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 2024-05-14.

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% 10% 3 8% 5% 5% 5% 4 5% 6% 32%	83398
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 31%	86117
Transparency	Day	50% 8% 12% 19% 10% 1	83103
	Night	53% 8% 12% 18% 9% 1	85946
Seeing	Day	22% 16% 31% 20% 9% 3	13781
	Night	22% 13% 30% 26% 9% 1	23015

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	20% 8% 2 7% 4 5% 4 4 6% 6% 34%	2660
	11th to 20th	Day	16% 7% 3 7% 6% 4 5% 3 5% 6% 39%	2520
	21st to end	Day	16% 7% 2 7% 6% 5% 5% 4 6% 7% 36%	2725
June	1st to 10th	Day	21% 8% 2 7% 7% 5% 5% 4 5% 7% 30%	2520
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 5% 6% 8% 38%	2520
	21st to end	Day	18% 7% 3 7% 5% 5% 6% 4 7% 8% 30%	2520
July	1st to 10th	Day	11% 6% 3 5% 4 5% 5% 5% 7% 8% 40%	2520
	11th to 20th	Day	12% 5% 2 7% 5% 5% 5% 4 7% 9% 38%	2520
	21st to end	Day	11% 6% 2 6% 6% 4 5% 5% 7% 10% 38%	2744
August	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 8% 39%	2476
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	2281
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 3 6% 9% 41%	2376
September	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 5% 8% 42%	2160
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 46%	2160
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 9% 38%	1921
October	1st to 10th	Day	20% 9% 3 7% 4 4 4 3 6% 6% 35%	1800
	11th to 20th	Day	23% 9% 3 8% 4 5% 5% 4 5% 7% 27%	1800
	21st to end	Day	26% 9% 4 6% 5% 5% 4 3 4 6% 27%	1832
November	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 7% 6% 34%	1671
	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% 8% 2 6% 4 4 4 4 6% 7% 36%	1520
	11th to 20th	Night	19% 7% 3 5% 4 4 4 3 4 6% 40%	1445
	21st to end	Night	13% 6% 2 7% 5% 3 4 4 5% 7% 44%	1550
June	1st to 10th	Night	16% 6% 2 7% 6% 5% 5% 5% 6% 8% 34%	1440
	11th to 20th	Night	14% 6% 2 7% 4 4 4 3 6% 8% 42%	1440
	21st to end	Night	17% 7% 3 6% 4 5% 5% 4 6% 8% 36%	1440
July	1st to 10th	Night	11% 4 2 5% 5% 5% 5% 4 6% 7% 46%	1440
	11th to 20th	Night	12% 6% 2 6% 3 4 5% 5% 6% 8% 43%	1440
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	1574
August	1st to 10th	Night	9% 7% 2 6% 4 4 5% 4 6% 10% 42%	1459
	11th to 20th	Night	10% 6% 2 7% 4 4 4 4 6% 8% 45%	1602
	21st to end	Night	11% 5% 2 7% 5% 5% 4 4 6% 7% 44%	1980
September	1st to 10th	Night	12% 5% 3 5% 4 5% 5% 4 5% 7% 46%	1800
	11th to 20th	Night	15% 5% 2 5% 4 3 4 3 5% 7% 48%	1800
	21st to end	Night	17% 6% 2 6% 5% 4 3 3 4 6% 44%	2037
October	1st to 10th	Night	19% 8% 2 6% 5% 4 4 3 5% 6% 38%	2160
	11th to 20th	Night	22% 8% 2 7% 6% 5% 5% 3 5% 6% 31%	2160
	21st to end	Night	24% 10% 3 8% 5% 5% 4 4 4 6% 28%	2523
November	1st to 10th	Night	20% 10% 2 6% 5% 4 4 2 6% 6% 33%	2407
	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	49% 7% 9% 19% 13% 3	2660
	11th to 20th	Day	45% 6% 9% 20% 16% 3	2524
	21st to end	Day	45% 8% 12% 20% 12% 3	2725
June	1st to 10th	Day	52% 9% 10% 18% 11% 1	2520
	11th to 20th	Day	41% 10% 12% 23% 13% 2	2520
	21st to end	Day	48% 10% 14% 18% 10% 1	2520
July	1st to 10th	Day	37% 10% 15% 25% 13% 1	2450
	11th to 20th	Day	38% 9% 15% 26% 11%	2445
	21st to end	Day	39% 10% 16% 24% 10%	2591
August	1st to 10th	Day	38% 11% 16% 24% 9%	2462
	11th to 20th	Day	39% 10% 17% 25% 8% 1	2281
	21st to end	Day	39% 8% 16% 26% 11% 1	2376
September	1st to 10th	Day	38% 9% 16% 26% 11%	2160
	11th to 20th	Day	36% 8% 13% 30% 12% 1	2160
	21st to end	Day	43% 8% 15% 26% 8%	1921
October	1st to 10th	Day	48% 8% 13% 21% 10% 1	1800
	11th to 20th	Day	54% 10% 16% 14% 6% 1	1800
	21st to end	Day	58% 7% 12% 14% 8%	1832
November	1st to 10th	Day	48% 8% 16% 21% 6% 1	1671
	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	46% 7% 9% 19% 14% 5%	1520
	11th to 20th	Night	44% 6% 8% 21% 18% 3	1446
	21st to end	Night	38% 9% 15% 18% 17% 2	1551
June	1st to 10th	Night	45% 8% 15% 22% 8% 1	1440
	11th to 20th	Night	38% 9% 12% 23% 16% 2	1440
	21st to end	Night	44% 10% 13% 20% 13% 1	1440
July	1st to 10th	Night	34% 9% 13% 26% 16% 2	1400
	11th to 20th	Night	36% 8% 14% 30% 12%	1392
	21st to end	Night	35% 10% 16% 25% 13%	1486
August	1st to 10th	Night	37% 10% 17% 22% 12% 1	1449
	11th to 20th	Night	36% 8% 16% 28% 11%	1602
	21st to end	Night	36% 9% 16% 28% 11%	1980
September	1st to 10th	Night	37% 7% 12% 28% 14% 1	1800
	11th to 20th	Night	35% 9% 13% 32% 9% 1	1800
	21st to end	Night	42% 6% 13% 30% 10%	2037
October	1st to 10th	Night	46% 8% 11% 22% 12% 1	2160
	11th to 20th	Night	53% 8% 12% 19% 8%	2160
	21st to end	Night	56% 8% 13% 15% 8%	2523
November	1st to 10th	Night	50% 6% 15% 20% 8%	2407
	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	18% 19% 34% 18% 9% 2	459
	21st to end	Day	19% 16% 31% 21% 11% 1	485
June	1st to 10th	Day	24% 12% 34% 20% 8% 2	450
	11th to 20th	Day	16% 16% 29% 26% 11% 3	450
	21st to end	Day	19% 13% 31% 23% 12% 3	450
July	1st to 10th	Day	11% 7% 29% 35% 13% 4	445
	11th to 20th	Day	12% 9% 34% 27% 15% 3	444
	21st to end	Day	12% 7% 33% 31% 13% 4	488
August	1st to 10th	Day	14% 8% 28% 30% 16% 4	446
	11th to 20th	Day	13% 8% 24% 30% 17% 8%	439
	21st to end	Day	13% 12% 31% 23% 14% 7%	495
September	1st to 10th	Day	16% 9% 32% 19% 16% 8%	450
	11th to 20th	Day	12% 11% 27% 22% 18% 10%	450
	21st to end	Day	18% 13% 29% 21% 14% 5%	450
October	1st to 10th	Day	24% 14% 25% 21% 12% 4	450
	11th to 20th	Day	24% 20% 30% 17% 7% 1	450
	21st to end	Day	33% 15% 26% 21% 5%	421
November	1st to 10th	Day	24% 18% 29% 20% 8% 1	365
	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	23% 12% 27% 29% 10%	362
	21st to end	Night	16% 8% 37% 27% 12%	389
June	1st to 10th	Night	21% 4 31% 34% 10%	360
	11th to 20th	Night	17% 8% 28% 33% 13% 2	360
	21st to end	Night	21% 5% 29% 31% 14% 1	360
July	1st to 10th	Night	10% 5% 28% 40% 17%	350
	11th to 20th	Night	11% 5% 22% 44% 16% 1	348
	21st to end	Night	8% 4 27% 44% 14% 2	394
August	1st to 10th	Night	13% 4 21% 41% 21% 1	385
	11th to 20th	Night	11% 4 19% 40% 23% 2	538
	21st to end	Night	13% 6% 25% 35% 18% 2	792
September	1st to 10th	Night	14% 7% 29% 32% 18% 1	720
	11th to 20th	Night	17% 7% 22% 33% 19% 3	720
	21st to end	Night	19% 9% 25% 26% 19% 2	720
October	1st to 10th	Night	23% 10% 27% 24% 15% 1	720
	11th to 20th	Night	25% 12% 30% 25% 7% 1	720
	21st to end	Night	27% 13% 29% 26% 5%	792
November	1st to 10th	Night	26% 13% 27% 24% 8% 1	740
	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 2024-05-11 05:13:28. Version ac07

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