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 2025-02-06.

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% 39%	111860
	Night	10% 12% 4 8% 5% 4 4 4 5% 7% 37%	114950
Transparency	Day	23% 46% 8% 12% 8% 3	111628
	Night	26% 45% 8% 12% 7% 2	114828
Seeing	Day	16% 14% 27% 26% 13% 4	14924
	Night	18% 7% 21% 33% 19% 3	24137

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	11% 22% 7% 14% 7% 7% 6% 4 5% 4 15%	3639
	11th to 20th	Day	10% 11% 2 6% 5% 6% 5% 4 6% 9% 36%	3616
	21st to end	Day	16% 22% 4 5% 3 3 4 3 5% 7% 27%	4004
February	1st to 10th	Day	10% 17% 4 9% 6% 5% 5% 4 6% 7% 27%	1598
	11th to 20th	Day	9% 16% 4 9% 6% 5% 5% 4 6% 6% 30%	1684
	21st to end	Day	8% 15% 4 11% 7% 6% 6% 4 7% 6% 26%	1570
March	1st to 10th	Day	8% 16% 6% 9% 6% 5% 6% 4 6% 7% 27%	3138
	11th to 20th	Day	7% 10% 3 6% 5% 4 3 4 4 7% 49%	3286
	21st to end	Day	11% 15% 5% 10% 7% 6% 6% 5% 6% 6% 22%	4156
April	1st to 10th	Day	11% 12% 5% 10% 8% 7% 5% 5% 5% 7% 25%	2280
	11th to 20th	Day	9% 11% 5% 10% 8% 6% 5% 5% 7% 8% 28%	2277
	21st to end	Day	9% 13% 3 9% 7% 6% 5% 5% 6% 8% 31%	2549
May	1st to 10th	Day	9% 8% 3 9% 6% 5% 5% 4 6% 8% 38%	4420
	11th to 20th	Day	7% 8% 3 8% 5% 5% 5% 5% 8% 10% 36%	4377
	21st to end	Day	12% 11% 3 9% 7% 7% 7% 5% 7% 7% 25%	4813
June	1st to 10th	Day	15% 9% 2 9% 6% 6% 6% 5% 5% 6% 30%	2831
	11th to 20th	Day	11% 8% 4 9% 8% 7% 6% 5% 7% 7% 29%	2850
	21st to end	Day	7% 6% 2 6% 6% 5% 5% 4 7% 8% 44%	2850
July	1st to 10th	Day	3 3 1 3 2 3 3 4 5% 6% 68%	4422
	11th to 20th	Day	2 3 1 3 3 2 2 2 3 4 74%	4420
	21st to end	Day	2 2 1 2 2 3 4 3 5% 8% 68%	4790
August	1st to 10th	Day	2 2 1 3 3 4 4 3 6% 8% 64%	4401
	11th to 20th	Day	3 3 2 4 3 4 5% 5% 6% 7% 58%	4296
	21st to end	Day	4 4 2 5% 4 5% 5% 4 6% 8% 53%	4180
September	1st to 10th	Day	5% 4 2 5% 3 3 5% 3 5% 7% 60%	2280
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 5% 8% 47%	2237
	21st to end	Day	8% 6% 2 6% 4 3 4 4 6% 7% 50%	2090
October	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 6% 8% 51%	3196
	11th to 20th	Day	9% 6% 3 8% 6% 7% 7% 4 6% 7% 39%	3044
	21st to end	Day	11% 10% 3 9% 5% 5% 7% 3 5% 6% 35%	3101
November	1st to 10th	Day	17% 13% 3 8% 5% 5% 6% 5% 5% 8% 26%	1727
	11th to 20th	Day	14% 18% 4 9% 6% 5% 4 3 4 5% 29%	1600
	21st to end	Day	15% 17% 5% 11% 6% 4 5% 4 5% 5% 22%	1500
December	1st to 10th	Day	8% 10% 4 10% 8% 5% 5% 5% 8% 9% 27%	2290
	11th to 20th	Day	11% 26% 9% 12% 7% 5% 4 3 4 4 15%	2642
	21st to end	Day	11% 25% 8% 16% 9% 6% 4 3 3 3 12%	3706
January	1st to 10th	Night	13% 26% 9% 13% 5% 4 4 3 5% 5% 13%	6306
	11th to 20th	Night	10% 12% 3 7% 4 4 4 3 6% 9% 39%	6282
	21st to end	Night	15% 21% 7% 9% 4 4 3 2 4 7% 23%	6952
February	1st to 10th	Night	12% 16% 5% 9% 6% 5% 6% 4 5% 7% 26%	2692
	11th to 20th	Night	10% 13% 4 8% 5% 5% 5% 4 5% 6% 35%	2383
	21st to end	Night	8% 14% 5% 10% 5% 5% 6% 5% 7% 7% 29%	1884
March	1st to 10th	Night	6% 12% 5% 8% 6% 6% 6% 4 7% 8% 33%	3736
	11th to 20th	Night	8% 10% 2 5% 3 3 3 3 5% 5% 52%	3689
	21st to end	Night	9% 16% 4 11% 6% 6% 5% 4 6% 6% 26%	3496
April	1st to 10th	Night	10% 10% 3 7% 5% 5% 6% 5% 6% 8% 35%	1900
	11th to 20th	Night	8% 8% 3 8% 6% 8% 7% 4 6% 7% 37%	1853
	21st to end	Night	6% 10% 3 7% 6% 6% 6% 4 7% 7% 39%	1629
May	1st to 10th	Night	7% 7% 3 6% 5% 5% 5% 4 5% 8% 44%	2540
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 6% 9% 49%	2516
	21st to end	Night	10% 7% 3 7% 5% 5% 6% 5% 7% 7% 38%	2762
June	1st to 10th	Night	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 38%	1348
	11th to 20th	Night	10% 6% 3 8% 6% 5% 5% 4 9% 8% 36%	1330
	21st to end	Night	6% 6% 2 5% 4 5% 5% 5% 5% 9% 48%	1244
July	1st to 10th	Night	2 2 1 3 2 2 2 2 3 5% 76%	2533
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 77%	2540
	21st to end	Night	1 1 1 3 2 3 3 4 5% 7% 70%	2762
August	1st to 10th	Night	2 2 1 5% 3 4 5% 3 4 6% 65%	2529
	11th to 20th	Night	2 2 1 3 2 3 4 4 7% 9% 63%	2519
	21st to end	Night	5% 4 2 4 4 4 4 3 6% 9% 55%	3496
September	1st to 10th	Night	3 4 1 4 3 3 4 3 7% 7% 62%	1900
	11th to 20th	Night	8% 6% 2 5% 3 4 3 4 5% 7% 54%	1943
	21st to end	Night	7% 4 2 6% 4 4 5% 3 4 8% 51%	2090
October	1st to 10th	Night	6% 4 1 5% 3 4 5% 5% 6% 7% 53%	3769
	11th to 20th	Night	11% 8% 2 8% 5% 5% 4 4 6% 7% 42%	3890
	21st to end	Night	14% 8% 3 6% 5% 5% 6% 4 6% 6% 39%	4457
November	1st to 10th	Night	19% 13% 4 7% 5% 4 5% 3 6% 6% 27%	2570
	11th to 20th	Night	17% 16% 4 8% 6% 4 4 3 5% 5% 29%	2800
	21st to end	Night	16% 18% 5% 12% 6% 5% 4 2 4 4 25%	2878
December	1st to 10th	Night	8% 11% 4 10% 7% 5% 4 4 9% 10% 29%	4890
	11th to 20th	Night	13% 24% 7% 14% 7% 5% 4 4 5% 5% 14%	5550
	21st to end	Night	14% 25% 6% 12% 6% 6% 6% 3 4 4 14%	7292

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	49% 33% 5% 11% 2	3639
	11th to 20th	Day	29% 31% 19% 13% 5% 3	3640
	21st to end	Day	42% 33% 10% 11% 5%	4004
February	1st to 10th	Day	22% 66% 3 5% 4 1	1598
	11th to 20th	Day	21% 65% 4 6% 3	1693
	21st to end	Day	17% 66% 5% 7% 5%	1570
March	1st to 10th	Day	34% 42% 6% 8% 10%	3138
	11th to 20th	Day	15% 42% 14% 26% 3	3286
	21st to end	Day	34% 47% 7% 7% 6%	4156
April	1st to 10th	Day	18% 67% 7% 6% 2	2280
	11th to 20th	Day	16% 67% 5% 9% 3	2277
	21st to end	Day	17% 66% 5% 6% 5%	2549
May	1st to 10th	Day	22% 43% 9% 14% 13% 1	4420
	11th to 20th	Day	21% 42% 10% 18% 9%	4380
	21st to end	Day	32% 45% 8% 12% 3	4814
June	1st to 10th	Day	16% 67% 5% 8% 4 1	2831
	11th to 20th	Day	16% 67% 5% 8% 4 1	2850
	21st to end	Day	13% 65% 5% 10% 7% 1	2850
July	1st to 10th	Day	7% 36% 5% 8% 22% 21%	4353
	11th to 20th	Day	7% 35% 3 6% 22% 27%	4345
	21st to end	Day	10% 36% 7% 15% 20% 12%	4639
August	1st to 10th	Day	9% 37% 4 16% 26% 8%	4384
	11th to 20th	Day	14% 41% 12% 24% 8% 1	4338
	21st to end	Day	17% 40% 10% 17% 12% 3	4179
September	1st to 10th	Day	12% 61% 7% 12% 7% 1	2280
	11th to 20th	Day	16% 63% 5% 9% 6% 1	2237
	21st to end	Day	16% 61% 7% 10% 6% 1	2090
October	1st to 10th	Day	16% 39% 15% 24% 6%	3196
	11th to 20th	Day	28% 40% 7% 19% 6%	3044
	21st to end	Day	27% 42% 10% 15% 6%	3101
November	1st to 10th	Day	24% 59% 8% 7% 2	1727
	11th to 20th	Day	26% 58% 6% 6% 3 1	1600
	21st to end	Day	26% 58% 5% 6% 3 1	1500
December	1st to 10th	Day	31% 42% 19% 7% 1	2290
	11th to 20th	Day	50% 37% 4 5% 3	2644
	21st to end	Day	59% 31% 3 4 2	3706
January	1st to 10th	Night	54% 32% 5% 8% 1	6306
	11th to 20th	Night	27% 33% 11% 18% 9% 3	6320
	21st to end	Night	44% 36% 9% 8% 3	6952
February	1st to 10th	Night	23% 65% 4 4 4 1	2692
	11th to 20th	Night	20% 65% 4 7% 3 1	2385
	21st to end	Night	16% 66% 5% 7% 6%	1884
March	1st to 10th	Night	29% 43% 8% 10% 10%	3736
	11th to 20th	Night	15% 40% 14% 24% 6%	3690
	21st to end	Night	35% 45% 6% 9% 4	3496
April	1st to 10th	Night	15% 67% 6% 8% 3 1	1900
	11th to 20th	Night	16% 67% 5% 7% 5%	1853
	21st to end	Night	14% 66% 4 8% 7%	1629
May	1st to 10th	Night	20% 42% 8% 14% 15%	2540
	11th to 20th	Night	16% 40% 12% 23% 7% 2	2516
	21st to end	Night	21% 47% 11% 16% 5%	2762
June	1st to 10th	Night	11% 67% 6% 11% 5% 1	1348
	11th to 20th	Night	15% 67% 5% 7% 6% 1	1330
	21st to end	Night	12% 65% 4 13% 6% 1	1244
July	1st to 10th	Night	6% 35% 3 11% 29% 17%	2493
	11th to 20th	Night	7% 35% 4 5% 20% 29%	2492
	21st to end	Night	9% 35% 5% 19% 21% 11%	2674
August	1st to 10th	Night	12% 37% 3 16% 27% 5%	2521
	11th to 20th	Night	10% 39% 14% 26% 10% 1	2543
	21st to end	Night	17% 39% 11% 20% 11% 1	3496
September	1st to 10th	Night	9% 62% 7% 12% 9% 1	1900
	11th to 20th	Night	13% 62% 6% 12% 7%	1943
	21st to end	Night	15% 60% 8% 11% 5%	2090
October	1st to 10th	Night	14% 40% 17% 21% 8%	3769
	11th to 20th	Night	26% 40% 8% 19% 7%	3890
	21st to end	Night	26% 41% 10% 20% 3	4457
November	1st to 10th	Night	25% 59% 7% 7% 2	2570
	11th to 20th	Night	24% 59% 6% 6% 3 1	2800
	21st to end	Night	25% 59% 6% 5% 4 1	2878
December	1st to 10th	Night	28% 43% 15% 12% 1	4890
	11th to 20th	Night	50% 37% 7% 4 2	5547
	21st to end	Night	52% 33% 5% 7% 3	7292

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% 15% 17% 30% 11% 3	200
	11th to 20th	Day	29% 7% 24% 24% 13% 4	200
	21st to end	Day	28% 11% 26% 23% 9% 2	220
February	1st to 10th	Day	18% 14% 36% 27% 4	228
	11th to 20th	Day	21% 16% 30% 24% 8% 1	374
	21st to end	Day	15% 14% 32% 28% 10% 1	382
March	1st to 10th	Day	23% 21% 29% 16% 9% 2	460
	11th to 20th	Day	20% 18% 23% 21% 14% 3	460
	21st to end	Day	22% 20% 25% 18% 12% 3	506
April	1st to 10th	Day	22% 20% 30% 19% 9% 1	460
	11th to 20th	Day	19% 21% 27% 19% 9% 4	452
	21st to end	Day	19% 20% 28% 20% 10% 3	460
May	1st to 10th	Day	16% 17% 30% 23% 11% 3	463
	11th to 20th	Day	19% 12% 27% 26% 13% 4	488
	21st to end	Day	18% 10% 28% 25% 15% 5%	529
June	1st to 10th	Day	19% 12% 30% 26% 11% 3	661
	11th to 20th	Day	15% 14% 29% 25% 14% 3	680
	21st to end	Day	10% 9% 27% 31% 19% 4	680
July	1st to 10th	Day	7% 12% 29% 35% 14% 3	488
	11th to 20th	Day	5% 15% 30% 32% 13% 5%	484
	21st to end	Day	5% 9% 28% 33% 20% 5%	531
August	1st to 10th	Day	6% 7% 26% 37% 18% 6%	487
	11th to 20th	Day	5% 8% 29% 31% 19% 8%	477
	21st to end	Day	10% 14% 25% 28% 17% 6%	539
September	1st to 10th	Day	10% 13% 27% 26% 18% 7%	490
	11th to 20th	Day	16% 9% 27% 29% 15% 4	490
	21st to end	Day	15% 16% 28% 24% 13% 3	490
October	1st to 10th	Day	13% 15% 28% 26% 14% 4	490
	11th to 20th	Day	18% 12% 27% 25% 15% 4	424
	21st to end	Day	26% 9% 29% 24% 11% 2	330
November	1st to 10th	Day	24% 14% 23% 28% 10%	281
	11th to 20th	Day	27% 16% 25% 18% 14% 2	200
	21st to end	Day	22% 13% 24% 24% 15% 4	200
December	1st to 10th	Day	24% 7% 18% 26% 16% 10%	200
	11th to 20th	Day	24% 11% 29% 25% 10% 2	200
	21st to end	Day	22% 12% 23% 29% 12% 2	220
January	1st to 10th	Night	29% 11% 21% 26% 11% 1	800
	11th to 20th	Night	26% 7% 22% 31% 13% 2	800
	21st to end	Night	22% 6% 20% 36% 14% 1	880
February	1st to 10th	Night	20% 7% 26% 32% 13% 2	780
	11th to 20th	Night	18% 7% 25% 35% 14% 1	744
	21st to end	Night	16% 7% 23% 33% 17% 3	628
March	1st to 10th	Night	19% 8% 22% 32% 17% 3	760
	11th to 20th	Night	17% 7% 25% 33% 17% 2	760
	21st to end	Night	17% 9% 26% 31% 15% 2	836
April	1st to 10th	Night	17% 7% 20% 34% 19% 3	760
	11th to 20th	Night	13% 9% 26% 32% 17% 3	725
	21st to end	Night	11% 10% 24% 36% 17% 2	489
May	1st to 10th	Night	9% 9% 21% 34% 23% 2	380
	11th to 20th	Night	11% 6% 20% 34% 25% 4	374
	21st to end	Night	13% 6% 22% 27% 27% 5%	411
June	1st to 10th	Night	16% 7% 24% 33% 18% 2	380
	11th to 20th	Night	15% 5% 24% 33% 20% 2	380
	21st to end	Night	8% 7% 23% 30% 30% 3	380
July	1st to 10th	Night	5% 4 22% 32% 35% 3	370
	11th to 20th	Night	5% 8% 18% 42% 25% 3	368
	21st to end	Night	3 5% 14% 41% 32% 4	416
August	1st to 10th	Night	3 3 16% 34% 37% 7%	378
	11th to 20th	Night	3 2 15% 36% 40% 4	385
	21st to end	Night	6% 6% 16% 37% 31% 4	834
September	1st to 10th	Night	5% 5% 19% 37% 27% 7%	760
	11th to 20th	Night	11% 5% 15% 29% 33% 7%	760
	21st to end	Night	11% 7% 17% 38% 24% 4	760
October	1st to 10th	Night	16% 4 18% 33% 26% 2	760
	11th to 20th	Night	19% 7% 20% 31% 21% 2	760
	21st to end	Night	23% 7% 20% 33% 14% 3	836
November	1st to 10th	Night	29% 7% 23% 31% 8% 1	780
	11th to 20th	Night	29% 9% 20% 26% 15% 2	800
	21st to end	Night	26% 8% 20% 29% 13% 3	878
December	1st to 10th	Night	22% 7% 20% 34% 15% 2	1000
	11th to 20th	Night	28% 8% 20% 33% 10% 1	1000
	21st to end	Night	28% 9% 23% 32% 8%	1025

Page generated at 2025-02-03 00:05:53. Version ac07

Copyright © 2025 Attilla Danko.