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 2025-03-12.

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% 8% 3 7% 4 4 4 4 5% 7% 39%	109074
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	109959
Transparency	Day	43% 3 10% 25% 16% 2	112770
	Night	43% 3 9% 24% 18% 3	114389
Seeing	Day	16% 9% 22% 25% 19% 9%	15678
	Night	17% 6% 24% 31% 19% 2	25546

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	14% 12% 8% 9% 4 4 5% 4 5% 6% 29%	3617
	11th to 20th	Day	14% 11% 4 13% 5% 3 5% 3 4 4 34%	3766
	21st to end	Day	20% 10% 7% 6% 4 3 3 3 5% 6% 33%	4586
February	1st to 10th	Day	19% 12% 4 8% 5% 4 3 3 4 6% 34%	2000
	11th to 20th	Day	19% 9% 4 8% 5% 4 4 3 5% 6% 33%	1948
	21st to end	Day	17% 10% 4 9% 5% 4 4 4 5% 6% 33%	1650
March	1st to 10th	Day	24% 14% 3 6% 3 3 3 3 4 4 32%	3220
	11th to 20th	Day	13% 8% 2 6% 3 3 4 3 5% 7% 46%	3412
	21st to end	Day	13% 8% 3 7% 5% 4 4 4 6% 7% 39%	3926
April	1st to 10th	Day	13% 7% 2 6% 5% 5% 6% 4 5% 8% 40%	2280
	11th to 20th	Day	15% 9% 3 6% 4 4 4 3 5% 6% 41%	2254
	21st to end	Day	17% 7% 3 6% 5% 4 5% 4 4 7% 38%	2413
May	1st to 10th	Day	19% 8% 2 6% 5% 5% 5% 4 6% 8% 30%	4082
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 4 5% 5% 38%	4081
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 5% 6% 9% 39%	4476
June	1st to 10th	Day	11% 5% 3 7% 5% 5% 5% 4 6% 7% 42%	2509
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 4 7% 10% 45%	2660
	21st to end	Day	7% 5% 2 5% 5% 5% 6% 5% 7% 9% 43%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 8% 9% 38%	4440
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 5% 7% 10% 45%	4376
	21st to end	Day	13% 10% 4 10% 7% 6% 6% 5% 6% 7% 26%	4466
August	1st to 10th	Day	5% 5% 2 5% 4 4 4 4 7% 11% 50%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 9% 41%	3774
	21st to end	Day	5% 3 1 5% 4 4 5% 3 6% 8% 56%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 5% 4 4 6% 7% 43%	2218
	11th to 20th	Day	12% 7% 2 5% 4 3 5% 3 6% 8% 45%	2090
	21st to end	Day	13% 5% 2 6% 4 4 4 3 6% 7% 47%	2090
October	1st to 10th	Day	10% 5% 2 5% 4 3 3 3 5% 6% 56%	2090
	11th to 20th	Day	13% 5% 2 5% 4 2 4 4 4 7% 51%	2019
	21st to end	Day	18% 7% 2 6% 4 3 3 3 4 5% 45%	1881
November	1st to 10th	Day	12% 5% 2 5% 3 4 3 2 4 6% 52%	1762
	11th to 20th	Day	15% 10% 2 5% 4 4 3 3 5% 5% 44%	1800
	21st to end	Day	17% 9% 3 7% 4 3 4 3 5% 6% 40%	1800
December	1st to 10th	Day	17% 10% 3 6% 4 3 3 3 4 4 41%	2930
	11th to 20th	Day	17% 11% 5% 10% 6% 5% 4 2 4 5% 30%	3373
	21st to end	Day	20% 15% 5% 9% 5% 4 4 3 5% 6% 24%	4141
January	1st to 10th	Night	15% 14% 5% 12% 5% 4 3 3 4 6% 28%	6348
	11th to 20th	Night	17% 12% 4 10% 6% 5% 4 3 3 4 32%	6201
	21st to end	Night	19% 11% 4 7% 3 3 3 2 4 5% 39%	6352
February	1st to 10th	Night	21% 11% 4 7% 5% 4 5% 3 5% 5% 29%	2400
	11th to 20th	Night	20% 9% 4 9% 5% 4 4 3 4 5% 35%	2338
	21st to end	Night	21% 10% 4 7% 4 4 4 3 4 5% 34%	1980
March	1st to 10th	Night	21% 14% 3 7% 4 3 3 3 4 5% 32%	3853
	11th to 20th	Night	13% 8% 3 5% 4 4 3 3 4 5% 48%	3576
	21st to end	Night	12% 9% 2 7% 5% 5% 4 3 5% 6% 41%	3734
April	1st to 10th	Night	11% 6% 2 7% 4 4 5% 3 7% 7% 43%	1900
	11th to 20th	Night	15% 8% 3 7% 4 3 4 3 4 6% 43%	1875
	21st to end	Night	15% 8% 3 7% 5% 4 5% 3 5% 7% 38%	1765
May	1st to 10th	Night	17% 9% 3 8% 6% 5% 5% 4 5% 7% 31%	2813
	11th to 20th	Night	15% 6% 2 7% 4 4 5% 3 6% 8% 40%	2812
	21st to end	Night	6% 6% 2 6% 6% 6% 5% 5% 8% 10% 40%	3079
June	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 6% 10% 48%	1665
	11th to 20th	Night	6% 5% 1 5% 4 4 4 4 7% 10% 50%	1520
	21st to end	Night	3 3 2 5% 5% 6% 5% 4 8% 12% 47%	1520
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	2530
	11th to 20th	Night	6% 4 1 4 3 3 4 4 7% 9% 54%	2587
	21st to end	Night	10% 9% 4 10% 6% 5% 6% 4 8% 8% 30%	3087
August	1st to 10th	Night	5% 4 2 5% 4 3 4 4 6% 9% 55%	2825
	11th to 20th	Night	5% 4 2 6% 4 4 4 5% 8% 10% 48%	3039
	21st to end	Night	3 3 1 4 2 3 4 3 5% 9% 64%	3476
September	1st to 10th	Night	10% 6% 2 4 3 4 5% 3 5% 7% 51%	1942
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 6% 7% 50%	2090
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 6% 52%	2090
October	1st to 10th	Night	9% 4 2 5% 3 3 2 4 4 6% 60%	2090
	11th to 20th	Night	13% 5% 1 5% 4 4 4 3 5% 6% 51%	2153
	21st to end	Night	16% 7% 2 5% 4 4 3 3 4 5% 47%	2717
November	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 5% 4 51%	2536
	11th to 20th	Night	14% 8% 3 6% 4 4 3 3 4 6% 45%	2600
	21st to end	Night	17% 9% 3 6% 4 3 3 4 5% 5% 42%	2600
December	1st to 10th	Night	17% 12% 3 7% 4 3 4 2 4 4 40%	4250
	11th to 20th	Night	21% 10% 3 6% 4 3 3 3 4 6% 35%	4822
	21st to end	Night	22% 15% 5% 10% 6% 5% 4 3 4 4 22%	6794

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	3618
	11th to 20th	Day	56% 2 5% 20% 14% 3	3766
	21st to end	Day	53% 2 8% 22% 12% 3	4586
February	1st to 10th	Day	53% 3 8% 23% 13% 2	2000
	11th to 20th	Day	53% 3 8% 21% 12% 3	1958
	21st to end	Day	53% 2 7% 20% 13% 4	1650
March	1st to 10th	Day	57% 2 5% 19% 13% 4	3220
	11th to 20th	Day	40% 2 10% 27% 19% 2	3412
	21st to end	Day	45% 2 12% 26% 14% 1	3926
April	1st to 10th	Day	42% 4 11% 24% 16% 2	2280
	11th to 20th	Day	44% 3 9% 23% 19% 3	2254
	21st to end	Day	47% 4 9% 26% 12% 2	2413
May	1st to 10th	Day	52% 3 14% 21% 9% 1	4120
	11th to 20th	Day	48% 2 11% 24% 14%	4085
	21st to end	Day	43% 4 12% 19% 21% 2	4489
June	1st to 10th	Day	40% 5% 10% 26% 17% 2	2509
	11th to 20th	Day	33% 5% 15% 27% 19% 2	2660
	21st to end	Day	34% 7% 15% 21% 19% 4	2660
July	1st to 10th	Day	41% 6% 13% 25% 15% 2	4371
	11th to 20th	Day	34% 6% 13% 33% 12% 2	4301
	21st to end	Day	55% 6% 12% 17% 8% 1	4324
August	1st to 10th	Day	27% 6% 14% 37% 15% 1	4091
	11th to 20th	Day	37% 5% 12% 27% 16% 1	3811
	21st to end	Day	27% 4 10% 40% 16% 3	4179
September	1st to 10th	Day	41% 5% 10% 22% 18% 5%	2218
	11th to 20th	Day	37% 4 9% 25% 22% 3	2090
	21st to end	Day	36% 3 8% 26% 23% 3	2090
October	1st to 10th	Day	20% 2 5% 26% 44% 3	3480
	11th to 20th	Day	24% 3 10% 27% 33% 3	3325
	21st to end	Day	32% 2 6% 30% 22% 8%	3081
November	1st to 10th	Day	34% 2 8% 29% 22% 5%	1762
	11th to 20th	Day	43% 2 7% 24% 20% 5%	1800
	21st to end	Day	45% 3 8% 23% 19% 2	1800
December	1st to 10th	Day	47% 2 7% 24% 19% 1	2930
	11th to 20th	Day	58% 2 7% 17% 13% 2	3370
	21st to end	Day	62% 2 10% 19% 7%	4141
January	1st to 10th	Night	59% 2 7% 22% 10% 1	6348
	11th to 20th	Night	58% 1 5% 19% 13% 3	6201
	21st to end	Night	50% 1 7% 21% 19% 2	6352
February	1st to 10th	Night	57% 3 9% 19% 12% 1	2400
	11th to 20th	Night	53% 3 6% 21% 14% 2	2339
	21st to end	Night	54% 2 9% 18% 13% 3	1980
March	1st to 10th	Night	58% 2 6% 19% 14% 2	3853
	11th to 20th	Night	39% 2 9% 28% 15% 7%	3576
	21st to end	Night	45% 2 10% 25% 17% 1	3734
April	1st to 10th	Night	38% 4 11% 25% 19% 3	1900
	11th to 20th	Night	43% 2 9% 19% 21% 6%	1875
	21st to end	Night	46% 4 9% 25% 15% 2	1765
May	1st to 10th	Night	54% 3 10% 24% 8% 2	2840
	11th to 20th	Night	44% 4 12% 27% 12% 1	2813
	21st to end	Night	38% 4 14% 24% 18% 3	3087
June	1st to 10th	Night	33% 4 13% 28% 18% 4	1665
	11th to 20th	Night	28% 5% 17% 24% 23% 4	1520
	21st to end	Night	28% 7% 17% 23% 19% 6%	1520
July	1st to 10th	Night	34% 6% 13% 29% 16% 2	2490
	11th to 20th	Night	26% 5% 13% 37% 16% 3	2539
	21st to end	Night	49% 8% 13% 21% 9% 1	2988
August	1st to 10th	Night	26% 5% 13% 38% 17% 1	2814
	11th to 20th	Night	30% 6% 15% 30% 18% 1	3067
	21st to end	Night	18% 4 14% 37% 24% 4	3476
September	1st to 10th	Night	33% 4 10% 23% 24% 5%	1942
	11th to 20th	Night	33% 3 9% 27% 23% 4	2090
	21st to end	Night	34% 3 7% 25% 24% 6%	2090
October	1st to 10th	Night	19% 2 6% 20% 48% 4	3480
	11th to 20th	Night	24% 2 6% 25% 37% 6%	3574
	21st to end	Night	31% 2 6% 22% 31% 8%	4467
November	1st to 10th	Night	37% 3 6% 26% 24% 5%	2536
	11th to 20th	Night	41% 2 6% 23% 21% 7%	2600
	21st to end	Night	45% 3 7% 25% 18% 2	2600
December	1st to 10th	Night	50% 2 8% 22% 17% 1	4250
	11th to 20th	Night	50% 2 9% 20% 16% 2	4824
	21st to end	Night	67% 2 7% 17% 7% 1	6794

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% 18% 32% 18% 8% 1	300
	11th to 20th	Day	27% 20% 35% 13% 5%	300
	21st to end	Day	22% 20% 38% 14% 6%	330
February	1st to 10th	Day	25% 19% 36% 16% 3 1	300
	11th to 20th	Day	29% 20% 34% 14% 3 1	300
	21st to end	Day	27% 19% 37% 15% 3	249
March	1st to 10th	Day	24% 19% 33% 19% 5% 1	300
	11th to 20th	Day	20% 17% 36% 18% 7% 1	411
	21st to end	Day	23% 18% 32% 19% 6% 1	506
April	1st to 10th	Day	17% 17% 27% 27% 12% 1	460
	11th to 20th	Day	19% 14% 27% 22% 14% 4	452
	21st to end	Day	21% 9% 29% 26% 10% 4	460
May	1st to 10th	Day	19% 11% 27% 26% 12% 5%	463
	11th to 20th	Day	18% 6% 22% 28% 18% 8%	487
	21st to end	Day	13% 4 17% 29% 21% 15%	529
June	1st to 10th	Day	12% 4 16% 30% 24% 14%	529
	11th to 20th	Day	7% 4 16% 33% 29% 12%	680
	21st to end	Day	9% 4 20% 29% 24% 14%	680
July	1st to 10th	Day	11% 3 11% 25% 29% 21%	671
	11th to 20th	Day	8% 3 13% 32% 27% 17%	630
	21st to end	Day	11% 3 10% 26% 32% 18%	532
August	1st to 10th	Day	10% 3 9% 24% 33% 21%	486
	11th to 20th	Day	11% 3 11% 29% 32% 13%	479
	21st to end	Day	10% 2 7% 23% 35% 23%	539
September	1st to 10th	Day	12% 3 12% 20% 39% 14%	490
	11th to 20th	Day	13% 2 9% 26% 33% 17%	490
	21st to end	Day	16% 4 17% 24% 25% 13%	490
October	1st to 10th	Day	11% 2 16% 33% 28% 9%	490
	11th to 20th	Day	13% 10% 29% 27% 15% 6%	485
	21st to end	Day	23% 12% 28% 24% 12% 1	330
November	1st to 10th	Day	18% 9% 25% 32% 12% 4	300
	11th to 20th	Day	20% 15% 28% 29% 7% 1	300
	21st to end	Day	24% 11% 32% 22% 10% 1	300
December	1st to 10th	Day	20% 22% 27% 21% 7% 2	300
	11th to 20th	Day	19% 17% 31% 23% 9% 1	300
	21st to end	Day	25% 19% 30% 20% 5%	330
January	1st to 10th	Night	21% 11% 37% 27% 5%	1000
	11th to 20th	Night	25% 12% 36% 23% 4	932
	21st to end	Night	22% 11% 39% 26% 2	880
February	1st to 10th	Night	28% 8% 36% 26% 3	800
	11th to 20th	Night	24% 11% 40% 23% 2	783
	21st to end	Night	27% 11% 38% 23% 2	660
March	1st to 10th	Night	22% 10% 34% 28% 7%	800
	11th to 20th	Night	22% 8% 34% 29% 7% 1	767
	21st to end	Night	20% 10% 29% 33% 8%	791
April	1st to 10th	Night	13% 8% 35% 33% 10%	570
	11th to 20th	Night	18% 8% 31% 29% 14% 1	563
	21st to end	Night	17% 6% 30% 35% 10% 1	570
May	1st to 10th	Night	20% 5% 26% 31% 16% 2	570
	11th to 20th	Night	13% 4 18% 40% 21% 4	561
	21st to end	Night	7% 1 16% 39% 30% 7%	617
June	1st to 10th	Night	11% 2 13% 38% 32% 4	570
	11th to 20th	Night	7% 2 12% 42% 34% 3	570
	21st to end	Night	5% 2 13% 43% 33% 5%	570
July	1st to 10th	Night	11% 1 7% 28% 42% 10%	555
	11th to 20th	Night	9% 2 7% 32% 45% 6%	552
	21st to end	Night	9% 2 7% 36% 39% 7%	624
August	1st to 10th	Night	7% 1 6% 31% 48% 7%	567
	11th to 20th	Night	8% 1 6% 37% 41% 7%	564
	21st to end	Night	6% 1 4 32% 49% 8%	627
September	1st to 10th	Night	12% 2 7% 33% 39% 6%	612
	11th to 20th	Night	11% 3 9% 33% 39% 5%	760
	21st to end	Night	12% 3 18% 31% 31% 5%	760
October	1st to 10th	Night	11% 3 15% 36% 30% 6%	760
	11th to 20th	Night	14% 5% 24% 36% 18% 2	760
	21st to end	Night	18% 5% 32% 30% 14% 1	836
November	1st to 10th	Night	13% 7% 26% 36% 16% 2	780
	11th to 20th	Night	16% 9% 33% 31% 12%	800
	21st to end	Night	22% 7% 29% 31% 11% 1	800
December	1st to 10th	Night	26% 8% 29% 31% 7% 1	800
	11th to 20th	Night	25% 7% 31% 29% 8% 1	800
	21st to end	Night	27% 13% 27% 29% 4	1015

Page generated at 2025-03-09 08:08:49. Version ac07

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