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 2024-10-25.

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 7% 5% 5% 5% 4 6% 7% 34%	101350
	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 36%	97473
Transparency	Day	48% 8% 13% 20% 10% 1	101109
	Night	47% 8% 13% 20% 11% 1	97333
Seeing	Day	23% 19% 28% 18% 10% 3	14796
	Night	21% 16% 29% 25% 9% 1	24041

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% 14% 6% 9% 6% 4 3 3 4 4 32%	2517
	11th to 20th	Day	13% 14% 7% 14% 6% 5% 4 3 4 5% 23%	2584
	21st to end	Day	17% 16% 6% 11% 7% 5% 4 3 4 5% 22%	3124
February	1st to 10th	Day	16% 12% 4 9% 6% 5% 5% 4 7% 5% 27%	1710
	11th to 20th	Day	13% 11% 5% 8% 7% 5% 4 3 6% 7% 32%	1735
	21st to end	Day	18% 11% 3 10% 6% 4 4 4 4 6% 29%	1570
March	1st to 10th	Day	24% 10% 3 9% 6% 5% 4 3 4 5% 28%	3198
	11th to 20th	Day	10% 9% 4 9% 5% 5% 4 3 5% 6% 38%	3480
	21st to end	Day	18% 11% 2 7% 4 3 4 3 4 5% 39%	3992
April	1st to 10th	Day	17% 12% 3 6% 6% 4 5% 3 5% 6% 32%	2280
	11th to 20th	Day	18% 9% 3 7% 4 4 5% 4 5% 6% 36%	2254
	21st to end	Day	16% 7% 3 8% 5% 4 5% 4 6% 7% 36%	2280
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	21% 9% 2 7% 5% 5% 4 4 5% 7% 32%	1978
November	1st to 10th	Day	13% 8% 3 6% 4 3 3 3 5% 6% 44%	1671
	11th to 20th	Day	16% 11% 3 8% 5% 4 3 3 4 7% 36%	1672
	21st to end	Day	17% 10% 3 6% 5% 5% 5% 3 5% 7% 33%	1520
December	1st to 10th	Day	18% 14% 5% 6% 6% 5% 5% 3 5% 5% 28%	1520
	11th to 20th	Day	22% 12% 5% 7% 4 3 4 3 5% 5% 31%	1906
	21st to end	Day	20% 16% 6% 10% 6% 6% 4 3 4 4 20%	2772
January	1st to 10th	Night	15% 11% 4 10% 6% 4 4 3 5% 6% 32%	4429
	11th to 20th	Night	15% 10% 5% 14% 5% 5% 5% 4 5% 6% 27%	4372
	21st to end	Night	21% 13% 4 11% 6% 5% 4 4 5% 5% 23%	4532
February	1st to 10th	Night	15% 11% 3 7% 6% 5% 4 4 5% 6% 34%	2470
	11th to 20th	Night	14% 10% 3 9% 7% 5% 5% 5% 5% 6% 31%	2329
	21st to end	Night	15% 11% 3 9% 6% 5% 4 4 4 5% 32%	1884
March	1st to 10th	Night	22% 14% 3 7% 4 4 4 3 5% 5% 30%	3639
	11th to 20th	Night	10% 11% 4 7% 4 5% 5% 5% 5% 6% 38%	3479
	21st to end	Night	13% 10% 3 7% 5% 4 4 3 4 5% 42%	3638
April	1st to 10th	Night	16% 12% 3 8% 5% 5% 5% 4 5% 5% 32%	1900
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 5% 7% 39%	1875
	21st to end	Night	18% 7% 2 7% 4 4 3 3 5% 5% 41%	1900
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	21% 7% 3 6% 4 5% 4 4 5% 6% 35%	2483
November	1st to 10th	Night	11% 8% 2 7% 5% 4 4 3 5% 6% 46%	2407
	11th to 20th	Night	18% 8% 3 6% 4 4 5% 3 4 6% 38%	2505
	21st to end	Night	14% 9% 3 7% 5% 5% 4 4 6% 6% 38%	2660
December	1st to 10th	Night	18% 12% 4 8% 5% 5% 5% 4 6% 6% 28%	2660
	11th to 20th	Night	19% 11% 3 8% 6% 4 4 3 5% 6% 30%	3288
	21st to end	Night	21% 14% 4 9% 6% 5% 4 3 4 5% 26%	4884

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	54% 7% 13% 20% 5% 1	2518
	11th to 20th	Day	64% 6% 10% 14% 5% 1	2584
	21st to end	Day	62% 7% 9% 16% 5% 1	3124
February	1st to 10th	Day	57% 6% 12% 15% 9% 1	1710
	11th to 20th	Day	52% 6% 11% 21% 9% 1	1744
	21st to end	Day	55% 5% 12% 19% 7% 2	1570
March	1st to 10th	Day	56% 7% 10% 19% 6% 1	3201
	11th to 20th	Day	43% 8% 15% 23% 9% 2	3480
	21st to end	Day	48% 6% 9% 17% 18% 2	3992
April	1st to 10th	Day	52% 6% 10% 19% 11% 3	2280
	11th to 20th	Day	47% 7% 11% 20% 13% 2	2254
	21st to end	Day	45% 8% 10% 21% 13% 4	2280
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	43% 8% 13% 21% 13% 2	1900
	11th to 20th	Day	44% 7% 11% 26% 12% 1	1900
	21st to end	Day	51% 6% 13% 17% 10% 2	1978
November	1st to 10th	Day	41% 6% 14% 22% 15% 2	1671
	11th to 20th	Day	48% 6% 12% 19% 12% 2	1672
	21st to end	Day	50% 8% 14% 21% 7% 1	1520
December	1st to 10th	Day	58% 7% 14% 13% 7% 1	1520
	11th to 20th	Day	56% 6% 15% 17% 5%	1906
	21st to end	Day	66% 8% 9% 13% 4	2772
January	1st to 10th	Night	51% 8% 15% 19% 6% 1	4429
	11th to 20th	Night	58% 7% 12% 16% 6% 1	4372
	21st to end	Night	61% 9% 9% 15% 6%	4532
February	1st to 10th	Night	51% 7% 13% 20% 8% 1	2470
	11th to 20th	Night	54% 7% 11% 18% 9% 1	2331
	21st to end	Night	53% 6% 11% 18% 11% 2	1884
March	1st to 10th	Night	55% 6% 10% 18% 9% 1	3649
	11th to 20th	Night	43% 9% 17% 23% 7% 1	3480
	21st to end	Night	44% 6% 10% 19% 17% 4	3638
April	1st to 10th	Night	52% 7% 11% 18% 11% 2	1900
	11th to 20th	Night	44% 9% 10% 21% 16% 1	1875
	21st to end	Night	44% 7% 11% 21% 15% 3	1900
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	42% 8% 13% 22% 15% 1	2280
	11th to 20th	Night	41% 6% 14% 24% 12% 2	2280
	21st to end	Night	48% 6% 13% 21% 11% 1	2483
November	1st to 10th	Night	39% 6% 15% 21% 16% 2	2407
	11th to 20th	Night	47% 5% 13% 21% 10% 2	2505
	21st to end	Night	45% 8% 14% 23% 9% 1	2660
December	1st to 10th	Night	55% 9% 13% 17% 6%	2660
	11th to 20th	Night	54% 7% 13% 17% 8% 1	3288
	21st to end	Night	59% 8% 12% 18% 3	4884

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% 25% 32% 9% 3 1	180
	11th to 20th	Day	37% 27% 27% 8% 1	198
	21st to end	Day	28% 31% 28% 8% 4 1	297
February	1st to 10th	Day	34% 30% 27% 7% 1	270
	11th to 20th	Day	35% 27% 30% 6% 1	342
	21st to end	Day	33% 26% 29% 9% 2	382
March	1st to 10th	Day	30% 24% 29% 13% 3	460
	11th to 20th	Day	31% 27% 26% 12% 3 1	460
	21st to end	Day	30% 28% 25% 11% 4 2	506
April	1st to 10th	Day	32% 24% 26% 13% 4	460
	11th to 20th	Day	27% 28% 24% 12% 7% 2	452
	21st to end	Day	25% 24% 31% 15% 5% 2	460
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	28% 18% 29% 19% 6% 1	490
November	1st to 10th	Day	22% 21% 25% 22% 9% 2	365
	11th to 20th	Day	26% 26% 27% 16% 5% 1	332
	21st to end	Day	32% 24% 33% 8% 3	180
December	1st to 10th	Day	33% 26% 28% 8% 3 1	180
	11th to 20th	Day	36% 25% 28% 11% 1	180
	21st to end	Day	29% 26% 35% 9% 2	198
January	1st to 10th	Night	23% 25% 36% 14% 3	760
	11th to 20th	Night	25% 21% 32% 20% 2	760
	21st to end	Night	19% 27% 35% 18% 2	836
February	1st to 10th	Night	25% 24% 32% 18% 2	760
	11th to 20th	Night	26% 25% 37% 11% 1	743
	21st to end	Night	29% 23% 32% 14% 3	628
March	1st to 10th	Night	28% 21% 32% 16% 3	760
	11th to 20th	Night	25% 24% 31% 16% 4	760
	21st to end	Night	23% 24% 31% 16% 5%	836
April	1st to 10th	Night	23% 23% 33% 18% 4	760
	11th to 20th	Night	22% 19% 33% 20% 6%	751
	21st to end	Night	21% 18% 30% 25% 5%	760
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	26% 14% 30% 24% 6%	812
November	1st to 10th	Night	17% 16% 32% 25% 9% 1	740
	11th to 20th	Night	24% 19% 29% 20% 6% 1	760
	21st to end	Night	21% 21% 32% 20% 5%	760
December	1st to 10th	Night	26% 23% 32% 16% 4	760
	11th to 20th	Night	24% 22% 31% 19% 3	760
	21st to end	Night	25% 20% 35% 18% 2	836

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