Ghost Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ghost Lake 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	8% 9% 3 8% 5% 5% 5% 4 6% 8% 39%	130657
	Night	6% 9% 4 8% 5% 5% 5% 4 6% 8% 40%	127105
Transparency	Day	39% 9% 16% 23% 11% 2	130363
	Night	38% 9% 17% 23% 11% 2	126976
Seeing	Day	14% 16% 32% 25% 10% 2	16475
	Night	11% 13% 35% 29% 11% 1	25412

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 5% 6% 11% 7% 8% 5% 5% 7% 8% 38%	3179
	11th to 20th	Day	1 9% 3 9% 7% 6% 6% 5% 6% 8% 39%	3311
	21st to end	Day	1 6% 3 8% 4 4 4 4 7% 11% 48%	4004
February	1st to 10th	Day	2 8% 3 9% 8% 6% 6% 6% 7% 10% 35%	1600
	11th to 20th	Day	1 9% 5% 10% 6% 7% 6% 6% 7% 9% 35%	1706
	21st to end	Day	2 9% 5% 14% 7% 6% 5% 3 6% 9% 35%	1638
March	1st to 10th	Day	2 8% 4 10% 5% 4 5% 4 7% 12% 40%	4518
	11th to 20th	Day	3 7% 4 9% 5% 4 5% 4 6% 8% 45%	4883
	21st to end	Day	5% 20% 8% 12% 5% 4 5% 4 5% 7% 25%	5655
April	1st to 10th	Day	6% 13% 4 11% 7% 5% 6% 4 7% 8% 29%	2428
	11th to 20th	Day	8% 17% 6% 10% 5% 4 4 4 6% 7% 29%	2570
	21st to end	Day	11% 14% 4 9% 6% 4 5% 4 5% 7% 31%	2600
May	1st to 10th	Day	12% 14% 3 6% 4 4 4 3 5% 6% 38%	6223
	11th to 20th	Day	12% 11% 4 10% 6% 5% 6% 5% 7% 8% 25%	6734
	21st to end	Day	13% 9% 3 9% 5% 5% 5% 4 5% 7% 36%	7405
June	1st to 10th	Day	16% 10% 3 8% 5% 5% 5% 5% 7% 7% 28%	3000
	11th to 20th	Day	21% 10% 3 8% 6% 6% 5% 4 6% 7% 24%	3000
	21st to end	Day	11% 5% 2 7% 5% 6% 5% 5% 7% 9% 38%	3000
July	1st to 10th	Day	9% 5% 2 5% 4 5% 6% 5% 7% 8% 44%	6780
	11th to 20th	Day	11% 5% 2 5% 4 4 6% 5% 8% 9% 41%	6780
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 10% 45%	7113
August	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 8% 40%	4541
	11th to 20th	Day	9% 4 2 6% 5% 6% 6% 5% 8% 10% 40%	4258
	21st to end	Day	9% 5% 2 5% 3 4 4 4 6% 8% 51%	4664
September	1st to 10th	Day	11% 5% 2 6% 4 3 4 4 6% 6% 47%	2457
	11th to 20th	Day	13% 7% 3 5% 3 4 5% 3 4 7% 46%	2192
	21st to end	Day	8% 8% 2 5% 4 4 4 5% 7% 7% 45%	2090
October	1st to 10th	Day	6% 8% 2 5% 4 3 5% 4 6% 9% 47%	3396
	11th to 20th	Day	5% 8% 3 7% 4 5% 5% 5% 8% 10% 41%	2852
	21st to end	Day	5% 8% 3 5% 5% 5% 5% 3 6% 8% 47%	3101
November	1st to 10th	Day	4 14% 4 7% 6% 4 5% 6% 6% 8% 36%	1669
	11th to 20th	Day	3 10% 3 6% 5% 5% 5% 5% 6% 7% 45%	1480
	21st to end	Day	2 10% 4 11% 7% 4 4 4 7% 8% 40%	1400
December	1st to 10th	Day	3 7% 3 10% 6% 5% 6% 5% 7% 10% 37%	2290
	11th to 20th	Day	2 7% 4 11% 6% 7% 6% 5% 7% 10% 37%	2642
	21st to end	Day	2 10% 5% 9% 5% 4 4 4 5% 7% 45%	3498
January	1st to 10th	Night	2 11% 6% 12% 7% 8% 6% 5% 7% 8% 29%	6766
	11th to 20th	Night	2 9% 4 11% 7% 5% 5% 5% 7% 9% 36%	6629
	21st to end	Night	2 8% 4 10% 5% 5% 5% 4 6% 8% 43%	6952
February	1st to 10th	Night	3 8% 4 10% 7% 6% 7% 5% 7% 8% 34%	2800
	11th to 20th	Night	2 10% 4 11% 7% 6% 6% 4 7% 8% 33%	2580
	21st to end	Night	1 8% 6% 11% 9% 7% 5% 3 5% 8% 37%	1990
March	1st to 10th	Night	2 8% 3 11% 6% 5% 6% 5% 7% 9% 36%	5376
	11th to 20th	Night	3 7% 4 9% 5% 5% 6% 5% 7% 9% 41%	5115
	21st to end	Night	7% 18% 6% 12% 5% 5% 5% 4 5% 8% 25%	5319
April	1st to 10th	Night	6% 15% 5% 10% 6% 4 5% 4 6% 7% 34%	1971
	11th to 20th	Night	7% 14% 5% 9% 5% 4 4 4 6% 7% 34%	1781
	21st to end	Night	9% 13% 4 8% 5% 4 5% 4 6% 8% 35%	1800
May	1st to 10th	Night	12% 14% 3 7% 4 3 4 2 4 6% 41%	3728
	11th to 20th	Night	13% 12% 4 9% 5% 5% 5% 4 6% 8% 30%	3159
	21st to end	Night	11% 7% 3 8% 5% 4 5% 4 5% 7% 41%	3470
June	1st to 10th	Night	16% 8% 3 5% 4 4 4 4 6% 8% 37%	1400
	11th to 20th	Night	20% 8% 3 6% 4 5% 5% 5% 6% 8% 30%	1400
	21st to end	Night	8% 4 2 7% 5% 6% 4 4 7% 9% 44%	1400
July	1st to 10th	Night	9% 6% 2 7% 4 4 5% 3 6% 7% 47%	3180
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 5% 8% 50%	3180
	21st to end	Night	10% 6% 2 5% 3 4 3 3 6% 8% 49%	3727
August	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 7% 9% 42%	2609
	11th to 20th	Night	10% 4 1 5% 4 4 4 5% 7% 11% 45%	2797
	21st to end	Night	9% 5% 1 4 3 3 4 3 6% 8% 54%	3245
September	1st to 10th	Night	10% 7% 2 5% 3 3 4 3 5% 8% 51%	1940
	11th to 20th	Night	13% 8% 2 5% 3 3 3 3 5% 6% 48%	2164
	21st to end	Night	9% 7% 3 5% 3 3 4 3 6% 7% 49%	2090
October	1st to 10th	Night	6% 8% 2 5% 3 4 4 3 5% 8% 51%	3580
	11th to 20th	Night	5% 7% 2 5% 3 4 4 4 7% 9% 50%	4076
	21st to end	Night	5% 10% 3 6% 4 4 4 4 6% 8% 47%	4457
November	1st to 10th	Night	4 12% 5% 8% 5% 4 5% 5% 7% 8% 38%	2610
	11th to 20th	Night	3 9% 3 8% 5% 4 5% 4 7% 9% 43%	2916
	21st to end	Night	2 10% 4 10% 7% 4 5% 3 7% 9% 39%	3000
December	1st to 10th	Night	2 8% 4 8% 6% 5% 6% 5% 7% 10% 39%	4890
	11th to 20th	Night	2 10% 4 11% 7% 6% 6% 5% 8% 9% 33%	5550
	21st to end	Night	3 11% 4 10% 6% 5% 4 4 6% 8% 39%	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	40% 9% 13% 23% 13% 2	3179
	11th to 20th	Day	39% 7% 15% 21% 16% 3	3332
	21st to end	Day	26% 10% 19% 30% 14% 1	4004
February	1st to 10th	Day	42% 9% 17% 19% 10% 4	1600
	11th to 20th	Day	42% 9% 17% 21% 9% 1	1715
	21st to end	Day	46% 7% 14% 19% 12% 2	1638
March	1st to 10th	Day	33% 8% 18% 22% 19% 1	4518
	11th to 20th	Day	34% 8% 17% 24% 16% 1	4884
	21st to end	Day	54% 11% 14% 13% 7% 1	5655
April	1st to 10th	Day	48% 11% 18% 16% 7%	2428
	11th to 20th	Day	50% 10% 14% 18% 8% 1	2570
	21st to end	Day	51% 8% 14% 19% 7% 1	2600
May	1st to 10th	Day	45% 7% 11% 23% 14%	6223
	11th to 20th	Day	48% 13% 14% 20% 5%	6738
	21st to end	Day	43% 10% 15% 23% 10%	7406
June	1st to 10th	Day	51% 11% 13% 15% 9% 1	3000
	11th to 20th	Day	55% 10% 12% 14% 7% 1	3000
	21st to end	Day	39% 11% 18% 22% 9% 2	3000
July	1st to 10th	Day	31% 11% 16% 20% 19% 3	6706
	11th to 20th	Day	33% 11% 19% 27% 9% 1	6699
	21st to end	Day	32% 8% 15% 25% 17% 4	6913
August	1st to 10th	Day	35% 12% 15% 30% 7% 1	4524
	11th to 20th	Day	33% 11% 24% 24% 7% 1	4299
	21st to end	Day	29% 8% 19% 32% 10% 2	4663
September	1st to 10th	Day	34% 8% 16% 24% 14% 4	2457
	11th to 20th	Day	37% 8% 15% 25% 13% 2	2192
	21st to end	Day	34% 9% 17% 25% 12% 3	2090
October	1st to 10th	Day	32% 7% 19% 32% 8% 1	3396
	11th to 20th	Day	34% 10% 21% 20% 12% 3	2852
	21st to end	Day	34% 7% 14% 32% 11% 1	3101
November	1st to 10th	Day	43% 11% 14% 20% 11% 1	1669
	11th to 20th	Day	37% 8% 19% 24% 11% 1	1480
	21st to end	Day	38% 10% 18% 23% 8% 3	1400
December	1st to 10th	Day	40% 9% 21% 20% 9% 1	2290
	11th to 20th	Day	37% 12% 21% 22% 8% 1	2644
	21st to end	Day	37% 7% 11% 25% 17% 3	3498
January	1st to 10th	Night	48% 11% 14% 19% 7% 2	6766
	11th to 20th	Night	41% 8% 14% 19% 13% 6%	6670
	21st to end	Night	34% 10% 18% 24% 13% 1	6952
February	1st to 10th	Night	42% 11% 13% 19% 12% 3	2800
	11th to 20th	Night	44% 9% 16% 17% 12% 2	2581
	21st to end	Night	43% 7% 12% 24% 12% 2	1990
March	1st to 10th	Night	37% 11% 18% 19% 14% 2	5376
	11th to 20th	Night	34% 10% 18% 24% 13% 1	5115
	21st to end	Night	52% 11% 15% 14% 8% 1	5319
April	1st to 10th	Night	47% 8% 14% 20% 10% 1	1971
	11th to 20th	Night	48% 8% 14% 20% 9% 1	1781
	21st to end	Night	44% 10% 16% 21% 7% 2	1800
May	1st to 10th	Night	44% 6% 12% 23% 14%	3725
	11th to 20th	Night	45% 12% 14% 25% 4	3159
	21st to end	Night	38% 11% 16% 27% 8% 1	3470
June	1st to 10th	Night	43% 11% 16% 22% 8% 1	1400
	11th to 20th	Night	47% 10% 15% 16% 9% 2	1400
	21st to end	Night	34% 10% 20% 26% 9% 1	1400
July	1st to 10th	Night	33% 10% 15% 26% 13% 3	3145
	11th to 20th	Night	30% 10% 22% 27% 8% 3	3138
	21st to end	Night	30% 10% 18% 25% 15% 3	3622
August	1st to 10th	Night	34% 10% 17% 27% 11% 1	2601
	11th to 20th	Night	28% 11% 29% 23% 8%	2822
	21st to end	Night	25% 8% 23% 32% 11% 1	3245
September	1st to 10th	Night	31% 7% 20% 27% 13% 3	1940
	11th to 20th	Night	35% 7% 16% 25% 12% 4	2164
	21st to end	Night	32% 7% 19% 25% 14% 3	2090
October	1st to 10th	Night	31% 7% 17% 27% 17% 1	3580
	11th to 20th	Night	28% 8% 22% 29% 12% 1	4076
	21st to end	Night	32% 8% 15% 35% 10%	4457
November	1st to 10th	Night	42% 9% 16% 21% 11% 1	2610
	11th to 20th	Night	36% 9% 18% 22% 14% 1	2916
	21st to end	Night	38% 9% 18% 21% 10% 4	3000
December	1st to 10th	Night	37% 9% 24% 21% 7% 2	4890
	11th to 20th	Night	42% 13% 20% 15% 10%	5547
	21st to end	Night	39% 8% 12% 23% 14% 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	7% 14% 40% 26% 10% 4	200
	11th to 20th	Day	7% 14% 43% 28% 7% 2	200
	21st to end	Day	10% 16% 47% 17% 7% 2	220
February	1st to 10th	Day	14% 19% 36% 21% 8% 4	200
	11th to 20th	Day	7% 20% 38% 26% 9%	276
	21st to end	Day	10% 22% 34% 25% 8% 1	402
March	1st to 10th	Day	11% 21% 35% 25% 5% 2	500
	11th to 20th	Day	10% 24% 36% 21% 6% 2	500
	21st to end	Day	15% 23% 35% 17% 8% 2	550
April	1st to 10th	Day	15% 23% 36% 19% 6% 1	500
	11th to 20th	Day	18% 23% 31% 19% 8% 1	492
	21st to end	Day	21% 25% 26% 20% 8% 1	500
May	1st to 10th	Day	21% 19% 28% 21% 10% 2	513
	11th to 20th	Day	18% 13% 29% 27% 12% 2	725
	21st to end	Day	21% 10% 25% 29% 13% 2	789
June	1st to 10th	Day	19% 17% 31% 22% 9% 2	730
	11th to 20th	Day	25% 15% 29% 20% 9% 2	730
	21st to end	Day	13% 13% 30% 26% 15% 3	730
July	1st to 10th	Day	14% 7% 30% 29% 18% 3	721
	11th to 20th	Day	14% 12% 27% 31% 14% 3	718
	21st to end	Day	10% 11% 31% 32% 13% 3	684
August	1st to 10th	Day	12% 9% 32% 32% 12% 4	527
	11th to 20th	Day	10% 9% 34% 31% 13% 2	517
	21st to end	Day	11% 12% 32% 27% 14% 4	583
September	1st to 10th	Day	13% 15% 32% 25% 12% 3	530
	11th to 20th	Day	16% 12% 32% 27% 9% 3	523
	21st to end	Day	16% 23% 30% 23% 6% 1	490
October	1st to 10th	Day	15% 18% 32% 22% 11% 2	435
	11th to 20th	Day	13% 18% 32% 27% 9% 1	300
	21st to end	Day	17% 19% 35% 21% 7% 1	330
November	1st to 10th	Day	16% 18% 39% 22% 4 1	300
	11th to 20th	Day	16% 19% 41% 20% 4 1	240
	21st to end	Day	9% 21% 35% 21% 11% 4	200
December	1st to 10th	Day	5% 20% 41% 28% 7% 1	200
	11th to 20th	Day	6% 19% 43% 26% 6% 1	200
	21st to end	Day	7% 14% 42% 26% 9% 2	220
January	1st to 10th	Night	10% 18% 46% 21% 5% 1	1000
	11th to 20th	Night	8% 17% 46% 23% 5% 1	955
	21st to end	Night	8% 23% 42% 21% 6%	880
February	1st to 10th	Night	8% 14% 40% 25% 13% 1	800
	11th to 20th	Night	8% 13% 41% 30% 8% 1	784
	21st to end	Night	6% 13% 40% 27% 13% 1	660
March	1st to 10th	Night	7% 13% 43% 27% 9% 1	800
	11th to 20th	Night	8% 14% 40% 30% 8% 1	800
	21st to end	Night	10% 12% 32% 35% 10% 1	880
April	1st to 10th	Night	12% 13% 35% 30% 9% 1	771
	11th to 20th	Night	16% 11% 37% 28% 8%	593
	21st to end	Night	16% 15% 31% 28% 10%	600
May	1st to 10th	Night	17% 10% 26% 36% 10% 1	459
	11th to 20th	Night	14% 6% 26% 37% 15% 1	394
	21st to end	Night	21% 7% 21% 31% 18% 1	433
June	1st to 10th	Night	20% 9% 26% 32% 12% 1	400
	11th to 20th	Night	22% 9% 26% 28% 16% 1	400
	21st to end	Night	10% 6% 20% 37% 26% 2	400
July	1st to 10th	Night	12% 5% 16% 41% 25% 1	390
	11th to 20th	Night	13% 3 22% 43% 17% 1	388
	21st to end	Night	6% 7% 16% 44% 25% 2	438
August	1st to 10th	Night	13% 3 14% 38% 30% 2	398
	11th to 20th	Night	11% 4 17% 42% 24% 2	526
	21st to end	Night	10% 6% 20% 44% 18% 1	660
September	1st to 10th	Night	16% 6% 22% 36% 18% 2	740
	11th to 20th	Night	18% 7% 19% 35% 19% 2	792
	21st to end	Night	14% 12% 32% 30% 12% 1	760
October	1st to 10th	Night	13% 10% 29% 35% 11% 1	760
	11th to 20th	Night	13% 10% 37% 31% 8% 1	760
	21st to end	Night	10% 18% 38% 27% 6% 1	836
November	1st to 10th	Night	14% 14% 44% 21% 6%	855
	11th to 20th	Night	9% 23% 45% 20% 3 1	1000
	21st to end	Night	8% 18% 43% 24% 6% 1	1000
December	1st to 10th	Night	7% 18% 43% 24% 7%	1000
	11th to 20th	Night	5% 18% 44% 26% 7%	1000
	21st to end	Night	6% 17% 44% 23% 8% 1	1100

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