StarEffects Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarEffects Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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% 7% 2 6% 4 4 5% 4 6% 8% 39%	127383
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 41%	123967
Transparency	Day	43% 5% 12% 23% 15% 2	130945
	Night	42% 5% 11% 23% 16% 3	128492
Seeing	Day	21% 13% 28% 23% 12% 4	16334
	Night	20% 10% 28% 30% 12% 1	25724

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	12% 6% 2 6% 4 5% 5% 5% 6% 6% 43%	3605
	11th to 20th	Day	13% 10% 5% 10% 6% 5% 5% 3 5% 6% 34%	3775
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 5% 5% 37%	4466
February	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 5% 6% 7% 34%	1800
	11th to 20th	Day	15% 9% 4 7% 4 5% 4 3 5% 8% 37%	1843
	21st to end	Day	18% 8% 3 7% 5% 4 5% 3 5% 6% 35%	1650
March	1st to 10th	Day	20% 5% 2 4 4 4 4 3 5% 6% 42%	4629
	11th to 20th	Day	11% 8% 2 7% 5% 5% 4 4 6% 7% 43%	4980
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 6% 42%	5691
April	1st to 10th	Day	17% 8% 2 7% 5% 4 4 4 6% 8% 36%	2400
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 4 5% 7% 40%	2374
	21st to end	Day	17% 5% 2 7% 4 5% 4 4 6% 7% 40%	2400
May	1st to 10th	Day	26% 6% 2 6% 4 4 5% 4 5% 8% 30%	5901
	11th to 20th	Day	25% 8% 2 6% 4 3 4 3 5% 7% 31%	6318
	21st to end	Day	19% 5% 2 6% 4 4 5% 4 6% 9% 36%	6949
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 38%	2800
	11th to 20th	Day	16% 5% 2 6% 4 5% 5% 5% 6% 8% 38%	2800
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 4 7% 11% 42%	2800
July	1st to 10th	Day	7% 6% 3 7% 6% 6% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 6% 6% 5% 7% 10% 32%	6360
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	6666
August	1st to 10th	Day	20% 6% 2 6% 5% 5% 5% 5% 6% 8% 32%	4234
	11th to 20th	Day	15% 5% 2 5% 3 4 5% 4 7% 8% 41%	3952
	21st to end	Day	14% 5% 2 4 3 3 3 3 5% 9% 48%	4312
September	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Day	12% 6% 2 4 4 3 4 4 5% 7% 51%	2400
	21st to end	Day	20% 8% 3 7% 5% 3 4 3 5% 5% 37%	2139
October	1st to 10th	Day	18% 6% 2 4 3 3 4 3 5% 7% 45%	2000
	11th to 20th	Day	17% 5% 2 6% 4 4 5% 3 5% 7% 45%	2000
	21st to end	Day	18% 6% 1 4 3 3 4 4 5% 8% 43%	2175
November	1st to 10th	Day	13% 7% 1 4 3 3 3 3 4 8% 51%	1851
	11th to 20th	Day	16% 6% 1 5% 4 4 3 4 6% 7% 43%	1879
	21st to end	Day	17% 6% 2 5% 4 4 4 4 6% 7% 41%	1680
December	1st to 10th	Day	15% 8% 2 6% 5% 4 4 4 6% 7% 40%	2680
	11th to 20th	Day	22% 8% 2 5% 2 4 4 4 5% 7% 38%	3066
	21st to end	Day	18% 7% 2 7% 4 4 5% 4 6% 8% 34%	4048
January	1st to 10th	Night	13% 6% 2 7% 4 4 4 3 6% 8% 44%	6364
	11th to 20th	Night	13% 7% 2 6% 4 4 5% 4 6% 7% 41%	6106
	21st to end	Night	17% 7% 2 7% 4 4 3 3 5% 7% 41%	6490
February	1st to 10th	Night	17% 9% 2 6% 4 4 5% 4 5% 6% 38%	2600
	11th to 20th	Night	17% 6% 3 8% 5% 3 5% 3 5% 6% 39%	2443
	21st to end	Night	18% 7% 3 6% 5% 4 4 4 5% 7% 38%	1980
March	1st to 10th	Night	17% 8% 2 6% 4 4 4 4 4 5% 42%	5208
	11th to 20th	Night	11% 7% 2 5% 4 4 3 3 4 6% 50%	4979
	21st to end	Night	15% 8% 2 6% 5% 4 4 3 5% 6% 43%	5239
April	1st to 10th	Night	19% 7% 3 6% 5% 3 4 3 6% 7% 36%	2000
	11th to 20th	Night	15% 7% 2 6% 5% 3 4 3 6% 8% 41%	1975
	21st to end	Night	19% 5% 2 5% 4 3 3 4 5% 5% 44%	2000
May	1st to 10th	Night	27% 8% 3 5% 4 4 4 4 5% 7% 29%	4068
	11th to 20th	Night	24% 8% 3 7% 5% 3 4 4 4 7% 32%	3575
	21st to end	Night	23% 6% 2 5% 4 4 4 3 5% 7% 37%	3926
June	1st to 10th	Night	16% 6% 2 6% 4 3 4 3 6% 8% 42%	1600
	11th to 20th	Night	14% 5% 2 5% 4 5% 5% 4 7% 8% 42%	1600
	21st to end	Night	12% 4 2 6% 3 4 5% 5% 6% 9% 44%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 6% 5% 6% 9% 39%	3600
	11th to 20th	Night	12% 7% 3 8% 5% 6% 5% 5% 7% 10% 31%	3600
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	4169
August	1st to 10th	Night	20% 7% 2 6% 5% 4 4 3 6% 7% 35%	2915
	11th to 20th	Night	14% 7% 2 8% 4 4 4 3 5% 7% 41%	3062
	21st to end	Night	13% 5% 2 4 3 4 4 4 5% 8% 48%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 3 3 3 5% 6% 49%	2000
	11th to 20th	Night	12% 5% 2 5% 4 4 3 4 5% 7% 51%	2000
	21st to end	Night	20% 7% 2 5% 4 4 5% 3 4 6% 40%	2259
October	1st to 10th	Night	20% 5% 2 4 3 3 3 3 4 6% 45%	2400
	11th to 20th	Night	17% 6% 2 5% 4 3 3 3 4 6% 48%	2400
	21st to end	Night	18% 5% 2 4 3 3 3 3 5% 7% 47%	2663
November	1st to 10th	Night	11% 5% 2 4 3 4 4 3 5% 6% 52%	2667
	11th to 20th	Night	15% 7% 3 5% 4 3 4 3 5% 5% 45%	2735
	21st to end	Night	15% 6% 2 6% 4 4 4 4 5% 8% 42%	2940
December	1st to 10th	Night	13% 8% 3 7% 4 4 4 4 5% 7% 41%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 4 3 5% 6% 39%	5348
	21st to end	Night	19% 9% 3 7% 4 4 4 4 5% 6% 35%	7150

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	41% 4 10% 25% 18% 2	3606
	11th to 20th	Day	52% 4 9% 17% 14% 4	3807
	21st to end	Day	49% 4 12% 21% 13% 1	4466
February	1st to 10th	Day	48% 6% 12% 21% 11% 1	1800
	11th to 20th	Day	47% 5% 10% 24% 13% 2	1852
	21st to end	Day	51% 4 11% 21% 10% 3	1650
March	1st to 10th	Day	43% 4 9% 25% 13% 7%	4632
	11th to 20th	Day	39% 5% 12% 27% 11% 5%	4980
	21st to end	Day	44% 3 10% 17% 22% 4	5691
April	1st to 10th	Day	46% 5% 10% 21% 14% 5%	2400
	11th to 20th	Day	43% 5% 11% 21% 15% 4	2374
	21st to end	Day	42% 5% 11% 21% 16% 5%	2400
May	1st to 10th	Day	51% 6% 12% 18% 10% 2	5910
	11th to 20th	Day	52% 5% 9% 22% 11% 1	6322
	21st to end	Day	44% 6% 11% 24% 14% 1	6949
June	1st to 10th	Day	40% 7% 13% 23% 15% 2	2800
	11th to 20th	Day	42% 7% 14% 24% 11% 2	2800
	21st to end	Day	37% 7% 14% 26% 16% 1	2800
July	1st to 10th	Day	40% 8% 13% 22% 16% 1	6290
	11th to 20th	Day	45% 6% 16% 22% 10% 1	6285
	21st to end	Day	42% 4 10% 21% 22% 1	6475
August	1st to 10th	Day	49% 6% 11% 23% 10%	4221
	11th to 20th	Day	40% 6% 11% 30% 13% 1	3991
	21st to end	Day	34% 5% 10% 29% 20% 2	4312
September	1st to 10th	Day	39% 6% 10% 21% 20% 4	2400
	11th to 20th	Day	33% 5% 10% 26% 23% 3	2400
	21st to end	Day	48% 5% 9% 17% 19% 2	2139
October	1st to 10th	Day	37% 3 8% 22% 25% 5%	3260
	11th to 20th	Day	30% 3 11% 28% 23% 5%	3240
	21st to end	Day	27% 3 14% 26% 27% 3	3489
November	1st to 10th	Day	34% 4 14% 25% 20% 3	1851
	11th to 20th	Day	40% 5% 11% 24% 18% 2	1879
	21st to end	Day	42% 5% 13% 29% 9% 2	1680
December	1st to 10th	Day	44% 4 15% 25% 10% 2	2680
	11th to 20th	Day	47% 3 15% 27% 8%	3066
	21st to end	Day	46% 4 15% 20% 12% 1	4048
January	1st to 10th	Night	38% 4 11% 24% 19% 4	6364
	11th to 20th	Night	41% 4 13% 21% 15% 5%	6149
	21st to end	Night	44% 3 11% 24% 17% 2	6490
February	1st to 10th	Night	46% 4 14% 25% 10% 2	2600
	11th to 20th	Night	47% 4 9% 21% 17% 3	2445
	21st to end	Night	45% 4 10% 20% 16% 3	1980
March	1st to 10th	Night	44% 4 10% 22% 13% 7%	5218
	11th to 20th	Night	35% 4 16% 28% 12% 5%	4980
	21st to end	Night	43% 3 8% 18% 25% 4	5239
April	1st to 10th	Night	46% 5% 11% 21% 15% 4	2000
	11th to 20th	Night	40% 6% 12% 19% 20% 2	1975
	21st to end	Night	40% 6% 10% 22% 18% 5%	2000
May	1st to 10th	Night	55% 6% 9% 17% 11% 2	4075
	11th to 20th	Night	52% 7% 11% 18% 12% 1	3576
	21st to end	Night	47% 6% 9% 23% 13% 3	3927
June	1st to 10th	Night	40% 7% 10% 22% 19% 2	1600
	11th to 20th	Night	38% 9% 12% 26% 14% 2	1600
	21st to end	Night	35% 8% 14% 27% 15% 2	1600
July	1st to 10th	Night	41% 7% 13% 20% 15% 3	3560
	11th to 20th	Night	48% 5% 15% 20% 11% 1	3552
	21st to end	Night	38% 5% 12% 23% 22% 1	4046
August	1st to 10th	Night	48% 5% 17% 20% 10% 1	2904
	11th to 20th	Night	43% 5% 10% 24% 16% 1	3089
	21st to end	Night	34% 5% 11% 30% 19% 1	3586
September	1st to 10th	Night	37% 5% 11% 23% 20% 4	2000
	11th to 20th	Night	34% 5% 8% 25% 25% 3	2000
	21st to end	Night	45% 5% 9% 18% 21% 2	2259
October	1st to 10th	Night	43% 3 9% 21% 22% 2	3920
	11th to 20th	Night	29% 3 9% 30% 25% 4	3899
	21st to end	Night	27% 3 11% 32% 24% 2	4299
November	1st to 10th	Night	33% 5% 12% 26% 20% 4	2667
	11th to 20th	Night	41% 4 9% 25% 18% 4	2735
	21st to end	Night	41% 4 13% 27% 14% 1	2940
December	1st to 10th	Night	43% 3 13% 28% 11% 1	4720
	11th to 20th	Night	46% 4 13% 24% 12% 2	5348
	21st to end	Night	48% 5% 11% 20% 15% 1	7150

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	26% 26% 29% 13% 5% 1	202
	11th to 20th	Day	30% 23% 33% 12% 2	245
	21st to end	Day	22% 22% 35% 15% 5% 2	330
February	1st to 10th	Day	25% 22% 33% 16% 3 1	300
	11th to 20th	Day	28% 23% 34% 14% 1	390
	21st to end	Day	29% 16% 31% 22% 2	414
March	1st to 10th	Day	24% 20% 32% 20% 4	500
	11th to 20th	Day	23% 24% 33% 15% 5% 1	500
	21st to end	Day	28% 19% 31% 15% 5% 1	550
April	1st to 10th	Day	27% 19% 31% 16% 6% 2	500
	11th to 20th	Day	24% 21% 27% 17% 8% 3	492
	21st to end	Day	25% 15% 32% 19% 6% 3	500
May	1st to 10th	Day	28% 13% 27% 19% 10% 3	503
	11th to 20th	Day	23% 14% 31% 20% 9% 3	527
	21st to end	Day	20% 7% 26% 27% 15% 6%	573
June	1st to 10th	Day	15% 12% 30% 23% 15% 5%	530
	11th to 20th	Day	20% 8% 30% 23% 16% 3	530
	21st to end	Day	16% 8% 28% 28% 16% 5%	530
July	1st to 10th	Day	13% 4 21% 30% 23% 9%	525
	11th to 20th	Day	9% 3 24% 31% 24% 8%	524
	21st to end	Day	13% 3 26% 33% 19% 5%	576
August	1st to 10th	Day	13% 3 20% 30% 21% 12%	526
	11th to 20th	Day	16% 4 18% 31% 21% 9%	519
	21st to end	Day	13% 5% 23% 29% 18% 12%	583
September	1st to 10th	Day	18% 7% 21% 24% 22% 8%	530
	11th to 20th	Day	14% 5% 19% 29% 21% 12%	530
	21st to end	Day	26% 10% 21% 22% 16% 5%	530
October	1st to 10th	Day	23% 5% 22% 25% 16% 8%	530
	11th to 20th	Day	19% 14% 28% 23% 13% 3	530
	21st to end	Day	20% 13% 31% 24% 11% 2	569
November	1st to 10th	Day	16% 14% 28% 26% 13% 2	425
	11th to 20th	Day	20% 17% 32% 24% 8%	419
	21st to end	Day	23% 21% 34% 15% 6% 1	220
December	1st to 10th	Day	25% 20% 33% 15% 6% 1	220
	11th to 20th	Day	26% 21% 32% 16% 4	220
	21st to end	Day	27% 19% 28% 18% 7% 1	242
January	1st to 10th	Night	20% 16% 40% 20% 3	805
	11th to 20th	Night	24% 17% 30% 26% 4	800
	21st to end	Night	17% 16% 38% 27% 2	880
February	1st to 10th	Night	22% 15% 32% 29% 2	800
	11th to 20th	Night	24% 20% 37% 18% 2	783
	21st to end	Night	24% 16% 34% 23% 3	660
March	1st to 10th	Night	22% 16% 33% 24% 4	800
	11th to 20th	Night	24% 17% 32% 21% 6% 1	800
	21st to end	Night	22% 16% 34% 23% 5%	880
April	1st to 10th	Night	25% 17% 32% 22% 4	800
	11th to 20th	Night	19% 15% 34% 25% 7%	791
	21st to end	Night	22% 12% 29% 29% 8%	800
May	1st to 10th	Night	26% 8% 27% 30% 9%	615
	11th to 20th	Night	20% 8% 28% 34% 10% 1	394
	21st to end	Night	18% 5% 25% 35% 17% 1	433
June	1st to 10th	Night	17% 5% 24% 35% 20%	400
	11th to 20th	Night	16% 5% 24% 42% 13% 1	400
	21st to end	Night	12% 3 22% 47% 16% 1	400
July	1st to 10th	Night	11% 3 20% 42% 21% 2	390
	11th to 20th	Night	11% 2 19% 40% 27% 2	388
	21st to end	Night	12% 2 15% 42% 27% 1	541
August	1st to 10th	Night	14% 3 11% 41% 29% 2	597
	11th to 20th	Night	15% 2 15% 34% 32% 2	703
	21st to end	Night	15% 3 18% 35% 28% 2	880
September	1st to 10th	Night	15% 3 21% 34% 23% 3	800
	11th to 20th	Night	14% 4 15% 36% 27% 5%	800
	21st to end	Night	23% 3 19% 33% 21% 2	800
October	1st to 10th	Night	23% 6% 22% 27% 19% 4	800
	11th to 20th	Night	20% 9% 24% 33% 13% 1	800
	21st to end	Night	21% 10% 26% 32% 10%	880
November	1st to 10th	Night	16% 8% 30% 33% 12% 1	820
	11th to 20th	Night	20% 14% 29% 29% 7% 1	840
	21st to end	Night	19% 14% 31% 30% 7%	840
December	1st to 10th	Night	25% 13% 33% 23% 6%	840
	11th to 20th	Night	22% 13% 32% 28% 5%	840
	21st to end	Night	24% 11% 33% 29% 3	924

Page generated at 2025-12-30 01:14:08. Version ac07

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