2077
	21st to end	Night	9% 4 2 4 3 3 4 3 5% 7% 56%	2200
October	1st to 10th	Night	6% 3 1 4 3 3 3 2 5% 7% 62%	2235
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 4 6% 57%	2377
	21st to end	Night	16% 5% 2 4 2 3 2 2 3 5% 56%	2508
November	1st to 10th	Night	13% 5% 1 4 3 2 4 3 4 5% 56%	2466
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 6% 54%	2600
	21st to end	Night	15% 6% 2 5% 4 3 3 4 4 6% 50%	2600
December	1st to 10th	Night	20% 7% 2 4 2 3 3 3 4 5% 46%	4250
	11th to 20th	Night	17% 9% 3 6% 3 3 3 2 4 4 45%	4822
	21st to end	Night	17% 9% 2 6% 4 3 3 3 5% 6% 41%	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	44% 3 3 28% 19% 3	3922
	11th to 20th	Day	37% 3 5% 23% 28% 5%	4080
	21st to end	Day	48% 2 6% 15% 20% 10%	4728
February	1st to 10th	Day	44% 4 8% 19% 19% 7%	2000
	11th to 20th	Day	50% 3 5% 18% 17% 8%	1958
	21st to end	Day	48% 4 6% 14% 20% 9%	1650
March	1st to 10th	Day	52% 4 6% 13% 17% 7%	4480
	11th to 20th	Day	41% 4 6% 16% 22% 12%	4930
	21st to end	Day	46% 4 7% 21% 15% 6%	5867
April	1st to 10th	Day	38% 4 10% 21% 20% 7%	2400
	11th to 20th	Day	40% 4 6% 19% 25% 7%	2374
	21st to end	Day	35% 5% 9% 25% 23% 3	2400
May	1st to 10th	Day	44% 6% 11% 27% 11% 2	5440
	11th to 20th	Day	44% 8% 14% 22% 11% 1	5631
	21st to end	Day	39% 8% 13% 25% 14% 1	6447
June	1st to 10th	Day	39% 8% 15% 20% 17% 1	2723
	11th to 20th	Day	34% 7% 17% 23% 17% 3	2800
	21st to end	Day	35% 8% 16% 23% 14% 3	2800
July	1st to 10th	Day	39% 11% 17% 22% 9% 1	6291
	11th to 20th	Day	37% 12% 26% 22% 3 1	5840
	21st to end	Day	43% 11% 19% 23% 5%	5914
August	1st to 10th	Day	21% 6% 12% 32% 28% 1	3893
	11th to 20th	Day	31% 8% 19% 27% 13% 2	3885
	21st to end	Day	33% 8% 13% 20% 22% 4	4311
September	1st to 10th	Day	37% 7% 11% 21% 18% 6%	2400
	11th to 20th	Day	39% 6% 11% 18% 21% 6%	2301
	21st to end	Day	32% 5% 10% 24% 20% 9%	2200
October	1st to 10th	Day	17% 3 6% 16% 28% 31%	3524
	11th to 20th	Day	18% 2 3 14% 38% 25%	3220
	21st to end	Day	25% 3 4 18% 35% 15%	3428
November	1st to 10th	Day	33% 3 6% 21% 28% 10%	1829
	11th to 20th	Day	34% 3 5% 18% 27% 13%	1800
	21st to end	Day	41% 3 6% 17% 25% 8%	1800
December	1st to 10th	Day	41% 4 6% 22% 23% 4	2930
	11th to 20th	Day	47% 3 6% 18% 19% 7%	3353
	21st to end	Day	40% 4 7% 21% 26% 2	3982
January	1st to 10th	Night	44% 3 4 23% 24% 3	6026
	11th to 20th	Night	42% 2 3 19% 26% 8%	5900
	21st to end	Night	48% 2 3 16% 24% 7%	6234
February	1st to 10th	Night	45% 4 6% 17% 20% 8%	2400
	11th to 20th	Night	47% 3 4 18% 18% 9%	2339
	21st to end	Night	47% 3 6% 14% 19% 10%	1980
March	1st to 10th	Night	51% 3 6% 13% 17% 10%	5373
	11th to 20th	Night	34% 4 6% 17% 20% 19%	5072
	21st to end	Night	44% 3 7% 21% 19% 7%	5062
April	1st to 10th	Night	37% 4 9% 21% 19% 9%	2000
	11th to 20th	Night	35% 3 8% 20% 25% 9%	1975
	21st to end	Night	33% 4 11% 25% 22% 5%	2000
May	1st to 10th	Night	40% 7% 13% 22% 15% 2	4520
	11th to 20th	Night	47% 6% 12% 24% 10% 1	4265
	21st to end	Night	38% 8% 14% 22% 17% 2	4429
June	1st to 10th	Night	33% 7% 15% 23% 20% 2	1671
	11th to 20th	Night	25% 7% 18% 29% 16% 6%	1600
	21st to end	Night	27% 7% 20% 25% 19% 3	1600
July	1st to 10th	Night	33% 12% 21% 22% 10% 2	3580
	11th to 20th	Night	29% 11% 31% 24% 4 1	3997
	21st to end	Night	39% 11% 19% 26% 5% 1	4615
August	1st to 10th	Night	15% 5% 16% 33% 29% 2	3232
	11th to 20th	Night	23% 7% 23% 30% 15% 2	3230
	21st to end	Night	29% 8% 14% 22% 21% 5%	3586
September	1st to 10th	Night	32% 6% 14% 23% 20% 6%	2000
	11th to 20th	Night	34% 7% 11% 18% 23% 7%	2077
	21st to end	Night	28% 4 9% 26% 22% 10%	2200
October	1st to 10th	Night	15% 2 7% 14% 29% 33%	3651
	11th to 20th	Night	19% 2 4 16% 29% 29%	3876
	21st to end	Night	22% 2 5% 17% 35% 19%	4120
November	1st to 10th	Night	32% 3 7% 18% 27% 13%	2466
	11th to 20th	Night	31% 4 7% 18% 27% 14%	2600
	21st to end	Night	37% 3 6% 22% 21% 11%	2600
December	1st to 10th	Night	42% 3 6% 20% 25% 4	4250
	11th to 20th	Night	44% 3 8% 17% 23% 4	4824
	21st to end	Night	43% 4 7% 20% 23% 2	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	25% 21% 37% 14% 3	300
	11th to 20th	Day	29% 19% 34% 11% 4 2	300
	21st to end	Day	27% 23% 36% 12% 2 1	330
February	1st to 10th	Day	24% 19% 36% 13% 6% 1	300
	11th to 20th	Day	37% 17% 34% 9% 2 1	300
	21st to end	Day	34% 17% 37% 9% 2	249
March	1st to 10th	Day	26% 20% 34% 16% 3	300
	11th to 20th	Day	29% 16% 40% 12% 3	300
	21st to end	Day	22% 18% 37% 16% 7%	500
April	1st to 10th	Day	23% 18% 29% 21% 8% 1	500
	11th to 20th	Day	24% 13% 33% 19% 9% 2	492
	21st to end	Day	15% 11% 31% 30% 12% 1	500
May	1st to 10th	Day	20% 7% 28% 31% 11% 3	503
	11th to 20th	Day	13% 8% 27% 31% 16% 7%	527
	21st to end	Day	10% 3 28% 30% 18% 11%	573
June	1st to 10th	Day	12% 3 19% 29% 27% 10%	653
	11th to 20th	Day	7% 2 18% 33% 30% 11%	730
	21st to end	Day	7% 2 13% 33% 28% 16%	730
July	1st to 10th	Day	8% 2 9% 33% 33% 15%	721
	11th to 20th	Day	9% 2 6% 26% 36% 21%	718
	21st to end	Day	8% 2 8% 26% 39% 17%	650
August	1st to 10th	Day	7% 2 9% 29% 33% 21%	526
	11th to 20th	Day	8% 1 9% 27% 32% 22%	519
	21st to end	Day	14% 2 8% 25% 29% 21%	583
September	1st to 10th	Day	10% 2 10% 32% 35% 11%	530
	11th to 20th	Day	12% 3 11% 25% 33% 16%	530
	21st to end	Day	12% 3 15% 32% 28% 10%	530
October	1st to 10th	Day	10% 4 22% 34% 22% 8%	490
	11th to 20th	Day	9% 8% 32% 33% 14% 3	324
	21st to end	Day	19% 9% 27% 29% 13% 3	330
November	1st to 10th	Day	18% 7% 29% 30% 12% 4	300
	11th to 20th	Day	16% 13% 39% 24% 8%	300
	21st to end	Day	17% 16% 38% 18% 10% 2	300
December	1st to 10th	Day	27% 18% 35% 17% 3 1	300
	11th to 20th	Day	25% 17% 35% 15% 8%	300
	21st to end	Day	30% 12% 38% 15% 4 1	330
January	1st to 10th	Night	24% 16% 35% 22% 4	1000
	11th to 20th	Night	22% 17% 31% 24% 6%	1000
	21st to end	Night	25% 14% 38% 21% 3	987
February	1st to 10th	Night	23% 14% 35% 24% 5%	800
	11th to 20th	Night	27% 17% 31% 22% 4	783
	21st to end	Night	25% 17% 34% 22% 2	660
March	1st to 10th	Night	21% 15% 31% 26% 7%	800
	11th to 20th	Night	19% 11% 37% 26% 7% 1	640
	21st to end	Night	16% 14% 33% 27% 10%	660
April	1st to 10th	Night	18% 13% 33% 26% 10% 1	600
	11th to 20th	Night	16% 12% 32% 28% 11% 2	593
	21st to end	Night	12% 9% 35% 32% 11% 1	600
May	1st to 10th	Night	14% 5% 30% 36% 15% 2	600
	11th to 20th	Night	13% 2 26% 36% 20% 4	591
	21st to end	Night	7% 2 19% 38% 28% 5%	650
June	1st to 10th	Night	7% 1 13% 39% 33% 7%	600
	11th to 20th	Night	6% 2 10% 33% 39% 11%	600
	21st to end	Night	5% 2 10% 32% 39% 12%	600
July	1st to 10th	Night	6% 1 7% 29% 42% 15%	585
	11th to 20th	Night	6% 1 5% 29% 39% 19%	582
	21st to end	Night	5% 1 6% 28% 46% 14%	657
August	1st to 10th	Night	5% 4 26% 47% 17%	597
	11th to 20th	Night	6% 1 4 30% 46% 13%	594
	21st to end	Night	10% 1 7% 28% 41% 13%	660
September	1st to 10th	Night	9% 3 11% 34% 37% 6%	600
	11th to 20th	Night	12% 2 10% 29% 40% 6%	677
	21st to end	Night	10% 2 14% 34% 34% 5%	800
October	1st to 10th	Night	8% 2 19% 35% 31% 6%	800
	11th to 20th	Night	13% 6% 21% 38% 20% 2	793
	21st to end	Night	19% 6% 22% 32% 20% 2	836
November	1st to 10th	Night	15% 6% 24% 32% 19% 4	780
	11th to 20th	Night	15% 8% 31% 34% 12% 1	800
	21st to end	Night	19% 9% 29% 29% 12% 2	800
December	1st to 10th	Night	23% 11% 31% 28% 8%	800
	11th to 20th	Night	22% 11% 33% 27% 8%	800
	21st to end	Night	25% 14% 32% 24% 5%	1100

Page generated at 2025-10-16 00:50:28. Version ac07

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