Rimrock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rimrock 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	12% 10% 3 6% 4 4 4 3 5% 6% 42%	129917
	Night	14% 11% 3 7% 4 4 4 3 5% 6% 38%	127824
Transparency	Day	43% 5% 12% 22% 13% 4	129643
	Night	46% 5% 12% 20% 12% 4	127675
Seeing	Day	20% 10% 24% 25% 16% 5%	16037
	Night	23% 5% 16% 32% 21% 3	25116

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	24% 28% 6% 10% 5% 3 3 3 4 4 9%	3639
	11th to 20th	Day	17% 13% 3 7% 5% 5% 5% 4 6% 8% 27%	3616
	21st to end	Day	22% 15% 3 7% 4 3 4 3 4 6% 29%	4004
February	1st to 10th	Day	23% 19% 5% 9% 5% 3 4 3 5% 4 20%	1600
	11th to 20th	Day	18% 20% 5% 9% 4 3 4 3 4 6% 23%	1865
	21st to end	Day	19% 22% 4 9% 6% 5% 4 3 4 4 22%	1650
March	1st to 10th	Day	11% 14% 3 7% 5% 5% 6% 4 7% 7% 31%	4518
	11th to 20th	Day	14% 13% 3 8% 4 4 4 3 5% 5% 36%	4734
	21st to end	Day	20% 18% 4 9% 6% 5% 4 3 4 7% 19%	5837
April	1st to 10th	Day	18% 14% 3 8% 6% 6% 5% 4 6% 7% 24%	2400
	11th to 20th	Day	13% 12% 3 8% 6% 5% 5% 4 5% 6% 33%	2467
	21st to end	Day	14% 10% 3 8% 7% 6% 5% 4 5% 6% 34%	2600
May	1st to 10th	Day	8% 7% 3 7% 4 4 4 4 6% 8% 43%	6134
	11th to 20th	Day	10% 9% 3 7% 5% 5% 5% 4 7% 11% 34%	6277
	21st to end	Day	9% 7% 2 6% 5% 5% 5% 4 6% 8% 42%	7101
June	1st to 10th	Day	16% 8% 3 6% 5% 4 4 4 5% 7% 38%	3000
	11th to 20th	Day	16% 7% 2 6% 6% 5% 5% 3 7% 8% 36%	3000
	21st to end	Day	7% 4 1 4 4 4 5% 4 6% 7% 54%	3000
July	1st to 10th	Day	4 2 1 3 2 3 3 3 4 5% 71%	6536
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 5% 78%	6320
	21st to end	Day	3 2 1 3 2 2 2 2 4 6% 73%	6874
August	1st to 10th	Day	4 3 1 3 3 2 3 3 5% 6% 68%	4541
	11th to 20th	Day	3 3 1 3 2 2 4 3 5% 8% 65%	4235
	21st to end	Day	4 3 1 4 2 4 4 3 6% 7% 62%	4454
September	1st to 10th	Day	6% 3 1 3 2 3 3 3 5% 6% 64%	2400
	11th to 20th	Day	10% 5% 2 6% 4 3 4 4 5% 6% 51%	2244
	21st to end	Day	10% 6% 3 6% 3 4 5% 4 5% 8% 46%	2090
October	1st to 10th	Day	10% 5% 1 4 4 3 4 3 4 6% 56%	3411
	11th to 20th	Day	14% 8% 3 5% 4 4 5% 3 5% 7% 42%	2920
	21st to end	Day	20% 13% 4 7% 5% 4 4 3 5% 6% 28%	3101
November	1st to 10th	Day	29% 14% 4 7% 4 3 4 3 4 5% 23%	1764
	11th to 20th	Day	26% 16% 5% 8% 5% 4 4 3 4 4 21%	1730
	21st to end	Day	21% 20% 6% 10% 5% 4 3 3 4 4 21%	1400
December	1st to 10th	Day	18% 13% 5% 11% 4 3 4 2 8% 8% 23%	2290
	11th to 20th	Day	23% 27% 6% 10% 5% 5% 5% 3 4 4 9%	2642
	21st to end	Day	26% 25% 5% 10% 5% 4 3 3 4 5% 12%	3523
January	1st to 10th	Night	27% 22% 5% 10% 4 4 5% 3 5% 5% 11%	6324
	11th to 20th	Night	15% 12% 3 9% 5% 5% 4 3 7% 9% 28%	6282
	21st to end	Night	22% 15% 3 5% 4 3 4 3 3 8% 30%	6952
February	1st to 10th	Night	22% 19% 5% 7% 5% 3 3 3 4 5% 24%	2800
	11th to 20th	Night	18% 16% 5% 9% 5% 3 3 3 4 6% 29%	2417
	21st to end	Night	16% 17% 4 8% 5% 4 4 3 5% 6% 28%	1980
March	1st to 10th	Night	10% 12% 3 7% 4 4 4 4 5% 7% 39%	5376
	11th to 20th	Night	15% 14% 3 7% 4 4 4 3 4 4 39%	5264
	21st to end	Night	17% 13% 4 9% 6% 6% 6% 6% 7% 7% 20%	5122
April	1st to 10th	Night	11% 12% 3 7% 5% 5% 5% 5% 7% 8% 32%	2000
	11th to 20th	Night	13% 10% 2 8% 5% 5% 5% 3 4 6% 40%	1883
	21st to end	Night	12% 9% 3 7% 5% 4 4 5% 6% 7% 39%	1800
May	1st to 10th	Night	10% 7% 2 6% 5% 4 5% 4 5% 8% 44%	3830
	11th to 20th	Night	14% 8% 3 7% 5% 4 4 4 5% 9% 37%	3616
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 6% 7% 44%	3770
June	1st to 10th	Night	12% 7% 2 6% 4 5% 5% 4 5% 6% 44%	1400
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 5% 7% 7% 42%	1400
	21st to end	Night	6% 5% 2 5% 4 3 4 4 6% 6% 54%	1400
July	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 75%	3406
	11th to 20th	Night	3 2 1 2 2 2 1 1 3 3 81%	3640
	21st to end	Night	4 3 1 3 2 2 2 3 4 5% 73%	3969
August	1st to 10th	Night	3 3 1 2 2 2 3 2 5% 6% 71%	2609
	11th to 20th	Night	3 4 1 5% 3 3 4 3 5% 6% 63%	2807
	21st to end	Night	3 3 1 4 3 4 4 4 5% 6% 63%	3459
September	1st to 10th	Night	5% 3 1 3 3 3 3 3 5% 7% 66%	2000
	11th to 20th	Night	8% 6% 1 5% 4 4 4 3 6% 8% 51%	2113
	21st to end	Night	12% 5% 1 5% 3 4 3 3 6% 7% 53%	2090
October	1st to 10th	Night	8% 4 1 4 3 3 4 2 5% 7% 58%	3569
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 47%	4010
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 39%	4457
November	1st to 10th	Night	24% 13% 3 7% 3 3 4 4 5% 7% 26%	2535
	11th to 20th	Night	22% 15% 4 8% 5% 4 4 3 4 5% 25%	2668
	21st to end	Night	24% 14% 4 6% 5% 4 4 3 5% 5% 25%	2978
December	1st to 10th	Night	15% 11% 4 10% 5% 4 4 4 8% 10% 26%	4890
	11th to 20th	Night	22% 23% 5% 10% 6% 5% 4 3 4 5% 13%	5550
	21st to end	Night	24% 21% 7% 9% 5% 5% 4 3 4 5% 12%	7458

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	77% 8% 8% 6% 1	3639
	11th to 20th	Day	54% 6% 18% 16% 5% 1	3640
	21st to end	Day	58% 5% 14% 18% 5%	4004
February	1st to 10th	Day	69% 6% 10% 10% 4 1	1600
	11th to 20th	Day	65% 6% 12% 10% 7%	1875
	21st to end	Day	68% 5% 8% 15% 4	1650
March	1st to 10th	Day	51% 5% 13% 17% 13%	4518
	11th to 20th	Day	52% 4 17% 25% 2	4735
	21st to end	Day	66% 6% 13% 11% 3	5837
April	1st to 10th	Day	60% 6% 16% 15% 3	2400
	11th to 20th	Day	52% 5% 14% 19% 9% 1	2467
	21st to end	Day	53% 6% 14% 19% 8% 1	2600
May	1st to 10th	Day	39% 5% 12% 27% 17%	6134
	11th to 20th	Day	43% 6% 17% 27% 7%	6280
	21st to end	Day	39% 8% 19% 27% 7%	7102
June	1st to 10th	Day	47% 5% 13% 23% 12% 1	3000
	11th to 20th	Day	46% 6% 13% 22% 10% 2	3000
	21st to end	Day	30% 6% 15% 30% 17% 3	3000
July	1st to 10th	Day	18% 3 8% 23% 31% 18%	6467
	11th to 20th	Day	12% 2 6% 25% 28% 27%	6245
	21st to end	Day	15% 3 5% 33% 30% 14%	6681
August	1st to 10th	Day	19% 2 6% 26% 35% 11%	4524
	11th to 20th	Day	19% 4 9% 41% 24% 3	4275
	21st to end	Day	22% 3 11% 35% 25% 3	4453
September	1st to 10th	Day	21% 4 12% 31% 22% 10%	2400
	11th to 20th	Day	34% 5% 14% 24% 21% 3	2244
	21st to end	Day	37% 7% 19% 23% 11% 3	2090
October	1st to 10th	Day	31% 3 13% 37% 16%	3411
	11th to 20th	Day	44% 4 18% 23% 11%	2920
	21st to end	Day	59% 5% 12% 19% 5%	3101
November	1st to 10th	Day	65% 6% 14% 12% 3	1764
	11th to 20th	Day	69% 4 13% 9% 4 1	1730
	21st to end	Day	68% 5% 11% 12% 4	1400
December	1st to 10th	Day	55% 8% 19% 16% 2	2290
	11th to 20th	Day	79% 5% 10% 5% 1	2644
	21st to end	Day	76% 5% 10% 6% 3	3523
January	1st to 10th	Night	73% 7% 10% 8% 1	6324
	11th to 20th	Night	50% 7% 18% 17% 7% 2	6320
	21st to end	Night	57% 5% 13% 20% 5%	6952
February	1st to 10th	Night	64% 5% 12% 12% 5% 1	2800
	11th to 20th	Night	59% 5% 16% 13% 6% 1	2418
	21st to end	Night	58% 6% 12% 17% 6% 1	1980
March	1st to 10th	Night	44% 6% 13% 22% 15% 1	5376
	11th to 20th	Night	50% 3 14% 25% 6%	5264
	21st to end	Night	60% 10% 12% 16% 3	5122
April	1st to 10th	Night	49% 6% 15% 23% 7%	2000
	11th to 20th	Night	47% 4 11% 24% 13% 1	1883
	21st to end	Night	45% 6% 12% 24% 12% 1	1800
May	1st to 10th	Night	37% 6% 11% 22% 23% 1	3827
	11th to 20th	Night	42% 6% 13% 26% 12% 1	3616
	21st to end	Night	35% 8% 15% 29% 12% 1	3770
June	1st to 10th	Night	40% 6% 11% 24% 17% 2	1400
	11th to 20th	Night	39% 5% 12% 24% 16% 4	1400
	21st to end	Night	29% 5% 12% 27% 21% 6%	1400
July	1st to 10th	Night	14% 2 6% 19% 34% 25%	3367
	11th to 20th	Night	12% 1 3 19% 28% 36%	3592
	21st to end	Night	15% 2 3 26% 32% 22%	3857
August	1st to 10th	Night	16% 2 5% 21% 36% 21%	2601
	11th to 20th	Night	23% 2 7% 32% 27% 7%	2832
	21st to end	Night	23% 4 8% 27% 31% 7%	3459
September	1st to 10th	Night	20% 4 9% 28% 28% 12%	2000
	11th to 20th	Night	32% 5% 11% 24% 22% 6%	2113
	21st to end	Night	32% 4 17% 27% 16% 5%	2090
October	1st to 10th	Night	27% 3 11% 40% 18% 1	3569
	11th to 20th	Night	38% 4 14% 24% 19% 1	4010
	21st to end	Night	45% 6% 13% 27% 8% 2	4457
November	1st to 10th	Night	58% 7% 16% 14% 4 1	2535
	11th to 20th	Night	64% 6% 14% 11% 4 1	2668
	21st to end	Night	61% 6% 15% 13% 4	2978
December	1st to 10th	Night	50% 9% 19% 19% 3	4890
	11th to 20th	Night	74% 6% 12% 8% 1	5547
	21st to end	Night	74% 7% 10% 7% 2	7458

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	36% 14% 17% 19% 12% 4	200
	11th to 20th	Day	38% 12% 15% 20% 11% 6%	200
	21st to end	Day	32% 7% 29% 18% 12% 2	220
February	1st to 10th	Day	35% 15% 28% 14% 9% 1	200
	11th to 20th	Day	34% 13% 22% 23% 9% 1	435
	21st to end	Day	33% 14% 23% 21% 8% 1	414
March	1st to 10th	Day	29% 17% 23% 20% 9% 2	500
	11th to 20th	Day	26% 12% 24% 20% 16% 2	500
	21st to end	Day	33% 13% 23% 21% 8% 1	550
April	1st to 10th	Day	29% 11% 27% 20% 9% 3	500
	11th to 20th	Day	21% 19% 25% 21% 11% 3	492
	21st to end	Day	20% 13% 26% 25% 11% 4	500
May	1st to 10th	Day	16% 11% 27% 28% 13% 4	503
	11th to 20th	Day	21% 10% 27% 23% 14% 5%	528
	21st to end	Day	15% 9% 21% 28% 19% 7%	654
June	1st to 10th	Day	19% 9% 22% 25% 19% 5%	730
	11th to 20th	Day	18% 15% 28% 23% 13% 3	730
	21st to end	Day	9% 8% 24% 30% 22% 6%	730
July	1st to 10th	Day	7% 6% 26% 34% 24% 4	625
	11th to 20th	Day	5% 9% 29% 35% 18% 5%	524
	21st to end	Day	4 7% 25% 35% 21% 7%	575
August	1st to 10th	Day	8% 4 24% 35% 22% 7%	527
	11th to 20th	Day	6% 4 22% 36% 22% 9%	517
	21st to end	Day	9% 8% 27% 28% 20% 9%	583
September	1st to 10th	Day	7% 9% 25% 27% 23% 9%	530
	11th to 20th	Day	14% 8% 26% 25% 20% 8%	523
	21st to end	Day	14% 13% 26% 21% 20% 5%	490
October	1st to 10th	Day	17% 8% 26% 26% 16% 7%	490
	11th to 20th	Day	23% 12% 19% 25% 13% 8%	352
	21st to end	Day	33% 9% 22% 23% 10% 3	330
November	1st to 10th	Day	41% 7% 18% 24% 10%	300
	11th to 20th	Day	39% 12% 15% 18% 15% 1	265
	21st to end	Day	31% 10% 27% 19% 13% 2	200
December	1st to 10th	Day	35% 9% 20% 19% 15% 4	200
	11th to 20th	Day	42% 8% 21% 18% 9% 2	200
	21st to end	Day	41% 10% 17% 21% 10% 1	220
January	1st to 10th	Night	42% 7% 15% 25% 10% 2	805
	11th to 20th	Night	32% 7% 15% 30% 14% 2	800
	21st to end	Night	30% 6% 16% 31% 16% 2	880
February	1st to 10th	Night	35% 7% 19% 26% 10% 2	800
	11th to 20th	Night	31% 6% 16% 31% 15% 1	784
	21st to end	Night	26% 8% 18% 27% 18% 2	660
March	1st to 10th	Night	25% 8% 17% 33% 16% 2	800
	11th to 20th	Night	26% 8% 16% 31% 18% 1	800
	21st to end	Night	26% 7% 20% 32% 13% 2	880
April	1st to 10th	Night	21% 6% 17% 37% 17% 2	800
	11th to 20th	Night	20% 7% 24% 33% 14% 2	695
	21st to end	Night	19% 6% 20% 35% 19% 2	600
May	1st to 10th	Night	13% 4 19% 35% 27% 3	486
	11th to 20th	Night	16% 4 16% 29% 27% 8%	394
	21st to end	Night	16% 4 15% 30% 30% 5%	433
June	1st to 10th	Night	17% 5% 13% 37% 25% 5%	400
	11th to 20th	Night	16% 5% 24% 33% 20% 3	400
	21st to end	Night	7% 4 13% 38% 34% 4	400
July	1st to 10th	Night	4 2 11% 41% 41% 3	390
	11th to 20th	Night	5% 1 14% 46% 32% 2	388
	21st to end	Night	2 2 9% 42% 40% 4	438
August	1st to 10th	Night	5% 2 10% 38% 39% 6%	398
	11th to 20th	Night	4 2 7% 39% 38% 10%	536
	21st to end	Night	6% 4 13% 35% 38% 3	783
September	1st to 10th	Night	7% 2 12% 39% 35% 6%	800
	11th to 20th	Night	12% 3 12% 31% 34% 7%	792
	21st to end	Night	15% 4 15% 35% 26% 5%	760
October	1st to 10th	Night	18% 4 12% 37% 27% 2	760
	11th to 20th	Night	23% 4 14% 30% 27% 3	760
	21st to end	Night	26% 5% 19% 30% 18% 3	836
November	1st to 10th	Night	36% 6% 12% 32% 14%	780
	11th to 20th	Night	33% 9% 17% 27% 14% 1	800
	21st to end	Night	34% 6% 14% 27% 16% 2	978
December	1st to 10th	Night	29% 6% 16% 31% 17% 1	1000
	11th to 20th	Night	38% 6% 18% 29% 10%	1000
	21st to end	Night	41% 7% 16% 26% 8% 1	1100

Page generated at 2025-09-15 22:49:14. Version ac07

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