Star
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star clear sky chart from 2005-11-06 to 2025-03-27.

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	12% 6% 2 7% 5% 4 5% 4 6% 7% 42%	108817
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	110598
Transparency	Day	38% 7% 11% 18% 19% 7%	112535
	Night	36% 6% 10% 18% 20% 9%	115020
Seeing	Day	14% 9% 22% 25% 21% 9%	15404
	Night	15% 8% 23% 30% 20% 4	25260

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% 11% 3 9% 3 3 3 3 4 5% 41%	4077
	11th to 20th	Day	13% 6% 2 6% 3 3 3 3 4 7% 50%	4080
	21st to end	Day	22% 10% 3 6% 4 3 3 3 3 5% 39%	4690
February	1st to 10th	Day	17% 11% 4 5% 4 4 4 3 4 5% 40%	2000
	11th to 20th	Day	21% 8% 3 6% 4 3 3 3 4 4 41%	1948
	21st to end	Day	17% 8% 3 8% 5% 5% 4 3 4 5% 39%	1650
March	1st to 10th	Day	23% 10% 3 9% 4 4 4 3 5% 7% 28%	3319
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	3590
	21st to end	Day	14% 8% 3 8% 5% 5% 4 4 5% 5% 40%	4093
April	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 6% 44%	2280
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 2 5% 7% 41%	2254
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	2280
May	1st to 10th	Day	11% 5% 2 6% 5% 5% 6% 4 7% 9% 39%	3765
	11th to 20th	Day	13% 7% 2 7% 4 4 4 4 5% 6% 44%	3764
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	4337
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 6% 8% 10% 35%	2578
	11th to 20th	Day	5% 5% 3 6% 6% 6% 7% 6% 8% 9% 39%	2660
	21st to end	Day	6% 5% 2 8% 6% 7% 7% 6% 8% 9% 36%	2635
July	1st to 10th	Day	7% 6% 3 9% 7% 8% 7% 6% 9% 10% 29%	4120
	11th to 20th	Day	9% 5% 2 7% 6% 7% 7% 7% 10% 12% 28%	4120
	21st to end	Day	12% 8% 4 10% 7% 6% 7% 5% 8% 9% 25%	4247
August	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 7% 62%	3785
	11th to 20th	Day	5% 4 2 6% 5% 4 6% 4 6% 11% 47%	3729
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 6% 8% 43%	4180
September	1st to 10th	Day	9% 5% 2 6% 6% 6% 7% 5% 6% 8% 39%	2280
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 5% 7% 42%	2220
	21st to end	Day	10% 4 2 5% 3 3 4 3 5% 8% 52%	2090
October	1st to 10th	Day	7% 4 1 5% 4 3 4 3 5% 6% 57%	1996
	11th to 20th	Day	10% 4 2 6% 3 3 3 3 4 6% 55%	1900
	21st to end	Day	12% 5% 1 5% 3 2 3 2 4 5% 58%	2090
November	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 4 6% 52%	1951
	11th to 20th	Day	11% 7% 2 5% 4 3 4 3 4 5% 53%	1800
	21st to end	Day	15% 6% 2 6% 3 4 4 3 4 5% 48%	1800
December	1st to 10th	Day	18% 7% 3 5% 3 3 4 3 5% 6% 43%	2930
	11th to 20th	Day	13% 9% 3 8% 5% 4 4 2 3 4 43%	3373
	21st to end	Day	15% 6% 2 6% 4 3 3 3 4 6% 47%	4206
January	1st to 10th	Night	18% 9% 4 7% 2 2 3 2 3 4 46%	5888
	11th to 20th	Night	16% 7% 4 5% 3 3 3 3 3 4 48%	5900
	21st to end	Night	24% 9% 2 5% 3 3 3 3 3 4 40%	6261
February	1st to 10th	Night	16% 9% 3 6% 4 3 3 3 4 5% 43%	2400
	11th to 20th	Night	21% 8% 2 5% 4 3 3 3 4 5% 43%	2338
	21st to end	Night	20% 8% 2 6% 4 4 3 2 5% 5% 40%	1980
March	1st to 10th	Night	21% 11% 3 6% 4 3 3 4 5% 6% 33%	3769
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 4 6% 8% 41%	3590
	21st to end	Night	16% 6% 2 6% 4 4 5% 4 5% 6% 41%	3674
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 6% 8% 46%	1875
	21st to end	Night	8% 5% 2 4 3 4 5% 3 6% 8% 53%	1900
May	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 6% 10% 46%	3130
	11th to 20th	Night	16% 6% 3 6% 4 4 4 3 5% 7% 43%	3129
	21st to end	Night	7% 4 3 7% 5% 5% 6% 4 7% 10% 43%	3214
June	1st to 10th	Night	7% 6% 2 6% 5% 4 5% 5% 7% 10% 42%	1595
	11th to 20th	Night	4 4 2 3 5% 5% 5% 5% 6% 11% 51%	1520
	21st to end	Night	5% 5% 2 5% 4 5% 5% 5% 8% 11% 45%	1536
July	1st to 10th	Night	5% 6% 3 8% 6% 5% 5% 4 8% 11% 38%	2850
	11th to 20th	Night	7% 4 2 8% 5% 6% 7% 7% 10% 13% 31%	2850
	21st to end	Night	10% 8% 4 9% 6% 6% 5% 4 6% 11% 31%	3307
August	1st to 10th	Night	2 2 1 3 3 3 4 4 6% 9% 61%	3144
	11th to 20th	Night	3 3 1 5% 3 5% 5% 5% 9% 13% 47%	3100
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 4 6% 8% 46%	3476
September	1st to 10th	Night	7% 4 2 6% 5% 5% 6% 5% 6% 8% 44%	1900
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 47%	1939
	21st to end	Night	8% 4 2 5% 3 4 4 4 5% 7% 54%	2090
October	1st to 10th	Night	6% 2 1 5% 4 3 3 3 5% 6% 62%	2178
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 4 6% 57%	2280
	21st to end	Night	13% 6% 1 4 3 3 3 2 3 4 58%	2508
November	1st to 10th	Night	12% 5% 1 4 3 3 3 3 5% 6% 56%	2345
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 54%	2600
	21st to end	Night	14% 6% 2 5% 3 3 3 3 5% 6% 49%	2600
December	1st to 10th	Night	20% 7% 2 5% 3 3 3 3 4 4 45%	4250
	11th to 20th	Night	16% 10% 2 6% 4 4 4 3 4 5% 44%	4822
	21st to end	Night	19% 10% 2 5% 3 3 2 2 3 5% 46%	6760

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	43% 4 4 23% 23% 4	4078
	11th to 20th	Day	34% 4 5% 20% 29% 7%	4080
	21st to end	Day	49% 3 6% 12% 18% 12%	4690
February	1st to 10th	Day	47% 5% 7% 14% 17% 10%	2000
	11th to 20th	Day	46% 5% 6% 16% 20% 8%	1958
	21st to end	Day	46% 6% 8% 14% 17% 10%	1650
March	1st to 10th	Day	55% 6% 8% 12% 14% 6%	3319
	11th to 20th	Day	45% 7% 9% 17% 16% 6%	3590
	21st to end	Day	45% 6% 7% 16% 19% 7%	4093
April	1st to 10th	Day	38% 6% 10% 19% 17% 8%	2280
	11th to 20th	Day	42% 5% 8% 17% 21% 7%	2254
	21st to end	Day	33% 8% 10% 22% 24% 3	2280
May	1st to 10th	Day	37% 11% 14% 21% 14% 3	3800
	11th to 20th	Day	39% 7% 13% 19% 20% 2	3768
	21st to end	Day	32% 10% 18% 27% 12% 1	4350
June	1st to 10th	Day	41% 11% 14% 20% 11% 2	2578
	11th to 20th	Day	37% 11% 16% 21% 13% 3	2660
	21st to end	Day	39% 13% 17% 19% 11% 2	2635
July	1st to 10th	Day	45% 13% 19% 16% 6% 1	4056
	11th to 20th	Day	40% 14% 27% 15% 3 1	4051
	21st to end	Day	53% 11% 12% 18% 6%	4112
August	1st to 10th	Day	21% 7% 13% 27% 28% 3	3773
	11th to 20th	Day	30% 11% 17% 29% 12% 2	3765
	21st to end	Day	38% 9% 13% 20% 16% 3	4179
September	1st to 10th	Day	39% 12% 14% 16% 14% 6%	2280
	11th to 20th	Day	41% 9% 10% 15% 18% 7%	2220
	21st to end	Day	30% 6% 12% 24% 19% 9%	2090
October	1st to 10th	Day	21% 4 7% 14% 26% 29%	3321
	11th to 20th	Day	18% 3 5% 13% 31% 30%	3140
	21st to end	Day	22% 3 4 16% 37% 17%	3428
November	1st to 10th	Day	33% 6% 7% 16% 28% 11%	1951
	11th to 20th	Day	33% 4 7% 14% 28% 14%	1800
	21st to end	Day	38% 4 8% 16% 24% 10%	1800
December	1st to 10th	Day	41% 5% 7% 17% 25% 5%	2930
	11th to 20th	Day	45% 4 8% 15% 20% 8%	3370
	21st to end	Day	38% 5% 6% 22% 26% 2	4206
January	1st to 10th	Night	42% 3 5% 20% 24% 6%	5888
	11th to 20th	Night	40% 4 5% 14% 26% 11%	5900
	21st to end	Night	49% 3 4 14% 21% 9%	6261
February	1st to 10th	Night	44% 4 7% 16% 20% 10%	2400
	11th to 20th	Night	44% 4 6% 15% 21% 10%	2339
	21st to end	Night	47% 5% 7% 14% 15% 12%	1980
March	1st to 10th	Night	50% 7% 9% 13% 14% 7%	3769
	11th to 20th	Night	39% 7% 10% 18% 18% 7%	3590
	21st to end	Night	42% 7% 10% 17% 20% 5%	3674
April	1st to 10th	Night	36% 8% 10% 18% 19% 10%	1900
	11th to 20th	Night	35% 6% 10% 20% 19% 9%	1875
	21st to end	Night	27% 6% 11% 23% 26% 6%	1900
May	1st to 10th	Night	32% 9% 13% 25% 18% 4	3160
	11th to 20th	Night	38% 8% 12% 24% 17% 1	3130
	21st to end	Night	31% 11% 17% 24% 15% 2	3222
June	1st to 10th	Night	34% 11% 16% 23% 15% 1	1595
	11th to 20th	Night	25% 10% 20% 27% 14% 4	1520
	21st to end	Night	29% 11% 21% 23% 14% 3	1536
July	1st to 10th	Night	37% 14% 23% 19% 6% 2	2805
	11th to 20th	Night	38% 15% 27% 15% 4 1	2796
	21st to end	Night	46% 11% 18% 19% 5% 1	3202
August	1st to 10th	Night	18% 9% 17% 31% 23% 2	3132
	11th to 20th	Night	25% 11% 21% 31% 10% 2	3130
	21st to end	Night	35% 9% 12% 21% 18% 4	3476
September	1st to 10th	Night	34% 12% 15% 17% 16% 6%	1900
	11th to 20th	Night	34% 8% 14% 16% 19% 9%	1939
	21st to end	Night	29% 6% 10% 25% 20% 10%	2090
October	1st to 10th	Night	18% 4 7% 13% 25% 33%	3633
	11th to 20th	Night	18% 4 4 17% 23% 34%	3779
	21st to end	Night	21% 3 5% 13% 36% 22%	4120
November	1st to 10th	Night	29% 5% 8% 17% 27% 13%	2345
	11th to 20th	Night	32% 4 7% 14% 28% 16%	2600
	21st to end	Night	36% 4 8% 19% 20% 13%	2600
December	1st to 10th	Night	43% 4 6% 16% 26% 5%	4250
	11th to 20th	Night	44% 4 8% 15% 24% 7%	4824
	21st to end	Night	43% 3 5% 19% 26% 4	6760

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	25% 24% 35% 13% 3 1	300
	11th to 20th	Day	25% 20% 37% 13% 6% 1	300
	21st to end	Day	28% 20% 35% 15% 2	330
February	1st to 10th	Day	25% 23% 35% 12% 5% 1	300
	11th to 20th	Day	37% 16% 33% 12% 1 1	300
	21st to end	Day	21% 29% 38% 9% 2	249
March	1st to 10th	Day	27% 18% 37% 14% 4	300
	11th to 20th	Day	27% 23% 31% 16% 3	300
	21st to end	Day	23% 16% 32% 24% 5%	438
April	1st to 10th	Day	18% 16% 34% 22% 10%	460
	11th to 20th	Day	22% 15% 29% 22% 10% 2	452
	21st to end	Day	11% 12% 33% 28% 15% 1	460
May	1st to 10th	Day	13% 11% 29% 29% 15% 3	463
	11th to 20th	Day	12% 5% 26% 27% 20% 9%	487
	21st to end	Day	7% 4 24% 31% 23% 10%	529
June	1st to 10th	Day	13% 3 13% 34% 26% 10%	598
	11th to 20th	Day	7% 3 13% 35% 32% 10%	680
	21st to end	Day	7% 2 13% 29% 34% 15%	680
July	1st to 10th	Day	5% 2 8% 32% 37% 16%	671
	11th to 20th	Day	7% 2 5% 22% 42% 22%	668
	21st to end	Day	8% 2 9% 23% 40% 18%	609
August	1st to 10th	Day	6% 1 7% 24% 38% 24%	486
	11th to 20th	Day	8% 2 6% 23% 38% 23%	479
	21st to end	Day	13% 2 7% 25% 29% 24%	539
September	1st to 10th	Day	10% 1 11% 27% 36% 16%	490
	11th to 20th	Day	13% 3 11% 20% 36% 17%	490
	21st to end	Day	11% 4 14% 29% 31% 11%	490
October	1st to 10th	Day	9% 6% 21% 38% 17% 10%	396
	11th to 20th	Day	10% 6% 28% 32% 19% 5%	300
	21st to end	Day	19% 8% 24% 29% 14% 5%	330
November	1st to 10th	Day	16% 10% 27% 33% 10% 4	300
	11th to 20th	Day	16% 14% 35% 24% 10% 1	300
	21st to end	Day	15% 20% 35% 16% 13% 2	300
December	1st to 10th	Day	24% 14% 36% 18% 6% 1	300
	11th to 20th	Day	19% 17% 37% 21% 5% 1	300
	21st to end	Day	24% 17% 33% 17% 8%	330
January	1st to 10th	Night	24% 15% 31% 25% 5%	1000
	11th to 20th	Night	23% 15% 30% 25% 6%	1000
	21st to end	Night	24% 15% 32% 25% 3	1001
February	1st to 10th	Night	21% 14% 33% 28% 4	800
	11th to 20th	Night	28% 15% 29% 25% 4	783
	21st to end	Night	25% 19% 35% 19% 2	660
March	1st to 10th	Night	20% 15% 31% 29% 5%	755
	11th to 20th	Night	15% 13% 39% 27% 6% 1	600
	21st to end	Night	16% 10% 40% 26% 9%	647
April	1st to 10th	Night	16% 11% 34% 27% 11% 1	570
	11th to 20th	Night	15% 11% 35% 26% 12% 1	563
	21st to end	Night	8% 9% 35% 35% 13%	570
May	1st to 10th	Night	10% 5% 30% 37% 16% 1	570
	11th to 20th	Night	11% 3 24% 39% 20% 3	561
	21st to end	Night	6% 3 20% 34% 31% 7%	617
June	1st to 10th	Night	9% 2 13% 40% 31% 6%	570
	11th to 20th	Night	4 1 9% 36% 41% 8%	570
	21st to end	Night	6% 2 9% 29% 41% 13%	570
July	1st to 10th	Night	6% 1 6% 28% 46% 13%	555
	11th to 20th	Night	6% 1 5% 24% 46% 18%	552
	21st to end	Night	5% 1 6% 28% 47% 14%	624
August	1st to 10th	Night	2 5% 24% 49% 19%	567
	11th to 20th	Night	6% 2 6% 25% 46% 15%	564
	21st to end	Night	11% 2 6% 25% 39% 18%	627
September	1st to 10th	Night	8% 1 10% 29% 45% 8%	570
	11th to 20th	Night	11% 3 11% 28% 41% 7%	609
	21st to end	Night	10% 2 12% 36% 35% 5%	760
October	1st to 10th	Night	7% 2 18% 37% 31% 6%	760
	11th to 20th	Night	12% 6% 23% 35% 22% 3	760
	21st to end	Night	15% 7% 22% 33% 21% 2	836
November	1st to 10th	Night	14% 6% 20% 38% 18% 3	780
	11th to 20th	Night	14% 8% 28% 35% 15% 1	800
	21st to end	Night	17% 8% 30% 30% 14% 1	800
December	1st to 10th	Night	20% 11% 28% 31% 10%	800
	11th to 20th	Night	22% 9% 31% 30% 8%	800
	21st to end	Night	24% 14% 30% 26% 6%	1089

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