Darkstar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Darkstar Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	8% 5% 2 7% 6% 6% 6% 5% 7% 9% 38%	126760
	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 43%	124264
Transparency	Day	40% 7% 16% 20% 13% 4	130442
	Night	36% 7% 17% 21% 13% 5%	128638
Seeing	Day	9% 5% 18% 25% 24% 20%	17199
	Night	9% 4 17% 33% 30% 8%	27278

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	8% 5% 1 5% 3 4 4 3 5% 7% 54%	4056
	11th to 20th	Day	19% 9% 4 8% 6% 5% 4 4 6% 6% 31%	4060
	21st to end	Day	16% 6% 2 6% 4 3 3 3 5% 8% 44%	4494
February	1st to 10th	Day	11% 7% 2 7% 5% 4 5% 4 6% 7% 43%	2000
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 3 5% 7% 50%	1948
	21st to end	Day	8% 6% 2 6% 4 6% 5% 5% 5% 6% 48%	1806
March	1st to 10th	Day	8% 6% 3 7% 5% 4 5% 4 6% 9% 43%	4935
	11th to 20th	Day	4 3 2 5% 3 3 4 2 5% 8% 60%	4980
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 8% 45%	5465
April	1st to 10th	Day	6% 3 2 7% 5% 6% 5% 4 7% 7% 48%	2200
	11th to 20th	Day	7% 6% 2 6% 5% 5% 6% 5% 6% 9% 43%	2253
	21st to end	Day	4 4 2 5% 5% 5% 6% 4 7% 9% 49%	2400
May	1st to 10th	Day	3 3 2 6% 5% 5% 6% 5% 7% 11% 46%	5584
	11th to 20th	Day	8% 5% 2 7% 5% 4 5% 5% 7% 8% 44%	5861
	21st to end	Day	5% 4 2 6% 5% 6% 6% 5% 8% 11% 43%	6433
June	1st to 10th	Day	13% 7% 3 10% 8% 7% 7% 6% 7% 9% 24%	2600
	11th to 20th	Day	7% 6% 3 9% 8% 9% 9% 6% 8% 10% 26%	2600
	21st to end	Day	9% 8% 3 9% 7% 8% 8% 6% 7% 9% 26%	2600
July	1st to 10th	Day	7% 7% 3 11% 8% 8% 9% 7% 10% 13% 19%	5900
	11th to 20th	Day	5% 5% 3 8% 7% 8% 8% 6% 10% 13% 28%	5900
	21st to end	Day	4 5% 2 8% 7% 7% 8% 6% 9% 13% 30%	6424
August	1st to 10th	Day	4 6% 3 10% 8% 7% 9% 7% 9% 13% 24%	4234
	11th to 20th	Day	4 5% 3 9% 7% 7% 7% 6% 9% 11% 32%	3996
	21st to end	Day	8% 6% 3 9% 7% 8% 8% 6% 10% 12% 23%	3966
September	1st to 10th	Day	9% 7% 3 10% 9% 9% 8% 6% 8% 10% 22%	2200
	11th to 20th	Day	10% 8% 3 8% 7% 8% 8% 6% 9% 9% 22%	2200
	21st to end	Day	10% 7% 3 10% 7% 7% 7% 5% 8% 9% 27%	2200
October	1st to 10th	Day	12% 6% 3 10% 7% 6% 7% 6% 7% 7% 28%	2139
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 44%	2000
	21st to end	Day	7% 5% 1 6% 5% 5% 5% 4 6% 9% 47%	2200
November	1st to 10th	Day	8% 5% 2 6% 6% 4 6% 5% 7% 9% 43%	2057
	11th to 20th	Day	9% 6% 3 7% 5% 5% 5% 4 5% 8% 44%	2100
	21st to end	Day	9% 5% 2 8% 5% 5% 5% 5% 5% 8% 43%	2047
December	1st to 10th	Day	5% 4 2 5% 4 4 4 3 6% 7% 55%	3020
	11th to 20th	Day	9% 7% 3 7% 5% 6% 6% 5% 7% 8% 37%	3436
	21st to end	Day	10% 7% 2 8% 6% 6% 5% 5% 7% 8% 36%	4466
January	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 5% 7% 53%	5889
	11th to 20th	Night	18% 10% 3 8% 5% 4 5% 4 6% 8% 30%	5900
	21st to end	Night	15% 6% 2 6% 4 4 4 4 6% 8% 42%	6444
February	1st to 10th	Night	10% 8% 2 6% 4 4 4 3 6% 7% 47%	2400
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 6% 7% 50%	2338
	21st to end	Night	8% 4 2 4 4 4 4 3 5% 8% 55%	1821
March	1st to 10th	Night	11% 5% 2 7% 4 5% 4 3 5% 7% 47%	4918
	11th to 20th	Night	5% 3 1 3 3 3 4 3 6% 10% 59%	4980
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 6% 7% 49%	5467
April	1st to 10th	Night	6% 4 1 5% 4 3 4 4 5% 9% 55%	2200
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 4 6% 8% 51%	2095
	21st to end	Night	5% 3 2 4 4 4 4 4 6% 8% 56%	2000
May	1st to 10th	Night	4 3 1 5% 5% 5% 5% 4 6% 9% 52%	4352
	11th to 20th	Night	8% 4 2 6% 3 5% 4 4 6% 8% 48%	4032
	21st to end	Night	4 4 2 6% 4 5% 5% 4 7% 9% 49%	4421
June	1st to 10th	Night	10% 8% 3 8% 5% 5% 6% 6% 8% 10% 32%	1800
	11th to 20th	Night	6% 7% 3 7% 7% 7% 7% 5% 9% 11% 31%	1800
	21st to end	Night	7% 8% 3 8% 6% 5% 5% 6% 9% 12% 32%	1800
July	1st to 10th	Night	8% 8% 4 9% 7% 7% 6% 5% 9% 12% 26%	4058
	11th to 20th	Night	4 4 2 7% 5% 6% 7% 6% 9% 14% 35%	4057
	21st to end	Night	4 5% 3 8% 6% 6% 7% 5% 9% 12% 35%	4427
August	1st to 10th	Night	4 5% 3 8% 7% 6% 7% 5% 8% 13% 34%	2915
	11th to 20th	Night	4 5% 2 8% 6% 6% 6% 6% 9% 11% 37%	3006
	21st to end	Night	8% 5% 2 8% 5% 6% 7% 6% 10% 13% 31%	3912
September	1st to 10th	Night	8% 6% 3 9% 6% 8% 7% 5% 9% 10% 29%	2200
	11th to 20th	Night	10% 8% 4 10% 6% 6% 7% 5% 7% 10% 29%	2200
	21st to end	Night	9% 6% 3 9% 6% 7% 6% 5% 8% 9% 34%	2200
October	1st to 10th	Night	10% 7% 3 8% 5% 5% 5% 5% 7% 9% 39%	2260
	11th to 20th	Night	6% 6% 2 6% 4 5% 5% 4 5% 7% 50%	2400
	21st to end	Night	9% 4 2 6% 4 4 4 4 5% 8% 50%	2640
November	1st to 10th	Night	8% 5% 2 6% 4 5% 4 4 5% 7% 49%	2461
	11th to 20th	Night	8% 7% 2 6% 5% 3 5% 4 6% 8% 47%	2520
	21st to end	Night	8% 5% 2 7% 5% 5% 4 4 5% 10% 45%	2567
December	1st to 10th	Night	5% 3 1 4 4 4 4 4 6% 8% 58%	4380
	11th to 20th	Night	10% 7% 3 7% 5% 4 4 3 6% 7% 44%	4914
	21st to end	Night	11% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	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	30% 4 9% 19% 27% 11%	4057
	11th to 20th	Day	54% 3 5% 17% 15% 6%	4060
	21st to end	Day	39% 4 6% 28% 16% 7%	4494
February	1st to 10th	Day	39% 6% 10% 15% 22% 8%	2000
	11th to 20th	Day	31% 6% 9% 22% 22% 10%	1958
	21st to end	Day	35% 5% 9% 18% 24% 9%	1806
March	1st to 10th	Day	38% 6% 12% 17% 21% 7%	4935
	11th to 20th	Day	23% 4 9% 25% 22% 17%	4980
	21st to end	Day	34% 5% 10% 27% 16% 8%	5465
April	1st to 10th	Day	33% 6% 12% 24% 17% 8%	2200
	11th to 20th	Day	36% 8% 11% 18% 20% 8%	2253
	21st to end	Day	29% 6% 12% 22% 23% 8%	2400
May	1st to 10th	Day	28% 7% 21% 27% 14% 3	5616
	11th to 20th	Day	34% 7% 17% 28% 12% 1	5865
	21st to end	Day	32% 7% 18% 30% 12% 1	6447
June	1st to 10th	Day	53% 9% 18% 16% 4	2600
	11th to 20th	Day	49% 11% 21% 17% 3	2600
	21st to end	Day	51% 10% 23% 13% 3 1	2600
July	1st to 10th	Day	52% 10% 27% 9% 2	5835
	11th to 20th	Day	44% 8% 35% 12% 1	5831
	21st to end	Day	41% 10% 27% 22% 1	6240
August	1st to 10th	Day	46% 11% 30% 11% 2	4221
	11th to 20th	Day	42% 10% 25% 15% 8%	4035
	21st to end	Day	50% 11% 26% 12% 2	3966
September	1st to 10th	Day	54% 13% 20% 9% 4	2200
	11th to 20th	Day	52% 12% 20% 13% 3	2200
	21st to end	Day	49% 9% 17% 13% 10% 2	2200
October	1st to 10th	Day	57% 8% 15% 10% 7% 3	3477
	11th to 20th	Day	30% 6% 7% 24% 28% 5%	3240
	21st to end	Day	29% 4 7% 24% 28% 7%	3538
November	1st to 10th	Day	36% 7% 11% 19% 20% 8%	2057
	11th to 20th	Day	38% 6% 8% 21% 19% 8%	2100
	21st to end	Day	38% 6% 9% 22% 19% 7%	2047
December	1st to 10th	Day	29% 4 16% 28% 15% 9%	3020
	11th to 20th	Day	43% 5% 11% 17% 18% 6%	3433
	21st to end	Day	43% 6% 11% 21% 13% 5%	4466
January	1st to 10th	Night	30% 4 9% 21% 25% 12%	5889
	11th to 20th	Night	53% 4 10% 17% 13% 4	5900
	21st to end	Night	39% 5% 9% 24% 16% 7%	6444
February	1st to 10th	Night	36% 6% 12% 18% 18% 11%	2400
	11th to 20th	Night	32% 5% 11% 22% 19% 10%	2339
	21st to end	Night	27% 5% 14% 22% 24% 8%	1821
March	1st to 10th	Night	37% 5% 12% 21% 18% 7%	4918
	11th to 20th	Night	20% 6% 17% 24% 17% 16%	4980
	21st to end	Night	32% 6% 11% 26% 20% 5%	5467
April	1st to 10th	Night	26% 6% 19% 26% 16% 8%	2200
	11th to 20th	Night	29% 7% 13% 24% 20% 7%	2095
	21st to end	Night	25% 7% 14% 25% 22% 7%	2000
May	1st to 10th	Night	26% 8% 21% 29% 12% 3	4382
	11th to 20th	Night	33% 7% 22% 28% 10% 1	4033
	21st to end	Night	29% 7% 21% 31% 12%	4429
June	1st to 10th	Night	44% 9% 23% 20% 5%	1800
	11th to 20th	Night	43% 12% 22% 22% 2	1800
	21st to end	Night	41% 13% 26% 16% 5%	1800
July	1st to 10th	Night	49% 11% 27% 13% 1	4015
	11th to 20th	Night	36% 13% 40% 11% 1	4006
	21st to end	Night	38% 10% 29% 22% 1	4297
August	1st to 10th	Night	40% 12% 35% 11% 2	2904
	11th to 20th	Night	37% 11% 29% 17% 7%	3033
	21st to end	Night	42% 12% 33% 12% 1	3912
September	1st to 10th	Night	46% 18% 20% 13% 4	2200
	11th to 20th	Night	49% 12% 21% 16% 2	2200
	21st to end	Night	43% 10% 19% 17% 8% 2	2200
October	1st to 10th	Night	55% 7% 15% 14% 7% 3	3689
	11th to 20th	Night	34% 5% 7% 26% 23% 4	3899
	21st to end	Night	31% 5% 9% 22% 28% 6%	4252
November	1st to 10th	Night	34% 6% 12% 20% 20% 9%	2461
	11th to 20th	Night	34% 6% 11% 22% 18% 8%	2520
	21st to end	Night	35% 6% 10% 24% 19% 7%	2567
December	1st to 10th	Night	24% 4 18% 31% 12% 11%	4380
	11th to 20th	Night	40% 4 13% 20% 15% 9%	4916
	21st to end	Night	43% 5% 13% 22% 12% 5%	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	12% 8% 45% 24% 9% 2	300
	11th to 20th	Day	9% 11% 38% 26% 12% 4	300
	21st to end	Day	17% 12% 39% 26% 6%	330
February	1st to 10th	Day	12% 10% 31% 25% 18% 3	300
	11th to 20th	Day	10% 6% 38% 30% 13% 5%	300
	21st to end	Day	10% 7% 29% 32% 20% 3	405
March	1st to 10th	Day	6% 11% 32% 34% 15% 1	500
	11th to 20th	Day	10% 7% 27% 32% 21% 4	500
	21st to end	Day	7% 8% 29% 30% 23% 4	550
April	1st to 10th	Day	8% 7% 29% 33% 18% 5%	500
	11th to 20th	Day	7% 8% 27% 29% 21% 9%	492
	21st to end	Day	6% 3 23% 32% 23% 14%	500
May	1st to 10th	Day	6% 3 20% 32% 25% 14%	503
	11th to 20th	Day	11% 2 16% 27% 25% 18%	527
	21st to end	Day	7% 2 10% 25% 29% 27%	573
June	1st to 10th	Day	13% 3 9% 23% 28% 24%	530
	11th to 20th	Day	8% 2 6% 20% 27% 37%	530
	21st to end	Day	8% 2 3 13% 28% 45%	530
July	1st to 10th	Day	7% 3 3 9% 27% 52%	525
	11th to 20th	Day	5% 2 2 9% 34% 48%	524
	21st to end	Day	6% 2 2 15% 26% 48%	576
August	1st to 10th	Day	6% 5% 4 15% 28% 42%	526
	11th to 20th	Day	4 2 1 14% 27% 52%	519
	21st to end	Day	11% 3 4 15% 29% 38%	583
September	1st to 10th	Day	9% 4 4 16% 29% 39%	530
	11th to 20th	Day	11% 3 6% 24% 26% 30%	530
	21st to end	Day	9% 3 8% 20% 32% 28%	530
October	1st to 10th	Day	16% 2 15% 33% 23% 11%	530
	11th to 20th	Day	6% 4 19% 31% 29% 10%	530
	21st to end	Day	7% 4 20% 36% 25% 7%	583
November	1st to 10th	Day	9% 6% 18% 34% 27% 7%	535
	11th to 20th	Day	10% 6% 23% 33% 22% 5%	540
	21st to end	Day	10% 6% 24% 32% 23% 4	487
December	1st to 10th	Day	6% 11% 36% 27% 15% 5%	330
	11th to 20th	Day	11% 6% 30% 33% 17% 2	321
	21st to end	Day	8% 8% 30% 36% 15% 3	330
January	1st to 10th	Night	12% 8% 28% 36% 14% 1	800
	11th to 20th	Night	12% 6% 27% 41% 14% 1	800
	21st to end	Night	13% 8% 33% 37% 10%	880
February	1st to 10th	Night	13% 6% 28% 37% 15% 1	800
	11th to 20th	Night	10% 5% 31% 39% 14% 1	783
	21st to end	Night	10% 4 32% 44% 10%	660
March	1st to 10th	Night	6% 6% 33% 42% 13% 1	800
	11th to 20th	Night	9% 6% 26% 43% 16% 1	800
	21st to end	Night	6% 5% 29% 43% 17%	880
April	1st to 10th	Night	7% 5% 27% 45% 16% 1	800
	11th to 20th	Night	7% 5% 25% 42% 20% 1	791
	21st to end	Night	5% 4 22% 44% 23% 2	800
May	1st to 10th	Night	6% 3 16% 47% 26% 3	740
	11th to 20th	Night	8% 3 10% 39% 35% 6%	591
	21st to end	Night	7% 2 7% 29% 49% 7%	650
June	1st to 10th	Night	13% 3 3 29% 46% 8%	600
	11th to 20th	Night	7% 3 4 20% 44% 23%	600
	21st to end	Night	7% 2 2 17% 52% 21%	600
July	1st to 10th	Night	8% 2 1 12% 50% 26%	585
	11th to 20th	Night	7% 1 1 9% 51% 32%	582
	21st to end	Night	6% 2 1 11% 50% 31%	657
August	1st to 10th	Night	5% 3 2 12% 53% 27%	597
	11th to 20th	Night	4 1 1 11% 49% 32%	673
	21st to end	Night	10% 3 4 16% 46% 22%	880
September	1st to 10th	Night	10% 3 4 17% 44% 22%	800
	11th to 20th	Night	13% 4 4 20% 44% 16%	800
	21st to end	Night	11% 2 5% 24% 46% 12%	800
October	1st to 10th	Night	15% 2 10% 36% 31% 7%	800
	11th to 20th	Night	9% 3 11% 34% 38% 5%	800
	21st to end	Night	11% 3 14% 38% 30% 4	880
November	1st to 10th	Night	9% 2 18% 36% 31% 4	820
	11th to 20th	Night	9% 4 19% 40% 25% 2	840
	21st to end	Night	9% 3 22% 44% 20% 2	840
December	1st to 10th	Night	6% 4 24% 39% 25% 2	840
	11th to 20th	Night	10% 5% 21% 40% 22% 1	829
	21st to end	Night	11% 5% 25% 38% 19% 1	880

Page generated at 2025-12-15 01:13:55. Version ac07

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