White Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the White Lake Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	13% 7% 3 6% 4 4 5% 4 6% 8% 41%	131459
	Night	13% 7% 3 6% 4 4 4 4 5% 7% 43%	130387
Transparency	Day	39% 7% 12% 20% 18% 5%	135245
	Night	38% 6% 10% 20% 20% 6%	134670
Seeing	Day	16% 11% 23% 23% 18% 9%	16630
	Night	16% 10% 23% 28% 19% 4	26731

By Month

Percentage Cloud Cover SE (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	20% 10% 3 6% 3 3 3 2 3 5% 42%	5258
	11th to 20th	Day	10% 8% 3 6% 3 2 3 2 3 4 56%	5280
	21st to end	Day	22% 11% 3 7% 3 4 4 3 4 5% 34%	6097
February	1st to 10th	Day	16% 9% 2 6% 4 3 4 3 6% 7% 39%	2100
	11th to 20th	Day	22% 7% 3 6% 3 4 3 3 4 5% 39%	2048
	21st to end	Day	20% 8% 3 6% 3 3 4 3 5% 5% 39%	1730
March	1st to 10th	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 34%	4669
	11th to 20th	Day	10% 6% 3 6% 4 4 4 3 5% 7% 49%	5090
	21st to end	Day	14% 6% 3 8% 5% 4 4 3 4 6% 43%	5586
April	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 7% 44%	2306
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 3 5% 6% 47%	2374
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 6% 8% 47%	2535
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	5862
	11th to 20th	Day	16% 8% 3 8% 5% 6% 6% 5% 7% 9% 28%	5861
	21st to end	Day	13% 7% 3 6% 4 5% 4 4 6% 7% 39%	6433
June	1st to 10th	Day	10% 6% 3 8% 5% 6% 5% 4 7% 10% 35%	2600
	11th to 20th	Day	5% 4 1 5% 5% 5% 6% 6% 9% 13% 42%	2600
	21st to end	Day	7% 4 2 7% 6% 6% 6% 5% 9% 10% 38%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 6% 9% 11% 35%	5900
	11th to 20th	Day	6% 4 2 7% 6% 5% 7% 6% 8% 13% 37%	5900
	21st to end	Day	9% 7% 3 9% 7% 7% 7% 6% 9% 11% 24%	6424
August	1st to 10th	Day	6% 4 2 7% 5% 5% 6% 6% 9% 11% 39%	4234
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 7% 11% 42%	3855
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 6% 9% 48%	4312
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 6% 9% 41%	2230
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 6% 8% 43%	2200
	21st to end	Day	12% 5% 2 6% 4 4 4 4 5% 7% 46%	2200
October	1st to 10th	Day	11% 5% 2 5% 4 4 3 3 4 6% 53%	2200
	11th to 20th	Day	11% 5% 2 3 3 3 3 3 4 6% 57%	2110
	21st to end	Day	14% 6% 2 5% 3 3 3 3 3 5% 54%	2185
November	1st to 10th	Day	12% 6% 3 5% 4 3 3 2 4 5% 54%	1852
	11th to 20th	Day	12% 6% 2 5% 3 3 3 2 5% 6% 52%	1890
	21st to end	Day	13% 6% 2 7% 3 4 4 3 6% 7% 46%	1890
December	1st to 10th	Day	18% 8% 2 5% 3 3 3 3 5% 6% 44%	3020
	11th to 20th	Day	14% 9% 3 9% 5% 4 4 3 4 5% 40%	3463
	21st to end	Day	19% 7% 3 6% 4 3 4 4 5% 6% 39%	4565
January	1st to 10th	Night	16% 8% 3 7% 4 3 3 3 5% 6% 42%	7635
	11th to 20th	Night	13% 8% 3 6% 3 3 3 2 4 5% 49%	7680
	21st to end	Night	21% 11% 3 6% 3 3 3 2 3 4 41%	8068
February	1st to 10th	Night	15% 8% 3 6% 5% 3 4 3 5% 7% 43%	2520
	11th to 20th	Night	21% 7% 2 7% 4 3 4 3 4 4 40%	2458
	21st to end	Night	19% 8% 3 5% 3 4 3 3 6% 6% 39%	2076
March	1st to 10th	Night	20% 11% 3 7% 4 4 4 3 5% 6% 33%	5403
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 5% 7% 51%	5090
	21st to end	Night	13% 6% 2 7% 5% 4 5% 3 5% 6% 45%	5588
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 7% 7% 47%	2132
	11th to 20th	Night	11% 6% 3 5% 3 3 3 3 6% 7% 50%	1975
	21st to end	Night	9% 5% 2 6% 4 4 4 3 6% 8% 50%	1862
May	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 4 7% 9% 36%	4033
	11th to 20th	Night	18% 8% 3 8% 5% 5% 5% 5% 7% 7% 30%	4032
	21st to end	Night	11% 6% 2 5% 4 5% 5% 4 7% 10% 41%	4421
June	1st to 10th	Night	6% 5% 2 7% 5% 6% 5% 6% 9% 11% 37%	1800
	11th to 20th	Night	4 4 2 6% 4 4 5% 5% 8% 12% 45%	1800
	21st to end	Night	6% 4 2 5% 5% 5% 4 5% 9% 12% 44%	1800
July	1st to 10th	Night	6% 6% 3 8% 6% 6% 7% 5% 7% 11% 34%	4058
	11th to 20th	Night	5% 4 2 7% 5% 6% 7% 6% 9% 13% 36%	4057
	21st to end	Night	8% 8% 4 9% 6% 6% 6% 6% 8% 10% 29%	4427
August	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 8% 13% 40%	2915
	11th to 20th	Night	7% 6% 3 7% 4 4 4 3 5% 8% 47%	3191
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 8% 51%	3586
September	1st to 10th	Night	8% 5% 3 7% 6% 6% 4 5% 7% 9% 42%	2150
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 7% 44%	2200
	21st to end	Night	12% 5% 2 5% 3 4 4 3 5% 8% 49%	2200
October	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 56%	2200
	11th to 20th	Night	9% 5% 2 5% 3 3 4 2 3 4 59%	2287
	21st to end	Night	12% 6% 2 4 3 3 3 3 3 5% 56%	2652
November	1st to 10th	Night	11% 6% 2 4 3 3 4 3 4 5% 54%	2666
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 6% 54%	2730
	21st to end	Night	14% 7% 2 5% 4 3 4 4 5% 6% 46%	2730
December	1st to 10th	Night	17% 7% 2 6% 3 3 3 3 4 5% 48%	4380
	11th to 20th	Night	17% 9% 3 6% 3 3 4 3 4 6% 43%	4952
	21st to end	Night	18% 8% 3 6% 4 3 4 3 4 6% 39%	6633

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	46% 4 6% 22% 18% 3	5259
	11th to 20th	Day	32% 4 5% 19% 31% 9%	5280
	21st to end	Day	52% 4 5% 14% 19% 6%	6097
February	1st to 10th	Day	44% 4 9% 19% 21% 3	2100
	11th to 20th	Day	46% 5% 6% 19% 18% 6%	2058
	21st to end	Day	46% 5% 7% 15% 19% 8%	1730
March	1st to 10th	Day	51% 4 9% 11% 18% 8%	4669
	11th to 20th	Day	35% 4 7% 15% 24% 16%	5090
	21st to end	Day	42% 5% 8% 22% 20% 4	5586
April	1st to 10th	Day	39% 5% 11% 21% 18% 5%	2306
	11th to 20th	Day	36% 6% 7% 19% 24% 7%	2374
	21st to end	Day	34% 6% 11% 26% 20% 3	2535
May	1st to 10th	Day	43% 8% 15% 20% 13% 1	5897
	11th to 20th	Day	49% 9% 15% 18% 8% 1	5865
	21st to end	Day	40% 8% 11% 22% 17% 2	6447
June	1st to 10th	Day	41% 10% 17% 20% 10% 2	2600
	11th to 20th	Day	29% 11% 20% 26% 13% 1	2600
	21st to end	Day	35% 12% 17% 22% 11% 2	2600
July	1st to 10th	Day	37% 12% 20% 23% 7% 1	5835
	11th to 20th	Day	34% 14% 29% 18% 4 1	5831
	21st to end	Day	48% 13% 18% 17% 4	6240
August	1st to 10th	Day	33% 9% 15% 31% 10%	4221
	11th to 20th	Day	33% 9% 18% 25% 13% 2	3891
	21st to end	Day	31% 9% 14% 23% 20% 3	4311
September	1st to 10th	Day	38% 9% 15% 16% 17% 5%	2230
	11th to 20th	Day	38% 8% 12% 21% 19% 2	2200
	21st to end	Day	37% 6% 13% 16% 22% 6%	2200
October	1st to 10th	Day	25% 4 7% 13% 37% 14%	3590
	11th to 20th	Day	20% 3 5% 16% 30% 26%	3415
	21st to end	Day	27% 3 6% 23% 29% 12%	3511
November	1st to 10th	Day	34% 3 7% 16% 31% 9%	1852
	11th to 20th	Day	34% 3 7% 19% 28% 10%	1890
	21st to end	Day	37% 5% 8% 19% 26% 6%	1890
December	1st to 10th	Day	42% 4 5% 18% 25% 6%	3020
	11th to 20th	Day	48% 4 7% 18% 18% 5%	3460
	21st to end	Day	43% 6% 9% 25% 15% 2	4565
January	1st to 10th	Night	42% 4 9% 22% 20% 3	7636
	11th to 20th	Night	38% 3 6% 18% 27% 7%	7680
	21st to end	Night	49% 3 4 15% 23% 7%	8068
February	1st to 10th	Night	41% 4 8% 20% 20% 7%	2520
	11th to 20th	Night	47% 5% 6% 17% 19% 7%	2459
	21st to end	Night	45% 4 7% 17% 18% 9%	2076
March	1st to 10th	Night	50% 5% 9% 14% 17% 5%	5403
	11th to 20th	Night	31% 4 8% 16% 25% 15%	5090
	21st to end	Night	39% 5% 7% 22% 20% 7%	5588
April	1st to 10th	Night	35% 6% 10% 19% 23% 7%	2132
	11th to 20th	Night	32% 5% 8% 22% 24% 9%	1975
	21st to end	Night	32% 6% 12% 27% 19% 5%	1862
May	1st to 10th	Night	42% 10% 16% 19% 12% 1	4060
	11th to 20th	Night	49% 8% 16% 18% 8%	4033
	21st to end	Night	35% 10% 14% 23% 17% 2	4429
June	1st to 10th	Night	34% 14% 18% 19% 13% 2	1800
	11th to 20th	Night	28% 10% 23% 25% 13% 2	1800
	21st to end	Night	30% 11% 21% 20% 15% 4	1800
July	1st to 10th	Night	41% 10% 18% 23% 7% 1	4015
	11th to 20th	Night	35% 15% 26% 18% 4 1	4006
	21st to end	Night	47% 13% 19% 17% 4 1	4297
August	1st to 10th	Night	33% 9% 17% 32% 8%	2904
	11th to 20th	Night	34% 8% 14% 28% 15% 1	3221
	21st to end	Night	30% 9% 13% 19% 26% 3	3586
September	1st to 10th	Night	37% 11% 15% 17% 15% 5%	2150
	11th to 20th	Night	37% 9% 12% 21% 19% 2	2200
	21st to end	Night	34% 7% 12% 20% 20% 7%	2200
October	1st to 10th	Night	24% 4 9% 13% 38% 12%	3590
	11th to 20th	Night	18% 3 5% 17% 34% 23%	3721
	21st to end	Night	23% 3 4 24% 30% 16%	4276
November	1st to 10th	Night	32% 5% 8% 18% 26% 11%	2666
	11th to 20th	Night	32% 4 6% 19% 28% 11%	2730
	21st to end	Night	38% 5% 6% 19% 25% 6%	2730
December	1st to 10th	Night	39% 4 6% 19% 26% 6%	4380
	11th to 20th	Night	43% 5% 9% 15% 23% 5%	4954
	21st to end	Night	44% 5% 7% 27% 15% 2	6633

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	26% 27% 33% 12% 2	330
	11th to 20th	Day	26% 25% 30% 14% 4 1	330
	21st to end	Day	26% 25% 35% 11% 3 1	363
February	1st to 10th	Day	22% 22% 35% 19% 2	330
	11th to 20th	Day	36% 21% 30% 11% 2	330
	21st to end	Day	27% 24% 36% 10% 3	273
March	1st to 10th	Day	26% 17% 32% 19% 4 1	367
	11th to 20th	Day	21% 20% 32% 21% 6% 1	540
	21st to end	Day	18% 19% 34% 22% 6% 1	594
April	1st to 10th	Day	18% 16% 33% 19% 12% 1	504
	11th to 20th	Day	13% 16% 35% 22% 13% 1	492
	21st to end	Day	15% 11% 29% 26% 17% 2	500
May	1st to 10th	Day	15% 11% 32% 24% 14% 5%	503
	11th to 20th	Day	15% 7% 23% 28% 18% 10%	527
	21st to end	Day	14% 8% 18% 29% 20% 12%	573
June	1st to 10th	Day	13% 5% 18% 28% 25% 12%	530
	11th to 20th	Day	7% 3 16% 26% 28% 20%	530
	21st to end	Day	9% 2 16% 27% 28% 18%	530
July	1st to 10th	Day	10% 2 8% 25% 29% 25%	525
	11th to 20th	Day	8% 1 9% 20% 35% 28%	524
	21st to end	Day	9% 3 7% 21% 40% 21%	576
August	1st to 10th	Day	13% 3 12% 25% 27% 21%	526
	11th to 20th	Day	11% 1 11% 30% 27% 19%	519
	21st to end	Day	13% 3 10% 26% 28% 21%	583
September	1st to 10th	Day	11% 4 13% 30% 26% 16%	530
	11th to 20th	Day	14% 6% 13% 24% 31% 12%	530
	21st to end	Day	13% 4 19% 28% 26% 11%	530
October	1st to 10th	Day	14% 5% 22% 33% 18% 8%	530
	11th to 20th	Day	12% 11% 23% 28% 24% 3	530
	21st to end	Day	19% 12% 23% 24% 19% 2	568
November	1st to 10th	Day	19% 13% 29% 22% 13% 4	330
	11th to 20th	Day	16% 18% 37% 20% 8% 2	330
	21st to end	Day	10% 22% 38% 21% 8% 1	330
December	1st to 10th	Day	28% 21% 30% 16% 3 1	330
	11th to 20th	Day	23% 18% 31% 21% 6% 1	330
	21st to end	Day	26% 17% 37% 17% 4	363
January	1st to 10th	Night	23% 22% 29% 21% 5%	840
	11th to 20th	Night	22% 19% 30% 23% 6%	840
	21st to end	Night	22% 19% 33% 23% 3	924
February	1st to 10th	Night	21% 15% 32% 28% 5%	840
	11th to 20th	Night	25% 20% 32% 22% 1	823
	21st to end	Night	24% 19% 35% 20% 3	692
March	1st to 10th	Night	18% 18% 31% 28% 5%	840
	11th to 20th	Night	16% 15% 34% 25% 10%	840
	21st to end	Night	17% 14% 33% 28% 8%	924
April	1st to 10th	Night	13% 15% 32% 27% 12%	725
	11th to 20th	Night	15% 13% 29% 31% 12% 1	593
	21st to end	Night	12% 9% 26% 36% 17% 1	600
May	1st to 10th	Night	10% 11% 25% 33% 21% 1	600
	11th to 20th	Night	14% 7% 22% 31% 22% 4	591
	21st to end	Night	12% 4 14% 34% 30% 6%	650
June	1st to 10th	Night	8% 3 13% 39% 32% 6%	600
	11th to 20th	Night	4 3 13% 30% 36% 13%	600
	21st to end	Night	6% 2 8% 34% 41% 9%	600
July	1st to 10th	Night	9% 3 8% 22% 45% 14%	585
	11th to 20th	Night	7% 1 7% 19% 50% 15%	582
	21st to end	Night	8% 2 9% 25% 46% 11%	657
August	1st to 10th	Night	10% 2 7% 31% 39% 12%	597
	11th to 20th	Night	10% 2 10% 29% 40% 9%	594
	21st to end	Night	14% 2 11% 27% 36% 10%	660
September	1st to 10th	Night	10% 2 12% 29% 37% 10%	750
	11th to 20th	Night	15% 3 14% 26% 37% 4	800
	21st to end	Night	15% 3 16% 31% 33% 2	800
October	1st to 10th	Night	11% 5% 22% 33% 22% 7%	800
	11th to 20th	Night	12% 8% 24% 32% 22% 3	800
	21st to end	Night	15% 10% 23% 35% 15% 2	880
November	1st to 10th	Night	15% 7% 26% 30% 20% 1	820
	11th to 20th	Night	15% 12% 31% 28% 12% 1	840
	21st to end	Night	19% 15% 28% 26% 12%	840
December	1st to 10th	Night	23% 17% 29% 24% 7% 1	840
	11th to 20th	Night	24% 13% 30% 25% 8%	840
	21st to end	Night	26% 15% 26% 27% 5%	924

Page generated at 2026-03-30 02:18:44. Version ac07

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