Bow Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bow Island clear sky chart from 2005-11-06 to 2025-02-25.

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% 8% 3 7% 5% 4 5% 4 6% 8% 44%	112054
	Night	7% 9% 3 8% 5% 5% 5% 4 6% 8% 40%	115304
Transparency	Day	37% 6% 14% 27% 13% 2	111774
	Night	40% 6% 15% 25% 12% 2	115173
Seeing	Day	13% 11% 30% 29% 15% 3	15778
	Night	12% 11% 35% 30% 11% 1	25212

By Month

Percentage Cloud Cover SW (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	5% 16% 7% 13% 7% 5% 5% 4 6% 8% 26%	3179
	11th to 20th	Day	5% 11% 5% 9% 5% 5% 9% 6% 6% 7% 31%	3180
	21st to end	Day	3 9% 6% 9% 5% 4 4 4 5% 7% 45%	3912
February	1st to 10th	Day	8% 13% 5% 11% 7% 6% 5% 4 6% 7% 26%	1755
	11th to 20th	Day	7% 12% 5% 11% 7% 6% 6% 6% 7% 8% 26%	1755
	21st to end	Day	5% 11% 5% 11% 8% 6% 5% 5% 6% 7% 31%	1470
March	1st to 10th	Day	5% 11% 3 9% 7% 6% 5% 4 5% 9% 36%	3258
	11th to 20th	Day	8% 8% 3 6% 5% 4 4 3 6% 9% 44%	3490
	21st to end	Day	10% 16% 5% 12% 5% 6% 4 5% 6% 6% 26%	3825
April	1st to 10th	Day	9% 11% 3 8% 6% 6% 5% 5% 6% 8% 33%	2307
	11th to 20th	Day	10% 12% 4 9% 6% 5% 4 3 5% 7% 32%	2437
	21st to end	Day	11% 9% 3 8% 6% 4 5% 4 6% 7% 36%	2470
May	1st to 10th	Day	18% 12% 4 8% 4 5% 4 3 5% 6% 31%	4302
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 9% 38%	4535
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 7% 8% 34%	5161
June	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 7% 8% 39%	2850
	11th to 20th	Day	16% 6% 2 6% 5% 6% 6% 5% 7% 8% 33%	2850
	21st to end	Day	7% 4 2 4 4 5% 4 4 7% 9% 50%	2850
July	1st to 10th	Day	5% 3 2 5% 4 4 5% 4 6% 8% 56%	4740
	11th to 20th	Day	5% 3 1 3 3 3 4 3 5% 7% 64%	4740
	21st to end	Day	2 2 1 2 2 2 2 2 4 7% 74%	5138
August	1st to 10th	Day	6% 4 2 5% 3 3 4 4 6% 9% 54%	4205
	11th to 20th	Day	5% 3 1 5% 3 4 4 4 7% 9% 55%	4028
	21st to end	Day	6% 3 1 4 2 3 4 3 5% 8% 61%	4521
September	1st to 10th	Day	10% 5% 2 4 3 3 3 2 4 7% 56%	2353
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 8% 48%	2106
	21st to end	Day	9% 5% 2 6% 4 3 4 3 5% 8% 52%	2090
October	1st to 10th	Day	7% 5% 2 6% 3 4 5% 4 7% 9% 47%	3211
	11th to 20th	Day	4 5% 2 6% 5% 4 4 3 6% 7% 53%	3138
	21st to end	Day	7% 7% 3 7% 3 4 5% 4 5% 6% 50%	3080
November	1st to 10th	Day	12% 12% 4 8% 6% 5% 4 3 6% 6% 33%	1568
	11th to 20th	Day	6% 9% 4 10% 7% 6% 5% 4 5% 8% 38%	1600
	21st to end	Day	6% 12% 5% 9% 6% 6% 4 3 6% 9% 34%	1517
December	1st to 10th	Day	8% 14% 4 9% 6% 4 5% 3 5% 7% 33%	2290
	11th to 20th	Day	5% 11% 4 10% 8% 7% 6% 5% 6% 6% 31%	2645
	21st to end	Day	3 9% 4 8% 5% 5% 5% 4 6% 7% 44%	3498
January	1st to 10th	Night	4 13% 5% 12% 7% 5% 5% 4 6% 8% 32%	6766
	11th to 20th	Night	4 12% 5% 12% 6% 4 6% 4 5% 9% 34%	6780
	21st to end	Night	3 9% 4 10% 6% 5% 5% 4 6% 8% 40%	7074
February	1st to 10th	Night	8% 12% 5% 9% 7% 6% 6% 4 6% 6% 31%	2644
	11th to 20th	Night	5% 12% 4 9% 5% 7% 7% 6% 7% 8% 30%	2532
	21st to end	Night	5% 12% 4 10% 6% 5% 6% 4 6% 9% 34%	2075
March	1st to 10th	Night	4 10% 5% 12% 7% 5% 5% 5% 6% 7% 34%	3615
	11th to 20th	Night	8% 8% 3 6% 4 5% 5% 4 7% 8% 43%	3490
	21st to end	Night	10% 16% 5% 12% 5% 5% 5% 4 7% 7% 22%	3828
April	1st to 10th	Night	10% 11% 4 9% 6% 5% 4 3 6% 9% 34%	1869
	11th to 20th	Night	10% 11% 4 9% 5% 5% 5% 3 7% 8% 34%	1691
	21st to end	Night	12% 9% 3 7% 5% 5% 5% 4 8% 7% 35%	1710
May	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 5% 5% 33%	2653
	11th to 20th	Night	7% 6% 3 7% 6% 6% 6% 5% 8% 10% 38%	2360
	21st to end	Night	14% 8% 4 10% 7% 5% 7% 4 6% 8% 28%	2414
June	1st to 10th	Night	15% 7% 2 7% 4 4 5% 4 7% 9% 36%	1330
	11th to 20th	Night	13% 5% 2 6% 5% 4 7% 5% 8% 9% 37%	1330
	21st to end	Night	6% 3 1 4 4 4 5% 3 7% 8% 53%	1330
July	1st to 10th	Night	5% 3 1 4 4 4 4 4 6% 8% 57%	2220
	11th to 20th	Night	5% 4 1 5% 3 3 4 4 6% 10% 56%	2220
	21st to end	Night	2 1 1 3 3 2 3 3 6% 8% 67%	2413
August	1st to 10th	Night	8% 4 2 4 3 4 4 4 7% 8% 53%	2728
	11th to 20th	Night	5% 2 2 5% 4 4 5% 5% 9% 11% 48%	2811
	21st to end	Night	9% 4 1 4 3 3 3 3 4 8% 59%	3146
September	1st to 10th	Night	10% 4 2 5% 3 4 4 3 5% 7% 53%	1826
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 5% 7% 49%	2071
	21st to end	Night	7% 6% 1 4 4 5% 4 4 7% 8% 50%	2090
October	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 6% 8% 51%	3759
	11th to 20th	Night	5% 6% 2 6% 4 4 4 3 6% 9% 50%	3778
	21st to end	Night	6% 8% 3 7% 4 4 5% 4 6% 7% 46%	4457
November	1st to 10th	Night	12% 12% 4 7% 5% 4 4 3 5% 7% 36%	2720
	11th to 20th	Night	6% 10% 3 8% 7% 5% 5% 4 6% 7% 39%	2800
	21st to end	Night	6% 12% 4 9% 6% 4 4 4 6% 8% 37%	2880
December	1st to 10th	Night	4 12% 4 11% 7% 6% 6% 5% 6% 7% 33%	4890
	11th to 20th	Night	5% 12% 5% 10% 5% 6% 6% 5% 6% 9% 30%	5546
	21st to end	Night	4 8% 3 8% 5% 4 5% 4 6% 8% 45%	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	56% 6% 13% 20% 4 1	3179
	11th to 20th	Day	51% 5% 14% 16% 14% 1	3180
	21st to end	Day	38% 6% 18% 30% 7% 1	3912
February	1st to 10th	Day	55% 6% 14% 16% 9%	1755
	11th to 20th	Day	52% 7% 14% 18% 8%	1764
	21st to end	Day	50% 7% 12% 20% 9% 1	1470
March	1st to 10th	Day	43% 6% 14% 25% 12%	3258
	11th to 20th	Day	36% 8% 22% 25% 10% 1	3490
	21st to end	Day	54% 11% 15% 15% 5%	3825
April	1st to 10th	Day	47% 8% 12% 23% 10% 1	2307
	11th to 20th	Day	50% 5% 11% 24% 9% 1	2440
	21st to end	Day	45% 6% 14% 24% 10% 1	2470
May	1st to 10th	Day	54% 6% 10% 24% 6%	4302
	11th to 20th	Day	37% 7% 15% 30% 11% 1	4539
	21st to end	Day	43% 9% 13% 25% 10% 1	5162
June	1st to 10th	Day	40% 7% 16% 23% 12% 2	2850
	11th to 20th	Day	45% 7% 13% 21% 12% 2	2850
	21st to end	Day	28% 7% 17% 31% 14% 3	2850
July	1st to 10th	Day	24% 7% 15% 27% 22% 4	4666
	11th to 20th	Day	20% 5% 14% 33% 22% 5%	4659
	21st to end	Day	12% 4 9% 30% 28% 18%	4976
August	1st to 10th	Day	26% 5% 13% 32% 22% 3	4188
	11th to 20th	Day	24% 6% 15% 39% 14% 2	4067
	21st to end	Day	21% 4 13% 40% 20% 3	4520
September	1st to 10th	Day	29% 3 13% 28% 18% 9%	2353
	11th to 20th	Day	34% 6% 12% 30% 15% 3	2106
	21st to end	Day	32% 5% 14% 32% 14% 3	2090
October	1st to 10th	Day	32% 6% 13% 32% 15% 2	3211
	11th to 20th	Day	30% 5% 16% 33% 15% 1	3138
	21st to end	Day	34% 5% 10% 38% 12% 1	3080
November	1st to 10th	Day	50% 7% 15% 19% 7% 2	1568
	11th to 20th	Day	44% 6% 19% 23% 8% 1	1600
	21st to end	Day	47% 6% 19% 20% 7% 1	1517
December	1st to 10th	Day	50% 5% 18% 22% 4 1	2290
	11th to 20th	Day	51% 7% 19% 20% 3	2644
	21st to end	Day	38% 5% 15% 26% 15% 1	3498
January	1st to 10th	Night	49% 7% 18% 21% 5% 1	6766
	11th to 20th	Night	48% 6% 15% 16% 14% 1	6780
	21st to end	Night	41% 6% 17% 28% 8% 1	7074
February	1st to 10th	Night	51% 6% 13% 18% 10% 1	2644
	11th to 20th	Night	48% 8% 14% 18% 10% 1	2533
	21st to end	Night	46% 6% 15% 22% 9% 2	2075
March	1st to 10th	Night	46% 7% 15% 23% 8%	3615
	11th to 20th	Night	36% 7% 19% 27% 11%	3490
	21st to end	Night	56% 11% 15% 14% 4	3828
April	1st to 10th	Night	46% 6% 15% 23% 10%	1869
	11th to 20th	Night	46% 6% 12% 23% 12% 1	1691
	21st to end	Night	45% 7% 13% 25% 10% 1	1710
May	1st to 10th	Night	52% 7% 10% 22% 9% 1	2653
	11th to 20th	Night	36% 10% 13% 30% 11% 1	2360
	21st to end	Night	50% 9% 12% 22% 6% 1	2414
June	1st to 10th	Night	42% 6% 15% 24% 11% 2	1330
	11th to 20th	Night	39% 9% 13% 25% 12% 2	1330
	21st to end	Night	27% 6% 16% 34% 15% 3	1330
July	1st to 10th	Night	23% 6% 18% 27% 22% 4	2185
	11th to 20th	Night	22% 6% 13% 34% 18% 7%	2178
	21st to end	Night	14% 4 8% 29% 29% 14%	2337
August	1st to 10th	Night	27% 6% 12% 33% 17% 5%	2721
	11th to 20th	Night	27% 8% 16% 29% 17% 3	2838
	21st to end	Night	24% 4 12% 36% 19% 4	3146
September	1st to 10th	Night	31% 5% 10% 27% 19% 9%	1826
	11th to 20th	Night	34% 5% 13% 28% 15% 5%	2071
	21st to end	Night	30% 6% 13% 31% 17% 3	2090
October	1st to 10th	Night	29% 5% 12% 35% 17% 2	3759
	11th to 20th	Night	30% 5% 15% 31% 16% 2	3778
	21st to end	Night	35% 5% 11% 35% 12% 1	4457
November	1st to 10th	Night	48% 5% 15% 21% 9% 1	2720
	11th to 20th	Night	43% 6% 19% 21% 11%	2800
	21st to end	Night	44% 6% 21% 19% 9% 1	2880
December	1st to 10th	Night	50% 6% 19% 18% 6% 1	4890
	11th to 20th	Night	49% 7% 26% 16% 3	5547
	21st to end	Night	37% 5% 13% 26% 16% 2	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	11% 14% 38% 24% 10% 5%	200
	11th to 20th	Day	15% 12% 38% 27% 7% 2	200
	21st to end	Day	11% 13% 42% 27% 5% 2	220
February	1st to 10th	Day	18% 17% 40% 17% 6% 1	277
	11th to 20th	Day	12% 13% 42% 24% 8% 1	300
	21st to end	Day	12% 17% 30% 29% 10% 2	258
March	1st to 10th	Day	12% 18% 37% 21% 10% 2	460
	11th to 20th	Day	11% 13% 37% 26% 10% 3	460
	21st to end	Day	14% 12% 40% 21% 11% 3	506
April	1st to 10th	Day	14% 20% 33% 28% 5% 1	460
	11th to 20th	Day	15% 15% 34% 26% 10%	452
	21st to end	Day	17% 15% 25% 30% 10% 3	460
May	1st to 10th	Day	15% 11% 25% 28% 17% 5%	577
	11th to 20th	Day	16% 7% 25% 31% 19% 3	675
	21st to end	Day	19% 7% 20% 29% 19% 7%	734
June	1st to 10th	Day	16% 11% 24% 31% 15% 3	680
	11th to 20th	Day	19% 12% 30% 26% 11% 2	680
	21st to end	Day	9% 8% 26% 32% 20% 4	680
July	1st to 10th	Day	6% 5% 23% 38% 23% 6%	671
	11th to 20th	Day	8% 4 28% 38% 19% 3	668
	21st to end	Day	4 5% 24% 38% 23% 6%	739
August	1st to 10th	Day	7% 5% 20% 38% 25% 5%	500
	11th to 20th	Day	7% 3 25% 34% 25% 7%	477
	21st to end	Day	8% 8% 26% 29% 23% 5%	539
September	1st to 10th	Day	11% 10% 28% 31% 18% 2	490
	11th to 20th	Day	16% 8% 28% 31% 14% 3	490
	21st to end	Day	14% 11% 32% 28% 11% 4	490
October	1st to 10th	Day	15% 8% 29% 27% 18% 3	328
	11th to 20th	Day	12% 14% 36% 25% 12% 2	300
	21st to end	Day	18% 17% 31% 24% 8% 1	330
November	1st to 10th	Day	23% 14% 35% 22% 6% 1	300
	11th to 20th	Day	14% 20% 39% 21% 5% 1	300
	21st to end	Day	10% 19% 38% 20% 9% 5%	257
December	1st to 10th	Day	11% 21% 41% 20% 8% 1	200
	11th to 20th	Day	11% 23% 40% 21% 6% 1	200
	21st to end	Day	16% 10% 36% 26% 8% 4	220
January	1st to 10th	Night	11% 15% 40% 24% 9% 1	1000
	11th to 20th	Night	13% 14% 39% 25% 9% 1	1000
	21st to end	Night	10% 15% 42% 24% 8% 1	937
February	1st to 10th	Night	16% 11% 34% 26% 12% 2	800
	11th to 20th	Night	11% 13% 38% 28% 9%	784
	21st to end	Night	11% 12% 31% 31% 14% 2	646
March	1st to 10th	Night	10% 11% 37% 30% 11% 1	760
	11th to 20th	Night	12% 12% 36% 30% 8% 1	760
	21st to end	Night	13% 12% 37% 28% 9% 1	836
April	1st to 10th	Night	14% 11% 39% 27% 8% 1	627
	11th to 20th	Night	13% 10% 38% 28% 10%	563
	21st to end	Night	17% 10% 31% 34% 8%	570
May	1st to 10th	Night	16% 8% 25% 35% 16% 1	570
	11th to 20th	Night	9% 7% 29% 34% 19% 1	470
	21st to end	Night	19% 8% 24% 31% 16% 2	411
June	1st to 10th	Night	19% 7% 31% 29% 12% 2	380
	11th to 20th	Night	16% 10% 33% 31% 9% 1	380
	21st to end	Night	8% 7% 29% 39% 17% 1	380
July	1st to 10th	Night	6% 5% 25% 42% 21%	370
	11th to 20th	Night	7% 5% 30% 44% 13% 1	368
	21st to end	Night	4 5% 26% 41% 23% 2	416
August	1st to 10th	Night	8% 4 26% 38% 22% 2	501
	11th to 20th	Night	7% 3 21% 45% 23% 1	564
	21st to end	Night	10% 5% 25% 40% 19% 1	627
September	1st to 10th	Night	13% 6% 28% 40% 12% 1	570
	11th to 20th	Night	14% 5% 25% 39% 16% 1	741
	21st to end	Night	9% 9% 35% 31% 14% 2	760
October	1st to 10th	Night	15% 8% 27% 36% 13% 1	760
	11th to 20th	Night	13% 11% 33% 31% 10% 1	760
	21st to end	Night	12% 16% 37% 28% 6%	836
November	1st to 10th	Night	18% 12% 39% 25% 6%	965
	11th to 20th	Night	12% 17% 42% 22% 5% 1	1000
	21st to end	Night	11% 14% 41% 27% 7% 1	1000
December	1st to 10th	Night	12% 12% 39% 26% 10% 1	1000
	11th to 20th	Night	11% 14% 40% 26% 7% 1	1000
	21st to end	Night	13% 13% 38% 28% 6% 2	1100

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