Lac Boswell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lac Boswell clear sky chart from 2005-11-06 to 2026-01-17.

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	18% 12% 4 10% 6% 5% 5% 3 5% 6% 26%	127130
	Night	17% 13% 5% 11% 6% 5% 5% 3 5% 6% 25%	124581
Transparency	Day	58% 5% 11% 17% 8% 1	130561
	Night	59% 5% 11% 17% 7% 1	129238
Seeing	Day	27% 18% 30% 16% 7% 1	16377
	Night	26% 14% 30% 23% 7% 1	25163

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	12% 21% 8% 20% 9% 6% 6% 2 3 3 10%	3677
	11th to 20th	Day	11% 16% 7% 20% 11% 6% 3 3 4 5% 16%	3618
	21st to end	Day	8% 12% 7% 16% 6% 5% 5% 4 6% 7% 23%	3960
February	1st to 10th	Day	10% 16% 6% 13% 11% 7% 5% 4 4 5% 19%	1633
	11th to 20th	Day	12% 20% 7% 12% 6% 6% 5% 3 4 6% 19%	1881
	21st to end	Day	14% 16% 6% 12% 7% 5% 5% 3 4 4 23%	1650
March	1st to 10th	Day	18% 18% 6% 13% 6% 4 3 2 3 4 23%	4476
	11th to 20th	Day	14% 15% 5% 12% 6% 5% 5% 3 4 5% 26%	4649
	21st to end	Day	16% 17% 4 9% 5% 4 3 3 4 4 31%	5962
April	1st to 10th	Day	21% 17% 5% 9% 4 5% 4 2 3 4 26%	2400
	11th to 20th	Day	21% 12% 4 8% 4 3 4 3 3 6% 32%	2428
	21st to end	Day	21% 13% 4 9% 5% 4 4 3 5% 6% 27%	2600
May	1st to 10th	Day	27% 11% 3 8% 5% 4 4 3 5% 6% 24%	6223
	11th to 20th	Day	22% 8% 3 6% 5% 5% 5% 3 5% 7% 30%	6318
	21st to end	Day	21% 10% 4 9% 6% 5% 5% 4 5% 6% 27%	6949
June	1st to 10th	Day	22% 9% 3 7% 6% 5% 5% 4 6% 7% 25%	2925
	11th to 20th	Day	21% 8% 3 7% 5% 4 5% 4 6% 8% 29%	3000
	21st to end	Day	24% 9% 3 8% 5% 6% 5% 3 6% 6% 25%	2934
July	1st to 10th	Day	18% 8% 3 9% 6% 6% 6% 4 7% 8% 25%	6360
	11th to 20th	Day	16% 8% 3 7% 5% 6% 6% 5% 7% 9% 29%	6360
	21st to end	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 9% 36%	6926
August	1st to 10th	Day	16% 8% 3 8% 6% 6% 6% 5% 7% 9% 27%	4563
	11th to 20th	Day	21% 7% 3 8% 5% 5% 5% 4 7% 8% 27%	4251
	21st to end	Day	20% 7% 3 8% 5% 5% 5% 4 5% 7% 31%	4380
September	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 5% 7% 36%	2400
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 4 6% 40%	2269
	21st to end	Day	24% 8% 2 7% 5% 3 4 4 4 6% 34%	2200
October	1st to 10th	Day	23% 9% 3 8% 4 5% 4 3 6% 6% 29%	2128
	11th to 20th	Day	23% 15% 4 10% 6% 5% 4 2 6% 5% 21%	1871
	21st to end	Day	29% 16% 4 8% 4 3 4 3 5% 5% 20%	1980
November	1st to 10th	Day	20% 18% 5% 8% 5% 5% 4 2 3 4 25%	1851
	11th to 20th	Day	19% 21% 7% 10% 6% 4 3 3 3 4 20%	1811
	21st to end	Day	19% 23% 6% 13% 6% 5% 4 3 4 4 13%	1595
December	1st to 10th	Day	18% 18% 6% 12% 10% 5% 5% 3 4 4 15%	2350
	11th to 20th	Day	20% 15% 6% 14% 8% 4 3 3 3 5% 19%	2688
	21st to end	Day	17% 20% 5% 10% 6% 5% 6% 4 5% 5% 17%	3864
January	1st to 10th	Night	12% 21% 9% 19% 8% 6% 6% 3 4 3 10%	6489
	11th to 20th	Night	10% 18% 7% 19% 11% 7% 5% 3 4 4 13%	6400
	21st to end	Night	7% 12% 6% 13% 8% 8% 6% 5% 7% 7% 22%	6996
February	1st to 10th	Night	11% 15% 5% 11% 8% 6% 5% 4 5% 6% 24%	2765
	11th to 20th	Night	12% 18% 6% 12% 7% 7% 5% 4 4 5% 20%	2404
	21st to end	Night	14% 16% 5% 10% 7% 6% 5% 4 4 5% 25%	1980
March	1st to 10th	Night	20% 15% 5% 12% 8% 5% 6% 3 4 4 20%	5364
	11th to 20th	Night	14% 14% 5% 10% 6% 6% 6% 5% 5% 6% 23%	5309
	21st to end	Night	15% 14% 5% 10% 5% 5% 4 3 5% 6% 28%	4967
April	1st to 10th	Night	21% 15% 5% 9% 5% 5% 4 3 5% 4 26%	2000
	11th to 20th	Night	20% 11% 3 7% 4 4 5% 3 5% 6% 32%	1919
	21st to end	Night	22% 12% 3 7% 4 4 3 3 4 6% 32%	1800
May	1st to 10th	Night	24% 10% 4 8% 6% 5% 4 3 4 5% 28%	3760
	11th to 20th	Night	25% 7% 2 7% 4 4 5% 3 5% 7% 30%	3575
	21st to end	Night	20% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	3926
June	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 4 6% 6% 30%	1470
	11th to 20th	Night	18% 7% 2 7% 5% 5% 5% 3 6% 7% 35%	1400
	21st to end	Night	22% 8% 2 8% 5% 5% 4 3 6% 7% 28%	1458
July	1st to 10th	Night	14% 6% 3 8% 6% 6% 6% 5% 8% 10% 28%	3600
	11th to 20th	Night	17% 9% 3 7% 4 5% 4 3 6% 7% 35%	3600
	21st to end	Night	13% 5% 2 6% 4 4 5% 4 6% 7% 44%	3926
August	1st to 10th	Night	14% 8% 3 7% 4 5% 5% 5% 6% 7% 34%	2586
	11th to 20th	Night	18% 8% 3 7% 4 5% 5% 5% 6% 7% 31%	2764
	21st to end	Night	18% 8% 3 7% 5% 5% 4 4 6% 6% 34%	3515
September	1st to 10th	Night	18% 7% 3 6% 3 4 4 3 4 6% 42%	2000
	11th to 20th	Night	18% 6% 2 6% 4 4 4 4 5% 6% 41%	2130
	21st to end	Night	19% 7% 2 6% 4 4 4 3 4 5% 40%	2200
October	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 5% 6% 37%	2270
	11th to 20th	Night	24% 11% 3 8% 4 4 4 3 4 6% 28%	2528
	21st to end	Night	27% 13% 3 8% 4 4 4 3 5% 5% 23%	2860
November	1st to 10th	Night	21% 16% 5% 9% 4 4 4 3 4 6% 25%	2667
	11th to 20th	Night	20% 14% 5% 11% 5% 4 5% 3 5% 5% 24%	2808
	21st to end	Night	22% 19% 6% 11% 6% 4 4 3 4 4 16%	3021
December	1st to 10th	Night	15% 16% 8% 18% 9% 5% 4 3 4 4 15%	5060
	11th to 20th	Night	21% 17% 5% 14% 9% 4 4 3 3 4 17%	5741
	21st to end	Night	18% 20% 7% 12% 6% 5% 4 2 3 4 20%	7323

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	79% 4 6% 9% 2	3678
	11th to 20th	Day	72% 4 5% 13% 6%	3648
	21st to end	Day	57% 5% 5% 18% 14%	3960
February	1st to 10th	Day	67% 4 7% 12% 8% 2	1633
	11th to 20th	Day	66% 4 9% 12% 9% 1	1890
	21st to end	Day	64% 4 6% 15% 8% 2	1650
March	1st to 10th	Day	67% 3 6% 10% 10% 3	4479
	11th to 20th	Day	60% 4 12% 13% 10% 1	4649
	21st to end	Day	57% 3 5% 16% 16% 3	5962
April	1st to 10th	Day	64% 3 8% 12% 10% 2	2400
	11th to 20th	Day	54% 4 8% 20% 12% 2	2428
	21st to end	Day	59% 4 8% 15% 13% 2	2600
May	1st to 10th	Day	58% 6% 7% 14% 13% 1	6234
	11th to 20th	Day	52% 7% 14% 18% 8% 2	6322
	21st to end	Day	56% 6% 8% 14% 12% 4	6949
June	1st to 10th	Day	56% 6% 10% 18% 9% 2	2925
	11th to 20th	Day	50% 6% 13% 19% 11% 2	3000
	21st to end	Day	58% 6% 12% 14% 10% 1	2934
July	1st to 10th	Day	54% 6% 13% 22% 5%	6290
	11th to 20th	Day	49% 7% 18% 22% 4	6285
	21st to end	Day	43% 6% 18% 27% 7%	6728
August	1st to 10th	Day	51% 7% 18% 19% 5%	4549
	11th to 20th	Day	52% 7% 20% 18% 4	4292
	21st to end	Day	52% 6% 13% 17% 12%	4380
September	1st to 10th	Day	46% 6% 16% 23% 9% 1	2400
	11th to 20th	Day	45% 5% 16% 25% 9%	2269
	21st to end	Day	51% 5% 13% 23% 8%	2200
October	1st to 10th	Day	56% 5% 11% 23% 5%	3466
	11th to 20th	Day	62% 4 13% 19% 1	3022
	21st to end	Day	59% 5% 17% 16% 3	3180
November	1st to 10th	Day	63% 6% 9% 17% 5% 1	1851
	11th to 20th	Day	68% 4 9% 14% 5%	1811
	21st to end	Day	74% 5% 9% 10% 2	1595
December	1st to 10th	Day	74% 3 8% 12% 3	2350
	11th to 20th	Day	68% 4 6% 17% 6%	2688
	21st to end	Day	67% 5% 6% 14% 7%	3864
January	1st to 10th	Night	77% 5% 7% 8% 3	6489
	11th to 20th	Night	73% 6% 7% 11% 3	6445
	21st to end	Night	59% 5% 9% 15% 12%	6996
February	1st to 10th	Night	59% 5% 8% 17% 10% 1	2765
	11th to 20th	Night	65% 5% 9% 13% 7% 1	2406
	21st to end	Night	60% 5% 7% 16% 11% 1	1980
March	1st to 10th	Night	69% 4 7% 11% 9% 1	5374
	11th to 20th	Night	60% 5% 11% 15% 8% 1	5310
	21st to end	Night	58% 3 7% 21% 9% 2	4967
April	1st to 10th	Night	61% 4 7% 14% 12% 2	2000
	11th to 20th	Night	51% 6% 8% 21% 12% 2	1919
	21st to end	Night	54% 4 7% 20% 13% 2	1800
May	1st to 10th	Night	58% 5% 8% 18% 10% 1	3766
	11th to 20th	Night	51% 7% 12% 21% 8% 1	3576
	21st to end	Night	53% 6% 10% 17% 12% 2	3927
June	1st to 10th	Night	51% 7% 11% 21% 9% 1	1470
	11th to 20th	Night	47% 5% 13% 20% 12% 3	1400
	21st to end	Night	53% 7% 12% 17% 11%	1458
July	1st to 10th	Night	48% 7% 20% 21% 4	3560
	11th to 20th	Night	48% 5% 21% 23% 3	3552
	21st to end	Night	38% 5% 21% 28% 7%	3809
August	1st to 10th	Night	46% 7% 19% 20% 7%	2576
	11th to 20th	Night	50% 7% 21% 18% 4	2789
	21st to end	Night	49% 6% 13% 23% 9%	3515
September	1st to 10th	Night	43% 6% 15% 26% 10% 1	2000
	11th to 20th	Night	43% 6% 15% 29% 6%	2130
	21st to end	Night	46% 5% 13% 28% 9%	2200
October	1st to 10th	Night	52% 4 13% 23% 7%	3712
	11th to 20th	Night	54% 4 15% 22% 4	4116
	21st to end	Night	56% 4 16% 16% 7%	4610
November	1st to 10th	Night	60% 5% 12% 17% 6% 1	2667
	11th to 20th	Night	60% 7% 9% 17% 6%	2808
	21st to end	Night	70% 5% 9% 12% 4	3021
December	1st to 10th	Night	73% 3 10% 10% 3	5060
	11th to 20th	Night	71% 4 5% 16% 4	5742
	21st to end	Night	69% 4 5% 14% 8%	7323

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% 20% 44% 10% 1	220
	11th to 20th	Day	26% 17% 37% 16% 4	212
	21st to end	Day	20% 19% 40% 15% 6%	220
February	1st to 10th	Day	24% 30% 37% 7% 1 1	217
	11th to 20th	Day	28% 26% 34% 10% 2	429
	21st to end	Day	27% 30% 29% 11% 4	414
March	1st to 10th	Day	25% 24% 32% 14% 5%	500
	11th to 20th	Day	29% 22% 32% 13% 4 1	500
	21st to end	Day	30% 24% 27% 13% 5% 1	550
April	1st to 10th	Day	33% 18% 32% 13% 4 1	500
	11th to 20th	Day	34% 21% 25% 13% 5% 3	492
	21st to end	Day	32% 23% 30% 10% 4 1	500
May	1st to 10th	Day	34% 16% 30% 13% 6% 1	503
	11th to 20th	Day	26% 18% 30% 20% 5% 1	527
	21st to end	Day	28% 23% 27% 16% 5%	573
June	1st to 10th	Day	24% 18% 34% 18% 6%	655
	11th to 20th	Day	25% 17% 29% 20% 8% 1	730
	21st to end	Day	27% 14% 28% 22% 8% 1	664
July	1st to 10th	Day	20% 14% 38% 20% 8% 1	525
	11th to 20th	Day	19% 15% 37% 22% 6% 2	524
	21st to end	Day	20% 13% 32% 24% 10% 1	576
August	1st to 10th	Day	20% 11% 32% 24% 10% 3	526
	11th to 20th	Day	19% 8% 32% 25% 11% 4	519
	21st to end	Day	25% 15% 26% 18% 13% 4	583
September	1st to 10th	Day	22% 15% 31% 19% 9% 3	530
	11th to 20th	Day	19% 14% 26% 23% 13% 4	530
	21st to end	Day	26% 16% 27% 15% 11% 5%	530
October	1st to 10th	Day	25% 19% 26% 18% 10% 1	530
	11th to 20th	Day	30% 22% 26% 12% 8% 2	458
	21st to end	Day	38% 16% 26% 15% 5%	462
November	1st to 10th	Day	35% 20% 28% 13% 4 1	425
	11th to 20th	Day	32% 18% 28% 17% 4 1	351
	21st to end	Day	33% 23% 31% 11% 1	220
December	1st to 10th	Day	30% 18% 33% 15% 4	220
	11th to 20th	Day	31% 22% 36% 7% 4	220
	21st to end	Day	33% 20% 31% 9% 7% 1	242
January	1st to 10th	Night	27% 17% 34% 18% 4	840
	11th to 20th	Night	26% 17% 33% 18% 5%	825
	21st to end	Night	17% 15% 36% 25% 7%	880
February	1st to 10th	Night	18% 22% 35% 20% 5%	800
	11th to 20th	Night	23% 20% 35% 18% 4	783
	21st to end	Night	24% 19% 31% 22% 4	660
March	1st to 10th	Night	24% 16% 37% 19% 5%	800
	11th to 20th	Night	26% 18% 34% 16% 5% 1	800
	21st to end	Night	24% 19% 31% 19% 7% 1	880
April	1st to 10th	Night	31% 16% 27% 21% 5% 1	800
	11th to 20th	Night	28% 15% 29% 22% 5% 1	735
	21st to end	Night	30% 20% 29% 17% 5%	600
May	1st to 10th	Night	28% 16% 29% 22% 5%	475
	11th to 20th	Night	24% 11% 31% 26% 8%	394
	21st to end	Night	26% 12% 32% 25% 5%	433
June	1st to 10th	Night	24% 10% 32% 28% 7%	400
	11th to 20th	Night	19% 11% 29% 33% 9%	400
	21st to end	Night	24% 9% 27% 32% 9% 1	400
July	1st to 10th	Night	17% 8% 30% 34% 12%	390
	11th to 20th	Night	17% 5% 27% 36% 15% 1	388
	21st to end	Night	16% 6% 27% 38% 12% 1	438
August	1st to 10th	Night	15% 5% 23% 42% 14%	398
	11th to 20th	Night	17% 7% 26% 33% 16% 1	520
	21st to end	Night	22% 10% 24% 29% 14% 1	835
September	1st to 10th	Night	23% 8% 26% 33% 10% 1	800
	11th to 20th	Night	23% 6% 25% 29% 16% 2	800
	21st to end	Night	23% 11% 27% 25% 13% 1	800
October	1st to 10th	Night	23% 13% 27% 27% 9% 1	800
	11th to 20th	Night	32% 12% 26% 21% 8% 1	800
	21st to end	Night	35% 13% 29% 18% 6%	880
November	1st to 10th	Night	32% 14% 29% 20% 5% 1	820
	11th to 20th	Night	29% 16% 31% 16% 7% 1	840
	21st to end	Night	35% 18% 25% 19% 4	883
December	1st to 10th	Night	30% 17% 28% 20% 5%	950
	11th to 20th	Night	29% 18% 30% 17% 4 1	950
	21st to end	Night	32% 15% 31% 17% 4 1	966

Page generated at 2026-01-14 01:14:32. Version ac07

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