The Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Ranch 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	6% 7% 3 6% 4 4 5% 4 6% 9% 46%	130373
	Night	4 7% 3 7% 4 4 5% 4 6% 9% 47%	127376
Transparency	Day	35% 6% 17% 25% 13% 3	130084
	Night	34% 7% 19% 24% 12% 3	127242
Seeing	Day	15% 18% 28% 23% 12% 3	16473
	Night	12% 19% 31% 27% 10% 2	25540

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	1 9% 5% 11% 7% 5% 5% 5% 8% 9% 36%	3179
	11th to 20th	Day	1 8% 3 11% 7% 10% 6% 5% 7% 6% 36%	3432
	21st to end	Day	1 5% 3 8% 4 4 4 3 5% 12% 50%	4004
February	1st to 10th	Day	2 8% 4 11% 7% 6% 7% 5% 7% 10% 33%	1656
	11th to 20th	Day	1 10% 5% 11% 6% 5% 5% 4 7% 8% 37%	1755
	21st to end	Day	2 8% 4 11% 7% 5% 5% 4 7% 9% 39%	1636
March	1st to 10th	Day	1 7% 3 7% 4 3 4 4 7% 11% 47%	4518
	11th to 20th	Day	1 7% 2 6% 5% 4 5% 5% 8% 11% 45%	4989
	21st to end	Day	3 13% 7% 13% 6% 5% 5% 5% 6% 9% 28%	5518
April	1st to 10th	Day	3 13% 4 8% 5% 6% 5% 5% 8% 8% 35%	2439
	11th to 20th	Day	5% 13% 4 9% 6% 5% 4 4 6% 9% 35%	2570
	21st to end	Day	8% 13% 4 8% 4 4 5% 5% 7% 8% 35%	2600
May	1st to 10th	Day	10% 14% 3 6% 4 3 3 3 4 6% 42%	5880
	11th to 20th	Day	10% 9% 2 7% 5% 6% 6% 5% 7% 9% 33%	6660
	21st to end	Day	10% 7% 2 5% 4 4 5% 4 7% 9% 44%	7405
June	1st to 10th	Day	12% 8% 2 7% 5% 5% 5% 5% 7% 8% 36%	3000
	11th to 20th	Day	14% 7% 3 7% 5% 6% 6% 5% 7% 8% 33%	3000
	21st to end	Day	8% 4 2 4 4 4 6% 5% 8% 8% 48%	3000
July	1st to 10th	Day	6% 3 1 5% 4 4 5% 4 6% 7% 54%	6780
	11th to 20th	Day	6% 3 1 3 3 3 5% 4 6% 8% 58%	6780
	21st to end	Day	7% 3 1 3 2 3 3 3 5% 7% 63%	6972
August	1st to 10th	Day	10% 6% 2 5% 4 4 5% 4 7% 8% 44%	4478
	11th to 20th	Day	5% 3 1 4 3 4 5% 5% 6% 9% 56%	4158
	21st to end	Day	7% 3 1 3 3 3 3 3 5% 7% 61%	4664
September	1st to 10th	Day	11% 5% 1 4 3 3 4 3 5% 7% 54%	2435
	11th to 20th	Day	9% 5% 2 5% 3 3 5% 4 7% 7% 50%	2179
	21st to end	Day	9% 7% 2 3 2 3 3 3 5% 8% 54%	2090
October	1st to 10th	Day	6% 6% 2 5% 4 3 4 3 6% 8% 54%	3366
	11th to 20th	Day	3 5% 2 5% 4 4 5% 4 7% 11% 52%	2976
	21st to end	Day	4 7% 3 6% 4 4 5% 3 7% 9% 49%	3101
November	1st to 10th	Day	5% 8% 3 7% 5% 4 4 4 7% 9% 43%	1698
	11th to 20th	Day	1 9% 3 5% 4 3 3 5% 6% 12% 48%	1600
	21st to end	Day	2 10% 4 9% 6% 5% 5% 3 5% 9% 43%	1425
December	1st to 10th	Day	1 5% 3 5% 4 3 4 4 6% 10% 56%	2290
	11th to 20th	Day	2 7% 2 7% 6% 5% 5% 4 6% 9% 44%	2642
	21st to end	Day	1 8% 3 8% 5% 5% 5% 4 6% 9% 45%	3498
January	1st to 10th	Night	2 8% 3 11% 7% 6% 6% 5% 7% 8% 38%	6766
	11th to 20th	Night	1 6% 3 10% 7% 7% 7% 5% 6% 9% 38%	6494
	21st to end	Night	1 6% 2 9% 4 4 5% 4 7% 9% 48%	6952
February	1st to 10th	Night	2 10% 4 12% 6% 5% 5% 4 5% 9% 36%	2743
	11th to 20th	Night	1 11% 4 10% 6% 5% 6% 4 6% 10% 38%	2532
	21st to end	Night	1 8% 3 11% 6% 7% 6% 5% 7% 9% 38%	1990
March	1st to 10th	Night	6% 3 9% 6% 5% 5% 4 6% 10% 45%	5376
	11th to 20th	Night	1 4 2 5% 3 4 5% 5% 8% 10% 53%	5010
	21st to end	Night	3 13% 7% 13% 5% 4 4 4 6% 10% 30%	5456
April	1st to 10th	Night	5% 11% 3 6% 4 4 4 4 6% 9% 44%	1960
	11th to 20th	Night	5% 12% 4 8% 4 3 4 4 6% 8% 41%	1781
	21st to end	Night	7% 11% 2 7% 6% 5% 4 4 6% 7% 41%	1800
May	1st to 10th	Night	11% 12% 3 6% 4 4 4 3 5% 6% 42%	4085
	11th to 20th	Night	8% 7% 3 7% 4 4 6% 5% 7% 8% 41%	3217
	21st to end	Night	10% 7% 2 5% 3 4 4 3 5% 7% 50%	3470
June	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 6% 8% 45%	1400
	11th to 20th	Night	14% 7% 1 5% 3 5% 5% 4 6% 8% 42%	1400
	21st to end	Night	5% 3 2 5% 4 3 4 3 7% 8% 57%	1400
July	1st to 10th	Night	5% 2 1 2 2 3 4 3 5% 7% 65%	3180
	11th to 20th	Night	6% 2 2 2 3 3 3 4 7% 66%	3180
	21st to end	Night	9% 5% 2 4 3 2 3 2 3 4 62%	3868
August	1st to 10th	Night	10% 4 2 3 3 3 4 3 5% 7% 55%	2674
	11th to 20th	Night	5% 2 1 5% 3 4 5% 5% 7% 10% 53%	2901
	21st to end	Night	5% 3 1 3 2 3 3 3 5% 8% 64%	3245
September	1st to 10th	Night	10% 4 2 2 3 3 4 3 6% 7% 56%	1962
	11th to 20th	Night	9% 6% 2 4 3 3 4 3 5% 8% 53%	2178
	21st to end	Night	8% 5% 2 4 3 3 4 3 5% 8% 55%	2090
October	1st to 10th	Night	5% 6% 1 3 2 2 3 3 5% 8% 60%	3609
	11th to 20th	Night	2 4 1 4 2 3 4 4 7% 10% 58%	3948
	21st to end	Night	3 5% 3 7% 4 3 3 3 6% 9% 55%	4457
November	1st to 10th	Night	4 10% 3 8% 5% 4 5% 4 7% 8% 42%	2580
	11th to 20th	Night	1 7% 2 7% 4 4 4 4 6% 10% 50%	2800
	21st to end	Night	1 9% 4 8% 6% 4 4 3 6% 11% 43%	2974
December	1st to 10th	Night	1 5% 3 7% 5% 3 4 5% 7% 12% 49%	4890
	11th to 20th	Night	1 7% 4 11% 6% 5% 6% 5% 8% 9% 37%	5550
	21st to end	Night	1 10% 5% 9% 6% 5% 4 3 6% 8% 42%	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	45% 7% 21% 18% 7% 1	3179
	11th to 20th	Day	46% 8% 17% 14% 11% 4	3456
	21st to end	Day	30% 12% 20% 26% 12% 1	4004
February	1st to 10th	Day	46% 11% 22% 13% 7% 2	1656
	11th to 20th	Day	43% 9% 21% 19% 7% 1	1764
	21st to end	Day	41% 10% 21% 18% 8% 2	1636
March	1st to 10th	Day	29% 7% 19% 27% 15% 3	4518
	11th to 20th	Day	31% 7% 22% 28% 11% 1	4990
	21st to end	Day	52% 6% 22% 13% 6% 1	5518
April	1st to 10th	Day	43% 8% 21% 23% 5%	2439
	11th to 20th	Day	46% 8% 17% 21% 8% 1	2570
	21st to end	Day	44% 8% 19% 20% 7% 1	2600
May	1st to 10th	Day	43% 4 12% 22% 16% 3	5880
	11th to 20th	Day	44% 6% 15% 27% 8% 1	6664
	21st to end	Day	35% 7% 19% 27% 11%	7406
June	1st to 10th	Day	44% 9% 16% 22% 9%	3000
	11th to 20th	Day	45% 7% 16% 19% 11% 2	3000
	21st to end	Day	29% 6% 21% 29% 13% 2	3000
July	1st to 10th	Day	27% 5% 12% 25% 23% 8%	6706
	11th to 20th	Day	24% 4 12% 29% 22% 9%	6699
	21st to end	Day	22% 3 12% 28% 21% 14%	6776
August	1st to 10th	Day	35% 4 15% 22% 19% 6%	4461
	11th to 20th	Day	26% 3 17% 33% 18% 2	4197
	21st to end	Day	23% 3 10% 41% 20% 3	4663
September	1st to 10th	Day	29% 4 13% 28% 17% 8%	2435
	11th to 20th	Day	30% 6% 16% 29% 15% 4	2179
	21st to end	Day	30% 5% 20% 30% 12% 3	2090
October	1st to 10th	Day	28% 3 16% 36% 16% 1	3366
	11th to 20th	Day	27% 4 23% 27% 17% 1	2976
	21st to end	Day	33% 6% 18% 29% 12% 2	3101
November	1st to 10th	Day	39% 11% 22% 21% 7% 1	1698
	11th to 20th	Day	30% 13% 26% 22% 9% 1	1600
	21st to end	Day	39% 10% 23% 17% 8% 2	1425
December	1st to 10th	Day	26% 9% 28% 25% 9% 2	2290
	11th to 20th	Day	35% 9% 27% 22% 6% 1	2644
	21st to end	Day	36% 9% 17% 21% 14% 3	3498
January	1st to 10th	Night	45% 8% 22% 18% 6% 1	6766
	11th to 20th	Night	43% 7% 16% 19% 9% 5%	6532
	21st to end	Night	32% 14% 19% 23% 10% 1	6952
February	1st to 10th	Night	45% 9% 18% 17% 9% 2	2743
	11th to 20th	Night	42% 10% 22% 17% 8% 1	2533
	21st to end	Night	42% 9% 21% 18% 9% 2	1990
March	1st to 10th	Night	34% 8% 17% 27% 10% 4	5376
	11th to 20th	Night	27% 8% 24% 27% 14% 1	5010
	21st to end	Night	51% 6% 23% 14% 6%	5456
April	1st to 10th	Night	36% 8% 21% 27% 7%	1960
	11th to 20th	Night	39% 9% 17% 27% 8% 1	1781
	21st to end	Night	42% 8% 18% 24% 8% 1	1800
May	1st to 10th	Night	43% 6% 11% 21% 16% 2	4082
	11th to 20th	Night	38% 6% 16% 28% 11% 1	3217
	21st to end	Night	35% 6% 17% 29% 12% 1	3470
June	1st to 10th	Night	37% 7% 20% 26% 10%	1400
	11th to 20th	Night	40% 6% 18% 21% 14% 2	1400
	21st to end	Night	24% 6% 22% 30% 16% 3	1400
July	1st to 10th	Night	19% 3 16% 27% 23% 12%	3145
	11th to 20th	Night	19% 3 11% 35% 20% 11%	3138
	21st to end	Night	26% 3 12% 25% 20% 14%	3759
August	1st to 10th	Night	28% 3 17% 24% 20% 9%	2666
	11th to 20th	Night	26% 3 18% 34% 16% 3	2928
	21st to end	Night	20% 3 10% 38% 24% 4	3245
September	1st to 10th	Night	26% 4 15% 30% 17% 7%	1962
	11th to 20th	Night	30% 5% 17% 29% 15% 4	2178
	21st to end	Night	27% 5% 22% 28% 15% 3	2090
October	1st to 10th	Night	23% 4 14% 34% 22% 3	3609
	11th to 20th	Night	22% 5% 23% 30% 18% 2	3948
	21st to end	Night	28% 7% 16% 34% 14% 1	4457
November	1st to 10th	Night	40% 9% 22% 19% 8% 2	2580
	11th to 20th	Night	30% 12% 24% 24% 8% 1	2800
	21st to end	Night	36% 12% 23% 19% 8% 3	2974
December	1st to 10th	Night	29% 9% 25% 25% 10% 2	4890
	11th to 20th	Night	42% 9% 25% 17% 6% 1	5547
	21st to end	Night	40% 7% 19% 18% 13% 4	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	15% 24% 30% 20% 8% 5%	200
	11th to 20th	Day	13% 22% 33% 23% 7% 2	200
	21st to end	Day	16% 32% 32% 13% 4 4	220
February	1st to 10th	Day	19% 16% 36% 21% 7% 1	228
	11th to 20th	Day	11% 24% 33% 23% 8%	300
	21st to end	Day	17% 30% 32% 16% 5% 1	400
March	1st to 10th	Day	13% 27% 34% 17% 7% 2	500
	11th to 20th	Day	13% 23% 32% 20% 9% 2	500
	21st to end	Day	15% 24% 34% 18% 7% 2	550
April	1st to 10th	Day	18% 25% 34% 16% 7%	500
	11th to 20th	Day	16% 25% 31% 17% 8% 2	492
	21st to end	Day	22% 17% 26% 22% 10% 3	500
May	1st to 10th	Day	20% 16% 25% 22% 14% 3	503
	11th to 20th	Day	24% 12% 24% 19% 16% 5%	690
	21st to end	Day	24% 10% 19% 22% 19% 5%	789
June	1st to 10th	Day	18% 17% 26% 23% 13% 3	730
	11th to 20th	Day	20% 21% 24% 21% 11% 3	730
	21st to end	Day	11% 12% 27% 28% 18% 5%	730
July	1st to 10th	Day	9% 10% 25% 30% 19% 6%	721
	11th to 20th	Day	9% 9% 29% 32% 17% 4	718
	21st to end	Day	7% 10% 29% 32% 17% 5%	621
August	1st to 10th	Day	12% 9% 24% 29% 19% 6%	527
	11th to 20th	Day	7% 7% 31% 32% 18% 5%	517
	21st to end	Day	11% 13% 28% 30% 14% 4	583
September	1st to 10th	Day	14% 16% 25% 22% 15% 8%	530
	11th to 20th	Day	12% 15% 31% 23% 16% 4	523
	21st to end	Day	21% 20% 27% 21% 9% 2	490
October	1st to 10th	Day	17% 17% 29% 21% 13% 3	418
	11th to 20th	Day	14% 22% 29% 21% 11% 4	300
	21st to end	Day	19% 26% 31% 14% 8% 2	330
November	1st to 10th	Day	20% 26% 31% 18% 5%	300
	11th to 20th	Day	19% 34% 30% 14% 3	300
	21st to end	Day	16% 35% 23% 14% 8% 4	213
December	1st to 10th	Day	13% 32% 32% 16% 6% 3	200
	11th to 20th	Day	23% 23% 29% 22% 4 1	200
	21st to end	Day	10% 26% 29% 21% 10% 4	220
January	1st to 10th	Night	12% 30% 33% 19% 5% 1	1000
	11th to 20th	Night	11% 28% 35% 20% 6% 1	895
	21st to end	Night	11% 29% 38% 16% 5% 2	880
February	1st to 10th	Night	11% 21% 34% 24% 8% 2	800
	11th to 20th	Night	11% 23% 34% 24% 7%	784
	21st to end	Night	10% 23% 33% 25% 8% 1	660
March	1st to 10th	Night	7% 21% 35% 27% 8% 2	800
	11th to 20th	Night	7% 24% 32% 26% 8% 2	800
	21st to end	Night	11% 19% 29% 28% 11% 2	880
April	1st to 10th	Night	11% 19% 33% 26% 10% 2	760
	11th to 20th	Night	13% 16% 36% 24% 10% 1	593
	21st to end	Night	13% 16% 32% 28% 11% 1	600
May	1st to 10th	Night	17% 13% 25% 29% 14% 1	598
	11th to 20th	Night	13% 7% 24% 37% 17% 2	394
	21st to end	Night	24% 9% 19% 28% 19% 2	433
June	1st to 10th	Night	16% 13% 24% 31% 15% 1	400
	11th to 20th	Night	17% 15% 28% 29% 10% 1	400
	21st to end	Night	7% 7% 30% 37% 19% 1	400
July	1st to 10th	Night	8% 8% 18% 41% 24% 2	390
	11th to 20th	Night	7% 5% 29% 41% 17% 1	388
	21st to end	Night	8% 4 21% 42% 23% 2	438
August	1st to 10th	Night	9% 2 20% 43% 25% 2	437
	11th to 20th	Night	6% 4 22% 42% 23% 3	594
	21st to end	Night	10% 8% 24% 42% 14% 2	660
September	1st to 10th	Night	13% 7% 22% 33% 21% 5%	723
	11th to 20th	Night	13% 7% 23% 34% 18% 5%	792
	21st to end	Night	14% 13% 30% 28% 12% 3	760
October	1st to 10th	Night	16% 15% 23% 31% 13% 2	760
	11th to 20th	Night	10% 20% 28% 29% 10% 2	760
	21st to end	Night	14% 24% 33% 22% 5% 2	836
November	1st to 10th	Night	16% 24% 35% 20% 4	825
	11th to 20th	Night	14% 31% 36% 16% 2 1	1000
	21st to end	Night	12% 29% 33% 20% 5% 1	1000
December	1st to 10th	Night	11% 24% 37% 20% 7% 1	1000
	11th to 20th	Night	9% 27% 38% 20% 5%	1000
	21st to end	Night	11% 26% 35% 21% 6% 1	1100

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