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 2025-04-26.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	113702
	Night	16% 9% 3 8% 5% 4 4 4 5% 6% 36%	114887
Transparency	Day	47% 7% 12% 20% 11% 2	117293
	Night	47% 7% 12% 20% 12% 2	119459
Seeing	Day	23% 19% 28% 18% 9% 3	15353
	Night	21% 16% 29% 24% 9% 1	24772

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	12% 12% 5% 13% 7% 5% 4 3 5% 5% 28%	3597
	11th to 20th	Day	12% 13% 6% 17% 8% 5% 4 3 4 6% 22%	3662
	21st to end	Day	15% 14% 5% 10% 7% 5% 4 3 4 5% 30%	4466
February	1st to 10th	Day	15% 12% 4 9% 6% 5% 5% 4 7% 6% 27%	1800
	11th to 20th	Day	13% 12% 5% 8% 7% 5% 4 3 6% 6% 32%	1829
	21st to end	Day	18% 11% 3 10% 6% 4 4 4 4 6% 30%	1650
March	1st to 10th	Day	20% 9% 3 8% 6% 5% 4 3 4 5% 33%	4586
	11th to 20th	Day	12% 8% 3 8% 5% 5% 4 3 5% 6% 41%	4980
	21st to end	Day	16% 9% 3 7% 5% 4 5% 4 5% 6% 38%	5726
April	1st to 10th	Day	18% 12% 3 6% 6% 4 5% 3 5% 6% 32%	2400
	11th to 20th	Day	17% 9% 3 7% 4 4 5% 3 5% 6% 36%	2374
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	2340
May	1st to 10th	Day	27% 7% 2 7% 4 3 4 3 4 7% 31%	4224
	11th to 20th	Day	24% 9% 3 8% 6% 5% 4 4 6% 7% 26%	4398
	21st to end	Day	14% 4 2 5% 4 4 5% 4 6% 9% 42%	4837
June	1st to 10th	Day	15% 6% 2 7% 7% 5% 6% 5% 6% 9% 33%	2660
	11th to 20th	Day	14% 7% 2 7% 5% 5% 7% 4 7% 8% 35%	2660
	21st to end	Day	13% 6% 2 6% 4 4 5% 5% 7% 9% 37%	2660
July	1st to 10th	Day	7% 5% 3 8% 6% 6% 6% 5% 8% 11% 35%	4440
	11th to 20th	Day	8% 5% 3 7% 5% 5% 5% 6% 8% 11% 37%	4440
	21st to end	Day	12% 8% 3 9% 5% 6% 6% 5% 6% 8% 33%	4733
August	1st to 10th	Day	21% 6% 3 7% 5% 6% 6% 4 6% 7% 29%	4104
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 9% 34%	3858
	21st to end	Day	16% 5% 2 5% 4 4 4 3 6% 10% 40%	4180
September	1st to 10th	Day	16% 6% 2 6% 4 5% 4 4 5% 7% 41%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 47%	2280
	21st to end	Day	20% 7% 3 6% 4 5% 4 3 5% 7% 36%	2033
October	1st to 10th	Day	18% 6% 2 7% 4 3 4 4 6% 8% 38%	1900
	11th to 20th	Day	17% 7% 3 7% 5% 5% 4 4 5% 7% 38%	1900
	21st to end	Day	20% 9% 2 7% 5% 5% 4 3 5% 7% 33%	2038
November	1st to 10th	Day	13% 8% 3 6% 4 3 4 3 5% 6% 45%	1761
	11th to 20th	Day	15% 11% 3 8% 5% 4 3 3 4 7% 37%	1760
	21st to end	Day	17% 11% 3 6% 5% 5% 5% 3 4 7% 33%	1600
December	1st to 10th	Day	15% 14% 5% 9% 5% 5% 4 3 4 5% 30%	2600
	11th to 20th	Day	22% 14% 4 7% 3 3 3 3 4 5% 32%	2986
	21st to end	Day	17% 13% 5% 11% 5% 5% 4 3 4 6% 24%	3960
January	1st to 10th	Night	12% 9% 4 12% 7% 5% 5% 4 5% 6% 31%	6349
	11th to 20th	Night	12% 10% 5% 15% 5% 5% 5% 4 5% 6% 28%	6205
	21st to end	Night	15% 10% 4 11% 6% 6% 5% 4 5% 6% 28%	6490
February	1st to 10th	Night	14% 11% 4 7% 6% 5% 4 4 5% 6% 34%	2600
	11th to 20th	Night	14% 9% 4 9% 7% 5% 5% 5% 5% 6% 31%	2455
	21st to end	Night	15% 11% 3 9% 6% 5% 4 4 4 5% 32%	1980
March	1st to 10th	Night	18% 12% 4 8% 5% 4 4 3 4 5% 34%	5237
	11th to 20th	Night	11% 9% 3 6% 4 5% 4 4 5% 6% 44%	4979
	21st to end	Night	15% 9% 3 8% 5% 4 4 3 4 5% 39%	5204
April	1st to 10th	Night	16% 12% 3 8% 5% 5% 5% 4 5% 5% 32%	2000
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 4 5% 7% 38%	1975
	21st to end	Night	17% 8% 2 7% 4 4 3 3 5% 6% 41%	1957
May	1st to 10th	Night	26% 8% 2 8% 3 4 4 3 5% 5% 32%	2746
	11th to 20th	Night	23% 8% 3 7% 4 4 4 3 5% 7% 30%	2495
	21st to end	Night	13% 6% 3 7% 4 5% 4 4 6% 8% 41%	2738
June	1st to 10th	Night	14% 5% 2 5% 5% 4 3 4 6% 9% 40%	1520
	11th to 20th	Night	12% 5% 2 6% 5% 4 5% 5% 7% 9% 39%	1520
	21st to end	Night	12% 5% 2 5% 3 4 4 4 7% 9% 44%	1520
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	2520
	11th to 20th	Night	10% 6% 2 7% 4 5% 6% 5% 8% 11% 37%	2520
	21st to end	Night	16% 7% 1 6% 5% 4 4 4 6% 7% 40%	2817
August	1st to 10th	Night	20% 6% 2 6% 4 4 5% 3 7% 8% 33%	2825
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 42%	2939
	21st to end	Night	14% 4 1 5% 4 4 4 3 5% 7% 49%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 4 5% 4 5% 6% 44%	1900
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 6% 7% 50%	1900
	21st to end	Night	20% 7% 2 6% 3 4 3 3 5% 6% 41%	2143
October	1st to 10th	Night	21% 6% 2 5% 3 4 4 3 4 7% 43%	2280
	11th to 20th	Night	16% 7% 2 5% 4 4 4 3 5% 6% 44%	2280
	21st to end	Night	20% 7% 3 6% 4 5% 4 4 5% 7% 36%	2559
November	1st to 10th	Night	11% 8% 2 6% 5% 4 4 3 5% 6% 46%	2537
	11th to 20th	Night	18% 9% 3 6% 4 4 5% 3 4 6% 38%	2637
	21st to end	Night	14% 9% 3 7% 5% 5% 4 4 6% 6% 37%	2800
December	1st to 10th	Night	13% 11% 6% 9% 4 5% 4 3 5% 7% 33%	4580
	11th to 20th	Night	23% 11% 3 7% 5% 3 3 3 4 5% 33%	5208
	21st to end	Night	19% 16% 4 8% 5% 4 4 3 4 5% 28%	6996

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	58% 6% 9% 19% 6% 2	3598
	11th to 20th	Day	64% 5% 8% 14% 7% 2	3692
	21st to end	Day	56% 6% 9% 20% 9%	4466
February	1st to 10th	Day	57% 6% 12% 15% 9% 1	1800
	11th to 20th	Day	53% 6% 11% 21% 9% 1	1838
	21st to end	Day	54% 5% 12% 20% 7% 2	1650
March	1st to 10th	Day	53% 6% 10% 17% 10% 5%	4589
	11th to 20th	Day	42% 6% 17% 23% 8% 3	4980
	21st to end	Day	46% 6% 11% 14% 19% 3	5726
April	1st to 10th	Day	53% 6% 9% 18% 11% 3	2400
	11th to 20th	Day	47% 7% 12% 19% 13% 2	2374
	21st to end	Day	45% 7% 10% 21% 13% 3	2340
May	1st to 10th	Day	53% 7% 10% 20% 9% 1	4236
	11th to 20th	Day	54% 10% 11% 17% 7% 1	4402
	21st to end	Day	36% 10% 14% 24% 16% 2	4837
June	1st to 10th	Day	44% 10% 14% 19% 12% 1	2660
	11th to 20th	Day	44% 9% 14% 21% 11% 1	2660
	21st to end	Day	40% 10% 15% 24% 11%	2660
July	1st to 10th	Day	40% 10% 16% 23% 11%	4370
	11th to 20th	Day	38% 7% 19% 23% 12%	4365
	21st to end	Day	48% 7% 13% 17% 14% 1	4582
August	1st to 10th	Day	52% 8% 13% 18% 8%	4091
	11th to 20th	Day	46% 7% 14% 26% 8%	3897
	21st to end	Day	38% 7% 12% 32% 10% 1	4180
September	1st to 10th	Day	42% 8% 11% 22% 15% 2	2280
	11th to 20th	Day	37% 6% 12% 22% 21% 2	2280
	21st to end	Day	47% 6% 11% 19% 15% 1	2033
October	1st to 10th	Day	42% 6% 9% 20% 20% 2	3160
	11th to 20th	Day	39% 6% 8% 28% 17% 3	3140
	21st to end	Day	34% 4 17% 27% 16% 2	3340
November	1st to 10th	Day	40% 6% 13% 23% 16% 2	1761
	11th to 20th	Day	48% 6% 12% 20% 12% 2	1760
	21st to end	Day	51% 7% 13% 21% 7% 1	1600
December	1st to 10th	Day	57% 5% 13% 19% 6% 1	2600
	11th to 20th	Day	56% 5% 14% 16% 9%	2986
	21st to end	Day	60% 6% 11% 14% 8%	3960
January	1st to 10th	Night	52% 8% 11% 16% 11% 2	6349
	11th to 20th	Night	57% 7% 12% 14% 7% 3	6250
	21st to end	Night	56% 8% 9% 14% 13% 1	6490
February	1st to 10th	Night	51% 7% 13% 20% 8% 1	2600
	11th to 20th	Night	54% 6% 11% 18% 9% 1	2457
	21st to end	Night	53% 6% 10% 18% 11% 2	1980
March	1st to 10th	Night	52% 5% 12% 16% 11% 4	5247
	11th to 20th	Night	39% 7% 20% 23% 8% 4	4980
	21st to end	Night	47% 5% 9% 16% 19% 4	5204
April	1st to 10th	Night	53% 6% 11% 17% 10% 2	2000
	11th to 20th	Night	45% 8% 10% 21% 15% 1	1975
	21st to end	Night	43% 7% 11% 21% 15% 3	1957
May	1st to 10th	Night	54% 6% 9% 22% 7% 2	2753
	11th to 20th	Night	50% 11% 13% 16% 8% 1	2496
	21st to end	Night	40% 12% 13% 19% 14% 2	2739
June	1st to 10th	Night	39% 8% 12% 26% 15%	1520
	11th to 20th	Night	38% 10% 16% 25% 11% 1	1520
	21st to end	Night	34% 9% 15% 28% 12% 1	1520
July	1st to 10th	Night	39% 9% 16% 19% 16% 1	2480
	11th to 20th	Night	39% 8% 16% 24% 12% 1	2472
	21st to end	Night	42% 7% 15% 19% 16% 1	2727
August	1st to 10th	Night	49% 9% 16% 19% 7%	2814
	11th to 20th	Night	42% 6% 16% 23% 13%	2966
	21st to end	Night	35% 7% 12% 32% 14% 1	3476
September	1st to 10th	Night	39% 7% 12% 24% 15% 2	1900
	11th to 20th	Night	34% 7% 11% 25% 21% 2	1900
	21st to end	Night	43% 5% 10% 23% 17% 1	2143
October	1st to 10th	Night	43% 7% 10% 21% 18% 1	3800
	11th to 20th	Night	34% 6% 12% 25% 20% 3	3779
	21st to end	Night	35% 5% 16% 27% 16% 1	4207
November	1st to 10th	Night	39% 6% 14% 22% 17% 2	2537
	11th to 20th	Night	48% 5% 13% 21% 11% 2	2637
	21st to end	Night	46% 8% 14% 22% 9% 1	2800
December	1st to 10th	Night	51% 7% 13% 25% 4	4580
	11th to 20th	Night	55% 5% 12% 17% 10% 1	5208
	21st to end	Night	58% 7% 9% 17% 9%	6996

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	32% 27% 31% 8% 3 1	200
	11th to 20th	Day	36% 29% 28% 7% 1	220
	21st to end	Day	27% 32% 28% 8% 4 1	330
February	1st to 10th	Day	33% 30% 28% 7% 1	300
	11th to 20th	Day	37% 26% 30% 6% 1	376
	21st to end	Day	32% 25% 31% 9% 2	414
March	1st to 10th	Day	31% 24% 30% 13% 3	500
	11th to 20th	Day	30% 26% 27% 13% 3 1	500
	21st to end	Day	30% 27% 27% 11% 4 2	550
April	1st to 10th	Day	34% 23% 26% 13% 4	500
	11th to 20th	Day	29% 27% 24% 12% 6% 2	492
	21st to end	Day	24% 23% 31% 15% 5% 2	480
May	1st to 10th	Day	30% 23% 27% 10% 8% 2	463
	11th to 20th	Day	23% 19% 32% 15% 9% 2	487
	21st to end	Day	19% 11% 30% 23% 14% 3	529
June	1st to 10th	Day	18% 16% 30% 21% 12% 2	490
	11th to 20th	Day	16% 19% 23% 29% 12% 2	490
	21st to end	Day	17% 13% 33% 26% 10% 1	490
July	1st to 10th	Day	11% 11% 29% 28% 18% 4	485
	11th to 20th	Day	9% 9% 31% 29% 18% 4	484
	21st to end	Day	14% 10% 32% 28% 13% 3	532
August	1st to 10th	Day	15% 8% 26% 24% 19% 7%	486
	11th to 20th	Day	15% 8% 27% 27% 17% 6%	479
	21st to end	Day	14% 13% 26% 24% 14% 9%	539
September	1st to 10th	Day	19% 13% 21% 21% 18% 7%	490
	11th to 20th	Day	13% 9% 26% 23% 19% 10%	490
	21st to end	Day	28% 10% 24% 21% 13% 4	490
October	1st to 10th	Day	22% 15% 23% 21% 15% 4	490
	11th to 20th	Day	20% 21% 28% 22% 7% 2	490
	21st to end	Day	27% 18% 28% 19% 7% 1	512
November	1st to 10th	Day	21% 22% 26% 21% 8% 2	395
	11th to 20th	Day	25% 25% 28% 16% 5% 1	360
	21st to end	Day	33% 26% 32% 7% 3	200
December	1st to 10th	Day	33% 26% 29% 8% 4 1	200
	11th to 20th	Day	36% 24% 28% 12% 1	200
	21st to end	Day	27% 25% 36% 10% 2	220
January	1st to 10th	Night	22% 26% 36% 14% 3	800
	11th to 20th	Night	25% 21% 32% 20% 2	800
	21st to end	Night	19% 26% 36% 18% 2	880
February	1st to 10th	Night	25% 24% 32% 18% 2	800
	11th to 20th	Night	27% 25% 36% 11% 1	783
	21st to end	Night	28% 22% 33% 14% 3	660
March	1st to 10th	Night	27% 22% 32% 16% 3	800
	11th to 20th	Night	24% 23% 31% 17% 5%	800
	21st to end	Night	23% 24% 32% 17% 5%	880
April	1st to 10th	Night	24% 23% 32% 17% 3	800
	11th to 20th	Night	23% 19% 32% 19% 6%	791
	21st to end	Night	21% 17% 30% 26% 5%	783
May	1st to 10th	Night	22% 17% 26% 25% 9% 1	522
	11th to 20th	Night	14% 11% 35% 33% 6% 1	374
	21st to end	Night	15% 9% 25% 37% 15%	411
June	1st to 10th	Night	16% 7% 24% 37% 16%	380
	11th to 20th	Night	13% 7% 28% 39% 13%	380
	21st to end	Night	11% 6% 28% 43% 12%	380
July	1st to 10th	Night	11% 5% 25% 38% 20% 1	370
	11th to 20th	Night	10% 4 21% 45% 17% 2	368
	21st to end	Night	14% 5% 21% 40% 17% 2	458
August	1st to 10th	Night	14% 3 17% 44% 20% 2	567
	11th to 20th	Night	14% 5% 15% 36% 27% 3	653
	21st to end	Night	13% 6% 20% 34% 25% 2	836
September	1st to 10th	Night	18% 5% 24% 32% 20% 2	760
	11th to 20th	Night	14% 7% 23% 27% 26% 5%	760
	21st to end	Night	22% 10% 20% 30% 17% 2	760
October	1st to 10th	Night	22% 10% 22% 27% 17% 2	760
	11th to 20th	Night	19% 15% 27% 31% 8% 1	760
	21st to end	Night	25% 14% 30% 25% 6%	836
November	1st to 10th	Night	17% 16% 32% 25% 9% 1	780
	11th to 20th	Night	25% 19% 29% 21% 5% 1	800
	21st to end	Night	22% 21% 33% 20% 5%	800
December	1st to 10th	Night	25% 22% 32% 17% 4	800
	11th to 20th	Night	25% 21% 31% 20% 3	800
	21st to end	Night	26% 19% 34% 18% 3	880

Page generated at 2025-04-23 09:25:03. Version ac07

Copyright © 2025 Attilla Danko.