Clearview Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clearview Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	21% 9% 3 7% 4 4 4 3 5% 6% 32%	131242
	Night	25% 11% 3 7% 4 4 4 3 4 5% 30%	127467
Transparency	Day	52% 6% 14% 18% 8% 1	130957
	Night	57% 6% 13% 17% 7% 1	127329
Seeing	Day	26% 6% 18% 26% 17% 6%	16370
	Night	32% 5% 12% 26% 21% 4	24668

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	38% 15% 4 10% 5% 5% 4 3 4 4 9%	3239
	11th to 20th	Day	21% 9% 3 9% 6% 6% 5% 4 6% 7% 24%	3616
	21st to end	Day	31% 12% 3 7% 4 3 3 3 4 4 26%	4078
February	1st to 10th	Day	31% 13% 4 8% 5% 4 4 4 5% 5% 16%	1800
	11th to 20th	Day	29% 14% 5% 8% 5% 4 4 3 4 5% 20%	1755
	21st to end	Day	29% 15% 4 7% 6% 5% 3 3 3 5% 20%	1555
March	1st to 10th	Day	23% 15% 4 9% 6% 5% 5% 4 5% 6% 18%	4601
	11th to 20th	Day	21% 14% 4 8% 5% 5% 4 3 5% 5% 25%	5000
	21st to end	Day	27% 12% 4 9% 6% 6% 7% 4 6% 6% 12%	5693
April	1st to 10th	Day	28% 12% 3 8% 6% 5% 5% 4 7% 7% 17%	2400
	11th to 20th	Day	24% 11% 3 7% 5% 5% 5% 4 4 7% 25%	2392
	21st to end	Day	22% 10% 3 9% 6% 6% 5% 5% 5% 6% 23%	2659
May	1st to 10th	Day	16% 8% 2 7% 4 4 4 3 5% 6% 39%	6320
	11th to 20th	Day	23% 10% 3 7% 5% 4 5% 4 6% 8% 26%	6277
	21st to end	Day	23% 9% 3 7% 4 4 4 4 6% 8% 28%	6903
June	1st to 10th	Day	23% 8% 2 6% 5% 5% 5% 3 5% 7% 30%	2850
	11th to 20th	Day	25% 9% 3 8% 6% 4 5% 5% 5% 6% 23%	3000
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 6% 7% 37%	3000
July	1st to 10th	Day	10% 4 2 4 2 3 3 3 4 6% 60%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 70%	6486
	21st to end	Day	10% 4 1 5% 3 3 3 3 5% 7% 56%	6874
August	1st to 10th	Day	7% 3 1 4 2 3 3 3 5% 7% 62%	4489
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	4156
	21st to end	Day	16% 8% 3 6% 4 4 4 3 5% 6% 42%	4458
September	1st to 10th	Day	12% 6% 3 6% 4 4 3 3 4 8% 47%	2400
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 3 5% 7% 38%	2400
	21st to end	Day	20% 8% 3 6% 4 4 4 3 5% 6% 37%	2118
October	1st to 10th	Day	17% 7% 2 6% 5% 5% 5% 4 7% 9% 34%	3280
	11th to 20th	Day	33% 9% 3 7% 4 4 3 3 4 6% 24%	3261
	21st to end	Day	30% 11% 4 9% 5% 5% 4 3 5% 6% 17%	3337
November	1st to 10th	Day	38% 11% 3 6% 4 3 5% 4 4 5% 17%	1630
	11th to 20th	Day	35% 11% 4 6% 4 4 4 3 4 4 21%	1600
	21st to end	Day	35% 12% 3 7% 4 4 3 3 5% 5% 19%	1600
December	1st to 10th	Day	25% 9% 3 7% 5% 6% 6% 3 5% 6% 24%	2620
	11th to 20th	Day	36% 16% 4 8% 4 5% 4 4 4 4 11%	3025
	21st to end	Day	33% 15% 4 11% 6% 5% 5% 3 4 4 11%	3590
January	1st to 10th	Night	37% 14% 4 11% 5% 4 4 3 4 4 10%	6701
	11th to 20th	Night	21% 9% 4 6% 4 4 4 4 6% 8% 29%	6282
	21st to end	Night	28% 11% 3 7% 4 4 4 3 4 5% 28%	6866
February	1st to 10th	Night	31% 12% 4 8% 6% 5% 4 3 4 6% 18%	2600
	11th to 20th	Night	29% 15% 3 7% 4 5% 4 3 4 5% 22%	2532
	21st to end	Night	30% 15% 3 6% 4 3 4 3 5% 5% 22%	2074
March	1st to 10th	Night	19% 15% 4 9% 6% 6% 6% 5% 6% 6% 18%	5290
	11th to 20th	Night	25% 14% 3 7% 4 4 4 3 4 5% 28%	4999
	21st to end	Night	33% 14% 3 9% 5% 4 5% 4 4 4 13%	5281
April	1st to 10th	Night	27% 12% 3 8% 6% 4 6% 5% 5% 6% 18%	2000
	11th to 20th	Night	22% 10% 2 8% 5% 5% 4 4 5% 5% 31%	1954
	21st to end	Night	25% 11% 3 8% 5% 5% 5% 4 5% 6% 25%	1740
May	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 3 4 42%	3640
	11th to 20th	Night	25% 9% 3 7% 4 3 4 3 4 5% 31%	3616
	21st to end	Night	26% 9% 3 7% 5% 5% 5% 3 6% 6% 24%	3972
June	1st to 10th	Night	21% 9% 3 6% 6% 4 5% 3 5% 6% 32%	1545
	11th to 20th	Night	25% 11% 4 9% 5% 4 3 3 6% 5% 26%	1400
	21st to end	Night	15% 6% 2 7% 5% 4 3 3 6% 7% 41%	1400
July	1st to 10th	Night	14% 6% 2 4 3 2 2 2 3 4 59%	3180
	11th to 20th	Night	4 3 1 3 2 1 2 2 4 3 75%	3470
	21st to end	Night	9% 4 1 4 2 3 4 3 5% 6% 59%	3969
August	1st to 10th	Night	6% 2 1 3 3 3 3 2 4 6% 68%	2659
	11th to 20th	Night	14% 6% 2 6% 3 3 3 2 4 6% 50%	2902
	21st to end	Night	19% 6% 2 5% 3 3 3 2 4 5% 48%	3443
September	1st to 10th	Night	10% 6% 2 5% 4 3 3 3 4 6% 53%	2000
	11th to 20th	Night	16% 8% 3 6% 5% 3 4 3 5% 5% 41%	2000
	21st to end	Night	19% 8% 3 7% 4 4 4 3 5% 5% 38%	2278
October	1st to 10th	Night	16% 7% 3 6% 5% 4 5% 4 6% 8% 37%	3920
	11th to 20th	Night	30% 8% 3 6% 4 4 4 3 5% 5% 29%	3897
	21st to end	Night	33% 12% 3 7% 4 4 3 3 4 5% 21%	4466
November	1st to 10th	Night	36% 10% 4 6% 4 4 4 4 4 5% 20%	2708
	11th to 20th	Night	37% 11% 3 7% 4 3 3 3 4 4 21%	2800
	21st to end	Night	33% 10% 3 8% 5% 4 4 3 4 5% 20%	2800
December	1st to 10th	Night	25% 9% 3 7% 5% 4 5% 5% 6% 6% 26%	4560
	11th to 20th	Night	34% 15% 4 9% 5% 4 4 3 4 4 12%	5180
	21st to end	Night	33% 19% 5% 9% 5% 4 4 3 4 4 11%	7343

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	79% 5% 6% 7% 2	3239
	11th to 20th	Day	54% 8% 12% 18% 7% 1	3640
	21st to end	Day	62% 4 8% 16% 10%	4078
February	1st to 10th	Day	70% 5% 10% 9% 4 1	1800
	11th to 20th	Day	69% 4 9% 9% 8%	1764
	21st to end	Day	69% 5% 9% 12% 5%	1555
March	1st to 10th	Day	64% 6% 11% 14% 5%	4601
	11th to 20th	Day	61% 7% 12% 18% 2	5000
	21st to end	Day	69% 8% 13% 9% 1	5693
April	1st to 10th	Day	68% 7% 14% 11% 1	2400
	11th to 20th	Day	62% 4 13% 14% 6% 1	2392
	21st to end	Day	63% 6% 12% 14% 4 1	2659
May	1st to 10th	Day	46% 5% 15% 24% 10%	6320
	11th to 20th	Day	55% 9% 18% 16% 3	6280
	21st to end	Day	54% 7% 24% 15% 1	6904
June	1st to 10th	Day	56% 5% 17% 16% 5% 1	2850
	11th to 20th	Day	62% 6% 15% 13% 4	3000
	21st to end	Day	46% 7% 20% 20% 7% 1	3000
July	1st to 10th	Day	26% 5% 18% 22% 19% 9%	6706
	11th to 20th	Day	19% 4 15% 31% 28% 2	6407
	21st to end	Day	28% 6% 23% 31% 10% 1	6681
August	1st to 10th	Day	23% 5% 14% 40% 17% 1	4472
	11th to 20th	Day	41% 8% 21% 22% 7% 2	4195
	21st to end	Day	43% 5% 13% 27% 11% 1	4457
September	1st to 10th	Day	38% 4 15% 26% 12% 4	2400
	11th to 20th	Day	47% 5% 13% 20% 14% 2	2400
	21st to end	Day	47% 6% 18% 18% 9% 2	2118
October	1st to 10th	Day	44% 6% 17% 27% 5%	3280
	11th to 20th	Day	63% 4 10% 15% 7%	3261
	21st to end	Day	66% 6% 10% 9% 7% 1	3337
November	1st to 10th	Day	70% 5% 11% 11% 4	1630
	11th to 20th	Day	68% 5% 9% 10% 7% 2	1600
	21st to end	Day	69% 5% 7% 11% 8% 1	1600
December	1st to 10th	Day	56% 6% 9% 17% 11%	2620
	11th to 20th	Day	74% 6% 10% 8% 1	3028
	21st to end	Day	76% 6% 9% 7% 2	3590
January	1st to 10th	Night	78% 6% 6% 7% 3	6701
	11th to 20th	Night	51% 6% 14% 21% 7% 1	6320
	21st to end	Night	60% 4 10% 17% 9%	6866
February	1st to 10th	Night	69% 5% 10% 10% 5% 1	2600
	11th to 20th	Night	66% 5% 10% 11% 7% 1	2534
	21st to end	Night	66% 5% 9% 15% 4 1	2074
March	1st to 10th	Night	61% 9% 12% 13% 5%	5290
	11th to 20th	Night	59% 6% 16% 17% 2 1	5000
	21st to end	Night	72% 8% 12% 8%	5281
April	1st to 10th	Night	66% 7% 13% 12% 2	2000
	11th to 20th	Night	56% 6% 15% 14% 8% 1	1954
	21st to end	Night	62% 6% 12% 14% 5% 1	1740
May	1st to 10th	Night	48% 4 12% 28% 8%	3637
	11th to 20th	Night	56% 6% 18% 17% 3	3616
	21st to end	Night	59% 9% 19% 12% 1	3972
June	1st to 10th	Night	53% 6% 16% 19% 6%	1545
	11th to 20th	Night	61% 7% 13% 16% 3	1400
	21st to end	Night	44% 5% 21% 22% 5% 3	1400
July	1st to 10th	Night	31% 4 15% 20% 18% 12%	3145
	11th to 20th	Night	16% 3 16% 29% 34% 1	3426
	21st to end	Night	26% 5% 21% 32% 14% 2	3857
August	1st to 10th	Night	20% 4 14% 40% 20% 2	2651
	11th to 20th	Night	38% 6% 22% 24% 9% 2	2929
	21st to end	Night	41% 5% 14% 30% 9% 1	3443
September	1st to 10th	Night	35% 5% 17% 28% 13% 4	2000
	11th to 20th	Night	46% 6% 11% 21% 13% 4	2000
	21st to end	Night	48% 5% 14% 22% 10% 2	2278
October	1st to 10th	Night	44% 8% 18% 26% 4	3920
	11th to 20th	Night	57% 5% 13% 18% 7%	3897
	21st to end	Night	67% 4 11% 11% 6%	4466
November	1st to 10th	Night	67% 5% 13% 11% 3 1	2708
	11th to 20th	Night	69% 4 10% 11% 4 2	2800
	21st to end	Night	68% 5% 8% 11% 8% 1	2800
December	1st to 10th	Night	56% 6% 11% 18% 9% 1	4560
	11th to 20th	Night	75% 5% 9% 9% 2	5176
	21st to end	Night	76% 8% 7% 7% 2	7343

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	42% 6% 13% 16% 17% 8%	200
	11th to 20th	Day	35% 7% 13% 24% 15% 8%	200
	21st to end	Day	43% 5% 14% 18% 13% 7%	255
February	1st to 10th	Day	47% 6% 17% 19% 9% 3	400
	11th to 20th	Day	45% 7% 20% 16% 10% 2	395
	21st to end	Day	42% 6% 19% 22% 9% 2	367
March	1st to 10th	Day	34% 8% 25% 17% 13% 2	500
	11th to 20th	Day	33% 9% 18% 21% 14% 5%	500
	21st to end	Day	38% 9% 23% 19% 10% 1	550
April	1st to 10th	Day	34% 9% 24% 19% 10% 4	500
	11th to 20th	Day	28% 11% 25% 20% 10% 6%	492
	21st to end	Day	26% 11% 23% 25% 12% 4	500
May	1st to 10th	Day	20% 7% 22% 31% 16% 3	503
	11th to 20th	Day	29% 6% 17% 24% 17% 7%	528
	21st to end	Day	30% 5% 15% 27% 16% 7%	573
June	1st to 10th	Day	24% 4 23% 29% 15% 5%	580
	11th to 20th	Day	24% 7% 22% 29% 15% 3	730
	21st to end	Day	15% 5% 18% 34% 22% 5%	730
July	1st to 10th	Day	12% 3 19% 36% 26% 4	721
	11th to 20th	Day	8% 5% 18% 35% 26% 8%	595
	21st to end	Day	10% 2 14% 38% 27% 9%	575
August	1st to 10th	Day	12% 3 13% 35% 25% 12%	527
	11th to 20th	Day	9% 3 13% 38% 26% 11%	517
	21st to end	Day	15% 3 20% 34% 18% 10%	583
September	1st to 10th	Day	17% 5% 18% 33% 20% 7%	530
	11th to 20th	Day	20% 6% 17% 28% 24% 6%	530
	21st to end	Day	22% 5% 21% 30% 15% 6%	530
October	1st to 10th	Day	25% 7% 17% 25% 18% 8%	530
	11th to 20th	Day	33% 6% 16% 21% 17% 6%	530
	21st to end	Day	36% 8% 17% 19% 14% 5%	450
November	1st to 10th	Day	45% 6% 15% 19% 11% 3	229
	11th to 20th	Day	46% 5% 14% 11% 14% 11%	200
	21st to end	Day	35% 9% 18% 17% 9% 14%	200
December	1st to 10th	Day	43% 5% 9% 12% 17% 16%	200
	11th to 20th	Day	47% 5% 16% 19% 13% 2	200
	21st to end	Day	46% 5% 13% 20% 13% 3	220
January	1st to 10th	Night	47% 7% 12% 17% 13% 3	886
	11th to 20th	Night	40% 5% 11% 21% 19% 3	800
	21st to end	Night	38% 6% 11% 26% 16% 4	880
February	1st to 10th	Night	40% 5% 13% 28% 11% 3	800
	11th to 20th	Night	41% 4 10% 27% 17% 2	784
	21st to end	Night	38% 5% 14% 28% 13% 2	660
March	1st to 10th	Night	35% 6% 15% 27% 15% 2	800
	11th to 20th	Night	37% 8% 13% 25% 15% 3	800
	21st to end	Night	40% 7% 15% 26% 11% 1	880
April	1st to 10th	Night	34% 6% 14% 27% 17% 2	800
	11th to 20th	Night	28% 7% 16% 30% 16% 3	766
	21st to end	Night	31% 5% 15% 30% 15% 4	540
May	1st to 10th	Night	24% 5% 12% 35% 22% 2	400
	11th to 20th	Night	24% 4 11% 29% 27% 5%	394
	21st to end	Night	27% 3 11% 27% 25% 6%	433
June	1st to 10th	Night	26% 3 16% 33% 19% 4	400
	11th to 20th	Night	30% 5% 14% 29% 19% 3	400
	21st to end	Night	18% 4 13% 33% 27% 5%	400
July	1st to 10th	Night	15% 3 11% 32% 35% 4	390
	11th to 20th	Night	10% 2 13% 34% 37% 4	388
	21st to end	Night	10% 2 9% 32% 41% 5%	438
August	1st to 10th	Night	11% 1 4 31% 43% 10%	422
	11th to 20th	Night	11% 3 5% 25% 43% 13%	594
	21st to end	Night	16% 2 10% 30% 34% 7%	780
September	1st to 10th	Night	13% 3 8% 32% 37% 7%	800
	11th to 20th	Night	21% 4 7% 25% 32% 11%	800
	21st to end	Night	24% 6% 10% 27% 27% 7%	800
October	1st to 10th	Night	24% 3 10% 29% 27% 7%	800
	11th to 20th	Night	33% 4 13% 24% 20% 6%	800
	21st to end	Night	37% 5% 14% 24% 16% 4	880
November	1st to 10th	Night	42% 6% 14% 24% 13% 1	788
	11th to 20th	Night	43% 5% 14% 21% 14% 3	800
	21st to end	Night	41% 5% 11% 23% 15% 5%	800
December	1st to 10th	Night	36% 5% 10% 25% 20% 5%	800
	11th to 20th	Night	47% 5% 12% 23% 12% 2	800
	21st to end	Night	50% 6% 13% 20% 10% 1	965

Page generated at 2025-10-30 23:02:38. Version ac07

Copyright © 2025 Attilla Danko.