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 2024-01-27.

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	13% 5% 2 4 4 3 4 4 5% 7% 47%	75012
	Night	13% 5% 2 4 4 2 3 4 5% 7% 51%	75060
Transparency	Day	36% 7% 10% 19% 19% 9%	74737
	Night	33% 6% 11% 20% 19% 11%	74855
Seeing	Day	14% 7% 21% 29% 23% 7%	13755
	Night	15% 6% 26% 35% 16% 1	23829

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	15% 5% 1 4 4 2 3 3 4 6% 53%	1707
	11th to 20th	Day	19% 6% 2 4 3 2 3 3 4 5% 50%	1710
	21st to end	Day	16% 5% 3 4 3 3 3 3 4 6% 50%	1945
February	1st to 10th	Day	21% 6% 3 5% 4 2 2 3 3 5% 46%	1800
	11th to 20th	Day	21% 6% 2 4 3 2 4 4 4 6% 45%	1749
	21st to end	Day	20% 6% 2 4 3 2 4 3 3 5% 48%	1480
March	1st to 10th	Day	20% 7% 2 6% 4 2 3 4 4 6% 41%	1965
	11th to 20th	Day	17% 7% 3 5% 4 3 4 4 4 6% 44%	1980
	21st to end	Day	15% 6% 3 5% 4 2 4 4 4 6% 47%	2178
April	1st to 10th	Day	16% 8% 3 4 5% 3 4 4 4 6% 42%	1980
	11th to 20th	Day	19% 7% 3 5% 4 2 4 3 4 5% 43%	1956
	21st to end	Day	16% 6% 3 5% 4 3 4 5% 4 5% 46%	2289
May	1st to 10th	Day	17% 6% 3 6% 5% 3 5% 4 5% 6% 40%	2340
	11th to 20th	Day	16% 7% 3 6% 6% 4 5% 5% 4 6% 38%	2302
	21st to end	Day	17% 5% 3 6% 6% 4 6% 6% 5% 9% 33%	2531
June	1st to 10th	Day	6% 4 2 5% 5% 4 5% 5% 7% 10% 49%	2340
	11th to 20th	Day	9% 3 2 5% 5% 3 5% 5% 7% 9% 47%	2340
	21st to end	Day	8% 6% 3 6% 6% 4 5% 7% 8% 10% 37%	2340
July	1st to 10th	Day	11% 5% 3 5% 7% 4 6% 6% 7% 9% 36%	2340
	11th to 20th	Day	5% 3 2 4 5% 4 6% 5% 7% 10% 51%	2340
	21st to end	Day	6% 3 2 3 4 4 5% 5% 8% 11% 50%	2546
August	1st to 10th	Day	2 2 1 3 3 2 5% 4 5% 9% 63%	2328
	11th to 20th	Day	7% 3 2 3 4 3 4 5% 6% 8% 53%	2147
	21st to end	Day	5% 4 1 5% 5% 5% 6% 7% 8% 11% 43%	2268
September	1st to 10th	Day	10% 5% 3 5% 7% 3 5% 5% 6% 8% 44%	1980
	11th to 20th	Day	13% 6% 2 5% 7% 3 5% 5% 6% 8% 41%	1980
	21st to end	Day	8% 4 2 4 4 4 4 6% 6% 8% 50%	1980
October	1st to 10th	Day	7% 5% 2 4 4 3 4 5% 5% 8% 54%	1980
	11th to 20th	Day	14% 4 2 4 4 2 4 4 5% 6% 51%	1846
	21st to end	Day	12% 4 2 4 3 2 3 3 3 6% 58%	1980
November	1st to 10th	Day	15% 7% 2 4 4 3 3 5% 5% 6% 46%	1858
	11th to 20th	Day	12% 6% 3 5% 4 2 3 4 4 5% 50%	1791
	21st to end	Day	18% 6% 2 5% 5% 3 3 3 4 5% 46%	1710
December	1st to 10th	Day	15% 6% 2 5% 4 2 3 3 4 7% 48%	1710
	11th to 20th	Day	15% 8% 3 5% 5% 2 4 5% 4 5% 44%	2161
	21st to end	Day	17% 5% 2 3 3 2 3 3 4 4 54%	3135
January	1st to 10th	Night	17% 4 2 3 3 2 3 3 3 7% 54%	2458
	11th to 20th	Night	18% 5% 2 4 3 2 3 3 3 5% 52%	2470
	21st to end	Night	18% 6% 3 4 3 2 3 3 4 5% 49%	2570
February	1st to 10th	Night	18% 7% 3 4 4 2 3 3 4 5% 48%	2160
	11th to 20th	Night	21% 6% 2 4 4 2 3 3 3 5% 49%	2098
	21st to end	Night	21% 7% 3 3 4 2 3 3 3 5% 45%	1776
March	1st to 10th	Night	19% 5% 2 4 4 3 4 4 4 7% 43%	1994
	11th to 20th	Night	17% 7% 2 5% 4 2 4 5% 3 5% 45%	1980
	21st to end	Night	16% 5% 3 4 4 2 3 3 4 6% 50%	2178
April	1st to 10th	Night	14% 7% 2 4 4 2 4 4 4 7% 47%	1980
	11th to 20th	Night	19% 6% 2 5% 4 2 3 4 5% 5% 45%	1953
	21st to end	Night	16% 6% 3 5% 4 3 3 3 5% 6% 47%	1665
May	1st to 10th	Night	15% 5% 2 5% 6% 2 4 4 5% 7% 45%	1620
	11th to 20th	Night	14% 6% 3 4 4 3 3 4 5% 8% 48%	1593
	21st to end	Night	14% 5% 2 3 4 3 4 6% 7% 9% 44%	1745
June	1st to 10th	Night	5% 3 2 3 3 2 3 3 5% 11% 60%	1620
	11th to 20th	Night	5% 2 1 4 5% 3 4 4 5% 10% 58%	1620
	21st to end	Night	7% 4 2 3 4 2 4 5% 6% 10% 52%	1620
July	1st to 10th	Night	9% 4 2 3 5% 2 5% 5% 7% 11% 47%	1620
	11th to 20th	Night	2 2 1 3 3 2 3 4 7% 11% 60%	1620
	21st to end	Night	4 2 1 3 3 2 4 3 6% 11% 59%	1772
August	1st to 10th	Night	1 2 1 2 2 2 2 4 5% 10% 70%	1611
	11th to 20th	Night	5% 3 2 4 4 3 4 4 7% 10% 53%	1750
	21st to end	Night	6% 3 2 3 5% 3 5% 6% 6% 11% 49%	2086
September	1st to 10th	Night	8% 4 2 4 4 3 3 4 5% 7% 54%	1980
	11th to 20th	Night	10% 6% 3 5% 6% 4 4 4 5% 7% 46%	1980
	21st to end	Night	9% 4 2 4 4 2 4 5% 6% 7% 53%	1980
October	1st to 10th	Night	7% 4 2 3 4 2 3 4 4 6% 61%	1980
	11th to 20th	Night	13% 5% 2 4 3 2 3 4 4 7% 53%	2093
	21st to end	Night	15% 4 1 3 3 2 3 3 3 4 60%	2376
November	1st to 10th	Night	14% 6% 2 4 3 2 4 3 5% 5% 53%	2220
	11th to 20th	Night	11% 5% 2 5% 4 2 3 4 4 6% 54%	2384
	21st to end	Night	18% 6% 1 4 4 3 4 3 4 7% 45%	2470
December	1st to 10th	Night	15% 6% 2 4 4 2 4 3 4 5% 52%	2470
	11th to 20th	Night	17% 8% 3 5% 4 2 3 3 3 5% 46%	3036
	21st to end	Night	19% 6% 1 3 3 2 2 2 3 4 54%	4532

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	35% 3 5% 20% 23% 14%	1708
	11th to 20th	Day	38% 3 6% 16% 22% 15%	1710
	21st to end	Day	36% 4 6% 13% 25% 15%	1945
February	1st to 10th	Day	42% 4 6% 12% 21% 15%	1800
	11th to 20th	Day	42% 5% 8% 16% 18% 12%	1759
	21st to end	Day	41% 4 7% 16% 18% 16%	1480
March	1st to 10th	Day	45% 6% 9% 12% 19% 10%	1965
	11th to 20th	Day	41% 6% 7% 12% 20% 14%	1980
	21st to end	Day	37% 5% 10% 16% 20% 11%	2178
April	1st to 10th	Day	43% 6% 7% 17% 16% 11%	1980
	11th to 20th	Day	43% 5% 8% 14% 19% 11%	1956
	21st to end	Day	39% 5% 6% 20% 19% 10%	2289
May	1st to 10th	Day	44% 7% 8% 16% 18% 8%	2340
	11th to 20th	Day	45% 7% 8% 18% 17% 4	2306
	21st to end	Day	46% 7% 12% 19% 14% 1	2531
June	1st to 10th	Day	29% 7% 14% 27% 19% 4	2340
	11th to 20th	Day	33% 8% 12% 26% 17% 5%	2340
	21st to end	Day	40% 10% 11% 21% 15% 3	2340
July	1st to 10th	Day	40% 11% 13% 23% 11% 2	2276
	11th to 20th	Day	26% 9% 19% 25% 19% 2	2271
	21st to end	Day	24% 9% 21% 33% 13% 1	2405
August	1st to 10th	Day	18% 7% 16% 38% 20% 1	2314
	11th to 20th	Day	26% 9% 17% 31% 13% 4	2147
	21st to end	Day	29% 13% 17% 25% 13% 3	2268
September	1st to 10th	Day	38% 8% 11% 21% 15% 8%	1980
	11th to 20th	Day	40% 9% 12% 18% 17% 5%	1980
	21st to end	Day	31% 8% 11% 20% 17% 12%	1980
October	1st to 10th	Day	28% 7% 11% 21% 22% 11%	1980
	11th to 20th	Day	32% 6% 10% 15% 20% 17%	1846
	21st to end	Day	29% 5% 6% 15% 24% 20%	1980
November	1st to 10th	Day	37% 7% 9% 17% 20% 10%	1858
	11th to 20th	Day	35% 5% 8% 17% 21% 14%	1791
	21st to end	Day	43% 4 6% 15% 20% 13%	1710
December	1st to 10th	Day	38% 4 9% 18% 22% 8%	1710
	11th to 20th	Day	40% 7% 7% 11% 21% 13%	2159
	21st to end	Day	35% 4 6% 15% 27% 14%	3135
January	1st to 10th	Night	33% 4 6% 17% 23% 18%	2458
	11th to 20th	Night	35% 3 7% 16% 23% 16%	2470
	21st to end	Night	39% 3 5% 14% 20% 18%	2570
February	1st to 10th	Night	40% 4 6% 12% 20% 18%	2160
	11th to 20th	Night	39% 4 6% 16% 19% 16%	2098
	21st to end	Night	43% 3 7% 14% 17% 17%	1776
March	1st to 10th	Night	41% 6% 9% 15% 19% 10%	1994
	11th to 20th	Night	42% 6% 8% 11% 19% 14%	1980
	21st to end	Night	37% 5% 10% 16% 19% 13%	2178
April	1st to 10th	Night	37% 6% 9% 18% 17% 13%	1980
	11th to 20th	Night	41% 6% 8% 14% 17% 14%	1953
	21st to end	Night	39% 5% 9% 19% 18% 10%	1665
May	1st to 10th	Night	39% 6% 10% 19% 17% 9%	1620
	11th to 20th	Night	37% 7% 11% 25% 16% 4	1594
	21st to end	Night	34% 9% 15% 27% 14% 1	1746
June	1st to 10th	Night	21% 7% 17% 28% 24% 3	1620
	11th to 20th	Night	22% 6% 16% 31% 19% 5%	1620
	21st to end	Night	27% 9% 18% 27% 15% 3	1620
July	1st to 10th	Night	29% 11% 21% 28% 10% 1	1575
	11th to 20th	Night	17% 9% 24% 30% 19% 1	1566
	21st to end	Night	17% 9% 26% 38% 10% 1	1673
August	1st to 10th	Night	10% 7% 25% 37% 19% 1	1601
	11th to 20th	Night	25% 10% 21% 29% 11% 3	1750
	21st to end	Night	26% 12% 21% 27% 12% 3	2086
September	1st to 10th	Night	30% 7% 16% 24% 16% 7%	1980
	11th to 20th	Night	36% 9% 14% 22% 14% 6%	1980
	21st to end	Night	28% 7% 16% 19% 18% 12%	1980
October	1st to 10th	Night	23% 6% 13% 24% 22% 12%	1980
	11th to 20th	Night	31% 6% 10% 15% 19% 19%	2093
	21st to end	Night	30% 3 6% 16% 23% 22%	2376
November	1st to 10th	Night	32% 6% 9% 17% 21% 13%	2220
	11th to 20th	Night	32% 6% 9% 18% 20% 15%	2384
	21st to end	Night	40% 5% 8% 15% 18% 15%	2470
December	1st to 10th	Night	36% 4 8% 20% 20% 12%	2470
	11th to 20th	Night	41% 5% 5% 11% 20% 18%	3037
	21st to end	Night	36% 4 5% 14% 29% 13%	4532

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% 21% 7% 1	270
	11th to 20th	Day	25% 14% 27% 25% 7% 1	270
	21st to end	Day	20% 13% 39% 22% 6%	282
February	1st to 10th	Day	29% 11% 30% 24% 6%	240
	11th to 20th	Day	28% 18% 34% 16% 5%	240
	21st to end	Day	28% 18% 38% 14% 2	198
March	1st to 10th	Day	23% 16% 35% 20% 5%	240
	11th to 20th	Day	21% 18% 33% 23% 5% 1	240
	21st to end	Day	21% 15% 33% 25% 5% 1	264
April	1st to 10th	Day	19% 15% 41% 20% 5%	240
	11th to 20th	Day	18% 16% 36% 21% 9%	234
	21st to end	Day	16% 14% 39% 21% 9% 1	549
May	1st to 10th	Day	18% 9% 35% 31% 7% 1	603
	11th to 20th	Day	18% 7% 27% 33% 13% 2	625
	21st to end	Day	16% 6% 22% 34% 19% 2	679
June	1st to 10th	Day	8% 2 14% 38% 32% 7%	630
	11th to 20th	Day	9% 1 13% 46% 27% 4	630
	21st to end	Day	7% 1 12% 37% 32% 10%	630
July	1st to 10th	Day	8% 1 11% 33% 34% 12%	621
	11th to 20th	Day	4 5% 33% 47% 11%	618
	21st to end	Day	4 5% 31% 47% 13%	684
August	1st to 10th	Day	4 5% 29% 47% 14%	626
	11th to 20th	Day	7% 4 31% 44% 13%	461
	21st to end	Day	6% 1 5% 24% 36% 27%	387
September	1st to 10th	Day	7% 4 24% 45% 20%	270
	11th to 20th	Day	9% 6% 22% 47% 16%	270
	21st to end	Day	10% 1 9% 22% 41% 16%	270
October	1st to 10th	Day	6% 3 14% 31% 33% 12%	270
	11th to 20th	Day	12% 3 23% 30% 24% 7%	270
	21st to end	Day	23% 11% 24% 22% 16% 4	297
November	1st to 10th	Day	17% 5% 28% 33% 13% 5%	270
	11th to 20th	Day	18% 11% 23% 33% 13% 2	270
	21st to end	Day	22% 16% 33% 17% 10% 1	270
December	1st to 10th	Day	18% 8% 33% 27% 10% 5%	270
	11th to 20th	Day	20% 19% 31% 20% 10%	270
	21st to end	Day	22% 10% 36% 24% 7% 1	297
January	1st to 10th	Night	20% 9% 30% 32% 8% 1	950
	11th to 20th	Night	21% 10% 31% 31% 7%	950
	21st to end	Night	22% 9% 31% 31% 7%	930
February	1st to 10th	Night	24% 11% 31% 28% 6%	720
	11th to 20th	Night	25% 10% 38% 24% 3	704
	21st to end	Night	26% 13% 40% 20% 2	592
March	1st to 10th	Night	20% 13% 36% 28% 3	554
	11th to 20th	Night	21% 9% 36% 30% 4	540
	21st to end	Night	16% 11% 40% 29% 3	594
April	1st to 10th	Night	17% 9% 40% 30% 4	540
	11th to 20th	Night	21% 10% 40% 25% 4	533
	21st to end	Night	17% 10% 41% 24% 8% 1	540
May	1st to 10th	Night	16% 10% 37% 31% 6%	540
	11th to 20th	Night	17% 5% 30% 35% 13%	531
	21st to end	Night	15% 4 32% 35% 14%	584
June	1st to 10th	Night	5% 2 21% 48% 23% 1	540
	11th to 20th	Night	5% 1 31% 46% 15% 1	540
	21st to end	Night	6% 2 18% 45% 25% 4	540
July	1st to 10th	Night	9% 1 13% 43% 29% 4	525
	11th to 20th	Night	3 1 11% 54% 29% 2	522
	21st to end	Night	4 1 8% 52% 33% 3	591
August	1st to 10th	Night	1 9% 54% 31% 4	537
	11th to 20th	Night	5% 8% 50% 32% 4	534
	21st to end	Night	6% 6% 41% 41% 6%	594
September	1st to 10th	Night	9% 1 9% 37% 39% 5%	540
	11th to 20th	Night	12% 1 8% 36% 39% 4	540
	21st to end	Night	11% 14% 34% 37% 4	540
October	1st to 10th	Night	7% 1 12% 43% 31% 5%	540
	11th to 20th	Night	14% 4 22% 37% 22% 2	653
	21st to end	Night	17% 8% 23% 30% 21% 1	792
November	1st to 10th	Night	14% 5% 20% 40% 19% 1	740
	11th to 20th	Night	13% 6% 28% 37% 14% 2	864
	21st to end	Night	21% 10% 27% 29% 11% 1	950
December	1st to 10th	Night	17% 8% 28% 36% 10% 1	950
	11th to 20th	Night	23% 10% 29% 32% 7%	950
	21st to end	Night	21% 8% 31% 33% 7%	1045

Page generated at 2024-01-24 08:37:30. Version ac07

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