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 2024-02-10.

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 6% 5% 3 5% 5% 5% 7% 39%	78611
	Night	14% 8% 3 6% 5% 3 4 4 5% 7% 40%	80339
Transparency	Day	45% 4 10% 23% 15% 2	78324
	Night	45% 3 10% 23% 16% 3	80146
Seeing	Day	16% 9% 21% 25% 20% 10%	14228
	Night	17% 6% 24% 32% 19% 3	24102

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% 7% 7% 6% 3 5% 6% 5% 5% 30%	2527
	11th to 20th	Day	18% 10% 4 11% 8% 3 5% 4 4 4 29%	2624
	21st to end	Day	26% 13% 9% 5% 4 2 3 3 4 6% 26%	3202
February	1st to 10th	Day	19% 11% 5% 7% 6% 3 3 3 4 6% 34%	1890
	11th to 20th	Day	19% 9% 4 7% 5% 3 4 5% 5% 7% 33%	1748
	21st to end	Day	18% 11% 4 9% 6% 3 4 5% 5% 6% 31%	1480
March	1st to 10th	Day	18% 8% 3 5% 4 2 5% 4 5% 4 41%	1800
	11th to 20th	Day	19% 11% 4 6% 6% 3 4 5% 4 7% 32%	1930
	21st to end	Day	17% 9% 4 8% 6% 3 5% 4 5% 7% 32%	2235
April	1st to 10th	Day	12% 6% 2 6% 5% 4 6% 6% 5% 8% 41%	2160
	11th to 20th	Day	16% 9% 3 5% 4 3 4 5% 5% 6% 41%	2134
	21st to end	Day	18% 7% 3 6% 6% 3 5% 5% 4 7% 37%	2286
May	1st to 10th	Day	18% 6% 2 5% 5% 3 5% 5% 6% 8% 36%	2340
	11th to 20th	Day	16% 8% 3 6% 6% 3 5% 6% 5% 6% 37%	2301
	21st to end	Day	8% 6% 3 5% 6% 5% 7% 6% 6% 10% 36%	2531
June	1st to 10th	Day	11% 4 3 6% 5% 3 5% 5% 6% 8% 43%	2376
	11th to 20th	Day	6% 4 2 5% 5% 4 6% 5% 7% 10% 45%	2520
	21st to end	Day	7% 5% 3 4 5% 4 6% 6% 7% 9% 43%	2520
July	1st to 10th	Day	11% 6% 3 5% 6% 4 6% 6% 9% 10% 34%	2520
	11th to 20th	Day	8% 4 3 5% 6% 4 7% 6% 6% 9% 41%	2484
	21st to end	Day	10% 4 2 5% 6% 4 7% 6% 7% 9% 41%	2547
August	1st to 10th	Day	7% 7% 2 6% 5% 3 6% 5% 9% 10% 40%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 5% 7% 10% 39%	2157
	21st to end	Day	8% 5% 2 5% 6% 3 6% 5% 5% 8% 47%	2376
September	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 5% 6% 7% 42%	2102
	11th to 20th	Day	12% 6% 3 4 5% 3 4 5% 6% 8% 45%	1980
	21st to end	Day	12% 4 2 4 4 3 4 4 5% 7% 50%	1980
October	1st to 10th	Day	11% 5% 2 4 4 2 3 3 5% 7% 55%	1980
	11th to 20th	Day	14% 4 3 4 5% 2 4 5% 4 7% 49%	1914
	21st to end	Day	19% 7% 3 5% 5% 2 3 3 4 5% 44%	1782
November	1st to 10th	Day	12% 4 3 4 4 3 4 3 4 6% 54%	1672
	11th to 20th	Day	16% 9% 2 5% 4 2 4 4 4 6% 45%	1710
	21st to end	Day	17% 7% 3 6% 4 2 4 4 5% 6% 41%	1710
December	1st to 10th	Day	21% 11% 5% 5% 5% 2 4 4 4 6% 33%	1710
	11th to 20th	Day	14% 11% 5% 9% 6% 4 5% 3 4 5% 34%	2153
	21st to end	Day	18% 16% 5% 7% 6% 3 4 4 5% 7% 25%	2903
January	1st to 10th	Night	16% 13% 5% 8% 6% 3 4 4 4 6% 30%	4428
	11th to 20th	Night	17% 12% 5% 10% 8% 4 5% 4 4 4 29%	4337
	21st to end	Night	23% 14% 5% 6% 4 3 4 3 4 5% 29%	4436
February	1st to 10th	Night	21% 11% 4 6% 5% 3 6% 4 5% 6% 29%	2275
	11th to 20th	Night	19% 8% 5% 7% 6% 3 4 4 4 5% 35%	2098
	21st to end	Night	23% 10% 4 6% 5% 3 4 4 4 5% 32%	1776
March	1st to 10th	Night	17% 9% 3 5% 5% 3 4 4 4 5% 41%	2160
	11th to 20th	Night	18% 10% 4 7% 5% 3 5% 4 4 5% 35%	2029
	21st to end	Night	16% 10% 3 7% 6% 3 4 4 5% 7% 35%	2136
April	1st to 10th	Night	10% 5% 3 6% 5% 3 5% 5% 6% 8% 44%	1800
	11th to 20th	Night	15% 8% 3 6% 5% 2 4 4 4 6% 43%	1775
	21st to end	Night	15% 7% 4 6% 5% 3 5% 4 5% 7% 38%	1673
May	1st to 10th	Night	17% 6% 3 6% 7% 3 5% 4 5% 7% 37%	1620
	11th to 20th	Night	12% 6% 2 7% 6% 4 6% 5% 6% 8% 39%	1592
	21st to end	Night	5% 4 2 5% 7% 5% 6% 7% 8% 11% 39%	1744
June	1st to 10th	Night	9% 5% 2 5% 4 2 4 5% 6% 9% 48%	1578
	11th to 20th	Night	6% 4 2 5% 4 3 5% 5% 7% 10% 51%	1440
	21st to end	Night	4 3 2 4 5% 4 5% 5% 8% 12% 48%	1440
July	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 6% 6% 8% 44%	1440
	11th to 20th	Night	7% 4 2 4 5% 2 5% 5% 7% 11% 49%	1471
	21st to end	Night	7% 4 2 5% 5% 3 6% 5% 7% 10% 47%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 3 5% 5% 7% 9% 49%	1611
	11th to 20th	Night	6% 4 2 5% 5% 3 4 6% 8% 10% 46%	1741
	21st to end	Night	6% 4 2 4 3 2 5% 5% 6% 10% 54%	1980
September	1st to 10th	Night	11% 5% 2 4 3 3 5% 5% 6% 7% 50%	1839
	11th to 20th	Night	9% 5% 2 5% 4 3 5% 5% 5% 7% 50%	1980
	21st to end	Night	9% 4 2 4 5% 2 4 4 5% 6% 55%	1980
October	1st to 10th	Night	9% 4 2 4 3 2 3 4 4 7% 58%	1980
	11th to 20th	Night	14% 5% 2 4 5% 3 4 4 5% 6% 49%	2039
	21st to end	Night	17% 7% 2 5% 4 3 4 3 4 6% 46%	2574
November	1st to 10th	Night	12% 6% 2 5% 5% 3 3 4 4 4 53%	2406
	11th to 20th	Night	14% 6% 3 6% 4 3 4 4 4 6% 46%	2470
	21st to end	Night	17% 7% 4 5% 4 3 4 4 5% 5% 42%	2470
December	1st to 10th	Night	19% 11% 4 7% 4 3 4 4 5% 6% 33%	2470
	11th to 20th	Night	18% 10% 4 7% 5% 3 4 4 4 6% 35%	3042
	21st to end	Night	17% 14% 5% 9% 8% 5% 6% 4 4 5% 24%	4738

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 8% 22% 10% 1	2528
	11th to 20th	Day	61% 1 7% 20% 9% 1	2624
	21st to end	Day	62% 1 7% 23% 6% 1	3202
February	1st to 10th	Day	53% 3 7% 23% 13% 1	1890
	11th to 20th	Day	52% 4 8% 22% 11% 3	1758
	21st to end	Day	55% 3 8% 18% 12% 4	1480
March	1st to 10th	Day	47% 3 8% 22% 19% 2	1800
	11th to 20th	Day	53% 3 10% 20% 12% 2	1930
	21st to end	Day	53% 3 11% 21% 10% 2	2235
April	1st to 10th	Day	41% 4 12% 24% 17% 2	2160
	11th to 20th	Day	44% 3 9% 22% 18% 3	2134
	21st to end	Day	47% 4 9% 26% 12% 2	2286
May	1st to 10th	Day	46% 4 14% 25% 11% 1	2340
	11th to 20th	Day	49% 3 12% 22% 13% 1	2305
	21st to end	Day	41% 6% 16% 22% 13% 2	2531
June	1st to 10th	Day	39% 5% 10% 27% 18% 2	2376
	11th to 20th	Day	33% 5% 15% 27% 18% 2	2520
	21st to end	Day	34% 6% 15% 22% 18% 4	2520
July	1st to 10th	Day	42% 6% 13% 23% 13% 3	2451
	11th to 20th	Day	38% 6% 16% 22% 14% 4	2409
	21st to end	Day	37% 7% 16% 25% 13% 2	2405
August	1st to 10th	Day	37% 6% 16% 26% 14% 1	2314
	11th to 20th	Day	40% 5% 13% 25% 15% 2	2157
	21st to end	Day	35% 5% 12% 27% 17% 5%	2376
September	1st to 10th	Day	42% 5% 10% 22% 17% 4	2102
	11th to 20th	Day	37% 4 9% 25% 22% 3	1980
	21st to end	Day	34% 3 8% 28% 24% 4	1980
October	1st to 10th	Day	31% 3 8% 26% 28% 4	1980
	11th to 20th	Day	36% 3 8% 23% 26% 4	1914
	21st to end	Day	44% 3 6% 26% 17% 3	1782
November	1st to 10th	Day	33% 3 8% 29% 22% 5%	1672
	11th to 20th	Day	42% 2 7% 24% 20% 5%	1710
	21st to end	Day	44% 3 8% 24% 19% 2	1710
December	1st to 10th	Day	53% 3 10% 23% 11% 2	1710
	11th to 20th	Day	55% 2 7% 19% 15% 2	2150
	21st to end	Day	60% 2 9% 20% 8%	2903
January	1st to 10th	Night	56% 2 7% 22% 12% 1	4428
	11th to 20th	Night	61% 1 6% 21% 10% 2	4337
	21st to end	Night	59% 1 7% 20% 11% 1	4436
February	1st to 10th	Night	56% 3 9% 19% 12% 1	2275
	11th to 20th	Night	52% 3 7% 22% 14% 2	2099
	21st to end	Night	56% 2 10% 16% 12% 3	1776
March	1st to 10th	Night	47% 3 7% 20% 20% 4	2160
	11th to 20th	Night	51% 3 10% 20% 13% 2	2029
	21st to end	Night	50% 4 11% 24% 10% 2	2136
April	1st to 10th	Night	37% 5% 11% 26% 19% 3	1800
	11th to 20th	Night	43% 3 9% 19% 21% 6%	1775
	21st to end	Night	46% 4 9% 24% 15% 2	1673
May	1st to 10th	Night	47% 3 9% 26% 11% 3	1620
	11th to 20th	Night	43% 6% 13% 22% 15% 1	1593
	21st to end	Night	35% 7% 16% 24% 16% 3	1745
June	1st to 10th	Night	32% 5% 13% 28% 19% 3	1578
	11th to 20th	Night	28% 5% 17% 24% 22% 4	1440
	21st to end	Night	28% 7% 17% 24% 18% 6%	1440
July	1st to 10th	Night	37% 5% 14% 27% 15% 3	1400
	11th to 20th	Night	29% 6% 17% 24% 18% 5%	1423
	21st to end	Night	30% 7% 16% 31% 15% 2	1672
August	1st to 10th	Night	30% 5% 16% 28% 17% 2	1600
	11th to 20th	Night	30% 6% 16% 27% 17% 3	1741
	21st to end	Night	26% 5% 17% 28% 20% 5%	1980
September	1st to 10th	Night	34% 4 10% 24% 22% 5%	1839
	11th to 20th	Night	34% 3 9% 28% 23% 4	1980
	21st to end	Night	32% 3 7% 26% 26% 6%	1980
October	1st to 10th	Night	27% 3 9% 26% 29% 6%	1980
	11th to 20th	Night	37% 3 7% 23% 24% 5%	2039
	21st to end	Night	42% 1 6% 25% 21% 5%	2574
November	1st to 10th	Night	35% 2 6% 27% 25% 5%	2406
	11th to 20th	Night	40% 2 6% 23% 21% 7%	2470
	21st to end	Night	45% 3 7% 25% 18% 2	2470
December	1st to 10th	Night	53% 3 10% 21% 12% 1	2470
	11th to 20th	Night	52% 2 7% 20% 16% 2	3044
	21st to end	Night	64% 2 8% 16% 8% 1	4738

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	22% 17% 33% 19% 8% 1	270
	11th to 20th	Day	26% 19% 35% 14% 6%	270
	21st to end	Day	24% 18% 37% 14% 6%	297
February	1st to 10th	Day	24% 18% 35% 18% 3 1	267
	11th to 20th	Day	30% 19% 35% 15% 3	240
	21st to end	Day	29% 16% 37% 15% 3	198
March	1st to 10th	Day	21% 19% 38% 18% 4	240
	11th to 20th	Day	22% 16% 35% 18% 8% 1	370
	21st to end	Day	24% 18% 31% 19% 6% 2	462
April	1st to 10th	Day	15% 16% 27% 28% 12% 1	420
	11th to 20th	Day	18% 14% 26% 22% 15% 4	412
	21st to end	Day	23% 10% 27% 26% 10% 4	420
May	1st to 10th	Day	19% 10% 28% 27% 11% 5%	423
	11th to 20th	Day	17% 6% 23% 29% 17% 7%	447
	21st to end	Day	13% 4 15% 30% 22% 16%	485
June	1st to 10th	Day	12% 4 15% 29% 25% 15%	486
	11th to 20th	Day	7% 3 16% 33% 29% 13%	630
	21st to end	Day	9% 4 19% 29% 25% 15%	630
July	1st to 10th	Day	12% 2 9% 24% 30% 22%	621
	11th to 20th	Day	9% 3 12% 30% 28% 18%	582
	21st to end	Day	10% 3 8% 25% 34% 20%	488
August	1st to 10th	Day	10% 3 9% 23% 34% 22%	446
	11th to 20th	Day	12% 2 10% 29% 34% 14%	439
	21st to end	Day	11% 1 7% 23% 34% 24%	495
September	1st to 10th	Day	13% 2 10% 20% 40% 15%	450
	11th to 20th	Day	13% 2 9% 25% 33% 19%	450
	21st to end	Day	15% 3 18% 24% 26% 14%	450
October	1st to 10th	Day	12% 2 16% 33% 27% 10%	450
	11th to 20th	Day	13% 9% 31% 28% 14% 5%	446
	21st to end	Day	25% 12% 26% 25% 11% 1	297
November	1st to 10th	Day	16% 8% 24% 34% 13% 4	270
	11th to 20th	Day	18% 15% 27% 31% 8% 1	270
	21st to end	Day	23% 9% 32% 24% 11% 1	270
December	1st to 10th	Day	20% 21% 27% 22% 8% 2	270
	11th to 20th	Day	17% 17% 32% 23% 10% 1	270
	21st to end	Day	24% 20% 30% 20% 5%	297
January	1st to 10th	Night	21% 10% 36% 27% 5%	950
	11th to 20th	Night	26% 10% 36% 23% 4	886
	21st to end	Night	23% 10% 39% 27% 2	836
February	1st to 10th	Night	28% 7% 35% 27% 3	759
	11th to 20th	Night	24% 10% 41% 23% 2	703
	21st to end	Night	28% 10% 37% 22% 3	592
March	1st to 10th	Night	21% 9% 36% 28% 6%	720
	11th to 20th	Night	23% 9% 33% 28% 7% 1	720
	21st to end	Night	20% 10% 28% 33% 8%	750
April	1st to 10th	Night	12% 7% 35% 34% 11%	540
	11th to 20th	Night	18% 8% 30% 29% 15% 1	533
	21st to end	Night	17% 6% 30% 35% 11% 1	540
May	1st to 10th	Night	20% 5% 26% 31% 16% 2	540
	11th to 20th	Night	14% 4 19% 40% 20% 4	531
	21st to end	Night	7% 1 15% 39% 31% 7%	584
June	1st to 10th	Night	11% 2 13% 38% 33% 4	540
	11th to 20th	Night	7% 2 12% 42% 34% 3	540
	21st to end	Night	5% 1 13% 42% 34% 5%	540
July	1st to 10th	Night	11% 1 6% 28% 43% 11%	525
	11th to 20th	Night	9% 2 7% 30% 47% 6%	522
	21st to end	Night	8% 2 6% 36% 40% 7%	591
August	1st to 10th	Night	7% 1 5% 31% 48% 7%	537
	11th to 20th	Night	7% 1 5% 36% 43% 8%	534
	21st to end	Night	7% 1 4 32% 49% 8%	594
September	1st to 10th	Night	13% 1 7% 33% 40% 6%	579
	11th to 20th	Night	11% 3 9% 32% 40% 6%	720
	21st to end	Night	11% 3 18% 31% 33% 5%	720
October	1st to 10th	Night	11% 3 15% 36% 29% 6%	720
	11th to 20th	Night	15% 5% 24% 37% 18% 2	720
	21st to end	Night	19% 5% 32% 30% 14% 1	792
November	1st to 10th	Night	12% 7% 26% 37% 17% 2	740
	11th to 20th	Night	15% 8% 33% 31% 13%	760
	21st to end	Night	21% 6% 29% 32% 11% 1	760
December	1st to 10th	Night	25% 8% 29% 31% 7% 1	760
	11th to 20th	Night	24% 6% 31% 30% 8% 1	760
	21st to end	Night	27% 12% 27% 29% 4	964

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