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 2024-01-17.

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	5% 6% 3 6% 7% 4 6% 7% 8% 9% 38%	74620
	Night	3 7% 4 8% 7% 4 6% 6% 7% 9% 40%	75204
Transparency	Day	42% 13% 17% 18% 8% 1	74312
	Night	42% 14% 17% 18% 8% 1	75029
Seeing	Day	11% 17% 30% 27% 13% 2	14018
	Night	8% 9% 29% 36% 16% 2	23279

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	2 8% 5% 12% 10% 5% 9% 9% 9% 10% 20%	1343
	11th to 20th	Day	2 5% 5% 9% 9% 6% 8% 10% 8% 12% 27%	1488
	21st to end	Day	1 7% 5% 11% 11% 7% 8% 9% 9% 10% 24%	1584
February	1st to 10th	Day	3 6% 3 9% 10% 5% 7% 8% 10% 12% 26%	1440
	11th to 20th	Day	1 7% 3 9% 10% 5% 6% 7% 10% 11% 30%	1596
	21st to end	Day	2 6% 5% 8% 8% 5% 6% 10% 11% 10% 29%	1480
March	1st to 10th	Day	2 6% 5% 7% 7% 5% 7% 9% 9% 11% 32%	1800
	11th to 20th	Day	2 7% 4 7% 7% 6% 8% 7% 8% 11% 34%	1897
	21st to end	Day	5% 8% 4 8% 7% 5% 7% 8% 10% 10% 28%	2283
April	1st to 10th	Day	5% 7% 4 8% 9% 5% 8% 8% 8% 10% 29%	2160
	11th to 20th	Day	5% 8% 5% 8% 8% 4 6% 7% 8% 8% 32%	2278
	21st to end	Day	7% 9% 4 7% 7% 5% 5% 7% 7% 9% 34%	2340
May	1st to 10th	Day	7% 8% 4 7% 6% 4 6% 7% 7% 9% 35%	2456
	11th to 20th	Day	9% 6% 2 6% 7% 4 6% 6% 7% 9% 38%	2534
	21st to end	Day	15% 9% 3 6% 6% 4 6% 7% 7% 8% 29%	2917
June	1st to 10th	Day	12% 7% 3 7% 7% 4 7% 6% 7% 9% 31%	2700
	11th to 20th	Day	9% 7% 3 6% 8% 5% 8% 8% 8% 10% 27%	2700
	21st to end	Day	5% 3 2 5% 6% 4 6% 7% 9% 9% 44%	2700
July	1st to 10th	Day	3 3 1 4 6% 4 6% 4 7% 8% 53%	2700
	11th to 20th	Day	4 2 1 3 4 4 5% 6% 6% 7% 58%	2642
	21st to end	Day	1 1 1 2 3 3 5% 5% 6% 8% 65%	2741
August	1st to 10th	Day	3 2 1 3 4 3 5% 6% 8% 10% 55%	2507
	11th to 20th	Day	3 2 1 3 3 2 4 5% 7% 9% 60%	2356
	21st to end	Day	4 3 1 3 4 3 5% 5% 7% 9% 55%	2547
September	1st to 10th	Day	4 3 2 4 4 3 5% 6% 6% 9% 55%	2160
	11th to 20th	Day	7% 4 2 3 6% 4 4 6% 7% 8% 50%	2016
	21st to end	Day	6% 4 1 4 5% 3 6% 6% 8% 10% 47%	1980
October	1st to 10th	Day	6% 5% 3 5% 5% 4 6% 6% 7% 10% 44%	1961
	11th to 20th	Day	3 4 2 4 7% 5% 6% 7% 8% 9% 46%	1620
	21st to end	Day	4 8% 3 6% 7% 4 6% 7% 7% 10% 39%	1782
November	1st to 10th	Day	5% 9% 4 9% 9% 4 9% 6% 8% 10% 29%	1674
	11th to 20th	Day	1 6% 3 7% 9% 5% 7% 9% 7% 9% 37%	1454
	21st to end	Day	1 8% 5% 9% 8% 5% 6% 8% 8% 12% 31%	1330
December	1st to 10th	Day	2 8% 5% 11% 8% 7% 8% 8% 10% 9% 24%	1330
	11th to 20th	Day	2 9% 7% 13% 11% 7% 8% 10% 9% 7% 18%	1682
	21st to end	Day	2 8% 5% 14% 12% 8% 9% 7% 8% 7% 19%	2442
January	1st to 10th	Night	1 10% 7% 13% 9% 6% 8% 8% 9% 9% 20%	2836
	11th to 20th	Night	1 10% 5% 11% 9% 6% 8% 7% 7% 8% 28%	2611
	21st to end	Night	2 9% 7% 11% 9% 6% 6% 7% 8% 9% 26%	2772
February	1st to 10th	Night	2 10% 4 8% 10% 4 6% 7% 9% 11% 29%	2520
	11th to 20th	Night	2 7% 4 7% 8% 4 6% 8% 8% 10% 36%	2249
	21st to end	Night	2 8% 4 8% 7% 5% 6% 7% 8% 9% 36%	1776
March	1st to 10th	Night	3 6% 3 7% 7% 4 6% 7% 8% 10% 39%	2160
	11th to 20th	Night	2 7% 2 5% 6% 4 6% 7% 9% 12% 42%	2062
	21st to end	Night	4 5% 3 6% 5% 3 7% 6% 9% 11% 39%	2070
April	1st to 10th	Night	4 6% 3 5% 5% 4 5% 6% 8% 11% 44%	1800
	11th to 20th	Night	4 7% 4 6% 5% 4 5% 6% 8% 8% 41%	1633
	21st to end	Night	5% 8% 3 6% 5% 3 6% 5% 7% 8% 44%	1620
May	1st to 10th	Night	5% 7% 3 6% 5% 4 7% 6% 7% 9% 42%	1503
	11th to 20th	Night	7% 5% 3 5% 6% 4 5% 5% 6% 6% 48%	1359
	21st to end	Night	11% 9% 4 6% 5% 4 4 6% 5% 8% 38%	1358
June	1st to 10th	Night	8% 4 2 6% 5% 4 6% 5% 7% 12% 41%	1260
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 7% 8% 8% 40%	1260
	21st to end	Night	3 2 2 4 4 3 5% 6% 7% 10% 54%	1260
July	1st to 10th	Night	2 1 1 3 3 2 5% 5% 5% 9% 64%	1260
	11th to 20th	Night	2 2 1 2 3 3 4 4 6% 7% 68%	1315
	21st to end	Night	1 1 1 2 2 2 3 4 4 7% 73%	1576
August	1st to 10th	Night	3 3 1 4 3 2 3 4 5% 8% 65%	1432
	11th to 20th	Night	3 2 1 3 2 2 3 4 5% 9% 67%	1543
	21st to end	Night	3 3 1 3 3 2 3 4 5% 8% 64%	1807
September	1st to 10th	Night	4 3 2 3 3 1 3 5% 5% 7% 64%	1800
	11th to 20th	Night	5% 3 2 5% 4 3 4 5% 5% 7% 57%	1944
	21st to end	Night	4 4 1 4 4 2 4 5% 6% 10% 56%	1980
October	1st to 10th	Night	5% 5% 2 4 5% 3 6% 6% 6% 7% 50%	1995
	11th to 20th	Night	2 4 2 5% 4 3 5% 7% 7% 10% 50%	2340
	21st to end	Night	4 6% 2 6% 5% 4 6% 6% 8% 9% 43%	2574
November	1st to 10th	Night	3 9% 4 6% 6% 4 6% 8% 8% 11% 35%	2405
	11th to 20th	Night	2 7% 5% 8% 6% 4 7% 7% 9% 10% 36%	2700
	21st to end	Night	2 10% 6% 9% 9% 4 6% 8% 8% 9% 29%	2850
December	1st to 10th	Night	2 10% 6% 11% 9% 5% 7% 7% 9% 7% 27%	2850
	11th to 20th	Night	1 10% 6% 17% 13% 6% 7% 8% 8% 7% 16%	3510
	21st to end	Night	2 9% 6% 15% 11% 6% 8% 9% 7% 7% 20%	5214

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	60% 16% 14% 6% 3 1	1343
	11th to 20th	Day	51% 19% 17% 9% 2 1	1488
	21st to end	Day	55% 18% 14% 9% 4	1584
February	1st to 10th	Day	51% 17% 16% 10% 5% 1	1440
	11th to 20th	Day	48% 18% 18% 12% 3	1605
	21st to end	Day	49% 16% 18% 12% 5%	1480
March	1st to 10th	Day	49% 15% 21% 12% 3	1800
	11th to 20th	Day	48% 14% 17% 17% 4	1898
	21st to end	Day	54% 16% 17% 10% 3	2283
April	1st to 10th	Day	51% 14% 19% 13% 3	2160
	11th to 20th	Day	48% 13% 18% 17% 4	2278
	21st to end	Day	48% 13% 16% 18% 4 1	2340
May	1st to 10th	Day	45% 13% 20% 16% 6% 1	2456
	11th to 20th	Day	42% 13% 18% 19% 6%	2537
	21st to end	Day	51% 12% 18% 14% 4	2918
June	1st to 10th	Day	51% 10% 15% 16% 6% 1	2700
	11th to 20th	Day	50% 13% 17% 14% 5% 1	2700
	21st to end	Day	33% 12% 19% 23% 11% 2	2700
July	1st to 10th	Day	26% 9% 17% 27% 16% 4	2626
	11th to 20th	Day	24% 9% 14% 22% 23% 7%	2561
	21st to end	Day	17% 7% 13% 29% 27% 7%	2589
August	1st to 10th	Day	22% 10% 19% 25% 18% 6%	2490
	11th to 20th	Day	20% 8% 19% 31% 19% 3	2356
	21st to end	Day	25% 11% 17% 29% 16% 2	2547
September	1st to 10th	Day	27% 9% 15% 30% 16% 3	2160
	11th to 20th	Day	32% 11% 17% 27% 12% 1	2016
	21st to end	Day	34% 12% 23% 21% 9% 1	1980
October	1st to 10th	Day	38% 14% 17% 26% 5%	1961
	11th to 20th	Day	37% 15% 21% 21% 6%	1620
	21st to end	Day	44% 15% 19% 17% 5%	1782
November	1st to 10th	Day	55% 16% 14% 11% 3	1674
	11th to 20th	Day	48% 19% 18% 10% 5%	1454
	21st to end	Day	48% 22% 20% 7% 2	1330
December	1st to 10th	Day	57% 15% 14% 11% 2	1330
	11th to 20th	Day	66% 14% 12% 7% 2	1684
	21st to end	Day	61% 17% 11% 6% 4 1	2442
January	1st to 10th	Night	61% 15% 12% 8% 3	2836
	11th to 20th	Night	57% 14% 15% 10% 3 1	2611
	21st to end	Night	55% 14% 15% 12% 5%	2772
February	1st to 10th	Night	53% 16% 15% 10% 6% 1	2520
	11th to 20th	Night	46% 15% 20% 14% 4	2250
	21st to end	Night	44% 13% 22% 15% 7% 1	1776
March	1st to 10th	Night	42% 17% 23% 13% 5%	2160
	11th to 20th	Night	39% 17% 18% 19% 6% 1	2062
	21st to end	Night	41% 16% 23% 15% 5%	2070
April	1st to 10th	Night	34% 17% 26% 16% 7%	1800
	11th to 20th	Night	39% 12% 21% 22% 6%	1633
	21st to end	Night	39% 12% 19% 24% 5% 1	1620
May	1st to 10th	Night	39% 13% 18% 20% 10%	1503
	11th to 20th	Night	35% 11% 21% 24% 8%	1359
	21st to end	Night	45% 12% 18% 21% 5%	1358
June	1st to 10th	Night	39% 13% 20% 19% 7% 2	1260
	11th to 20th	Night	40% 14% 17% 20% 7% 1	1260
	21st to end	Night	23% 12% 21% 27% 15% 2	1260
July	1st to 10th	Night	18% 9% 21% 30% 16% 6%	1225
	11th to 20th	Night	17% 8% 16% 25% 27% 7%	1273
	21st to end	Night	12% 7% 12% 32% 29% 8%	1488
August	1st to 10th	Night	19% 8% 16% 29% 21% 8%	1424
	11th to 20th	Night	15% 7% 16% 37% 23% 2	1543
	21st to end	Night	19% 9% 14% 33% 22% 3	1807
September	1st to 10th	Night	21% 7% 16% 33% 18% 5%	1800
	11th to 20th	Night	27% 8% 19% 30% 14% 2	1944
	21st to end	Night	27% 12% 22% 25% 13% 1	1980
October	1st to 10th	Night	32% 12% 19% 29% 8%	1995
	11th to 20th	Night	32% 13% 23% 24% 7% 1	2340
	21st to end	Night	41% 14% 21% 19% 4	2574
November	1st to 10th	Night	45% 20% 17% 13% 3 1	2405
	11th to 20th	Night	47% 16% 19% 12% 6%	2700
	21st to end	Night	54% 16% 17% 10% 3	2850
December	1st to 10th	Night	59% 12% 14% 12% 3	2850
	11th to 20th	Night	69% 15% 10% 6% 1	3507
	21st to end	Night	60% 17% 11% 7% 4	5214

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	9% 19% 27% 27% 13% 4	180
	11th to 20th	Day	6% 12% 34% 27% 18% 3	172
	21st to end	Day	3 10% 45% 28% 9% 5%	176
February	1st to 10th	Day	8% 18% 51% 19% 5%	160
	11th to 20th	Day	9% 14% 38% 28% 11%	287
	21st to end	Day	8% 16% 34% 31% 10% 2	346
March	1st to 10th	Day	11% 28% 32% 18% 10% 2	420
	11th to 20th	Day	11% 25% 25% 27% 11% 2	420
	21st to end	Day	12% 23% 35% 20% 7% 2	462
April	1st to 10th	Day	15% 23% 32% 22% 7% 1	420
	11th to 20th	Day	13% 26% 28% 21% 9% 2	412
	21st to end	Day	14% 23% 29% 21% 10% 3	420
May	1st to 10th	Day	13% 18% 34% 24% 8% 3	423
	11th to 20th	Day	16% 14% 27% 26% 16% 1	505
	21st to end	Day	18% 11% 24% 27% 18% 2	679
June	1st to 10th	Day	16% 18% 27% 27% 11% 2	630
	11th to 20th	Day	14% 19% 29% 24% 12% 1	630
	21st to end	Day	7% 11% 28% 34% 16% 3	630
July	1st to 10th	Day	6% 11% 27% 32% 23% 2	621
	11th to 20th	Day	7% 15% 30% 31% 16% 2	560
	21st to end	Day	4 14% 34% 31% 14% 3	487
August	1st to 10th	Day	5% 11% 32% 33% 16% 3	447
	11th to 20th	Day	5% 10% 29% 33% 19% 5%	437
	21st to end	Day	7% 15% 31% 31% 12% 4	495
September	1st to 10th	Day	8% 17% 32% 25% 14% 3	450
	11th to 20th	Day	10% 17% 27% 27% 15% 5%	450
	21st to end	Day	13% 23% 26% 24% 13% 1	450
October	1st to 10th	Day	13% 15% 31% 27% 13% 1	449
	11th to 20th	Day	12% 17% 31% 22% 15% 3	270
	21st to end	Day	19% 16% 32% 24% 6% 2	297
November	1st to 10th	Day	16% 19% 28% 30% 8%	270
	11th to 20th	Day	12% 20% 33% 27% 8% 1	225
	21st to end	Day	8% 13% 30% 32% 13% 3	180
December	1st to 10th	Day	9% 16% 23% 33% 12% 7%	180
	11th to 20th	Day	8% 16% 32% 31% 12% 3	180
	21st to end	Day	7% 9% 29% 40% 14% 2	198
January	1st to 10th	Night	9% 18% 29% 33% 9% 2	950
	11th to 20th	Night	7% 9% 38% 33% 11% 2	747
	21st to end	Night	7% 9% 33% 37% 11% 3	792
February	1st to 10th	Night	8% 11% 35% 35% 11% 1	720
	11th to 20th	Night	7% 10% 32% 38% 13% 1	704
	21st to end	Night	7% 8% 32% 34% 16% 3	592
March	1st to 10th	Night	8% 8% 36% 33% 14% 1	720
	11th to 20th	Night	8% 13% 32% 30% 15% 2	720
	21st to end	Night	8% 9% 34% 33% 14% 2	792
April	1st to 10th	Night	8% 9% 30% 35% 18% 1	720
	11th to 20th	Night	8% 12% 35% 33% 12%	565
	21st to end	Night	9% 11% 34% 32% 14%	540
May	1st to 10th	Night	9% 10% 26% 36% 18% 1	423
	11th to 20th	Night	9% 7% 22% 34% 26% 2	354
	21st to end	Night	15% 8% 21% 30% 25% 1	389
June	1st to 10th	Night	13% 8% 21% 39% 19% 1	360
	11th to 20th	Night	11% 7% 28% 36% 18%	360
	21st to end	Night	3 7% 21% 44% 23% 1	360
July	1st to 10th	Night	3 3 15% 48% 29% 2	350
	11th to 20th	Night	2 3 23% 50% 22% 1	348
	21st to end	Night	2 3 20% 43% 30% 1	394
August	1st to 10th	Night	5% 3 13% 42% 35% 2	358
	11th to 20th	Night	4 3 16% 41% 33% 2	475
	21st to end	Night	5% 5% 21% 43% 25% 2	619
September	1st to 10th	Night	6% 3 26% 39% 22% 4	720
	11th to 20th	Night	7% 5% 19% 35% 29% 5%	720
	21st to end	Night	8% 8% 24% 38% 20% 3	720
October	1st to 10th	Night	8% 8% 26% 38% 20% 1	720
	11th to 20th	Night	8% 9% 27% 36% 19% 2	720
	21st to end	Night	12% 11% 30% 34% 12% 2	792
November	1st to 10th	Night	12% 11% 31% 39% 6% 1	740
	11th to 20th	Night	9% 18% 31% 30% 11% 1	900
	21st to end	Night	9% 15% 30% 33% 12% 1	950
December	1st to 10th	Night	9% 9% 32% 35% 13% 2	950
	11th to 20th	Night	8% 10% 32% 39% 10% 1	950
	21st to end	Night	8% 11% 32% 37% 11%	1045

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