Rock Creek
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rock Creek clear sky chart from 2005-11-06 to 2026-06-01.

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	8% 10% 3 7% 5% 5% 5% 4 6% 7% 40%	135559
	Night	9% 12% 4 8% 5% 5% 5% 4 6% 7% 38%	136205
Transparency	Day	25% 39% 9% 15% 9% 3	135286
	Night	28% 39% 9% 15% 8% 2	136056
Seeing	Day	16% 13% 27% 26% 14% 4	16639
	Night	17% 7% 21% 33% 19% 3	25772

By Month

Percentage Cloud Cover NW (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	9% 22% 7% 13% 7% 6% 5% 4 6% 7% 14%	4723
	11th to 20th	Day	7% 9% 2 7% 5% 5% 5% 4 6% 10% 39%	4716
	21st to end	Day	15% 20% 5% 6% 3 3 5% 3 6% 7% 28%	5186
February	1st to 10th	Day	10% 17% 5% 9% 6% 5% 5% 4 5% 7% 28%	1722
	11th to 20th	Day	9% 16% 4 8% 6% 5% 5% 4 6% 7% 30%	1866
	21st to end	Day	7% 15% 4 11% 7% 6% 6% 5% 7% 6% 26%	1730
March	1st to 10th	Day	6% 12% 4 8% 6% 5% 6% 5% 8% 9% 32%	4618
	11th to 20th	Day	6% 13% 4 8% 5% 4 4 4 4 7% 40%	4808
	21st to end	Day	10% 15% 5% 11% 7% 6% 6% 5% 6% 7% 23%	6092
April	1st to 10th	Day	10% 12% 5% 10% 7% 6% 5% 5% 6% 8% 27%	2520
	11th to 20th	Day	8% 10% 4 9% 7% 6% 5% 5% 7% 8% 30%	2520
	21st to end	Day	8% 12% 3 8% 6% 6% 5% 5% 6% 9% 33%	2817
May	1st to 10th	Day	7% 6% 2 7% 5% 6% 6% 4 6% 7% 44%	6460
	11th to 20th	Day	8% 10% 4 9% 6% 6% 6% 5% 8% 9% 30%	6417
	21st to end	Day	9% 8% 2 8% 5% 6% 6% 5% 7% 9% 35%	7057
June	1st to 10th	Day	14% 9% 2 8% 6% 6% 6% 4 6% 6% 32%	2981
	11th to 20th	Day	11% 8% 4 8% 7% 7% 6% 5% 7% 7% 29%	3000
	21st to end	Day	7% 6% 2 6% 6% 5% 5% 4 7% 8% 43%	3000
July	1st to 10th	Day	2 2 1 3 2 3 4 4 5% 6% 69%	6322
	11th to 20th	Day	2 2 1 3 2 2 2 3 4 5% 73%	6320
	21st to end	Day	2 3 2 3 3 4 4 3 5% 8% 62%	6874
August	1st to 10th	Day	2 2 1 3 3 4 4 4 6% 8% 63%	4541
	11th to 20th	Day	3 3 2 4 3 4 5% 5% 6% 8% 58%	4435
	21st to end	Day	4 4 2 5% 4 5% 5% 4 6% 8% 53%	4312
September	1st to 10th	Day	4 4 2 5% 3 3 5% 3 5% 7% 60%	2400
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 5% 8% 48%	2355
	21st to end	Day	8% 6% 2 6% 4 3 4 4 6% 7% 50%	2200
October	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 6% 9% 51%	3297
	11th to 20th	Day	9% 6% 3 8% 6% 7% 6% 4 6% 7% 39%	3140
	21st to end	Day	11% 10% 3 9% 5% 5% 7% 3 5% 6% 35%	3200
November	1st to 10th	Day	17% 13% 3 8% 5% 5% 6% 5% 5% 8% 27%	1815
	11th to 20th	Day	14% 18% 4 9% 6% 5% 4 3 4 5% 28%	1680
	21st to end	Day	15% 18% 5% 11% 6% 5% 5% 4 5% 5% 22%	1575
December	1st to 10th	Day	8% 10% 4 10% 8% 5% 6% 5% 8% 9% 26%	2360
	11th to 20th	Day	11% 25% 8% 12% 7% 5% 4 4 4 4 15%	2712
	21st to end	Day	11% 24% 8% 16% 9% 6% 4 3 3 3 12%	3788
January	1st to 10th	Night	12% 26% 8% 12% 5% 4 5% 3 5% 6% 13%	8172
	11th to 20th	Night	8% 10% 2 8% 5% 5% 4 4 7% 10% 39%	8182
	21st to end	Night	13% 19% 6% 9% 4 4 4 3 5% 7% 26%	8994
February	1st to 10th	Night	11% 16% 5% 10% 6% 5% 6% 4 5% 7% 27%	2897
	11th to 20th	Night	9% 14% 4 8% 5% 5% 5% 4 5% 7% 34%	2641
	21st to end	Night	8% 14% 5% 9% 5% 6% 6% 5% 7% 7% 28%	2076
March	1st to 10th	Night	5% 10% 4 7% 6% 6% 6% 5% 7% 8% 36%	5496
	11th to 20th	Night	7% 13% 4 7% 4 4 4 4 5% 6% 42%	5407
	21st to end	Night	9% 16% 5% 12% 7% 6% 6% 5% 6% 6% 23%	5124
April	1st to 10th	Night	9% 9% 3 6% 4 5% 6% 5% 7% 9% 36%	2100
	11th to 20th	Night	7% 8% 3 8% 5% 7% 6% 4 6% 7% 39%	2050
	21st to end	Night	6% 9% 3 7% 5% 6% 6% 4 7% 7% 41%	1801
May	1st to 10th	Night	5% 6% 2 5% 5% 5% 6% 3 5% 7% 50%	3720
	11th to 20th	Night	6% 7% 3 8% 5% 5% 5% 4 5% 8% 42%	3696
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 7% 9% 46%	4060
June	1st to 10th	Night	12% 6% 2 5% 5% 5% 5% 4 7% 9% 39%	1419
	11th to 20th	Night	10% 6% 3 8% 6% 5% 5% 4 9% 8% 37%	1400
	21st to end	Night	6% 6% 2 5% 4 5% 5% 5% 5% 9% 48%	1309
July	1st to 10th	Night	2 3 1 3 2 2 2 2 4 5% 76%	3633
	11th to 20th	Night	2 2 1 3 2 2 2 3 4 5% 75%	3640
	21st to end	Night	2 2 1 3 2 3 3 3 6% 8% 68%	3969
August	1st to 10th	Night	2 2 1 5% 4 4 5% 3 4 6% 64%	2609
	11th to 20th	Night	2 2 1 3 2 3 4 4 7% 9% 62%	2600
	21st to end	Night	5% 4 2 4 4 4 4 3 6% 9% 55%	3606
September	1st to 10th	Night	3 4 1 4 3 3 4 3 7% 7% 62%	2000
	11th to 20th	Night	8% 6% 2 5% 3 3 3 4 5% 7% 54%	2045
	21st to end	Night	7% 4 2 6% 4 5% 5% 3 4 8% 52%	2200
October	1st to 10th	Night	6% 4 1 5% 3 4 5% 5% 6% 7% 53%	3888
	11th to 20th	Night	10% 8% 2 7% 5% 5% 4 4 5% 7% 42%	4014
	21st to end	Night	14% 8% 3 6% 5% 5% 6% 4 6% 6% 39%	4600
November	1st to 10th	Night	19% 13% 4 7% 5% 4 5% 4 6% 6% 27%	2702
	11th to 20th	Night	17% 17% 4 8% 5% 4 4 3 5% 5% 28%	2940
	21st to end	Night	15% 18% 5% 12% 6% 5% 4 3 4 4 24%	3022
December	1st to 10th	Night	8% 11% 4 10% 7% 5% 4 4 9% 10% 29%	5040
	11th to 20th	Night	13% 23% 7% 14% 7% 5% 4 4 5% 5% 14%	5700
	21st to end	Night	14% 25% 6% 12% 6% 6% 6% 3 4 4 14%	7453

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	50% 29% 7% 11% 2	4723
	11th to 20th	Day	28% 27% 19% 18% 6% 2	4740
	21st to end	Day	43% 28% 10% 14% 5%	5187
February	1st to 10th	Day	24% 62% 3 5% 6% 1	1722
	11th to 20th	Day	23% 60% 5% 8% 4	1875
	21st to end	Day	20% 61% 7% 7% 5%	1730
March	1st to 10th	Day	31% 33% 11% 13% 13%	4618
	11th to 20th	Day	29% 30% 11% 23% 6%	4808
	21st to end	Day	39% 35% 7% 11% 7%	6092
April	1st to 10th	Day	18% 61% 9% 9% 3	2520
	11th to 20th	Day	17% 61% 7% 11% 4 1	2520
	21st to end	Day	18% 60% 6% 8% 7% 1	2817
May	1st to 10th	Day	22% 31% 7% 19% 20% 1	6460
	11th to 20th	Day	35% 31% 10% 17% 7%	6420
	21st to end	Day	28% 32% 13% 22% 5%	7058
June	1st to 10th	Day	15% 63% 5% 9% 6% 1	2981
	11th to 20th	Day	17% 63% 5% 9% 5% 1	3000
	21st to end	Day	14% 62% 5% 10% 7% 1	3000
July	1st to 10th	Day	9% 26% 9% 15% 24% 17%	6253
	11th to 20th	Day	8% 26% 7% 20% 20% 19%	6245
	21st to end	Day	15% 27% 8% 24% 17% 9%	6681
August	1st to 10th	Day	10% 36% 4 16% 26% 7%	4524
	11th to 20th	Day	14% 40% 12% 24% 9% 1	4477
	21st to end	Day	18% 39% 10% 18% 12% 4	4311
September	1st to 10th	Day	12% 58% 7% 13% 8% 2	2400
	11th to 20th	Day	16% 60% 5% 11% 7% 1	2355
	21st to end	Day	17% 58% 8% 11% 6% 1	2200
October	1st to 10th	Day	16% 38% 15% 24% 6%	3297
	11th to 20th	Day	28% 39% 7% 19% 7%	3140
	21st to end	Day	28% 41% 11% 15% 6%	3200
November	1st to 10th	Day	24% 57% 9% 8% 2	1815
	11th to 20th	Day	28% 56% 6% 6% 3 1	1680
	21st to end	Day	28% 56% 5% 6% 4 1	1575
December	1st to 10th	Day	32% 41% 19% 7% 1	2360
	11th to 20th	Day	51% 37% 4 5% 3	2714
	21st to end	Day	59% 30% 3 4 2	3788
January	1st to 10th	Night	55% 29% 7% 7% 2	8172
	11th to 20th	Night	28% 29% 13% 21% 7% 2	8220
	21st to end	Night	45% 30% 8% 13% 4	8994
February	1st to 10th	Night	25% 61% 4 4 5% 1	2897
	11th to 20th	Night	23% 59% 5% 8% 4 1	2643
	21st to end	Night	19% 61% 6% 8% 6%	2076
March	1st to 10th	Night	27% 34% 10% 17% 12%	5496
	11th to 20th	Night	28% 29% 13% 21% 9%	5408
	21st to end	Night	44% 34% 7% 10% 5%	5124
April	1st to 10th	Night	16% 61% 8% 11% 3 1	2100
	11th to 20th	Night	17% 61% 6% 10% 6%	2050
	21st to end	Night	14% 61% 5% 11% 8% 1	1801
May	1st to 10th	Night	19% 31% 7% 23% 19% 1	3717
	11th to 20th	Night	28% 30% 10% 25% 6% 2	3696
	21st to end	Night	19% 34% 13% 28% 6% 1	4060
June	1st to 10th	Night	11% 63% 6% 11% 7% 1	1419
	11th to 20th	Night	15% 63% 5% 8% 8% 1	1400
	21st to end	Night	13% 62% 5% 13% 6% 1	1309
July	1st to 10th	Night	8% 25% 8% 16% 28% 15%	3593
	11th to 20th	Night	9% 26% 8% 20% 17% 20%	3592
	21st to end	Night	12% 26% 7% 27% 20% 8%	3857
August	1st to 10th	Night	13% 36% 3 16% 26% 5%	2601
	11th to 20th	Night	11% 38% 14% 26% 10% 1	2624
	21st to end	Night	17% 38% 11% 20% 12% 2	3606
September	1st to 10th	Night	10% 59% 8% 12% 10% 1	2000
	11th to 20th	Night	14% 59% 6% 13% 8%	2045
	21st to end	Night	16% 58% 8% 13% 5%	2200
October	1st to 10th	Night	14% 39% 18% 21% 8%	3888
	11th to 20th	Night	26% 39% 8% 18% 8%	4014
	21st to end	Night	26% 40% 11% 20% 3	4600
November	1st to 10th	Night	27% 57% 8% 7% 2	2702
	11th to 20th	Night	26% 57% 6% 6% 3 1	2940
	21st to end	Night	27% 56% 6% 5% 4 1	3022
December	1st to 10th	Night	29% 42% 15% 12% 1	5040
	11th to 20th	Night	50% 37% 7% 4 2	5697
	21st to end	Night	51% 32% 5% 7% 3	7453

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% 14% 19% 28% 12% 3	220
	11th to 20th	Day	26% 7% 25% 26% 13% 3	220
	21st to end	Day	26% 11% 25% 24% 11% 2	242
February	1st to 10th	Day	16% 14% 34% 27% 6% 2	262
	11th to 20th	Day	19% 15% 29% 26% 9% 2	436
	21st to end	Day	15% 15% 32% 28% 9% 1	446
March	1st to 10th	Day	21% 20% 29% 18% 11% 2	540
	11th to 20th	Day	21% 19% 23% 21% 13% 3	540
	21st to end	Day	22% 20% 25% 19% 11% 3	594
April	1st to 10th	Day	21% 19% 27% 20% 10% 2	540
	11th to 20th	Day	18% 21% 27% 21% 8% 4	532
	21st to end	Day	18% 18% 27% 21% 11% 4	540
May	1st to 10th	Day	15% 17% 31% 23% 11% 3	543
	11th to 20th	Day	18% 13% 28% 25% 12% 4	568
	21st to end	Day	17% 11% 27% 26% 15% 5%	617
June	1st to 10th	Day	17% 12% 29% 26% 12% 3	711
	11th to 20th	Day	14% 13% 29% 26% 15% 3	730
	21st to end	Day	9% 9% 26% 32% 19% 5%	730
July	1st to 10th	Day	7% 12% 29% 35% 14% 3	528
	11th to 20th	Day	5% 16% 30% 32% 14% 5%	524
	21st to end	Day	4 9% 28% 33% 20% 6%	575
August	1st to 10th	Day	6% 7% 26% 36% 18% 7%	527
	11th to 20th	Day	6% 8% 30% 30% 19% 8%	517
	21st to end	Day	10% 13% 23% 28% 18% 8%	583
September	1st to 10th	Day	9% 12% 25% 25% 19% 9%	530
	11th to 20th	Day	15% 9% 26% 29% 17% 5%	530
	21st to end	Day	15% 15% 29% 25% 13% 4	530
October	1st to 10th	Day	12% 14% 29% 26% 15% 3	530
	11th to 20th	Day	18% 12% 28% 25% 14% 3	460
	21st to end	Day	24% 9% 31% 23% 11% 2	363
November	1st to 10th	Day	24% 13% 23% 30% 10%	309
	11th to 20th	Day	27% 14% 24% 19% 14% 2	220
	21st to end	Day	21% 13% 25% 24% 13% 4	220
December	1st to 10th	Day	22% 9% 20% 26% 14% 9%	220
	11th to 20th	Day	23% 12% 30% 25% 9% 2	220
	21st to end	Day	20% 12% 24% 29% 12% 2	242
January	1st to 10th	Night	29% 12% 21% 27% 11% 1	840
	11th to 20th	Night	25% 6% 21% 33% 13% 1	840
	21st to end	Night	22% 6% 20% 37% 15% 1	924
February	1st to 10th	Night	20% 8% 25% 32% 13% 2	840
	11th to 20th	Night	17% 7% 24% 37% 14% 1	824
	21st to end	Night	16% 8% 24% 33% 17% 2	692
March	1st to 10th	Night	17% 7% 21% 34% 18% 2	840
	11th to 20th	Night	17% 8% 25% 33% 16% 2	840
	21st to end	Night	18% 9% 25% 31% 15% 2	924
April	1st to 10th	Night	16% 7% 20% 35% 20% 2	840
	11th to 20th	Night	13% 9% 26% 32% 17% 2	802
	21st to end	Night	11% 10% 22% 36% 18% 2	541
May	1st to 10th	Night	9% 9% 21% 36% 24% 2	420
	11th to 20th	Night	11% 6% 21% 34% 24% 4	414
	21st to end	Night	12% 6% 22% 29% 26% 5%	455
June	1st to 10th	Night	15% 7% 24% 34% 19% 2	400
	11th to 20th	Night	15% 6% 24% 35% 19% 2	400
	21st to end	Night	8% 8% 22% 31% 30% 3	400
July	1st to 10th	Night	5% 4 23% 32% 34% 3	390
	11th to 20th	Night	5% 8% 18% 41% 26% 3	388
	21st to end	Night	3 5% 14% 40% 33% 4	438
August	1st to 10th	Night	3 3 16% 34% 37% 7%	398
	11th to 20th	Night	4 2 15% 37% 39% 4	406
	21st to end	Night	6% 5% 15% 37% 32% 4	878
September	1st to 10th	Night	5% 5% 18% 36% 29% 8%	800
	11th to 20th	Night	11% 5% 14% 30% 33% 7%	800
	21st to end	Night	10% 7% 17% 39% 23% 4	800
October	1st to 10th	Night	15% 5% 18% 35% 26% 2	800
	11th to 20th	Night	19% 6% 21% 32% 20% 2	800
	21st to end	Night	23% 7% 20% 34% 14% 3	880
November	1st to 10th	Night	29% 7% 23% 32% 8% 1	820
	11th to 20th	Night	29% 9% 20% 26% 15% 2	840
	21st to end	Night	26% 8% 20% 30% 13% 3	922
December	1st to 10th	Night	21% 8% 20% 35% 14% 2	1050
	11th to 20th	Night	27% 8% 22% 33% 10% 1	1050
	21st to end	Night	27% 9% 23% 32% 8%	1076

Page generated at 2026-05-28 22:53:29. Version ac07

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