The Friedman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Friedman Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	16% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	144436
	Night	16% 9% 3 8% 5% 4 4 4 5% 6% 36%	137772
Transparency	Day	48% 6% 12% 21% 12% 1	147980
	Night	48% 6% 12% 20% 12% 1	142315
Seeing	Day	23% 18% 28% 18% 9% 3	17145
	Night	21% 16% 29% 25% 9% 1	26354

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	13% 13% 5% 13% 7% 5% 4 3 5% 5% 27%	4661
	11th to 20th	Day	11% 13% 6% 17% 7% 5% 4 3 4 6% 25%	4770
	21st to end	Day	13% 12% 4 9% 7% 5% 4 3 5% 6% 32%	5780
February	1st to 10th	Day	15% 12% 4 9% 6% 5% 6% 4 7% 6% 28%	1890
	11th to 20th	Day	14% 12% 6% 8% 7% 5% 4 3 5% 6% 31%	1923
	21st to end	Day	17% 11% 3 10% 6% 4 4 4 4 6% 30%	1730
March	1st to 10th	Day	20% 9% 3 8% 6% 5% 4 3 4 5% 33%	4688
	11th to 20th	Day	13% 8% 3 8% 5% 5% 4 3 5% 6% 40%	5090
	21st to end	Day	15% 9% 3 7% 5% 4 5% 4 5% 6% 37%	5852
April	1st to 10th	Day	18% 12% 4 6% 6% 4 5% 3 5% 6% 32%	2520
	11th to 20th	Day	17% 9% 3 7% 4 4 5% 4 5% 7% 36%	2494
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 5% 7% 37%	2520
May	1st to 10th	Day	24% 8% 2 7% 4 4 5% 4 5% 7% 29%	7868
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 4 5% 7% 33%	8221
	21st to end	Day	23% 6% 2 6% 4 5% 5% 5% 6% 7% 30%	9060
June	1st to 10th	Day	14% 6% 2 7% 6% 4 6% 5% 6% 9% 35%	2940
	11th to 20th	Day	15% 6% 2 7% 4 5% 6% 5% 7% 8% 35%	2940
	21st to end	Day	13% 6% 2 6% 4 4 5% 5% 7% 9% 37%	2940
July	1st to 10th	Day	11% 6% 3 8% 5% 6% 6% 4 6% 9% 36%	8280
	11th to 20th	Day	8% 6% 3 7% 5% 5% 6% 6% 8% 11% 36%	8280
	21st to end	Day	13% 6% 2 7% 4 5% 5% 5% 6% 8% 38%	8842
August	1st to 10th	Day	20% 6% 3 7% 5% 6% 5% 4 6% 7% 30%	4364
	11th to 20th	Day	16% 7% 2 6% 5% 5% 5% 5% 6% 9% 34%	4106
	21st to end	Day	16% 5% 2 5% 4 4 4 3 6% 10% 41%	4420
September	1st to 10th	Day	15% 6% 2 6% 4 5% 4 4 5% 7% 42%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 7% 47%	2400
	21st to end	Day	20% 8% 3 6% 4 5% 5% 3 5% 7% 35%	2139
October	1st to 10th	Day	17% 6% 2 7% 4 3 4 4 6% 8% 40%	2000
	11th to 20th	Day	16% 7% 3 7% 5% 5% 4 4 5% 7% 38%	2000
	21st to end	Day	20% 9% 2 7% 5% 5% 4 4 5% 7% 33%	2145
November	1st to 10th	Day	12% 8% 3 6% 4 3 4 3 5% 6% 44%	1851
	11th to 20th	Day	15% 11% 3 8% 5% 4 3 4 4 7% 36%	1848
	21st to end	Day	17% 12% 3 6% 5% 5% 5% 3 4 7% 32%	1680
December	1st to 10th	Day	15% 13% 5% 9% 5% 5% 4 3 4 5% 30%	2680
	11th to 20th	Day	21% 13% 4 7% 4 3 3 3 4 5% 32%	3066
	21st to end	Day	17% 14% 5% 12% 5% 5% 4 3 4 6% 24%	4048
January	1st to 10th	Night	11% 12% 5% 13% 7% 5% 5% 4 5% 5% 28%	8234
	11th to 20th	Night	12% 10% 5% 15% 6% 5% 5% 4 5% 6% 27%	8097
	21st to end	Night	13% 9% 4 9% 6% 6% 5% 4 5% 6% 33%	8401
February	1st to 10th	Night	14% 11% 3 7% 6% 5% 4 4 5% 6% 34%	2730
	11th to 20th	Night	14% 10% 4 9% 7% 5% 5% 5% 5% 6% 30%	2581
	21st to end	Night	14% 12% 3 9% 6% 5% 4 3 4 5% 32%	2076
March	1st to 10th	Night	18% 12% 4 8% 5% 4 4 3 4 5% 34%	5355
	11th to 20th	Night	12% 9% 3 6% 4 5% 4 4 5% 6% 43%	5089
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 4 5% 39%	5320
April	1st to 10th	Night	17% 12% 3 8% 5% 5% 5% 4 5% 5% 32%	2100
	11th to 20th	Night	16% 8% 2 7% 4 4 4 4 5% 7% 38%	2075
	21st to end	Night	17% 8% 2 7% 3 4 3 3 6% 6% 41%	2100
May	1st to 10th	Night	26% 8% 3 7% 4 4 4 3 5% 7% 30%	5088
	11th to 20th	Night	21% 7% 3 6% 4 4 5% 3 5% 7% 35%	4639
	21st to end	Night	23% 6% 2 7% 4 4 4 3 5% 7% 35%	5114
June	1st to 10th	Night	14% 5% 2 6% 5% 4 3 4 6% 9% 42%	1680
	11th to 20th	Night	13% 5% 3 6% 5% 5% 5% 5% 6% 9% 39%	1680
	21st to end	Night	13% 5% 2 5% 3 4 4 4 7% 9% 42%	1680
July	1st to 10th	Night	13% 6% 2 7% 4 5% 5% 4 6% 9% 40%	4680
	11th to 20th	Night	10% 7% 2 7% 4 5% 5% 4 8% 10% 37%	4680
	21st to end	Night	12% 7% 2 6% 5% 4 4 4 6% 8% 43%	5218
August	1st to 10th	Night	20% 6% 2 6% 4 4 5% 3 7% 8% 33%	3005
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 6% 42%	3129
	21st to end	Night	13% 4 1 5% 3 4 4 3 6% 7% 48%	3677
September	1st to 10th	Night	15% 6% 2 5% 3 4 5% 4 5% 6% 45%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 6% 7% 50%	2000
	21st to end	Night	20% 7% 2 6% 3 4 3 3 5% 6% 40%	2256
October	1st to 10th	Night	20% 6% 2 5% 3 3 4 3 4 6% 45%	2400
	11th to 20th	Night	16% 7% 2 5% 4 4 3 3 5% 6% 44%	2400
	21st to end	Night	20% 7% 3 6% 4 5% 4 4 5% 7% 36%	2694
November	1st to 10th	Night	11% 8% 3 6% 5% 4 4 3 5% 6% 45%	2667
	11th to 20th	Night	17% 9% 3 7% 4 4 5% 3 4 6% 37%	2769
	21st to end	Night	14% 10% 3 7% 5% 5% 4 4 6% 6% 36%	2940
December	1st to 10th	Night	12% 11% 6% 9% 4 5% 4 3 5% 7% 33%	4720
	11th to 20th	Night	22% 11% 3 7% 5% 3 3 3 4 5% 33%	5348
	21st to end	Night	19% 16% 4 8% 5% 4 4 3 4 5% 28%	7150

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	59% 5% 11% 18% 6% 1	4662
	11th to 20th	Day	62% 5% 9% 14% 9% 2	4800
	21st to end	Day	53% 5% 11% 20% 11%	5780
February	1st to 10th	Day	55% 6% 12% 16% 9% 1	1890
	11th to 20th	Day	54% 6% 11% 21% 9% 1	1932
	21st to end	Day	54% 5% 12% 20% 7% 2	1730
March	1st to 10th	Day	53% 6% 10% 18% 9% 5%	4691
	11th to 20th	Day	42% 6% 17% 23% 8% 3	5090
	21st to end	Day	47% 6% 11% 15% 19% 3	5852
April	1st to 10th	Day	53% 6% 9% 18% 11% 3	2520
	11th to 20th	Day	47% 7% 13% 19% 13% 2	2494
	21st to end	Day	45% 7% 10% 22% 13% 3	2520
May	1st to 10th	Day	55% 5% 13% 18% 8% 1	7877
	11th to 20th	Day	49% 7% 10% 23% 9% 1	8225
	21st to end	Day	51% 7% 11% 18% 11% 1	9061
June	1st to 10th	Day	43% 10% 13% 20% 13% 1	2940
	11th to 20th	Day	44% 9% 14% 20% 11% 1	2940
	21st to end	Day	41% 9% 15% 23% 11%	2940
July	1st to 10th	Day	44% 8% 13% 22% 13%	8210
	11th to 20th	Day	40% 5% 18% 26% 10%	8205
	21st to end	Day	43% 6% 11% 22% 18%	8646
August	1st to 10th	Day	51% 8% 14% 18% 9%	4351
	11th to 20th	Day	46% 6% 14% 26% 8%	4145
	21st to end	Day	38% 6% 12% 33% 10% 1	4420
September	1st to 10th	Day	41% 8% 11% 23% 15% 2	2400
	11th to 20th	Day	36% 6% 12% 23% 21% 2	2400
	21st to end	Day	48% 6% 11% 18% 15% 1	2139
October	1st to 10th	Day	41% 6% 9% 21% 20% 2	3260
	11th to 20th	Day	40% 6% 8% 27% 16% 3	3240
	21st to end	Day	34% 4 17% 27% 16% 2	3447
November	1st to 10th	Day	41% 6% 13% 23% 16% 2	1851
	11th to 20th	Day	48% 6% 11% 21% 11% 2	1848
	21st to end	Day	52% 7% 13% 21% 7% 1	1680
December	1st to 10th	Day	57% 5% 13% 19% 6% 1	2680
	11th to 20th	Day	56% 5% 14% 16% 9%	3066
	21st to end	Day	60% 6% 11% 14% 8%	4048
January	1st to 10th	Night	57% 7% 12% 15% 8% 1	8234
	11th to 20th	Night	58% 7% 10% 13% 9% 3	8142
	21st to end	Night	51% 7% 8% 17% 16% 1	8401
February	1st to 10th	Night	50% 7% 13% 20% 9% 1	2730
	11th to 20th	Night	55% 6% 11% 18% 9% 1	2583
	21st to end	Night	54% 5% 10% 18% 11% 2	2076
March	1st to 10th	Night	52% 5% 12% 16% 11% 4	5365
	11th to 20th	Night	40% 7% 19% 23% 8% 4	5090
	21st to end	Night	47% 5% 9% 16% 19% 4	5320
April	1st to 10th	Night	53% 6% 11% 17% 11% 2	2100
	11th to 20th	Night	45% 8% 11% 21% 15% 1	2075
	21st to end	Night	44% 6% 11% 21% 15% 3	2100
May	1st to 10th	Night	55% 5% 11% 18% 9% 1	5095
	11th to 20th	Night	48% 8% 13% 20% 10% 1	4640
	21st to end	Night	50% 8% 11% 20% 11% 1	5115
June	1st to 10th	Night	38% 8% 11% 27% 16%	1680
	11th to 20th	Night	40% 9% 15% 24% 11% 1	1680
	21st to end	Night	36% 9% 15% 27% 12% 1	1680
July	1st to 10th	Night	42% 7% 15% 21% 15% 1	4640
	11th to 20th	Night	41% 6% 15% 26% 10% 1	4632
	21st to end	Night	40% 6% 12% 23% 17% 1	5099
August	1st to 10th	Night	48% 8% 16% 20% 8% 1	2994
	11th to 20th	Night	42% 6% 16% 23% 13%	3156
	21st to end	Night	35% 7% 12% 32% 13% 1	3677
September	1st to 10th	Night	39% 7% 12% 25% 16% 2	2000
	11th to 20th	Night	33% 6% 11% 26% 21% 2	2000
	21st to end	Night	44% 5% 11% 22% 17% 1	2256
October	1st to 10th	Night	42% 7% 10% 21% 19% 1	3920
	11th to 20th	Night	34% 6% 12% 25% 20% 2	3899
	21st to end	Night	35% 5% 15% 27% 16% 1	4342
November	1st to 10th	Night	39% 6% 14% 22% 16% 2	2667
	11th to 20th	Night	48% 5% 13% 21% 11% 2	2769
	21st to end	Night	48% 8% 14% 21% 9% 1	2940
December	1st to 10th	Night	51% 7% 13% 25% 4	4720
	11th to 20th	Night	55% 5% 12% 17% 10% 1	5348
	21st to end	Night	58% 7% 9% 17% 9% 1	7150

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	31% 26% 29% 11% 2	220
	11th to 20th	Day	36% 29% 28% 6% 1	242
	21st to end	Day	28% 33% 28% 8% 3 1	363
February	1st to 10th	Day	32% 29% 28% 9% 1	330
	11th to 20th	Day	36% 25% 30% 8%	410
	21st to end	Day	32% 25% 32% 9% 2	446
March	1st to 10th	Day	31% 23% 30% 13% 3	540
	11th to 20th	Day	32% 25% 26% 13% 3 1	540
	21st to end	Day	31% 27% 26% 11% 4 2	594
April	1st to 10th	Day	34% 23% 26% 13% 4 1	540
	11th to 20th	Day	29% 27% 25% 11% 6% 2	532
	21st to end	Day	26% 22% 32% 15% 5% 2	540
May	1st to 10th	Day	30% 21% 31% 10% 7% 2	543
	11th to 20th	Day	23% 19% 32% 15% 9% 2	567
	21st to end	Day	23% 12% 29% 21% 12% 2	617
June	1st to 10th	Day	18% 16% 28% 23% 12% 2	570
	11th to 20th	Day	18% 19% 26% 26% 11% 2	570
	21st to end	Day	18% 12% 33% 25% 11% 1	570
July	1st to 10th	Day	11% 10% 28% 29% 18% 4	565
	11th to 20th	Day	9% 8% 32% 29% 18% 4	564
	21st to end	Day	14% 9% 30% 30% 14% 3	620
August	1st to 10th	Day	15% 8% 25% 25% 20% 7%	566
	11th to 20th	Day	16% 8% 26% 28% 16% 6%	559
	21st to end	Day	14% 12% 28% 25% 14% 8%	619
September	1st to 10th	Day	18% 13% 21% 23% 18% 7%	530
	11th to 20th	Day	12% 8% 26% 23% 21% 9%	530
	21st to end	Day	28% 10% 24% 22% 13% 4	530
October	1st to 10th	Day	21% 14% 23% 21% 15% 5%	530
	11th to 20th	Day	21% 21% 27% 21% 7% 2	530
	21st to end	Day	26% 18% 29% 20% 7% 1	553
November	1st to 10th	Day	22% 22% 27% 21% 8% 1	425
	11th to 20th	Day	25% 24% 29% 16% 5% 1	388
	21st to end	Day	33% 26% 31% 6% 3	220
December	1st to 10th	Day	32% 27% 28% 8% 4 1	220
	11th to 20th	Day	35% 24% 28% 12% 1	220
	21st to end	Day	28% 25% 36% 9% 2	242
January	1st to 10th	Night	22% 26% 36% 14% 3	840
	11th to 20th	Night	25% 21% 32% 19% 2	840
	21st to end	Night	18% 26% 36% 18% 2	924
February	1st to 10th	Night	24% 23% 32% 19% 2	840
	11th to 20th	Night	28% 24% 36% 12% 1	823
	21st to end	Night	28% 22% 34% 14% 2	692
March	1st to 10th	Night	28% 21% 32% 16% 3	840
	11th to 20th	Night	25% 23% 31% 17% 4	840
	21st to end	Night	23% 24% 32% 16% 4	924
April	1st to 10th	Night	25% 22% 32% 17% 3	840
	11th to 20th	Night	23% 18% 33% 20% 6%	831
	21st to end	Night	22% 17% 30% 26% 5%	840
May	1st to 10th	Night	23% 16% 27% 25% 8% 1	576
	11th to 20th	Night	15% 11% 35% 33% 6% 1	414
	21st to end	Night	17% 8% 25% 36% 13%	455
June	1st to 10th	Night	16% 7% 25% 37% 15%	420
	11th to 20th	Night	15% 7% 29% 36% 12%	420
	21st to end	Night	12% 5% 27% 42% 13%	420
July	1st to 10th	Night	11% 5% 24% 40% 20% 1	410
	11th to 20th	Night	10% 4 21% 46% 17% 2	408
	21st to end	Night	14% 5% 19% 42% 18% 2	506
August	1st to 10th	Night	15% 3 16% 43% 22% 2	627
	11th to 20th	Night	14% 4 15% 38% 27% 2	723
	21st to end	Night	13% 6% 21% 35% 23% 1	917
September	1st to 10th	Night	17% 5% 23% 32% 20% 2	800
	11th to 20th	Night	13% 6% 22% 28% 26% 5%	800
	21st to end	Night	22% 9% 20% 31% 17% 2	800
October	1st to 10th	Night	22% 10% 22% 27% 18% 2	800
	11th to 20th	Night	19% 15% 26% 31% 8% 1	800
	21st to end	Night	25% 14% 30% 25% 5%	880
November	1st to 10th	Night	18% 16% 32% 26% 8% 1	820
	11th to 20th	Night	25% 19% 30% 20% 5% 1	840
	21st to end	Night	23% 21% 32% 20% 4	840
December	1st to 10th	Night	25% 21% 33% 17% 4	840
	11th to 20th	Night	25% 21% 31% 20% 3	840
	21st to end	Night	27% 19% 34% 17% 3	924

Page generated at 2026-08-27 04:07:11. Version ac07

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