Friendship Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Friendship Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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% 6% 2 5% 4 4 4 3 5% 7% 46%	109049
	Night	15% 6% 2 4 3 3 3 3 4 6% 50%	109439
Transparency	Day	35% 6% 10% 20% 19% 9%	112855
	Night	34% 5% 10% 19% 19% 12%	113760
Seeing	Day	14% 7% 22% 29% 23% 6%	15121
	Night	16% 7% 26% 35% 15% 1	25174

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	17% 8% 2 5% 2 3 3 2 4 6% 48%	4077
	11th to 20th	Day	17% 5% 2 4 2 2 3 2 6% 6% 53%	4080
	21st to end	Day	26% 7% 2 5% 3 3 2 2 3 6% 41%	4764
February	1st to 10th	Day	21% 6% 2 6% 3 3 3 3 4 4 46%	1950
	11th to 20th	Day	20% 7% 2 4 3 3 4 3 4 6% 45%	1849
	21st to end	Day	19% 6% 2 4 3 2 4 2 4 5% 49%	1570
March	1st to 10th	Day	16% 7% 2 7% 4 3 4 3 4 6% 43%	3421
	11th to 20th	Day	21% 10% 3 7% 5% 5% 4 3 4 5% 32%	3480
	21st to end	Day	15% 7% 2 6% 4 3 3 3 5% 7% 45%	3814
April	1st to 10th	Day	16% 9% 3 5% 5% 4 4 4 4 6% 42%	2090
	11th to 20th	Day	19% 7% 2 6% 3 4 4 3 5% 5% 42%	2066
	21st to end	Day	15% 7% 2 6% 4 4 4 4 5% 6% 44%	2417
May	1st to 10th	Day	26% 8% 2 7% 4 4 4 3 5% 6% 31%	4082
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 5% 6% 42%	4082
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 6% 7% 10% 35%	4477
June	1st to 10th	Day	6% 5% 2 5% 4 5% 5% 4 7% 10% 48%	2470
	11th to 20th	Day	9% 4 2 6% 4 5% 5% 4 7% 9% 46%	2470
	21st to end	Day	8% 7% 2 6% 6% 5% 5% 5% 8% 9% 38%	2470
July	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 4 6% 9% 48%	4120
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 7% 11% 50%	4120
	21st to end	Day	10% 6% 2 5% 5% 5% 5% 5% 8% 10% 38%	4465
August	1st to 10th	Day	1 1 1 2 2 2 3 3 4 9% 72%	4104
	11th to 20th	Day	4 2 1 3 3 3 3 4 7% 10% 60%	3763
	21st to end	Day	4 3 1 5% 4 5% 6% 5% 8% 11% 48%	3988
September	1st to 10th	Day	9% 6% 3 7% 5% 4 5% 4 5% 7% 44%	2090
	11th to 20th	Day	12% 6% 2 6% 6% 5% 5% 3 6% 8% 42%	2090
	21st to end	Day	8% 4 2 5% 4 6% 4 5% 6% 7% 51%	2090
October	1st to 10th	Day	7% 5% 2 4 4 4 4 4 5% 7% 56%	2090
	11th to 20th	Day	13% 4 1 5% 4 3 4 3 5% 5% 53%	1948
	21st to end	Day	12% 4 2 5% 3 3 3 2 4 6% 57%	2090
November	1st to 10th	Day	16% 7% 2 5% 3 3 3 4 5% 6% 46%	1958
	11th to 20th	Day	12% 6% 2 6% 3 3 3 4 4 6% 51%	1885
	21st to end	Day	17% 6% 2 6% 4 4 2 3 4 5% 47%	1800
December	1st to 10th	Day	24% 7% 2 7% 3 3 3 2 4 6% 39%	2940
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 5% 43%	3391
	21st to end	Day	16% 5% 1 4 2 2 3 2 3 4 58%	4488
January	1st to 10th	Night	20% 6% 2 4 2 2 2 2 3 5% 51%	5888
	11th to 20th	Night	13% 3 1 2 2 2 2 3 3 5% 64%	5900
	21st to end	Night	29% 7% 2 5% 2 2 3 2 4 4 40%	6242
February	1st to 10th	Night	18% 7% 2 5% 3 2 3 2 4 5% 48%	2340
	11th to 20th	Night	20% 6% 2 5% 3 3 3 2 3 5% 49%	2218
	21st to end	Night	20% 7% 2 4 3 3 4 2 4 5% 45%	1884
March	1st to 10th	Night	18% 8% 2 5% 4 5% 4 3 5% 6% 39%	3433
	11th to 20th	Night	23% 10% 3 6% 4 4 4 3 4 5% 33%	3480
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 5% 44%	3817
April	1st to 10th	Night	14% 7% 2 5% 3 3 4 4 4 7% 46%	2090
	11th to 20th	Night	20% 6% 2 5% 4 3 4 3 5% 5% 44%	2063
	21st to end	Night	18% 7% 2 5% 3 4 3 2 5% 6% 46%	1757
May	1st to 10th	Night	26% 8% 2 5% 4 2 4 2 4 5% 36%	2823
	11th to 20th	Night	13% 6% 3 5% 3 3 3 3 4 6% 50%	2823
	21st to end	Night	14% 6% 2 5% 4 4 4 4 7% 9% 42%	3090
June	1st to 10th	Night	5% 4 2 3 3 3 3 3 5% 11% 59%	1710
	11th to 20th	Night	6% 2 1 5% 4 4 4 3 6% 10% 56%	1710
	21st to end	Night	7% 5% 1 4 3 3 5% 4 7% 9% 52%	1710
July	1st to 10th	Night	6% 3 2 3 3 3 4 4 6% 10% 55%	2850
	11th to 20th	Night	4 3 1 5% 3 4 4 3 7% 10% 55%	2850
	21st to end	Night	11% 8% 2 6% 4 4 5% 3 7% 9% 41%	3098
August	1st to 10th	Night	1 1 1 2 2 2 2 2 5% 8% 75%	2836
	11th to 20th	Night	3 2 1 3 3 3 4 3 7% 11% 60%	3072
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 11% 55%	3671
September	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 5% 7% 53%	2090
	11th to 20th	Night	10% 6% 2 5% 5% 5% 4 4 5% 7% 46%	2090
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 7% 54%	2090
October	1st to 10th	Night	7% 4 1 4 3 3 3 3 4 6% 62%	2090
	11th to 20th	Night	13% 5% 1 4 2 3 3 3 4 6% 55%	2210
	21st to end	Night	15% 4 1 4 2 2 2 2 3 4 61%	2508
November	1st to 10th	Night	15% 6% 2 5% 2 3 3 2 5% 5% 51%	2340
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 6% 56%	2510
	21st to end	Night	17% 6% 1 5% 3 4 3 3 4 7% 47%	2600
December	1st to 10th	Night	23% 8% 2 6% 3 3 3 2 4 4 41%	4250
	11th to 20th	Night	21% 9% 2 5% 3 2 2 2 3 4 47%	4816
	21st to end	Night	18% 6% 1 4 2 2 2 2 3 4 57%	6490

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	40% 3 4 22% 21% 10%	4078
	11th to 20th	Day	33% 2 5% 16% 25% 18%	4080
	21st to end	Day	46% 3 3 9% 24% 14%	4764
February	1st to 10th	Day	42% 5% 6% 12% 21% 15%	1950
	11th to 20th	Day	41% 5% 7% 16% 19% 12%	1859
	21st to end	Day	40% 4 7% 17% 18% 15%	1570
March	1st to 10th	Day	42% 6% 8% 13% 21% 10%	3421
	11th to 20th	Day	52% 9% 7% 8% 16% 8%	3480
	21st to end	Day	37% 7% 10% 14% 22% 9%	3814
April	1st to 10th	Day	42% 7% 7% 16% 15% 12%	2090
	11th to 20th	Day	43% 6% 8% 13% 19% 11%	2066
	21st to end	Day	40% 6% 6% 20% 18% 9%	2417
May	1st to 10th	Day	51% 9% 9% 13% 13% 5%	4120
	11th to 20th	Day	40% 9% 8% 17% 20% 6%	4086
	21st to end	Day	37% 12% 15% 25% 11% 1	4489
June	1st to 10th	Day	29% 8% 14% 27% 19% 4	2470
	11th to 20th	Day	33% 8% 12% 25% 17% 5%	2470
	21st to end	Day	38% 10% 11% 22% 16% 3	2470
July	1st to 10th	Day	31% 7% 16% 38% 7% 1	4056
	11th to 20th	Day	28% 6% 19% 30% 16% 1	4051
	21st to end	Day	39% 6% 18% 28% 8%	4325
August	1st to 10th	Day	12% 4 11% 42% 29% 1	4090
	11th to 20th	Day	19% 5% 22% 39% 11% 2	3800
	21st to end	Day	28% 9% 19% 25% 17% 2	3987
September	1st to 10th	Day	38% 7% 10% 20% 15% 9%	2090
	11th to 20th	Day	39% 8% 11% 18% 18% 6%	2090
	21st to end	Day	30% 8% 11% 20% 17% 14%	2090
October	1st to 10th	Day	17% 4 6% 12% 23% 37%	3480
	11th to 20th	Day	21% 4 9% 18% 27% 20%	3214
	21st to end	Day	27% 4 4 20% 32% 14%	3428
November	1st to 10th	Day	38% 7% 9% 17% 19% 11%	1958
	11th to 20th	Day	34% 5% 7% 17% 20% 15%	1885
	21st to end	Day	42% 3 7% 16% 19% 14%	1800
December	1st to 10th	Day	49% 3 6% 17% 16% 9%	2940
	11th to 20th	Day	42% 5% 8% 11% 23% 12%	3389
	21st to end	Day	32% 3 5% 16% 27% 17%	4488
January	1st to 10th	Night	37% 3 4 19% 24% 13%	5888
	11th to 20th	Night	25% 2 4 18% 30% 21%	5900
	21st to end	Night	48% 3 4 10% 20% 16%	6242
February	1st to 10th	Night	40% 4 6% 12% 20% 18%	2340
	11th to 20th	Night	40% 4 6% 16% 19% 16%	2218
	21st to end	Night	42% 4 7% 14% 17% 16%	1884
March	1st to 10th	Night	43% 8% 9% 14% 17% 9%	3433
	11th to 20th	Night	51% 9% 9% 10% 13% 9%	3480
	21st to end	Night	40% 8% 9% 15% 19% 9%	3817
April	1st to 10th	Night	38% 7% 8% 17% 16% 14%	2090
	11th to 20th	Night	42% 6% 8% 13% 17% 14%	2063
	21st to end	Night	40% 6% 9% 19% 17% 9%	1757
May	1st to 10th	Night	48% 8% 9% 16% 13% 6%	2850
	11th to 20th	Night	34% 8% 11% 23% 17% 7%	2824
	21st to end	Night	35% 10% 14% 28% 11% 2	3099
June	1st to 10th	Night	21% 7% 18% 28% 23% 3	1710
	11th to 20th	Night	23% 6% 15% 30% 20% 5%	1710
	21st to end	Night	27% 9% 18% 28% 16% 3	1710
July	1st to 10th	Night	24% 7% 25% 37% 6% 1	2805
	11th to 20th	Night	24% 7% 20% 34% 15% 1	2796
	21st to end	Night	38% 6% 22% 26% 6%	2999
August	1st to 10th	Night	9% 4 16% 45% 26% 1	2826
	11th to 20th	Night	20% 6% 30% 32% 10% 2	3101
	21st to end	Night	24% 11% 21% 27% 16% 2	3671
September	1st to 10th	Night	31% 7% 16% 23% 16% 8%	2090
	11th to 20th	Night	35% 9% 14% 22% 14% 6%	2090
	21st to end	Night	28% 7% 15% 20% 17% 13%	2090
October	1st to 10th	Night	16% 4 7% 14% 21% 39%	3480
	11th to 20th	Night	20% 4 10% 16% 30% 20%	3670
	21st to end	Night	26% 2 5% 19% 32% 16%	4120
November	1st to 10th	Night	34% 6% 9% 16% 20% 14%	2340
	11th to 20th	Night	31% 5% 9% 18% 20% 17%	2510
	21st to end	Night	38% 5% 8% 16% 18% 15%	2600
December	1st to 10th	Night	49% 3 6% 17% 16% 10%	4250
	11th to 20th	Night	44% 3 6% 8% 22% 17%	4817
	21st to end	Night	34% 3 4 14% 29% 16%	6490

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	20% 14% 38% 20% 6% 1	300
	11th to 20th	Day	24% 14% 29% 24% 7% 1	300
	21st to end	Day	21% 13% 38% 22% 6%	330
February	1st to 10th	Day	30% 11% 28% 24% 6%	285
	11th to 20th	Day	27% 18% 35% 16% 4	270
	21st to end	Day	27% 17% 42% 13% 2	225
March	1st to 10th	Day	22% 17% 37% 19% 5%	270
	11th to 20th	Day	21% 18% 34% 21% 5% 1	270
	21st to end	Day	21% 16% 35% 23% 4 1	297
April	1st to 10th	Day	20% 16% 40% 20% 4	270
	11th to 20th	Day	18% 17% 36% 20% 8%	264
	21st to end	Day	16% 14% 39% 22% 8% 1	597
May	1st to 10th	Day	19% 9% 35% 29% 7% 1	653
	11th to 20th	Day	18% 8% 28% 33% 12% 2	675
	21st to end	Day	15% 7% 24% 34% 18% 2	734
June	1st to 10th	Day	7% 2 15% 38% 31% 7%	680
	11th to 20th	Day	10% 1 13% 45% 27% 4	680
	21st to end	Day	7% 1 12% 37% 33% 10%	680
July	1st to 10th	Day	8% 1 11% 33% 34% 13%	671
	11th to 20th	Day	4 6% 33% 46% 10%	668
	21st to end	Day	5% 6% 32% 45% 12%	739
August	1st to 10th	Day	3 5% 28% 48% 16%	676
	11th to 20th	Day	7% 4 32% 45% 12%	502
	21st to end	Day	6% 1 6% 25% 36% 27%	425
September	1st to 10th	Day	8% 7% 25% 41% 18%	300
	11th to 20th	Day	8% 6% 24% 46% 16%	300
	21st to end	Day	9% 1 9% 25% 40% 15%	300
October	1st to 10th	Day	5% 3 13% 33% 33% 13%	300
	11th to 20th	Day	11% 3 22% 33% 24% 7%	300
	21st to end	Day	20% 10% 25% 23% 18% 5%	330
November	1st to 10th	Day	18% 6% 28% 31% 12% 4	300
	11th to 20th	Day	17% 11% 26% 30% 15% 2	300
	21st to end	Day	20% 15% 36% 17% 11% 2	300
December	1st to 10th	Day	21% 8% 33% 25% 9% 4	300
	11th to 20th	Day	19% 18% 31% 21% 11%	300
	21st to end	Day	21% 10% 37% 24% 7% 2	330
January	1st to 10th	Night	20% 9% 31% 32% 8% 1	1000
	11th to 20th	Night	21% 10% 32% 30% 7%	1000
	21st to end	Night	23% 8% 31% 30% 7%	995
February	1st to 10th	Night	24% 11% 31% 28% 6%	780
	11th to 20th	Night	26% 9% 38% 24% 3	744
	21st to end	Night	25% 14% 40% 20% 1	628
March	1st to 10th	Night	21% 13% 36% 27% 3	584
	11th to 20th	Night	22% 10% 35% 29% 4	570
	21st to end	Night	17% 12% 40% 28% 3	627
April	1st to 10th	Night	18% 10% 38% 29% 4	570
	11th to 20th	Night	21% 10% 40% 24% 4	563
	21st to end	Night	19% 10% 40% 24% 8% 1	570
May	1st to 10th	Night	18% 11% 36% 29% 6%	570
	11th to 20th	Night	17% 5% 31% 34% 12%	561
	21st to end	Night	14% 6% 33% 34% 13%	617
June	1st to 10th	Night	6% 2 21% 47% 23% 1	570
	11th to 20th	Night	6% 2 31% 45% 15% 1	570
	21st to end	Night	6% 2 18% 45% 25% 4	570
July	1st to 10th	Night	9% 1 13% 44% 29% 4	555
	11th to 20th	Night	3 1 11% 54% 29% 2	552
	21st to end	Night	5% 1 8% 51% 32% 3	624
August	1st to 10th	Night	1 8% 53% 33% 4	567
	11th to 20th	Night	5% 8% 51% 32% 4	564
	21st to end	Night	6% 6% 41% 41% 6%	627
September	1st to 10th	Night	9% 1 9% 38% 37% 5%	570
	11th to 20th	Night	12% 2 8% 37% 38% 3	570
	21st to end	Night	11% 13% 36% 35% 4	570
October	1st to 10th	Night	6% 1 12% 44% 32% 5%	570
	11th to 20th	Night	13% 3 22% 38% 22% 2	690
	21st to end	Night	16% 8% 23% 31% 21% 1	836
November	1st to 10th	Night	16% 5% 20% 39% 18% 1	780
	11th to 20th	Night	13% 6% 28% 38% 14% 1	910
	21st to end	Night	21% 10% 28% 29% 11% 1	1000
December	1st to 10th	Night	19% 8% 28% 36% 9% 1	1000
	11th to 20th	Night	24% 9% 28% 32% 7%	1000
	21st to end	Night	21% 8% 31% 32% 8%	1100

Page generated at 2025-02-03 02:05:49. Version ac07

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