Freeman Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Freeman Lake Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	14% 8% 3 7% 5% 4 5% 4 6% 7% 38%	137753
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	134958
Transparency	Day	44% 4 10% 24% 16% 2	141407
	Night	44% 3 9% 23% 17% 3	139365
Seeing	Day	16% 10% 23% 24% 18% 8%	17574
	Night	17% 7% 25% 31% 18% 2	27258

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	16% 11% 7% 9% 5% 4 5% 4 5% 6% 30%	4691
	11th to 20th	Day	12% 10% 4 12% 5% 3 4 3 4 4 40%	4895
	21st to end	Day	21% 11% 6% 6% 5% 4 4 3 4 6% 31%	5942
February	1st to 10th	Day	18% 12% 5% 8% 5% 4 3 3 4 6% 35%	2100
	11th to 20th	Day	19% 9% 4 8% 5% 4 4 4 5% 6% 33%	2048
	21st to end	Day	17% 11% 4 9% 5% 4 4 4 5% 5% 33%	1730
March	1st to 10th	Day	19% 12% 4 7% 4 3 3 3 4 5% 36%	4580
	11th to 20th	Day	13% 10% 3 6% 4 4 4 3 5% 7% 43%	4967
	21st to end	Day	13% 8% 3 8% 5% 5% 4 4 5% 7% 39%	5740
April	1st to 10th	Day	14% 6% 2 6% 4 5% 6% 4 5% 8% 40%	2520
	11th to 20th	Day	14% 9% 3 6% 4 4 4 3 5% 6% 41%	2494
	21st to end	Day	17% 7% 3 6% 5% 4 5% 4 4 7% 38%	2667
May	1st to 10th	Day	18% 8% 2 7% 5% 6% 5% 5% 7% 8% 30%	7616
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 6% 7% 38%	7626
	21st to end	Day	18% 8% 3 7% 5% 6% 5% 5% 6% 9% 28%	8391
June	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 4 6% 8% 42%	2773
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	2940
	21st to end	Day	7% 5% 2 5% 5% 5% 6% 5% 7% 9% 43%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 40%	6500
	11th to 20th	Day	8% 5% 3 7% 5% 6% 6% 6% 8% 11% 37%	6339
	21st to end	Day	10% 8% 3 9% 6% 7% 7% 5% 8% 9% 28%	6424
August	1st to 10th	Day	5% 5% 2 5% 4 4 4 4 7% 11% 49%	4234
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 5% 8% 10% 40%	3896
	21st to end	Day	5% 3 1 5% 4 4 5% 3 6% 8% 56%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 5% 4 4 6% 7% 43%	2335
	11th to 20th	Day	11% 7% 2 5% 4 3 5% 3 6% 8% 46%	2200
	21st to end	Day	14% 5% 2 6% 4 4 4 3 6% 7% 47%	2200
October	1st to 10th	Day	10% 5% 2 5% 4 3 3 2 5% 6% 56%	2200
	11th to 20th	Day	12% 5% 2 5% 4 2 4 4 4 6% 52%	2125
	21st to end	Day	18% 7% 3 6% 3 3 3 3 4 5% 44%	1980
November	1st to 10th	Day	12% 5% 2 5% 3 4 3 2 4 6% 51%	1852
	11th to 20th	Day	15% 10% 2 5% 4 4 3 3 5% 5% 44%	1890
	21st to end	Day	16% 9% 3 7% 4 3 4 3 5% 6% 39%	1890
December	1st to 10th	Day	17% 10% 4 7% 4 3 3 3 4 4 40%	3020
	11th to 20th	Day	17% 11% 5% 10% 6% 5% 4 2 4 5% 30%	3463
	21st to end	Day	20% 15% 5% 9% 5% 4 4 3 5% 6% 24%	4233
January	1st to 10th	Night	14% 13% 4 11% 4 4 3 3 4 6% 32%	8232
	11th to 20th	Night	15% 11% 4 9% 5% 5% 4 3 4 4 36%	8065
	21st to end	Night	18% 11% 4 8% 4 3 4 2 4 5% 37%	8221
February	1st to 10th	Night	21% 11% 4 8% 5% 4 5% 3 5% 5% 29%	2520
	11th to 20th	Night	20% 9% 4 8% 5% 4 4 3 4 5% 35%	2458
	21st to end	Night	21% 9% 4 8% 4 4 4 3 4 5% 34%	2076
March	1st to 10th	Night	19% 14% 3 7% 5% 4 3 3 4 4 35%	5493
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 5% 47%	5198
	21st to end	Night	13% 9% 3 8% 5% 5% 5% 4 5% 6% 36%	5461
April	1st to 10th	Night	12% 6% 2 7% 4 4 5% 3 7% 7% 43%	2100
	11th to 20th	Night	14% 8% 3 7% 4 3 4 4 4 6% 43%	2075
	21st to end	Night	15% 8% 3 7% 5% 4 5% 3 5% 7% 38%	1951
May	1st to 10th	Night	16% 9% 3 8% 6% 5% 5% 4 6% 8% 29%	5237
	11th to 20th	Night	12% 7% 2 9% 6% 5% 6% 4 6% 8% 35%	5234
	21st to end	Night	16% 9% 3 8% 5% 5% 5% 4 7% 8% 30%	5761
June	1st to 10th	Night	9% 5% 2 6% 4 3 4 4 6% 9% 47%	1840
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 7% 10% 49%	1680
	21st to end	Night	4 3 2 5% 5% 6% 4 4 8% 12% 47%	1680
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 6% 8% 47%	3698
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 8% 11% 45%	3740
	21st to end	Night	8% 7% 3 9% 6% 5% 6% 5% 8% 10% 33%	4427
August	1st to 10th	Night	5% 4 2 5% 4 3 4 4 6% 9% 54%	2915
	11th to 20th	Night	5% 4 2 6% 4 4 4 5% 8% 10% 48%	3137
	21st to end	Night	3 3 1 4 2 2 4 3 5% 9% 64%	3586
September	1st to 10th	Night	10% 6% 2 5% 3 4 5% 3 5% 7% 51%	2044
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 6% 7% 51%	2200
	21st to end	Night	11% 5% 2 5% 4 4 4 3 4 6% 52%	2200
October	1st to 10th	Night	9% 4 2 4 2 3 2 3 4 6% 60%	2200
	11th to 20th	Night	12% 5% 1 5% 4 4 4 3 4 6% 52%	2267
	21st to end	Night	17% 7% 2 5% 4 4 3 3 4 5% 47%	2860
November	1st to 10th	Night	12% 6% 2 5% 4 4 3 3 4 4 51%	2666
	11th to 20th	Night	13% 8% 3 6% 4 4 3 3 4 6% 45%	2730
	21st to end	Night	17% 9% 3 6% 4 3 3 4 5% 5% 41%	2730
December	1st to 10th	Night	17% 12% 3 8% 4 4 3 2 4 4 39%	4380
	11th to 20th	Night	21% 10% 3 7% 3 3 3 3 4 6% 36%	4952
	21st to end	Night	22% 15% 5% 10% 6% 5% 4 3 4 4 22%	6944

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	57% 2 9% 23% 8% 1	4692
	11th to 20th	Day	49% 2 5% 24% 17% 2	4895
	21st to end	Day	55% 2 7% 22% 10% 4	5942
February	1st to 10th	Day	53% 3 8% 24% 12% 1	2100
	11th to 20th	Day	52% 3 8% 21% 12% 3	2058
	21st to end	Day	53% 2 7% 20% 13% 4	1730
March	1st to 10th	Day	53% 2 4 18% 19% 5%	4580
	11th to 20th	Day	42% 2 8% 25% 20% 2	4967
	21st to end	Day	45% 3 11% 28% 13% 1	5742
April	1st to 10th	Day	42% 4 13% 23% 16% 2	2520
	11th to 20th	Day	43% 3 10% 23% 18% 3	2494
	21st to end	Day	46% 4 9% 27% 13% 2	2667
May	1st to 10th	Day	50% 4 13% 22% 10% 1	7651
	11th to 20th	Day	46% 4 11% 24% 15%	7630
	21st to end	Day	53% 5% 11% 16% 14% 1	8405
June	1st to 10th	Day	40% 5% 10% 25% 18% 2	2773
	11th to 20th	Day	34% 5% 15% 26% 18% 2	2940
	21st to end	Day	34% 7% 15% 22% 18% 4	2940
July	1st to 10th	Day	39% 6% 12% 25% 17% 1	6431
	11th to 20th	Day	39% 8% 16% 27% 9% 2	6264
	21st to end	Day	49% 8% 15% 19% 8% 1	6240
August	1st to 10th	Day	28% 6% 14% 36% 15% 1	4221
	11th to 20th	Day	37% 6% 13% 28% 16% 1	3933
	21st to end	Day	26% 4 10% 40% 16% 3	4311
September	1st to 10th	Day	40% 5% 10% 22% 19% 4	2335
	11th to 20th	Day	36% 4 9% 26% 23% 3	2200
	21st to end	Day	37% 4 8% 25% 23% 4	2200
October	1st to 10th	Day	20% 2 5% 26% 43% 3	3590
	11th to 20th	Day	23% 3 10% 27% 33% 4	3431
	21st to end	Day	32% 2 6% 30% 22% 8%	3180
November	1st to 10th	Day	35% 2 8% 29% 21% 4	1852
	11th to 20th	Day	43% 2 7% 24% 19% 5%	1890
	21st to end	Day	46% 3 8% 23% 18% 2	1890
December	1st to 10th	Day	48% 2 7% 24% 18% 1	3020
	11th to 20th	Day	58% 2 7% 18% 13% 2	3460
	21st to end	Day	62% 3 10% 18% 7%	4233
January	1st to 10th	Night	54% 3 9% 24% 9% 1	8233
	11th to 20th	Night	53% 2 4 21% 17% 3	8065
	21st to end	Night	52% 2 7% 21% 17% 2	8221
February	1st to 10th	Night	56% 3 9% 19% 11% 1	2520
	11th to 20th	Night	53% 3 7% 22% 14% 2	2459
	21st to end	Night	54% 2 9% 19% 13% 3	2076
March	1st to 10th	Night	55% 2 5% 20% 14% 4	5493
	11th to 20th	Night	39% 2 8% 25% 20% 5%	5198
	21st to end	Night	48% 3 9% 25% 14% 1	5461
April	1st to 10th	Night	39% 4 11% 25% 18% 3	2100
	11th to 20th	Night	43% 3 10% 20% 19% 5%	2075
	21st to end	Night	45% 4 9% 25% 15% 2	1951
May	1st to 10th	Night	53% 3 10% 22% 10% 1	5264
	11th to 20th	Night	47% 5% 11% 21% 15%	5235
	21st to end	Night	51% 4 12% 20% 11% 1	5771
June	1st to 10th	Night	33% 4 13% 27% 20% 3	1840
	11th to 20th	Night	30% 5% 16% 23% 23% 4	1680
	21st to end	Night	28% 8% 16% 24% 19% 5%	1680
July	1st to 10th	Night	34% 6% 13% 28% 18% 1	3660
	11th to 20th	Night	33% 8% 18% 29% 11% 2	3695
	21st to end	Night	43% 9% 16% 22% 10% 1	4297
August	1st to 10th	Night	26% 5% 13% 38% 17% 1	2904
	11th to 20th	Night	30% 6% 16% 30% 17% 1	3165
	21st to end	Night	18% 4 13% 37% 24% 4	3586
September	1st to 10th	Night	33% 4 10% 23% 24% 5%	2044
	11th to 20th	Night	33% 3 8% 28% 24% 4	2200
	21st to end	Night	34% 3 7% 24% 25% 6%	2200
October	1st to 10th	Night	19% 2 5% 21% 48% 5%	3590
	11th to 20th	Night	24% 2 6% 25% 37% 7%	3688
	21st to end	Night	31% 2 6% 21% 31% 8%	4610
November	1st to 10th	Night	37% 3 6% 26% 24% 4	2666
	11th to 20th	Night	40% 2 6% 23% 21% 7%	2730
	21st to end	Night	46% 3 7% 25% 18% 2	2730
December	1st to 10th	Night	51% 2 8% 21% 16% 1	4380
	11th to 20th	Night	50% 2 9% 20% 16% 2	4954
	21st to end	Night	67% 2 7% 17% 7% 1	6944

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	23% 19% 32% 18% 8% 1	330
	11th to 20th	Day	28% 21% 34% 12% 5%	330
	21st to end	Day	23% 21% 36% 13% 5%	363
February	1st to 10th	Day	25% 19% 36% 16% 3 1	330
	11th to 20th	Day	29% 19% 35% 13% 3 1	330
	21st to end	Day	26% 19% 36% 15% 3	273
March	1st to 10th	Day	24% 19% 35% 17% 4 1	330
	11th to 20th	Day	21% 19% 37% 16% 6% 1	482
	21st to end	Day	23% 19% 34% 18% 5% 1	594
April	1st to 10th	Day	17% 16% 30% 24% 10% 1	540
	11th to 20th	Day	20% 15% 29% 21% 12% 3	532
	21st to end	Day	21% 10% 30% 26% 9% 4	540
May	1st to 10th	Day	19% 12% 29% 25% 11% 4	543
	11th to 20th	Day	17% 8% 25% 27% 17% 7%	567
	21st to end	Day	16% 5% 18% 29% 19% 13%	617
June	1st to 10th	Day	12% 5% 18% 29% 23% 13%	613
	11th to 20th	Day	8% 4 19% 31% 27% 11%	780
	21st to end	Day	9% 4 20% 28% 24% 14%	780
July	1st to 10th	Day	12% 3 10% 25% 31% 19%	771
	11th to 20th	Day	8% 3 14% 32% 27% 16%	704
	21st to end	Day	11% 3 9% 26% 34% 18%	576
August	1st to 10th	Day	9% 4 10% 24% 33% 21%	526
	11th to 20th	Day	10% 3 12% 29% 32% 14%	519
	21st to end	Day	9% 2 8% 25% 34% 22%	583
September	1st to 10th	Day	12% 3 13% 21% 38% 14%	530
	11th to 20th	Day	12% 2 9% 25% 34% 18%	530
	21st to end	Day	16% 5% 18% 25% 25% 12%	530
October	1st to 10th	Day	11% 2 15% 34% 28% 9%	530
	11th to 20th	Day	12% 9% 28% 29% 16% 6%	525
	21st to end	Day	25% 13% 27% 23% 11% 1	363
November	1st to 10th	Day	19% 9% 28% 30% 11% 3	330
	11th to 20th	Day	22% 16% 27% 27% 7% 1	330
	21st to end	Day	24% 12% 33% 21% 9% 1	330
December	1st to 10th	Day	21% 22% 30% 20% 7% 2	330
	11th to 20th	Day	19% 18% 32% 22% 8% 1	330
	21st to end	Day	26% 21% 31% 18% 4	363
January	1st to 10th	Night	21% 11% 36% 27% 5%	1050
	11th to 20th	Night	25% 13% 36% 22% 4	978
	21st to end	Night	22% 12% 39% 25% 2	924
February	1st to 10th	Night	27% 8% 36% 27% 3	840
	11th to 20th	Night	25% 11% 39% 23% 2	823
	21st to end	Night	26% 11% 38% 23% 2	692
March	1st to 10th	Night	22% 11% 34% 27% 6%	840
	11th to 20th	Night	22% 10% 35% 27% 6% 1	840
	21st to end	Night	19% 11% 30% 32% 7%	874
April	1st to 10th	Night	15% 8% 36% 31% 10%	630
	11th to 20th	Night	18% 8% 32% 29% 13%	623
	21st to end	Night	16% 7% 31% 35% 10% 1	630
May	1st to 10th	Night	20% 7% 25% 32% 15% 1	630
	11th to 20th	Night	14% 5% 19% 39% 19% 3	621
	21st to end	Night	9% 3 16% 38% 27% 6%	683
June	1st to 10th	Night	11% 2 15% 38% 30% 3	630
	11th to 20th	Night	8% 2 13% 41% 32% 3	630
	21st to end	Night	5% 3 12% 43% 33% 5%	630
July	1st to 10th	Night	11% 1 7% 28% 43% 10%	615
	11th to 20th	Night	9% 3 7% 32% 44% 6%	597
	21st to end	Night	9% 2 7% 35% 40% 7%	657
August	1st to 10th	Night	7% 1 7% 31% 47% 7%	597
	11th to 20th	Night	8% 1 6% 36% 42% 8%	594
	21st to end	Night	6% 1 5% 34% 47% 8%	660
September	1st to 10th	Night	12% 2 7% 34% 39% 6%	644
	11th to 20th	Night	10% 3 9% 33% 40% 5%	800
	21st to end	Night	12% 3 18% 32% 31% 4	800
October	1st to 10th	Night	11% 3 14% 36% 31% 6%	800
	11th to 20th	Night	14% 5% 24% 37% 18% 2	800
	21st to end	Night	19% 6% 32% 29% 13% 1	880
November	1st to 10th	Night	13% 8% 27% 35% 15% 2	820
	11th to 20th	Night	15% 9% 33% 31% 11%	840
	21st to end	Night	22% 7% 29% 31% 10% 1	840
December	1st to 10th	Night	25% 8% 29% 31% 6% 1	840
	11th to 20th	Night	25% 7% 31% 29% 8%	840
	21st to end	Night	28% 12% 28% 28% 4	1066

Page generated at 2026-07-13 03:14:52. Version ac07

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