Boreal Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boreal Skies Observatory clear sky chart from 2005-11-06 to 2024-11-06.

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	17% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101583
	Night	18% 11% 3 8% 5% 5% 4 3 5% 6% 32%	97484
Transparency	Day	49% 8% 13% 20% 9% 1	105066
	Night	51% 7% 12% 19% 9% 1	102090
Seeing	Day	22% 16% 30% 20% 9% 3	14472
	Night	22% 13% 29% 26% 9% 1	23536

By Month

Percentage Cloud Cover NE (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	16% 16% 6% 12% 6% 5% 5% 4 5% 5% 20%	2517
	11th to 20th	Day	15% 12% 6% 15% 7% 6% 5% 3 5% 5% 22%	2520
	21st to end	Day	14% 17% 6% 13% 6% 7% 5% 4 5% 5% 18%	2877
February	1st to 10th	Day	13% 15% 5% 13% 7% 6% 5% 4 4 5% 24%	1710
	11th to 20th	Day	12% 15% 5% 10% 5% 5% 4 5% 5% 5% 28%	1665
	21st to end	Day	19% 13% 4 8% 6% 5% 5% 4 6% 5% 25%	1570
March	1st to 10th	Day	21% 14% 4 7% 6% 5% 4 4 4 5% 25%	3116
	11th to 20th	Day	12% 14% 5% 11% 6% 5% 5% 4 5% 5% 27%	3476
	21st to end	Day	14% 8% 3 6% 7% 4 4 3 5% 6% 40%	4028
April	1st to 10th	Day	22% 11% 3 6% 4 4 3 4 4 6% 32%	2280
	11th to 20th	Day	22% 8% 3 6% 4 4 4 3 4 5% 36%	2254
	21st to end	Day	22% 10% 3 9% 5% 4 4 3 4 4 31%	2419
May	1st to 10th	Day	21% 8% 3 7% 4 5% 5% 4 6% 6% 30%	4426
	11th to 20th	Day	17% 8% 3 7% 6% 4 5% 3 6% 7% 33%	4398
	21st to end	Day	15% 7% 2 7% 5% 5% 4 4 6% 8% 37%	4837
June	1st to 10th	Day	21% 8% 3 7% 6% 5% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 5% 6% 8% 38%	2660
	21st to end	Day	19% 6% 3 7% 5% 5% 6% 4 7% 8% 30%	2660
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4440
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 6% 8% 41%	4440
	21st to end	Day	11% 5% 2 5% 5% 4 5% 4 6% 9% 44%	4814
August	1st to 10th	Day	14% 6% 2 8% 4 5% 6% 5% 6% 8% 36%	4365
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 7% 9% 38%	3994
	21st to end	Day	14% 5% 2 6% 4 5% 5% 3 6% 9% 41%	4180
September	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 5% 8% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 6% 7% 47%	2280
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 9% 39%	2027
October	1st to 10th	Day	20% 9% 3 7% 4 4 4 4 6% 6% 35%	1900
	11th to 20th	Day	22% 9% 3 8% 4 5% 5% 4 6% 7% 29%	1900
	21st to end	Day	25% 9% 4 6% 5% 5% 4 3 4 6% 28%	1933
November	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 7% 6% 34%	1717
	11th to 20th	Day	20% 12% 3 7% 5% 5% 5% 5% 5% 6% 26%	1522
	21st to end	Day	22% 15% 4 8% 8% 5% 5% 4 5% 5% 21%	1520
December	1st to 10th	Day	22% 17% 5% 11% 6% 4 4 3 5% 6% 17%	1520
	11th to 20th	Day	21% 14% 4 10% 6% 4 5% 3 5% 5% 24%	1906
	21st to end	Day	27% 19% 5% 10% 4 3 3 3 2 4 18%	2772
January	1st to 10th	Night	17% 18% 6% 13% 6% 4 4 3 4 5% 20%	4429
	11th to 20th	Night	13% 15% 5% 12% 8% 6% 5% 3 5% 5% 23%	4440
	21st to end	Night	16% 21% 6% 11% 5% 5% 4 3 4 5% 21%	4774
February	1st to 10th	Night	14% 14% 5% 12% 6% 6% 5% 4 5% 5% 24%	2470
	11th to 20th	Night	14% 13% 6% 10% 7% 5% 4 3 5% 6% 29%	2399
	21st to end	Night	17% 13% 4 8% 6% 4 5% 4 5% 5% 29%	1884
March	1st to 10th	Night	22% 12% 4 9% 5% 4 4 3 3 5% 30%	3724
	11th to 20th	Night	16% 16% 4 11% 6% 4 4 3 5% 5% 26%	3479
	21st to end	Night	16% 8% 3 7% 4 3 4 3 5% 7% 40%	3604
April	1st to 10th	Night	20% 11% 3 7% 5% 4 4 3 5% 5% 33%	1900
	11th to 20th	Night	19% 8% 3 7% 5% 4 4 4 4 6% 35%	1875
	21st to end	Night	21% 10% 3 7% 4 4 4 3 5% 6% 34%	1775
May	1st to 10th	Night	20% 9% 3 8% 5% 5% 5% 5% 6% 6% 29%	2514
	11th to 20th	Night	20% 7% 3 6% 4 5% 4 4 5% 7% 35%	2495
	21st to end	Night	15% 6% 3 7% 5% 4 5% 5% 5% 7% 38%	2738
June	1st to 10th	Night	16% 7% 2 8% 6% 5% 5% 5% 6% 8% 34%	1520
	11th to 20th	Night	13% 6% 2 7% 4 4 4 3 6% 8% 42%	1520
	21st to end	Night	18% 7% 3 6% 4 5% 5% 4 6% 8% 35%	1520
July	1st to 10th	Night	14% 6% 3 8% 6% 6% 6% 4 6% 7% 35%	2520
	11th to 20th	Night	14% 5% 2 7% 4 4 4 4 6% 7% 42%	2520
	21st to end	Night	11% 5% 2 6% 5% 5% 5% 5% 6% 8% 44%	2739
August	1st to 10th	Night	12% 8% 2 7% 4 5% 4 4 5% 9% 39%	2561
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 5% 7% 42%	2803
	21st to end	Night	14% 5% 2 7% 5% 5% 5% 4 6% 7% 41%	3476
September	1st to 10th	Night	12% 5% 2 5% 4 5% 5% 4 5% 7% 47%	1900
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 49%	1900
	21st to end	Night	17% 6% 2 6% 5% 4 3 3 4 6% 44%	2151
October	1st to 10th	Night	19% 8% 2 6% 5% 4 4 3 5% 6% 39%	2280
	11th to 20th	Night	21% 9% 2 6% 6% 5% 5% 3 5% 6% 32%	2280
	21st to end	Night	23% 10% 3 8% 5% 5% 5% 4 4 6% 28%	2664
November	1st to 10th	Night	20% 10% 3 7% 5% 4 4 2 6% 6% 33%	2483
	11th to 20th	Night	19% 11% 4 8% 5% 5% 5% 4 4 5% 29%	2655
	21st to end	Night	22% 12% 4 9% 6% 5% 5% 3 5% 6% 24%	2660
December	1st to 10th	Night	21% 17% 5% 11% 6% 4 5% 4 5% 5% 18%	2660
	11th to 20th	Night	22% 14% 5% 9% 6% 4 3 3 3 5% 27%	3288
	21st to end	Night	30% 18% 4 9% 5% 4 4 2 4 3 18%	4884

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	62% 12% 11% 9% 5% 1	2518
	11th to 20th	Day	62% 8% 13% 11% 6%	2520
	21st to end	Day	67% 9% 10% 9% 5%	2877
February	1st to 10th	Day	61% 6% 11% 17% 4 2	1710
	11th to 20th	Day	56% 7% 12% 16% 9% 1	1674
	21st to end	Day	58% 8% 11% 13% 9% 2	1570
March	1st to 10th	Day	58% 8% 9% 16% 9% 1	3119
	11th to 20th	Day	54% 10% 12% 14% 8% 2	3476
	21st to end	Day	41% 6% 11% 23% 16% 3	4028
April	1st to 10th	Day	53% 6% 8% 19% 11% 3	2280
	11th to 20th	Day	50% 5% 8% 21% 14% 2	2254
	21st to end	Day	55% 6% 8% 14% 14% 4	2419
May	1st to 10th	Day	48% 9% 11% 20% 10% 2	4440
	11th to 20th	Day	46% 11% 14% 18% 10% 2	4402
	21st to end	Day	43% 9% 12% 21% 14% 2	4837
June	1st to 10th	Day	51% 9% 10% 18% 11% 1	2660
	11th to 20th	Day	41% 9% 12% 23% 12% 2	2660
	21st to end	Day	48% 9% 14% 19% 10% 1	2660
July	1st to 10th	Day	42% 7% 16% 24% 11%	4370
	11th to 20th	Day	39% 7% 15% 30% 9%	4365
	21st to end	Day	36% 8% 15% 31% 11%	4661
August	1st to 10th	Day	44% 8% 16% 23% 9%	4351
	11th to 20th	Day	41% 7% 16% 28% 8%	4033
	21st to end	Day	40% 5% 17% 26% 11% 1	4180
September	1st to 10th	Day	37% 8% 16% 27% 11%	2280
	11th to 20th	Day	35% 8% 14% 30% 12% 1	2280
	21st to end	Day	43% 8% 15% 26% 9%	2027
October	1st to 10th	Day	46% 6% 12% 28% 7%	3160
	11th to 20th	Day	46% 7% 15% 24% 8%	3140
	21st to end	Day	49% 6% 18% 20% 7%	3158
November	1st to 10th	Day	47% 8% 16% 22% 6% 1	1717
	11th to 20th	Day	56% 8% 13% 14% 8% 1	1522
	21st to end	Day	64% 8% 11% 10% 6% 1	1520
December	1st to 10th	Day	67% 8% 13% 9% 3 1	1520
	11th to 20th	Day	61% 9% 13% 13% 4	1906
	21st to end	Day	71% 5% 7% 13% 3	2772
January	1st to 10th	Night	64% 9% 14% 10% 3	4429
	11th to 20th	Night	59% 11% 15% 11% 5%	4440
	21st to end	Night	66% 8% 10% 10% 6% 1	4774
February	1st to 10th	Night	61% 7% 11% 15% 6% 1	2470
	11th to 20th	Night	56% 6% 11% 18% 8% 1	2401
	21st to end	Night	55% 6% 11% 18% 10% 1	1884
March	1st to 10th	Night	55% 7% 10% 18% 9% 2	3734
	11th to 20th	Night	58% 8% 12% 14% 7% 1	3480
	21st to end	Night	42% 7% 9% 22% 15% 4	3604
April	1st to 10th	Night	52% 6% 10% 17% 13% 2	1900
	11th to 20th	Night	49% 7% 8% 20% 15% 1	1875
	21st to end	Night	51% 6% 8% 17% 15% 4	1775
May	1st to 10th	Night	48% 12% 10% 18% 9% 3	2520
	11th to 20th	Night	45% 10% 12% 20% 11% 2	2496
	21st to end	Night	40% 11% 13% 18% 17% 2	2739
June	1st to 10th	Night	45% 8% 15% 22% 9% 1	1520
	11th to 20th	Night	38% 8% 12% 24% 16% 2	1520
	21st to end	Night	45% 10% 12% 19% 13% 1	1520
July	1st to 10th	Night	47% 7% 14% 18% 13% 1	2480
	11th to 20th	Night	40% 6% 14% 32% 8%	2472
	21st to end	Night	38% 7% 15% 25% 15%	2651
August	1st to 10th	Night	43% 7% 14% 25% 10%	2551
	11th to 20th	Night	41% 6% 15% 30% 8%	2830
	21st to end	Night	40% 7% 15% 24% 12% 2	3476
September	1st to 10th	Night	37% 7% 12% 29% 14% 1	1900
	11th to 20th	Night	34% 8% 13% 33% 10% 1	1900
	21st to end	Night	42% 6% 13% 28% 12%	2151
October	1st to 10th	Night	40% 5% 14% 29% 11%	3800
	11th to 20th	Night	45% 6% 16% 25% 9%	3779
	21st to end	Night	48% 7% 17% 19% 9%	4389
November	1st to 10th	Night	51% 6% 15% 20% 8%	2483
	11th to 20th	Night	55% 7% 12% 16% 8% 1	2655
	21st to end	Night	60% 9% 12% 14% 5%	2660
December	1st to 10th	Night	67% 7% 12% 11% 3	2660
	11th to 20th	Night	61% 6% 10% 16% 7% 1	3288
	21st to end	Night	71% 6% 7% 12% 3	4884

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	25% 21% 29% 20% 4	180
	11th to 20th	Day	30% 16% 36% 16% 2 1	180
	21st to end	Day	23% 17% 36% 19% 5%	229
February	1st to 10th	Day	31% 22% 33% 11% 2	270
	11th to 20th	Day	27% 28% 37% 7% 1	272
	21st to end	Day	31% 20% 31% 15% 2	382
March	1st to 10th	Day	25% 18% 35% 18% 4	460
	11th to 20th	Day	26% 19% 35% 16% 3 1	460
	21st to end	Day	28% 22% 28% 15% 6% 1	506
April	1st to 10th	Day	28% 23% 28% 15% 5%	460
	11th to 20th	Day	33% 19% 23% 15% 8% 2	452
	21st to end	Day	30% 20% 31% 12% 6% 1	460
May	1st to 10th	Day	24% 16% 32% 18% 8% 2	463
	11th to 20th	Day	17% 19% 33% 19% 10% 2	487
	21st to end	Day	19% 17% 31% 22% 11% 1	529
June	1st to 10th	Day	24% 12% 32% 22% 8% 1	490
	11th to 20th	Day	15% 16% 30% 26% 11% 3	490
	21st to end	Day	22% 13% 31% 21% 11% 2	490
July	1st to 10th	Day	12% 7% 30% 34% 13% 4	485
	11th to 20th	Day	12% 9% 34% 27% 14% 3	484
	21st to end	Day	12% 8% 32% 31% 14% 4	532
August	1st to 10th	Day	15% 8% 27% 30% 16% 4	486
	11th to 20th	Day	14% 8% 24% 30% 17% 7%	479
	21st to end	Day	13% 11% 30% 25% 14% 7%	539
September	1st to 10th	Day	17% 10% 32% 19% 16% 7%	490
	11th to 20th	Day	11% 10% 25% 21% 20% 12%	490
	21st to end	Day	19% 12% 28% 21% 14% 5%	490
October	1st to 10th	Day	23% 15% 25% 22% 11% 4	490
	11th to 20th	Day	24% 20% 31% 17% 8% 1	490
	21st to end	Day	32% 14% 28% 21% 5%	456
November	1st to 10th	Day	24% 18% 30% 20% 7% 1	381
	11th to 20th	Day	29% 23% 32% 12% 4 1	182
	21st to end	Day	28% 23% 31% 13% 4 1	180
December	1st to 10th	Day	31% 17% 36% 14% 2	180
	11th to 20th	Day	29% 24% 34% 11% 1 1	180
	21st to end	Day	33% 20% 35% 10% 2 1	198
January	1st to 10th	Night	26% 15% 34% 21% 3	760
	11th to 20th	Night	26% 19% 29% 21% 5% 1	760
	21st to end	Night	19% 13% 37% 25% 5%	836
February	1st to 10th	Night	22% 16% 35% 24% 3	760
	11th to 20th	Night	21% 17% 39% 19% 4	743
	21st to end	Night	25% 17% 29% 25% 4	628
March	1st to 10th	Night	23% 15% 35% 23% 3	760
	11th to 20th	Night	23% 20% 33% 19% 5%	760
	21st to end	Night	23% 17% 28% 24% 7%	836
April	1st to 10th	Night	26% 14% 32% 23% 4	760
	11th to 20th	Night	25% 17% 30% 21% 8%	751
	21st to end	Night	25% 14% 29% 24% 7% 1	635
May	1st to 10th	Night	23% 9% 27% 30% 9% 2	380
	11th to 20th	Night	22% 11% 27% 30% 10%	374
	21st to end	Night	15% 8% 38% 27% 12%	411
June	1st to 10th	Night	19% 5% 31% 34% 11%	380
	11th to 20th	Night	16% 8% 27% 34% 12% 2	380
	21st to end	Night	22% 5% 28% 31% 13% 1	380
July	1st to 10th	Night	11% 5% 27% 41% 16%	370
	11th to 20th	Night	11% 6% 22% 45% 15% 1	368
	21st to end	Night	8% 4 27% 45% 15% 1	416
August	1st to 10th	Night	13% 3 21% 41% 21%	407
	11th to 20th	Night	12% 5% 18% 39% 23% 2	569
	21st to end	Night	13% 6% 24% 36% 18% 2	836
September	1st to 10th	Night	14% 7% 29% 33% 18% 1	760
	11th to 20th	Night	16% 6% 20% 31% 22% 4	760
	21st to end	Night	20% 9% 24% 26% 19% 2	760
October	1st to 10th	Night	23% 10% 26% 25% 16% 1	760
	11th to 20th	Night	25% 12% 29% 25% 8% 1	760
	21st to end	Night	27% 13% 29% 26% 5%	836
November	1st to 10th	Night	26% 13% 28% 24% 8% 1	764
	11th to 20th	Night	24% 17% 33% 16% 10% 1	760
	21st to end	Night	29% 17% 27% 22% 6%	760
December	1st to 10th	Night	30% 16% 31% 21% 3	760
	11th to 20th	Night	28% 16% 30% 21% 5% 1	760
	21st to end	Night	34% 15% 30% 17% 3	836

Page generated at 2024-11-03 13:19:02. Version ac07

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