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 2024-11-05.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	101206
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 35%	97860
Transparency	Day	47% 6% 12% 21% 13% 1	104833
	Night	47% 6% 11% 21% 13% 1	102373
Seeing	Day	17% 11% 28% 24% 14% 6%	15683
	Night	19% 9% 29% 29% 13% 1	25508

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	16% 17% 6% 12% 6% 6% 5% 3 4 6% 21%	2527
	11th to 20th	Day	16% 16% 6% 14% 8% 5% 6% 5% 5% 6% 13%	2530
	21st to end	Day	26% 21% 7% 12% 5% 4 3 2 2 4 14%	3001
February	1st to 10th	Day	13% 16% 5% 12% 9% 5% 5% 4 5% 6% 20%	1710
	11th to 20th	Day	15% 11% 4 13% 7% 7% 5% 3 5% 5% 25%	1843
	21st to end	Day	16% 14% 4 11% 6% 4 5% 4 5% 6% 25%	1570
March	1st to 10th	Day	16% 13% 5% 11% 6% 5% 4 3 5% 6% 25%	3146
	11th to 20th	Day	12% 11% 4 10% 6% 5% 5% 3 5% 7% 33%	3480
	21st to end	Day	19% 12% 5% 10% 5% 4 5% 4 5% 5% 26%	3815
April	1st to 10th	Day	15% 12% 4 8% 5% 5% 4 4 6% 6% 30%	2251
	11th to 20th	Day	16% 9% 3 8% 4 4 5% 4 6% 7% 34%	2371
	21st to end	Day	17% 9% 3 8% 5% 3 5% 3 5% 7% 34%	2470
May	1st to 10th	Day	17% 9% 2 7% 5% 4 5% 4 6% 6% 35%	4082
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 9% 34%	4081
	21st to end	Day	10% 6% 3 6% 5% 4 5% 4 5% 8% 45%	4528
June	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	2660
	11th to 20th	Day	10% 6% 2 6% 5% 4 6% 4 8% 9% 40%	2660
	21st to end	Day	8% 6% 3 6% 5% 5% 6% 5% 7% 10% 38%	2660
July	1st to 10th	Day	10% 6% 3 7% 5% 6% 5% 5% 7% 10% 37%	4440
	11th to 20th	Day	7% 4 1 6% 4 4 5% 4 6% 11% 48%	4440
	21st to end	Day	5% 6% 2 6% 6% 6% 6% 5% 8% 9% 41%	4600
August	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 44%	4104
	11th to 20th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 9% 41%	4044
	21st to end	Day	8% 5% 3 7% 5% 5% 5% 4 7% 9% 42%	4384
September	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 7% 9% 41%	2176
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 5% 6% 9% 47%	2090
	21st to end	Day	14% 6% 3 6% 4 5% 4 4 6% 9% 38%	2090
October	1st to 10th	Day	11% 7% 3 6% 5% 4 4 3 6% 8% 42%	2071
	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% 7% 3 9% 5% 5% 4 4 5% 6% 40%	1708
	11th to 20th	Day	14% 13% 4 9% 7% 5% 4 4 5% 6% 29%	1710
	21st to end	Day	18% 13% 4 8% 5% 5% 5% 3 5% 6% 27%	1710
December	1st to 10th	Day	13% 15% 7% 13% 6% 6% 4 3 4 6% 22%	1710
	11th to 20th	Day	14% 15% 6% 11% 6% 4 5% 4 7% 6% 21%	1961
	21st to end	Day	22% 21% 6% 14% 5% 5% 4 3 4 5% 11%	2783
January	1st to 10th	Night	15% 15% 7% 12% 6% 4 5% 3 4 6% 22%	4428
	11th to 20th	Night	14% 17% 7% 14% 6% 5% 5% 6% 6% 6% 14%	4440
	21st to end	Night	23% 17% 8% 12% 6% 4 3 3 3 5% 15%	4656
February	1st to 10th	Night	16% 15% 4 11% 8% 6% 4 3 4 5% 23%	2470
	11th to 20th	Night	13% 13% 4 9% 6% 6% 5% 4 6% 5% 28%	2221
	21st to end	Night	18% 12% 4 10% 5% 5% 4 4 4 6% 29%	1884
March	1st to 10th	Night	15% 12% 4 8% 5% 3 3 3 5% 6% 35%	3706
	11th to 20th	Night	10% 7% 2 5% 4 5% 5% 4 6% 7% 45%	3480
	21st to end	Night	17% 10% 3 8% 5% 5% 4 3 5% 6% 34%	3817
April	1st to 10th	Night	15% 9% 3 7% 5% 4 4 5% 5% 7% 36%	1945
	11th to 20th	Night	19% 8% 2 6% 4 3 5% 4 5% 7% 37%	1758
	21st to end	Night	16% 10% 3 6% 5% 5% 4 4 5% 7% 36%	1710
May	1st to 10th	Night	16% 8% 3 8% 6% 5% 5% 3 6% 7% 33%	2813
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 3 5% 6% 40%	2812
	21st to end	Night	10% 7% 2 5% 4 4 4 4 6% 9% 47%	3025
June	1st to 10th	Night	9% 5% 2 5% 5% 3 4 4 7% 9% 46%	1520
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 4 6% 9% 43%	1520
	21st to end	Night	8% 5% 2 6% 5% 4 4 4 8% 9% 47%	1520
July	1st to 10th	Night	8% 3 2 6% 4 4 4 5% 6% 8% 49%	2530
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 8% 56%	2530
	21st to end	Night	5% 5% 2 6% 5% 4 5% 5% 8% 10% 47%	2955
August	1st to 10th	Night	10% 6% 2 7% 4 4 4 3 5% 9% 47%	2825
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 6% 8% 48%	2785
	21st to end	Night	9% 6% 2 6% 5% 5% 4 4 5% 7% 46%	3270
September	1st to 10th	Night	9% 6% 2 5% 5% 3 4 4 5% 7% 49%	1985
	11th to 20th	Night	9% 5% 1 4 4 4 3 3 5% 7% 55%	2090
	21st to end	Night	13% 7% 3 6% 4 4 5% 4 6% 7% 42%	2090
October	1st to 10th	Night	11% 7% 3 6% 4 4 3 3 5% 7% 46%	2107
	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	11% 9% 4 6% 5% 4 4 3 5% 6% 42%	2470
	11th to 20th	Night	15% 11% 3 7% 6% 5% 4 4 5% 7% 32%	2470
	21st to end	Night	17% 13% 4 9% 5% 4 4 3 5% 6% 29%	2470
December	1st to 10th	Night	16% 15% 6% 11% 6% 4 5% 3 6% 6% 21%	2470
	11th to 20th	Night	15% 14% 5% 11% 6% 5% 5% 3 6% 6% 23%	3231
	21st to end	Night	26% 21% 6% 10% 5% 5% 3 3 5% 4 12%	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	64% 7% 11% 14% 5%	2528
	11th to 20th	Day	71% 5% 11% 8% 4	2530
	21st to end	Day	76% 5% 8% 7% 3	3001
February	1st to 10th	Day	62% 7% 9% 14% 8% 1	1710
	11th to 20th	Day	60% 5% 10% 16% 9% 1	1853
	21st to end	Day	59% 6% 12% 16% 6% 2	1570
March	1st to 10th	Day	59% 5% 11% 17% 7%	3146
	11th to 20th	Day	50% 5% 14% 21% 8% 2	3480
	21st to end	Day	58% 6% 12% 14% 9% 1	3815
April	1st to 10th	Day	54% 5% 11% 18% 12%	2251
	11th to 20th	Day	48% 6% 11% 21% 13% 2	2371
	21st to end	Day	49% 6% 11% 17% 14% 1	2470
May	1st to 10th	Day	46% 7% 11% 22% 12% 1	4120
	11th to 20th	Day	45% 8% 13% 23% 10% 1	4085
	21st to end	Day	35% 8% 10% 20% 23% 4	4541
June	1st to 10th	Day	38% 7% 12% 26% 14% 3	2660
	11th to 20th	Day	37% 8% 13% 22% 18% 2	2660
	21st to end	Day	39% 9% 14% 20% 16% 2	2660
July	1st to 10th	Day	41% 5% 14% 22% 16% 3	4371
	11th to 20th	Day	30% 5% 12% 34% 18% 1	4365
	21st to end	Day	37% 6% 15% 20% 21% 1	4454
August	1st to 10th	Day	37% 6% 13% 27% 17% 1	4091
	11th to 20th	Day	39% 6% 9% 32% 13% 1	4083
	21st to end	Day	38% 5% 13% 30% 13% 2	4383
September	1st to 10th	Day	38% 8% 15% 22% 16% 2	2176
	11th to 20th	Day	32% 7% 11% 31% 18% 1	2090
	21st to end	Day	42% 7% 12% 23% 16%	2090
October	1st to 10th	Day	31% 4 10% 33% 22% 1	3448
	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	43% 6% 12% 24% 14% 1	1708
	11th to 20th	Day	54% 6% 12% 18% 8% 2	1710
	21st to end	Day	57% 6% 13% 17% 7% 1	1710
December	1st to 10th	Day	64% 7% 11% 13% 5%	1710
	11th to 20th	Day	60% 7% 13% 16% 4	1958
	21st to end	Day	77% 5% 9% 6% 4	2783
January	1st to 10th	Night	61% 6% 11% 15% 6% 1	4428
	11th to 20th	Night	69% 6% 11% 10% 5%	4440
	21st to end	Night	72% 5% 9% 10% 3	4656
February	1st to 10th	Night	63% 6% 10% 13% 7% 1	2470
	11th to 20th	Night	56% 5% 11% 18% 9% 1	2222
	21st to end	Night	56% 4 13% 17% 8% 2	1884
March	1st to 10th	Night	50% 5% 12% 25% 8% 1	3706
	11th to 20th	Night	36% 6% 15% 21% 20% 2	3480
	21st to end	Night	50% 6% 12% 21% 10% 1	3817
April	1st to 10th	Night	46% 7% 10% 22% 14% 1	1945
	11th to 20th	Night	46% 6% 11% 18% 18% 1	1758
	21st to end	Night	46% 6% 12% 18% 15% 3	1710
May	1st to 10th	Night	47% 7% 10% 21% 13% 2	2840
	11th to 20th	Night	43% 7% 9% 27% 13% 1	2813
	21st to end	Night	32% 8% 13% 22% 21% 3	3033
June	1st to 10th	Night	32% 7% 13% 28% 19% 2	1520
	11th to 20th	Night	38% 7% 12% 25% 17% 2	1520
	21st to end	Night	32% 8% 13% 26% 20% 1	1520
July	1st to 10th	Night	32% 5% 12% 29% 16% 6%	2490
	11th to 20th	Night	28% 5% 13% 30% 22% 2	2482
	21st to end	Night	31% 6% 19% 18% 24% 2	2861
August	1st to 10th	Night	36% 5% 13% 29% 16% 1	2814
	11th to 20th	Night	32% 6% 11% 35% 14% 2	2812
	21st to end	Night	37% 5% 12% 29% 16% 1	3270
September	1st to 10th	Night	33% 7% 12% 27% 19% 3	1985
	11th to 20th	Night	29% 6% 9% 31% 23% 2	2090
	21st to end	Night	40% 7% 11% 26% 15% 1	2090
October	1st to 10th	Night	27% 3 9% 33% 27% 1	3510
	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	2470
	11th to 20th	Night	49% 6% 14% 20% 9% 1	2470
	21st to end	Night	56% 6% 12% 19% 7%	2470
December	1st to 10th	Night	62% 6% 11% 14% 6%	2470
	11th to 20th	Night	59% 7% 13% 14% 7% 1	3233
	21st to end	Night	75% 5% 10% 7% 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	23% 17% 36% 18% 4 1	270
	11th to 20th	Day	21% 15% 41% 17% 3 1	270
	21st to end	Day	28% 16% 33% 18% 5%	297
February	1st to 10th	Day	22% 18% 36% 18% 4 1	270
	11th to 20th	Day	24% 19% 37% 16% 3	270
	21st to end	Day	27% 23% 38% 11% 2	225
March	1st to 10th	Day	24% 20% 35% 17% 4	283
	11th to 20th	Day	23% 12% 35% 21% 7% 1	460
	21st to end	Day	25% 17% 30% 21% 6% 1	506
April	1st to 10th	Day	19% 15% 34% 23% 8% 1	460
	11th to 20th	Day	25% 17% 31% 15% 8% 4	452
	21st to end	Day	22% 13% 32% 22% 9% 2	460
May	1st to 10th	Day	22% 14% 31% 19% 12% 3	463
	11th to 20th	Day	20% 10% 28% 22% 15% 7%	487
	21st to end	Day	12% 7% 24% 26% 20% 11%	555
June	1st to 10th	Day	11% 8% 29% 31% 17% 4	680
	11th to 20th	Day	13% 6% 28% 30% 17% 5%	680
	21st to end	Day	11% 7% 30% 30% 16% 5%	680
July	1st to 10th	Day	10% 4 19% 27% 26% 13%	671
	11th to 20th	Day	11% 4 24% 29% 21% 11%	668
	21st to end	Day	8% 3 22% 31% 25% 11%	614
August	1st to 10th	Day	10% 2 19% 28% 23% 18%	486
	11th to 20th	Day	12% 4 17% 28% 26% 12%	479
	21st to end	Day	10% 3 20% 29% 25% 13%	539
September	1st to 10th	Day	13% 5% 23% 23% 25% 12%	490
	11th to 20th	Day	10% 2 17% 26% 29% 16%	490
	21st to end	Day	18% 10% 21% 24% 19% 9%	490
October	1st to 10th	Day	14% 8% 28% 26% 15% 9%	490
	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	15% 16% 33% 25% 9% 2	282
	11th to 20th	Day	21% 22% 29% 21% 5% 1	270
	21st to end	Day	26% 14% 41% 15% 4	270
December	1st to 10th	Day	24% 23% 30% 15% 7% 1	270
	11th to 20th	Day	21% 20% 42% 13% 4	270
	21st to end	Day	27% 22% 33% 16% 2	297
January	1st to 10th	Night	22% 15% 37% 21% 5% 1	950
	11th to 20th	Night	24% 12% 38% 21% 5% 1	950
	21st to end	Night	20% 11% 37% 26% 5%	908
February	1st to 10th	Night	25% 12% 34% 25% 3	760
	11th to 20th	Night	22% 15% 36% 24% 3	743
	21st to end	Night	25% 14% 39% 20% 3	628
March	1st to 10th	Night	20% 12% 34% 27% 6%	760
	11th to 20th	Night	20% 15% 34% 25% 5% 1	760
	21st to end	Night	24% 10% 31% 27% 8%	836
April	1st to 10th	Night	18% 13% 37% 26% 5% 1	615
	11th to 20th	Night	21% 9% 37% 24% 8% 1	563
	21st to end	Night	21% 9% 32% 30% 8% 1	570
May	1st to 10th	Night	19% 8% 29% 34% 9% 1	570
	11th to 20th	Night	23% 2 29% 32% 13% 1	561
	21st to end	Night	13% 4 24% 36% 22% 2	617
June	1st to 10th	Night	11% 3 24% 44% 17% 1	570
	11th to 20th	Night	13% 3 23% 41% 20% 1	570
	21st to end	Night	10% 3 21% 42% 22% 2	570
July	1st to 10th	Night	9% 2 16% 41% 28% 5%	555
	11th to 20th	Night	10% 4 15% 42% 26% 3	552
	21st to end	Night	6% 1 18% 44% 30% 1	624
August	1st to 10th	Night	8% 2 16% 43% 27% 4	567
	11th to 20th	Night	10% 1 17% 37% 30% 5%	564
	21st to end	Night	12% 1 15% 36% 31% 4	627
September	1st to 10th	Night	10% 4 19% 35% 27% 4	655
	11th to 20th	Night	10% 3 17% 34% 31% 4	760
	21st to end	Night	16% 6% 21% 31% 21% 5%	760
October	1st to 10th	Night	15% 7% 25% 33% 18% 2	760
	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	18% 11% 31% 27% 12% 1	760
	11th to 20th	Night	21% 14% 35% 22% 7% 1	760
	21st to end	Night	24% 10% 34% 26% 5%	760
December	1st to 10th	Night	23% 15% 36% 20% 6%	760
	11th to 20th	Night	24% 12% 38% 21% 5%	902
	21st to end	Night	31% 12% 34% 21% 2	1045

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