Spruce Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spruce Ridge Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	4 6% 2 7% 5% 6% 6% 5% 8% 10% 42%	135289
	Night	3 6% 3 8% 6% 6% 6% 5% 7% 9% 42%	135596
Transparency	Day	37% 12% 19% 20% 10% 2	135005
	Night	39% 13% 20% 19% 8% 1	135458
Seeing	Day	10% 16% 29% 28% 14% 3	16928
	Night	8% 9% 28% 37% 16% 2	26192

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 7% 5% 14% 9% 9% 9% 7% 10% 10% 20%	4167
	11th to 20th	Day	1 6% 4 9% 6% 6% 7% 5% 7% 11% 38%	4716
	21st to end	Day	5% 3 8% 7% 8% 9% 8% 11% 9% 33%	5186
February	1st to 10th	Day	3 7% 3 10% 7% 7% 7% 7% 10% 12% 27%	1680
	11th to 20th	Day	1 8% 4 10% 8% 7% 6% 6% 10% 11% 30%	1870
	21st to end	Day	1 6% 4 9% 7% 7% 7% 7% 11% 10% 32%	1730
March	1st to 10th	Day	2 8% 3 6% 5% 5% 6% 6% 9% 14% 38%	4618
	11th to 20th	Day	1 5% 2 7% 5% 6% 6% 6% 9% 13% 41%	4918
	21st to end	Day	3 7% 3 9% 6% 7% 8% 6% 10% 12% 29%	5873
April	1st to 10th	Day	5% 8% 3 9% 7% 6% 8% 6% 8% 10% 31%	2520
	11th to 20th	Day	4 9% 4 9% 7% 6% 6% 6% 8% 8% 33%	2612
	21st to end	Day	6% 9% 3 7% 6% 6% 6% 5% 8% 8% 35%	2600
May	1st to 10th	Day	6% 9% 4 8% 5% 5% 5% 4 6% 8% 41%	6178
	11th to 20th	Day	8% 8% 3 8% 6% 7% 7% 6% 9% 10% 28%	6432
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 7% 9% 42%	7405
June	1st to 10th	Day	11% 8% 2 7% 6% 6% 6% 5% 7% 8% 34%	3000
	11th to 20th	Day	9% 7% 2 7% 7% 7% 7% 6% 9% 10% 29%	3000
	21st to end	Day	5% 3 1 6% 6% 6% 6% 6% 9% 9% 44%	3000
July	1st to 10th	Day	3 2 1 4 4 4 4 3 6% 7% 61%	6780
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 7% 66%	6624
	21st to end	Day	4 2 1 3 2 3 4 3 6% 9% 63%	6874
August	1st to 10th	Day	3 3 1 3 3 4 5% 5% 7% 10% 56%	4541
	11th to 20th	Day	5% 2 1 3 3 3 5% 4 7% 9% 59%	4254
	21st to end	Day	3 3 1 3 3 4 5% 4 7% 10% 57%	4609
September	1st to 10th	Day	4 3 2 4 4 4 5% 4 6% 8% 57%	2400
	11th to 20th	Day	7% 4 2 5% 4 5% 4 5% 6% 8% 50%	2240
	21st to end	Day	5% 4 1 4 4 4 6% 4 8% 10% 49%	2200
October	1st to 10th	Day	3 3 2 4 3 4 4 4 7% 10% 56%	3553
	11th to 20th	Day	2 3 1 4 4 5% 6% 5% 9% 11% 50%	2932
	21st to end	Day	4 7% 3 7% 4 5% 5% 5% 7% 9% 43%	3200
November	1st to 10th	Day	4 10% 3 10% 7% 7% 8% 5% 8% 9% 29%	1854
	11th to 20th	Day	2 7% 3 8% 7% 7% 7% 7% 7% 10% 35%	1606
	21st to end	Day	1 9% 3 11% 7% 6% 6% 6% 9% 12% 29%	1470
December	1st to 10th	Day	1 5% 3 10% 9% 10% 8% 7% 10% 9% 27%	2360
	11th to 20th	Day	1 8% 6% 16% 9% 9% 8% 8% 9% 8% 19%	2712
	21st to end	Day	1 8% 5% 16% 10% 10% 8% 5% 8% 10% 20%	3575
January	1st to 10th	Night	1 10% 6% 13% 8% 10% 9% 6% 8% 9% 21%	8768
	11th to 20th	Night	1 6% 3 9% 8% 8% 8% 5% 6% 11% 35%	8183
	21st to end	Night	1 7% 3 9% 6% 8% 9% 8% 9% 9% 31%	8994
February	1st to 10th	Night	2 11% 4 10% 9% 6% 6% 6% 9% 11% 28%	2940
	11th to 20th	Night	1 7% 4 9% 7% 6% 6% 6% 9% 10% 35%	2634
	21st to end	Night	1 8% 4 9% 6% 6% 6% 5% 9% 9% 37%	2076
March	1st to 10th	Night	1 6% 2 7% 5% 5% 5% 5% 9% 11% 45%	5496
	11th to 20th	Night	1 4 2 7% 6% 6% 7% 6% 9% 11% 41%	5301
	21st to end	Night	4 8% 3 7% 6% 6% 7% 6% 9% 11% 35%	5341
April	1st to 10th	Night	4 7% 2 6% 4 4 5% 4 8% 10% 45%	2100
	11th to 20th	Night	4 8% 3 7% 4 5% 6% 5% 8% 9% 41%	1872
	21st to end	Night	5% 8% 3 6% 5% 5% 5% 5% 7% 8% 44%	1800
May	1st to 10th	Night	6% 8% 3 7% 4 4 5% 3 6% 7% 45%	3787
	11th to 20th	Night	7% 6% 3 7% 5% 6% 6% 4 8% 9% 40%	3461
	21st to end	Night	6% 7% 2 5% 4 4 4 4 6% 8% 51%	3470
June	1st to 10th	Night	8% 4 2 6% 5% 5% 6% 4 6% 12% 43%	1400
	11th to 20th	Night	8% 6% 2 6% 5% 7% 7% 5% 8% 8% 40%	1400
	21st to end	Night	4 3 2 5% 4 4 5% 5% 7% 9% 54%	1400
July	1st to 10th	Night	4 2 1 4 2 2 3 3 4 7% 68%	3180
	11th to 20th	Night	4 2 1 3 3 3 3 2 5% 5% 69%	3333
	21st to end	Night	3 1 1 2 2 2 3 3 4 6% 72%	3969
August	1st to 10th	Night	4 3 1 4 2 2 3 3 5% 9% 64%	2609
	11th to 20th	Night	3 2 1 4 3 3 4 3 6% 10% 61%	2800
	21st to end	Night	3 3 1 3 3 3 3 4 7% 9% 62%	3300
September	1st to 10th	Night	4 3 1 4 2 2 4 3 5% 7% 65%	2000
	11th to 20th	Night	5% 4 2 5% 4 4 4 3 5% 8% 57%	2160
	21st to end	Night	4 4 1 4 3 3 4 4 6% 9% 57%	2200
October	1st to 10th	Night	3 3 1 3 3 3 3 4 5% 8% 64%	3642
	11th to 20th	Night	1 4 1 4 3 3 4 5% 6% 9% 59%	4216
	21st to end	Night	2 5% 2 6% 3 4 5% 4 8% 9% 52%	4600
November	1st to 10th	Night	3 9% 3 7% 5% 5% 6% 7% 8% 11% 35%	2665
	11th to 20th	Night	2 8% 4 9% 5% 7% 6% 6% 9% 9% 35%	2986
	21st to end	Night	1 13% 4 11% 7% 6% 7% 6% 8% 9% 28%	3150
December	1st to 10th	Night	1 6% 3 11% 10% 8% 6% 5% 8% 10% 30%	5040
	11th to 20th	Night	1 9% 5% 17% 13% 9% 8% 6% 8% 7% 19%	5700
	21st to end	Night	1 9% 5% 15% 9% 9% 8% 6% 8% 9% 20%	7623

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	56% 15% 15% 9% 5%	4167
	11th to 20th	Day	40% 15% 24% 15% 3 2	4740
	21st to end	Day	46% 18% 22% 12% 3	5187
February	1st to 10th	Day	49% 16% 16% 10% 7% 1	1680
	11th to 20th	Day	47% 18% 18% 12% 5%	1879
	21st to end	Day	46% 17% 19% 12% 6%	1730
March	1st to 10th	Day	37% 12% 24% 19% 8%	4618
	11th to 20th	Day	34% 15% 26% 18% 7%	4919
	21st to end	Day	45% 19% 18% 13% 5%	5873
April	1st to 10th	Day	47% 14% 21% 15% 3	2520
	11th to 20th	Day	46% 12% 19% 18% 4	2612
	21st to end	Day	46% 13% 17% 19% 5% 1	2600
May	1st to 10th	Day	41% 8% 13% 24% 13%	6178
	11th to 20th	Day	44% 13% 18% 21% 4	6435
	21st to end	Day	36% 12% 22% 24% 6%	7406
June	1st to 10th	Day	48% 10% 15% 17% 9% 1	3000
	11th to 20th	Day	47% 13% 17% 16% 6% 1	3000
	21st to end	Day	33% 12% 19% 23% 12% 2	3000
July	1st to 10th	Day	20% 7% 15% 26% 26% 5%	6706
	11th to 20th	Day	17% 7% 15% 25% 27% 9%	6543
	21st to end	Day	18% 6% 19% 28% 18% 11%	6681
August	1st to 10th	Day	21% 8% 22% 27% 17% 5%	4524
	11th to 20th	Day	20% 8% 25% 33% 12% 2	4295
	21st to end	Day	21% 9% 22% 33% 13% 1	4608
September	1st to 10th	Day	25% 9% 16% 32% 16% 3	2400
	11th to 20th	Day	32% 10% 17% 28% 11% 1	2240
	21st to end	Day	32% 12% 24% 22% 10% 1	2200
October	1st to 10th	Day	25% 9% 26% 34% 5%	3553
	11th to 20th	Day	28% 14% 26% 25% 7%	2932
	21st to end	Day	38% 13% 23% 19% 7%	3200
November	1st to 10th	Day	54% 16% 15% 11% 3	1854
	11th to 20th	Day	48% 19% 18% 11% 5%	1606
	21st to end	Day	49% 21% 21% 7% 2	1470
December	1st to 10th	Day	52% 19% 17% 11% 1	2360
	11th to 20th	Day	61% 14% 15% 7% 3	2714
	21st to end	Day	58% 15% 15% 8% 3 1	3575
January	1st to 10th	Night	57% 16% 16% 9% 3	8769
	11th to 20th	Night	43% 15% 21% 12% 6% 2	8221
	21st to end	Night	48% 17% 19% 14% 2	8994
February	1st to 10th	Night	52% 15% 15% 11% 6% 1	2940
	11th to 20th	Night	46% 14% 19% 14% 6%	2635
	21st to end	Night	42% 15% 22% 15% 6%	2076
March	1st to 10th	Night	34% 13% 30% 16% 8%	5496
	11th to 20th	Night	36% 13% 25% 19% 6%	5301
	21st to end	Night	40% 19% 22% 16% 3	5341
April	1st to 10th	Night	33% 16% 27% 16% 7%	2100
	11th to 20th	Night	38% 12% 21% 23% 6%	1872
	21st to end	Night	39% 12% 18% 25% 6% 1	1800
May	1st to 10th	Night	37% 9% 15% 26% 12% 1	3784
	11th to 20th	Night	36% 11% 21% 25% 7%	3461
	21st to end	Night	32% 9% 23% 28% 7%	3470
June	1st to 10th	Night	37% 12% 20% 20% 8% 2	1400
	11th to 20th	Night	39% 14% 18% 21% 8% 1	1400
	21st to end	Night	25% 11% 21% 27% 15% 2	1400
July	1st to 10th	Night	17% 7% 20% 25% 24% 7%	3145
	11th to 20th	Night	19% 6% 17% 21% 27% 11%	3291
	21st to end	Night	15% 5% 20% 29% 19% 12%	3857
August	1st to 10th	Night	18% 7% 20% 31% 19% 5%	2601
	11th to 20th	Night	18% 7% 29% 29% 16% 2	2825
	21st to end	Night	17% 7% 25% 35% 15% 2	3300
September	1st to 10th	Night	20% 7% 17% 34% 18% 5%	2000
	11th to 20th	Night	26% 9% 20% 30% 13% 2	2160
	21st to end	Night	25% 11% 22% 27% 13% 1	2200
October	1st to 10th	Night	20% 8% 26% 36% 10%	3642
	11th to 20th	Night	25% 11% 24% 32% 7% 1	4216
	21st to end	Night	30% 14% 25% 27% 3	4600
November	1st to 10th	Night	44% 20% 18% 13% 3 1	2665
	11th to 20th	Night	47% 16% 19% 12% 5%	2986
	21st to end	Night	55% 16% 17% 9% 3	3150
December	1st to 10th	Night	52% 19% 16% 11% 2	5040
	11th to 20th	Night	65% 14% 11% 8% 1	5697
	21st to end	Night	58% 17% 14% 9% 3	7623

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	7% 17% 25% 33% 14% 4	220
	11th to 20th	Day	5% 11% 31% 35% 16% 2	220
	21st to end	Day	3 8% 37% 34% 13% 4	242
February	1st to 10th	Day	7% 14% 42% 25% 10% 2	220
	11th to 20th	Day	7% 12% 34% 31% 14% 1	381
	21st to end	Day	10% 17% 32% 30% 9% 2	446
March	1st to 10th	Day	11% 25% 29% 21% 12% 2	540
	11th to 20th	Day	11% 23% 26% 27% 11% 2	540
	21st to end	Day	12% 23% 35% 21% 7% 2	594
April	1st to 10th	Day	14% 22% 29% 23% 8% 3	540
	11th to 20th	Day	13% 24% 28% 23% 10% 2	517
	21st to end	Day	13% 22% 28% 22% 11% 4	500
May	1st to 10th	Day	13% 19% 32% 23% 9% 4	503
	11th to 20th	Day	15% 14% 28% 26% 15% 2	592
	21st to end	Day	16% 12% 23% 26% 19% 3	789
June	1st to 10th	Day	15% 19% 27% 26% 11% 2	730
	11th to 20th	Day	13% 19% 28% 25% 13% 2	730
	21st to end	Day	7% 11% 28% 34% 16% 4	730
July	1st to 10th	Day	5% 10% 26% 33% 23% 4	721
	11th to 20th	Day	7% 13% 28% 30% 19% 3	653
	21st to end	Day	5% 13% 31% 30% 16% 5%	575
August	1st to 10th	Day	5% 9% 29% 32% 19% 5%	527
	11th to 20th	Day	4 9% 27% 34% 20% 6%	517
	21st to end	Day	7% 14% 28% 30% 15% 7%	583
September	1st to 10th	Day	7% 15% 28% 23% 17% 10%	530
	11th to 20th	Day	9% 16% 24% 26% 18% 8%	530
	21st to end	Day	12% 22% 26% 25% 14% 1	530
October	1st to 10th	Day	11% 14% 29% 29% 14% 3	528
	11th to 20th	Day	11% 15% 31% 22% 15% 5%	330
	21st to end	Day	17% 15% 31% 26% 8% 3	363
November	1st to 10th	Day	15% 17% 27% 30% 11%	330
	11th to 20th	Day	13% 19% 31% 27% 8% 1	275
	21st to end	Day	9% 13% 28% 35% 13% 3	220
December	1st to 10th	Day	8% 13% 23% 36% 14% 6%	220
	11th to 20th	Day	8% 15% 31% 30% 13% 2	220
	21st to end	Day	6% 8% 28% 42% 14% 2	242
January	1st to 10th	Night	8% 17% 29% 34% 10% 2	1050
	11th to 20th	Night	6% 9% 36% 37% 10% 2	841
	21st to end	Night	6% 9% 31% 41% 11% 2	924
February	1st to 10th	Night	7% 11% 34% 35% 11% 2	840
	11th to 20th	Night	6% 9% 30% 39% 14% 1	824
	21st to end	Night	8% 10% 32% 33% 15% 2	692
March	1st to 10th	Night	7% 9% 34% 34% 15% 1	840
	11th to 20th	Night	8% 14% 32% 31% 14% 2	840
	21st to end	Night	8% 10% 34% 34% 13% 2	924
April	1st to 10th	Night	8% 8% 30% 36% 17% 1	840
	11th to 20th	Night	8% 11% 35% 34% 12%	648
	21st to end	Night	9% 11% 33% 32% 15% 1	600
May	1st to 10th	Night	9% 10% 26% 36% 19% 1	469
	11th to 20th	Night	8% 7% 23% 34% 26% 2	394
	21st to end	Night	14% 8% 21% 30% 25% 2	433
June	1st to 10th	Night	13% 8% 21% 39% 19% 1	400
	11th to 20th	Night	10% 6% 28% 38% 18% 1	400
	21st to end	Night	4 6% 21% 44% 24% 1	400
July	1st to 10th	Night	4 3 15% 47% 30% 2	390
	11th to 20th	Night	3 2 21% 49% 23% 1	388
	21st to end	Night	3 3 19% 43% 31% 1	438
August	1st to 10th	Night	6% 3 11% 42% 36% 2	398
	11th to 20th	Night	4 3 16% 42% 33% 2	529
	21st to end	Night	5% 5% 19% 44% 25% 2	689
September	1st to 10th	Night	5% 3 23% 38% 26% 5%	800
	11th to 20th	Night	7% 5% 18% 36% 30% 5%	800
	21st to end	Night	7% 7% 25% 40% 19% 2	800
October	1st to 10th	Night	7% 8% 26% 39% 20% 1	800
	11th to 20th	Night	7% 10% 27% 36% 18% 2	800
	21st to end	Night	11% 11% 30% 35% 11% 2	880
November	1st to 10th	Night	11% 11% 30% 40% 7% 1	820
	11th to 20th	Night	9% 18% 31% 30% 11% 1	996
	21st to end	Night	9% 14% 30% 35% 12% 1	1050
December	1st to 10th	Night	8% 9% 31% 36% 13% 2	1050
	11th to 20th	Night	8% 11% 33% 37% 11% 1	1050
	21st to end	Night	8% 11% 32% 38% 11% 1	1155

Page generated at 2026-04-14 00:44:26. Version ac07

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