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 2024-12-24.

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% 3 7% 5% 6% 6% 5% 8% 9% 41%	105834
	Night	3 7% 3 8% 5% 5% 6% 5% 7% 9% 42%	102856
Transparency	Day	39% 12% 18% 19% 10% 2	105591
	Night	40% 13% 18% 19% 8% 2	102744
Seeing	Day	11% 17% 29% 27% 13% 3	15274
	Night	8% 9% 28% 36% 16% 2	24447

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 10% 8% 18% 9% 9% 9% 7% 8% 8% 15%	2233
	11th to 20th	Day	1 11% 5% 11% 6% 7% 7% 6% 7% 9% 29%	2516
	21st to end	Day	1 9% 5% 12% 9% 8% 8% 7% 10% 10% 21%	2794
February	1st to 10th	Day	3 8% 3 10% 8% 7% 7% 7% 11% 12% 25%	1520
	11th to 20th	Day	1 7% 3 10% 9% 7% 6% 6% 10% 10% 30%	1686
	21st to end	Day	1 7% 4 9% 7% 7% 7% 7% 11% 10% 29%	1570
March	1st to 10th	Day	1 8% 3 7% 5% 6% 7% 7% 10% 11% 35%	3138
	11th to 20th	Day	1 5% 2 6% 5% 5% 5% 5% 8% 12% 46%	3355
	21st to end	Day	4 9% 4 10% 7% 7% 8% 6% 10% 11% 25%	4007
April	1st to 10th	Day	5% 8% 3 9% 7% 6% 8% 6% 8% 10% 29%	2280
	11th to 20th	Day	4 9% 4 9% 7% 5% 6% 6% 8% 8% 33%	2407
	21st to end	Day	7% 10% 4 8% 6% 7% 5% 5% 8% 8% 33%	2470
May	1st to 10th	Day	8% 12% 5% 11% 6% 6% 6% 4 6% 7% 30%	4317
	11th to 20th	Day	6% 6% 2 7% 5% 7% 7% 5% 9% 11% 35%	4490
	21st to end	Day	10% 9% 3 7% 5% 6% 7% 6% 8% 9% 30%	5161
June	1st to 10th	Day	11% 8% 2 8% 6% 6% 6% 5% 7% 8% 31%	2850
	11th to 20th	Day	9% 8% 2 7% 7% 7% 7% 6% 8% 10% 28%	2850
	21st to end	Day	5% 3 1 6% 6% 6% 6% 5% 9% 9% 44%	2850
July	1st to 10th	Day	2 2 1 4 4 4 5% 3 6% 7% 61%	4740
	11th to 20th	Day	2 1 1 2 2 3 4 3 5% 6% 70%	4626
	21st to end	Day	1 2 1 3 3 4 4 3 7% 8% 65%	4790
August	1st to 10th	Day	3 2 1 3 3 4 5% 5% 7% 10% 57%	4401
	11th to 20th	Day	5% 2 1 3 3 3 5% 4 7% 9% 59%	4121
	21st to end	Day	3 3 1 3 3 4 5% 4 8% 10% 57%	4468
September	1st to 10th	Day	4 3 2 4 4 4 5% 4 6% 8% 56%	2280
	11th to 20th	Day	7% 4 2 5% 4 5% 4 5% 7% 8% 49%	2128
	21st to end	Day	5% 4 1 5% 4 4 6% 4 8% 10% 48%	2090
October	1st to 10th	Day	3 3 2 4 3 4 4 4 7% 10% 55%	3444
	11th to 20th	Day	2 3 1 4 4 5% 6% 5% 9% 11% 50%	2842
	21st to end	Day	4 7% 3 6% 4 5% 5% 5% 7% 9% 43%	3101
November	1st to 10th	Day	4 9% 3 10% 7% 7% 8% 5% 8% 10% 28%	1764
	11th to 20th	Day	2 7% 3 8% 8% 7% 7% 7% 7% 9% 36%	1530
	21st to end	Day	1 9% 3 10% 7% 7% 6% 6% 9% 12% 30%	1400
December	1st to 10th	Day	2 9% 5% 12% 7% 8% 7% 7% 10% 9% 24%	1400
	11th to 20th	Day	2 9% 6% 15% 9% 10% 8% 8% 9% 6% 18%	1752
	21st to end	Day	1 8% 5% 17% 10% 11% 8% 5% 8% 7% 19%	2463
January	1st to 10th	Night	1 14% 8% 17% 8% 9% 8% 6% 8% 7% 14%	4726
	11th to 20th	Night	1 11% 4 11% 7% 8% 9% 6% 7% 8% 29%	4383
	21st to end	Night	2 13% 6% 12% 7% 8% 8% 6% 9% 8% 22%	4862
February	1st to 10th	Night	2 12% 4 9% 9% 6% 6% 6% 9% 10% 28%	2660
	11th to 20th	Night	2 7% 4 8% 6% 6% 6% 6% 9% 9% 36%	2378
	21st to end	Night	1 9% 4 9% 6% 7% 6% 5% 9% 9% 36%	1884
March	1st to 10th	Night	1 5% 2 7% 5% 5% 5% 5% 10% 11% 42%	3736
	11th to 20th	Night	2 5% 2 6% 4 4 6% 5% 8% 11% 49%	3624
	21st to end	Night	5% 11% 4 8% 6% 5% 6% 5% 8% 9% 33%	3643
April	1st to 10th	Night	5% 7% 2 6% 4 5% 5% 4 8% 10% 44%	1900
	11th to 20th	Night	4 8% 4 7% 4 5% 6% 5% 8% 9% 40%	1724
	21st to end	Night	5% 9% 3 7% 5% 5% 5% 4 7% 8% 43%	1710
May	1st to 10th	Night	9% 11% 4 9% 5% 5% 6% 4 5% 7% 34%	2645
	11th to 20th	Night	5% 5% 2 5% 4 5% 5% 4 8% 9% 48%	2403
	21st to end	Night	8% 10% 3 7% 5% 5% 5% 5% 7% 9% 37%	2414
June	1st to 10th	Night	8% 4 2 7% 5% 5% 6% 3 7% 12% 41%	1330
	11th to 20th	Night	8% 6% 2 6% 5% 7% 7% 5% 8% 8% 39%	1330
	21st to end	Night	3 2 2 5% 4 4 5% 5% 7% 9% 55%	1330
July	1st to 10th	Night	2 2 1 5% 3 3 4 4 5% 8% 65%	2220
	11th to 20th	Night	2 2 1 2 2 3 3 3 5% 5% 75%	2331
	21st to end	Night	1 1 1 2 2 2 3 2 4 7% 77%	2762
August	1st to 10th	Night	4 3 1 4 2 2 3 3 5% 9% 64%	2529
	11th to 20th	Night	3 2 1 4 3 3 4 3 6% 10% 62%	2713
	21st to end	Night	3 2 1 3 3 3 3 4 7% 10% 61%	3199
September	1st to 10th	Night	4 3 1 4 3 2 4 3 5% 7% 64%	1900
	11th to 20th	Night	5% 4 1 5% 4 4 4 3 5% 8% 57%	2052
	21st to end	Night	4 4 1 5% 3 3 4 5% 6% 9% 56%	2090
October	1st to 10th	Night	3 3 1 3 3 3 3 4 5% 8% 64%	3531
	11th to 20th	Night	1 3 1 4 3 3 4 5% 6% 9% 59%	4086
	21st to end	Night	2 5% 2 6% 3 4 5% 4 8% 9% 52%	4457
November	1st to 10th	Night	3 9% 3 7% 5% 5% 7% 7% 8% 11% 35%	2535
	11th to 20th	Night	2 8% 4 9% 5% 6% 6% 6% 9% 9% 35%	2843
	21st to end	Night	1 14% 4 11% 7% 6% 7% 6% 8% 9% 28%	3000
December	1st to 10th	Night	2 11% 5% 13% 9% 6% 7% 5% 9% 7% 27%	3000
	11th to 20th	Night	1 11% 6% 19% 12% 8% 6% 7% 8% 7% 16%	3660
	21st to end	Night	1 10% 5% 17% 10% 8% 8% 7% 7% 7% 20%	5266

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	66% 15% 11% 5% 2	2233
	11th to 20th	Day	51% 12% 17% 11% 3 5%	2540
	21st to end	Day	60% 21% 12% 6% 2	2794
February	1st to 10th	Day	52% 17% 16% 9% 5% 1	1520
	11th to 20th	Day	48% 18% 18% 11% 5%	1695
	21st to end	Day	49% 16% 18% 12% 5%	1570
March	1st to 10th	Day	45% 13% 19% 15% 8%	3138
	11th to 20th	Day	34% 18% 29% 17% 3	3356
	21st to end	Day	52% 18% 16% 11% 3	4007
April	1st to 10th	Day	50% 14% 19% 13% 3	2280
	11th to 20th	Day	46% 13% 18% 19% 4	2407
	21st to end	Day	48% 13% 16% 17% 4 1	2470
May	1st to 10th	Day	53% 10% 15% 18% 4	4317
	11th to 20th	Day	39% 14% 18% 24% 5%	4493
	21st to end	Day	47% 15% 18% 15% 4	5162
June	1st to 10th	Day	50% 10% 15% 16% 7% 1	2850
	11th to 20th	Day	49% 13% 17% 15% 5% 1	2850
	21st to end	Day	33% 12% 19% 23% 12% 2	2850
July	1st to 10th	Day	20% 8% 14% 26% 26% 7%	4666
	11th to 20th	Day	15% 6% 9% 20% 38% 13%	4545
	21st to end	Day	15% 6% 12% 25% 25% 16%	4639
August	1st to 10th	Day	20% 8% 22% 27% 18% 5%	4384
	11th to 20th	Day	20% 8% 25% 33% 12% 2	4162
	21st to end	Day	21% 9% 23% 33% 12% 1	4467
September	1st to 10th	Day	26% 9% 15% 31% 16% 3	2280
	11th to 20th	Day	33% 11% 17% 27% 11% 1	2128
	21st to end	Day	33% 12% 23% 22% 9% 1	2090
October	1st to 10th	Day	26% 9% 26% 34% 5%	3444
	11th to 20th	Day	28% 14% 27% 25% 6%	2842
	21st to end	Day	38% 13% 22% 19% 7%	3101
November	1st to 10th	Day	55% 16% 15% 11% 3	1764
	11th to 20th	Day	48% 19% 17% 11% 5%	1530
	21st to end	Day	49% 21% 20% 7% 2	1400
December	1st to 10th	Day	57% 16% 14% 11% 2	1400
	11th to 20th	Day	65% 14% 12% 7% 2	1754
	21st to end	Day	61% 16% 11% 6% 4 1	2463
January	1st to 10th	Night	69% 13% 9% 6% 2	4726
	11th to 20th	Night	53% 12% 13% 14% 5% 3	4421
	21st to end	Night	62% 15% 12% 8% 3	4862
February	1st to 10th	Night	53% 15% 15% 10% 5% 1	2660
	11th to 20th	Night	46% 15% 20% 14% 6%	2379
	21st to end	Night	44% 14% 21% 15% 6%	1884
March	1st to 10th	Night	37% 15% 25% 16% 7%	3736
	11th to 20th	Night	32% 16% 31% 17% 3	3624
	21st to end	Night	47% 17% 20% 13% 3	3643
April	1st to 10th	Night	35% 16% 26% 15% 7%	1900
	11th to 20th	Night	39% 12% 21% 23% 6%	1724
	21st to end	Night	40% 12% 18% 24% 6% 1	1710
May	1st to 10th	Night	49% 11% 15% 20% 6%	2645
	11th to 20th	Night	30% 11% 21% 28% 9%	2403
	21st to end	Night	44% 12% 19% 19% 6%	2414
June	1st to 10th	Night	39% 12% 21% 19% 7% 2	1330
	11th to 20th	Night	40% 14% 18% 20% 7% 1	1330
	21st to end	Night	24% 12% 21% 27% 15% 2	1330
July	1st to 10th	Night	18% 8% 19% 25% 21% 9%	2185
	11th to 20th	Night	14% 5% 10% 21% 35% 16%	2289
	21st to end	Night	10% 5% 9% 30% 28% 17%	2674
August	1st to 10th	Night	18% 7% 20% 31% 19% 5%	2521
	11th to 20th	Night	18% 7% 29% 28% 16% 2	2738
	21st to end	Night	17% 7% 25% 35% 15% 2	3199
September	1st to 10th	Night	20% 7% 16% 33% 19% 5%	1900
	11th to 20th	Night	26% 9% 20% 29% 13% 2	2052
	21st to end	Night	26% 11% 22% 26% 13% 1	2090
October	1st to 10th	Night	20% 8% 25% 36% 10%	3531
	11th to 20th	Night	24% 12% 24% 33% 7%	4086
	21st to end	Night	31% 13% 25% 27% 3	4457
November	1st to 10th	Night	45% 20% 17% 14% 3 1	2535
	11th to 20th	Night	47% 16% 19% 13% 5%	2843
	21st to end	Night	55% 16% 16% 10% 3	3000
December	1st to 10th	Night	58% 13% 15% 12% 3	3000
	11th to 20th	Night	68% 15% 10% 6% 1	3657
	21st to end	Night	60% 18% 11% 7% 4	5266

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% 28% 17% 3	180
	21st to end	Day	4 10% 42% 29% 11% 5%	198
February	1st to 10th	Day	8% 16% 47% 21% 7% 1	180
	11th to 20th	Day	8% 14% 36% 30% 12% 1	317
	21st to end	Day	11% 17% 32% 29% 10% 2	382
March	1st to 10th	Day	11% 28% 30% 19% 11% 2	460
	11th to 20th	Day	10% 23% 24% 28% 12% 2	460
	21st to end	Day	12% 23% 35% 21% 8% 2	506
April	1st to 10th	Day	15% 24% 31% 22% 7% 1	460
	11th to 20th	Day	13% 26% 27% 22% 10% 2	452
	21st to end	Day	14% 23% 29% 22% 9% 3	460
May	1st to 10th	Day	14% 18% 33% 25% 8% 3	463
	11th to 20th	Day	15% 14% 28% 26% 15% 1	549
	21st to end	Day	17% 13% 24% 27% 17% 2	734
June	1st to 10th	Day	16% 20% 27% 26% 10% 2	680
	11th to 20th	Day	14% 19% 28% 24% 12% 2	680
	21st to end	Day	7% 11% 28% 34% 16% 3	680
July	1st to 10th	Day	5% 10% 26% 33% 23% 3	671
	11th to 20th	Day	7% 14% 28% 30% 18% 3	606
	21st to end	Day	4 13% 33% 31% 15% 4	531
August	1st to 10th	Day	5% 10% 31% 32% 18% 4	487
	11th to 20th	Day	5% 9% 27% 33% 21% 6%	477
	21st to end	Day	7% 15% 30% 30% 13% 5%	539
September	1st to 10th	Day	8% 16% 30% 24% 15% 7%	490
	11th to 20th	Day	9% 17% 26% 27% 16% 6%	490
	21st to end	Day	12% 23% 26% 25% 13% 1	490
October	1st to 10th	Day	12% 15% 30% 28% 14% 2	488
	11th to 20th	Day	11% 17% 30% 21% 16% 5%	300
	21st to end	Day	18% 16% 32% 25% 7% 3	330
November	1st to 10th	Day	14% 17% 27% 31% 10%	300
	11th to 20th	Day	13% 19% 32% 27% 8% 1	250
	21st to end	Day	9% 13% 28% 33% 14% 4	200
December	1st to 10th	Day	9% 14% 23% 35% 14% 7%	200
	11th to 20th	Day	7% 15% 30% 32% 15% 3	200
	21st to end	Day	6% 8% 28% 41% 14% 2	204
January	1st to 10th	Night	9% 18% 29% 33% 9% 2	950
	11th to 20th	Night	7% 10% 38% 33% 11% 2	761
	21st to end	Night	7% 9% 33% 38% 10% 3	836
February	1st to 10th	Night	8% 11% 34% 35% 11% 1	760
	11th to 20th	Night	6% 9% 32% 38% 13% 1	744
	21st to end	Night	9% 9% 31% 34% 15% 3	628
March	1st to 10th	Night	8% 9% 34% 34% 14% 1	760
	11th to 20th	Night	8% 13% 31% 31% 15% 2	760
	21st to end	Night	8% 9% 33% 34% 13% 2	836
April	1st to 10th	Night	9% 9% 31% 34% 17% 1	760
	11th to 20th	Night	8% 12% 35% 33% 12%	596
	21st to end	Night	9% 12% 34% 32% 14%	570
May	1st to 10th	Night	9% 10% 26% 36% 18% 1	446
	11th to 20th	Night	8% 7% 23% 34% 25% 2	374
	21st to end	Night	15% 9% 22% 29% 25% 1	411
June	1st to 10th	Night	14% 8% 21% 38% 18% 1	380
	11th to 20th	Night	11% 7% 28% 36% 18% 1	380
	21st to end	Night	4 7% 21% 44% 23% 1	380
July	1st to 10th	Night	3 3 15% 47% 30% 2	370
	11th to 20th	Night	2 2 21% 49% 23% 1	368
	21st to end	Night	2 3 20% 43% 31% 1	416
August	1st to 10th	Night	5% 3 12% 42% 35% 2	378
	11th to 20th	Night	4 2 17% 41% 33% 3	502
	21st to end	Night	5% 5% 20% 43% 24% 2	654
September	1st to 10th	Night	6% 3 24% 38% 24% 5%	760
	11th to 20th	Night	7% 5% 19% 36% 29% 4	760
	21st to end	Night	8% 8% 25% 38% 19% 2	760
October	1st to 10th	Night	7% 8% 26% 38% 20% 1	760
	11th to 20th	Night	7% 10% 26% 36% 19% 2	760
	21st to end	Night	11% 10% 29% 35% 12% 2	836
November	1st to 10th	Night	11% 11% 30% 40% 7% 1	780
	11th to 20th	Night	9% 18% 31% 30% 11% 1	948
	21st to end	Night	9% 14% 30% 34% 12% 1	1000
December	1st to 10th	Night	8% 9% 31% 36% 14% 2	1000
	11th to 20th	Night	8% 10% 32% 38% 11% 1	1000
	21st to end	Night	8% 11% 32% 38% 11%	1063

Page generated at 2024-12-21 02:13:50. Version ac07

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