Seven Sisters Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seven Sisters Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	126211
	Night	14% 11% 4 9% 5% 4 4 4 5% 7% 34%	123319
Transparency	Day	48% 5% 12% 21% 13% 1	129794
	Night	48% 5% 11% 21% 13% 1	127806
Seeing	Day	17% 11% 28% 23% 14% 6%	16946
	Night	19% 9% 29% 29% 13% 1	26723

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	11% 14% 6% 17% 6% 6% 4 3 4 6% 20%	3617
	11th to 20th	Day	14% 14% 6% 16% 11% 7% 8% 5% 5% 5% 11%	3620
	21st to end	Day	21% 17% 7% 14% 5% 4 4 3 3 5% 17%	4304
February	1st to 10th	Day	14% 15% 5% 12% 9% 5% 5% 4 5% 6% 20%	1800
	11th to 20th	Day	15% 11% 5% 13% 8% 7% 5% 3 5% 5% 24%	1943
	21st to end	Day	16% 14% 4 11% 6% 4 4 4 6% 6% 25%	1650
March	1st to 10th	Day	14% 11% 5% 13% 7% 5% 4 3 4 5% 29%	4520
	11th to 20th	Day	11% 11% 3 9% 5% 4 4 3 5% 7% 36%	4980
	21st to end	Day	17% 14% 5% 10% 5% 5% 5% 4 5% 6% 26%	5465
April	1st to 10th	Day	16% 13% 4 8% 5% 5% 4 4 5% 6% 29%	2370
	11th to 20th	Day	16% 9% 3 8% 4 4 5% 4 6% 7% 34%	2497
	21st to end	Day	16% 9% 3 8% 5% 3 5% 3 5% 7% 35%	2600
May	1st to 10th	Day	19% 10% 3 7% 5% 4 5% 4 5% 6% 34%	5862
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 9% 34%	5861
	21st to end	Day	14% 6% 3 6% 4 4 5% 4 5% 7% 41%	6513
June	1st to 10th	Day	10% 6% 3 7% 4 5% 5% 4 7% 9% 41%	2800
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 10% 39%	2800
	21st to end	Day	8% 6% 3 6% 6% 5% 6% 6% 7% 9% 39%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 11% 39%	6360
	11th to 20th	Day	6% 5% 2 7% 5% 5% 5% 4 6% 11% 45%	6360
	21st to end	Day	6% 6% 3 6% 5% 5% 5% 5% 7% 9% 44%	6614
August	1st to 10th	Day	11% 5% 2 5% 4 5% 5% 4 6% 8% 45%	4234
	11th to 20th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 9% 41%	4174
	21st to end	Day	7% 5% 2 7% 5% 5% 5% 4 7% 9% 42%	4522
September	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 7% 9% 42%	2290
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 5% 6% 8% 48%	2200
	21st to end	Day	14% 7% 3 6% 4 5% 4 4 6% 9% 38%	2200
October	1st to 10th	Day	11% 7% 3 6% 5% 5% 5% 3 6% 8% 43%	2104
	11th to 20th	Day	17% 9% 3 7% 6% 5% 5% 4 5% 6% 34%	1900
	21st to end	Day	19% 10% 3 8% 5% 5% 5% 4 5% 6% 31%	1900
November	1st to 10th	Day	12% 8% 3 9% 5% 5% 4 4 6% 6% 40%	1762
	11th to 20th	Day	15% 13% 4 10% 7% 5% 4 3 4 6% 28%	1800
	21st to end	Day	18% 14% 4 9% 6% 5% 5% 3 5% 6% 26%	1800
December	1st to 10th	Day	11% 14% 8% 14% 6% 6% 5% 4 5% 7% 20%	2930
	11th to 20th	Day	18% 18% 7% 11% 7% 5% 5% 3 5% 4 17%	3077
	21st to end	Day	24% 21% 6% 13% 5% 5% 3 2 3 4 13%	3982
January	1st to 10th	Night	13% 14% 6% 15% 6% 5% 5% 3 4 7% 22%	6348
	11th to 20th	Night	11% 16% 7% 15% 8% 6% 6% 5% 6% 6% 15%	6360
	21st to end	Night	18% 15% 7% 11% 6% 4 4 3 4 6% 22%	6656
February	1st to 10th	Night	15% 15% 4 12% 8% 6% 4 3 4 5% 23%	2600
	11th to 20th	Night	13% 13% 4 10% 6% 6% 5% 4 6% 5% 27%	2341
	21st to end	Night	18% 12% 4 10% 4 5% 4 3 5% 7% 29%	1980
March	1st to 10th	Night	14% 11% 4 8% 4 4 4 3 5% 7% 36%	5332
	11th to 20th	Night	10% 7% 2 5% 4 5% 5% 4 6% 7% 44%	4980
	21st to end	Night	16% 11% 3 8% 5% 4 4 3 4 6% 35%	5467
April	1st to 10th	Night	15% 10% 3 7% 5% 4 4 4 5% 7% 35%	2047
	11th to 20th	Night	18% 8% 2 6% 4 4 5% 4 5% 7% 38%	1852
	21st to end	Night	15% 10% 3 6% 5% 5% 4 4 5% 7% 37%	1800
May	1st to 10th	Night	19% 10% 3 9% 6% 4 5% 3 5% 6% 30%	4033
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 6% 39%	4032
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 8% 44%	4341
June	1st to 10th	Night	10% 5% 2 5% 5% 3 4 4 7% 9% 46%	1600
	11th to 20th	Night	11% 7% 2 7% 5% 4 4 4 6% 10% 42%	1600
	21st to end	Night	8% 5% 2 6% 5% 4 4 4 8% 9% 47%	1600
July	1st to 10th	Night	7% 4 2 6% 4 4 4 4 7% 9% 49%	3618
	11th to 20th	Night	6% 6% 2 6% 4 4 4 4 5% 9% 51%	3617
	21st to end	Night	8% 6% 2 6% 4 4 4 4 6% 9% 48%	4243
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 3 5% 9% 47%	2915
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	2875
	21st to end	Night	8% 6% 2 6% 5% 5% 4 4 5% 7% 47%	3373
September	1st to 10th	Night	9% 6% 2 5% 5% 4 4 4 5% 7% 50%	2090
	11th to 20th	Night	8% 5% 1 4 4 4 3 3 5% 7% 56%	2200
	21st to end	Night	13% 7% 3 6% 4 3 5% 4 6% 6% 43%	2200
October	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 5% 7% 46%	2141
	11th to 20th	Night	16% 10% 3 8% 5% 5% 4 3 4 5% 37%	2280
	21st to end	Night	19% 9% 3 8% 4 4 4 5% 5% 6% 34%	2693
November	1st to 10th	Night	12% 9% 4 6% 5% 4 4 3 5% 5% 42%	2536
	11th to 20th	Night	15% 11% 3 7% 6% 5% 4 4 5% 7% 32%	2600
	21st to end	Night	17% 14% 5% 10% 5% 4 4 3 5% 6% 28%	2600
December	1st to 10th	Night	12% 13% 5% 14% 6% 5% 5% 4 6% 6% 24%	4250
	11th to 20th	Night	15% 18% 6% 13% 7% 5% 5% 3 5% 5% 19%	5123
	21st to end	Night	25% 21% 6% 10% 5% 4 4 3 4 4 14%	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	64% 5% 11% 16% 5%	3618
	11th to 20th	Day	73% 5% 9% 7% 6%	3620
	21st to end	Day	70% 4 9% 10% 7%	4304
February	1st to 10th	Day	63% 7% 9% 14% 8% 1	1800
	11th to 20th	Day	61% 5% 10% 15% 8% 1	1953
	21st to end	Day	59% 6% 12% 16% 7% 2	1650
March	1st to 10th	Day	57% 4 11% 17% 9% 1	4520
	11th to 20th	Day	47% 4 16% 23% 8% 2	4980
	21st to end	Day	59% 4 11% 15% 11% 1	5465
April	1st to 10th	Day	54% 5% 11% 18% 12%	2370
	11th to 20th	Day	48% 6% 11% 21% 12% 2	2497
	21st to end	Day	48% 6% 11% 19% 14% 1	2600
May	1st to 10th	Day	50% 5% 10% 19% 14% 1	5897
	11th to 20th	Day	46% 6% 12% 22% 12% 1	5865
	21st to end	Day	41% 6% 9% 23% 18% 3	6527
June	1st to 10th	Day	38% 7% 12% 26% 15% 3	2800
	11th to 20th	Day	38% 8% 13% 22% 17% 2	2800
	21st to end	Day	38% 8% 14% 21% 16% 2	2800
July	1st to 10th	Day	38% 4 13% 28% 16% 2	6291
	11th to 20th	Day	33% 4 14% 30% 18% 1	6285
	21st to end	Day	36% 5% 14% 24% 20% 1	6426
August	1st to 10th	Day	36% 5% 14% 27% 17% 1	4221
	11th to 20th	Day	39% 6% 10% 32% 13% 1	4213
	21st to end	Day	37% 5% 13% 31% 12% 2	4521
September	1st to 10th	Day	37% 7% 15% 24% 16% 2	2290
	11th to 20th	Day	31% 6% 12% 31% 18% 1	2200
	21st to end	Day	42% 7% 12% 23% 17%	2200
October	1st to 10th	Day	31% 4 10% 33% 22% 1	3481
	11th to 20th	Day	42% 4 9% 24% 21%	3140
	21st to end	Day	46% 4 13% 24% 12% 2	3112
November	1st to 10th	Day	44% 6% 11% 24% 14% 1	1762
	11th to 20th	Day	55% 5% 12% 18% 8% 2	1800
	21st to end	Day	58% 6% 12% 17% 7% 1	1800
December	1st to 10th	Day	63% 6% 15% 12% 4	2930
	11th to 20th	Day	69% 4 10% 14% 3	3074
	21st to end	Day	76% 4 8% 8% 5%	3982
January	1st to 10th	Night	62% 5% 8% 17% 8% 1	6348
	11th to 20th	Night	68% 5% 10% 10% 7%	6360
	21st to end	Night	64% 4 12% 11% 9%	6656
February	1st to 10th	Night	64% 5% 10% 13% 7% 1	2600
	11th to 20th	Night	57% 5% 11% 17% 8% 1	2342
	21st to end	Night	56% 4 13% 17% 8% 2	1980
March	1st to 10th	Night	48% 4 12% 24% 11% 1	5332
	11th to 20th	Night	38% 5% 14% 27% 16% 1	4980
	21st to end	Night	50% 4 11% 19% 14% 2	5467
April	1st to 10th	Night	47% 7% 10% 21% 14% 2	2047
	11th to 20th	Night	46% 5% 11% 18% 18% 1	1852
	21st to end	Night	46% 6% 12% 19% 16% 3	1800
May	1st to 10th	Night	54% 5% 9% 16% 13% 3	4060
	11th to 20th	Night	46% 5% 9% 26% 12% 2	4033
	21st to end	Night	38% 6% 12% 23% 19% 3	4349
June	1st to 10th	Night	33% 6% 12% 27% 20% 2	1600
	11th to 20th	Night	38% 6% 12% 26% 16% 2	1600
	21st to end	Night	32% 8% 14% 26% 20% 1	1600
July	1st to 10th	Night	32% 4 13% 29% 17% 5%	3580
	11th to 20th	Night	32% 4 15% 28% 20% 2	3572
	21st to end	Night	33% 5% 15% 21% 25% 2	4118
August	1st to 10th	Night	36% 5% 13% 29% 16% 1	2904
	11th to 20th	Night	32% 6% 11% 36% 13% 2	2902
	21st to end	Night	36% 5% 12% 29% 16% 1	3373
September	1st to 10th	Night	33% 6% 11% 29% 18% 3	2090
	11th to 20th	Night	28% 5% 10% 32% 23% 2	2200
	21st to end	Night	40% 6% 12% 26% 15% 1	2200
October	1st to 10th	Night	26% 3 9% 33% 27% 1	3544
	11th to 20th	Night	42% 4 10% 23% 21%	3779
	21st to end	Night	38% 4 9% 33% 15% 1	4431
November	1st to 10th	Night	42% 6% 11% 24% 16% 1	2536
	11th to 20th	Night	50% 6% 14% 20% 10% 2	2600
	21st to end	Night	57% 6% 12% 18% 7%	2600
December	1st to 10th	Night	60% 4 16% 16% 4	4250
	11th to 20th	Night	66% 5% 10% 13% 5%	5125
	21st to end	Night	74% 4 9% 8% 6%	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	22% 17% 39% 18% 4 1	300
	11th to 20th	Day	21% 16% 43% 16% 3 1	300
	21st to end	Day	26% 17% 35% 17% 5%	330
February	1st to 10th	Day	23% 18% 37% 17% 4 1	300
	11th to 20th	Day	24% 20% 38% 16% 3	300
	21st to end	Day	28% 22% 39% 10% 2	249
March	1st to 10th	Day	25% 19% 37% 16% 3	314
	11th to 20th	Day	23% 13% 35% 21% 7% 1	500
	21st to end	Day	25% 18% 30% 21% 5% 1	550
April	1st to 10th	Day	21% 15% 34% 22% 8% 1	500
	11th to 20th	Day	24% 18% 31% 16% 7% 4	492
	21st to end	Day	22% 12% 32% 22% 9% 2	500
May	1st to 10th	Day	23% 14% 31% 18% 12% 2	503
	11th to 20th	Day	20% 10% 28% 22% 14% 6%	527
	21st to end	Day	13% 8% 25% 25% 18% 10%	601
June	1st to 10th	Day	11% 8% 29% 31% 16% 4	730
	11th to 20th	Day	13% 7% 28% 31% 16% 5%	730
	21st to end	Day	10% 7% 30% 31% 17% 5%	730
July	1st to 10th	Day	10% 4 20% 27% 26% 13%	721
	11th to 20th	Day	11% 4 24% 29% 21% 10%	718
	21st to end	Day	8% 3 22% 31% 25% 11%	662
August	1st to 10th	Day	9% 2 18% 28% 26% 17%	526
	11th to 20th	Day	12% 4 16% 29% 26% 13%	519
	21st to end	Day	10% 3 20% 29% 25% 13%	583
September	1st to 10th	Day	12% 5% 23% 22% 24% 14%	530
	11th to 20th	Day	9% 2 16% 25% 30% 18%	530
	21st to end	Day	18% 9% 20% 24% 20% 9%	530
October	1st to 10th	Day	14% 8% 28% 26% 16% 9%	502
	11th to 20th	Day	18% 16% 28% 27% 10% 2	490
	21st to end	Day	26% 16% 36% 15% 6% 1	349
November	1st to 10th	Day	14% 18% 34% 24% 8% 2	300
	11th to 20th	Day	22% 21% 30% 20% 5% 1	300
	21st to end	Day	25% 15% 41% 15% 4	300
December	1st to 10th	Day	23% 23% 31% 16% 7% 1	300
	11th to 20th	Day	22% 20% 40% 13% 4	300
	21st to end	Day	28% 22% 32% 16% 2	330
January	1st to 10th	Night	22% 16% 36% 21% 5% 1	1000
	11th to 20th	Night	24% 12% 38% 21% 5% 1	1000
	21st to end	Night	20% 12% 37% 25% 5%	955
February	1st to 10th	Night	25% 13% 35% 24% 3	800
	11th to 20th	Night	23% 15% 36% 24% 3	783
	21st to end	Night	24% 14% 40% 19% 3	660
March	1st to 10th	Night	21% 12% 34% 27% 6%	800
	11th to 20th	Night	20% 16% 34% 25% 5% 1	800
	21st to end	Night	24% 11% 31% 27% 7%	880
April	1st to 10th	Night	19% 14% 36% 26% 4 1	647
	11th to 20th	Night	20% 10% 37% 24% 8% 1	593
	21st to end	Night	20% 10% 32% 30% 8% 1	600
May	1st to 10th	Night	19% 8% 30% 33% 9% 1	600
	11th to 20th	Night	23% 3 29% 31% 12% 1	591
	21st to end	Night	14% 4 24% 36% 21% 2	650
June	1st to 10th	Night	12% 3 26% 43% 16% 1	600
	11th to 20th	Night	13% 4 23% 41% 19% 1	600
	21st to end	Night	10% 3 21% 42% 23% 2	600
July	1st to 10th	Night	9% 2 16% 41% 28% 5%	585
	11th to 20th	Night	9% 3 16% 42% 26% 3	582
	21st to end	Night	7% 1 18% 44% 29% 1	657
August	1st to 10th	Night	8% 2 15% 42% 29% 3	597
	11th to 20th	Night	10% 1 17% 36% 30% 5%	594
	21st to end	Night	12% 1 16% 36% 31% 4	660
September	1st to 10th	Night	10% 4 19% 34% 29% 4	690
	11th to 20th	Night	10% 3 16% 34% 33% 5%	800
	21st to end	Night	16% 6% 21% 31% 21% 5%	800
October	1st to 10th	Night	14% 7% 25% 33% 18% 2	773
	11th to 20th	Night	21% 13% 29% 27% 10% 1	760
	21st to end	Night	24% 11% 33% 25% 6%	836
November	1st to 10th	Night	19% 11% 31% 27% 11% 1	780
	11th to 20th	Night	21% 14% 35% 22% 6% 1	800
	21st to end	Night	24% 11% 34% 25% 5%	800
December	1st to 10th	Night	23% 15% 36% 20% 6%	800
	11th to 20th	Night	25% 12% 38% 21% 4	950
	21st to end	Night	31% 12% 34% 21% 2	1100

Page generated at 2025-10-01 01:39:03. Version ac07

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