Starfront Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starfront Observatories clear sky chart from 2005-11-06 to 2026-05-02.

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	9% 4 2 4 3 3 3 3 5% 6% 57%	132111
	Night	9% 4 1 4 3 3 3 3 4 6% 59%	130749
Transparency	Day	28% 5% 9% 22% 24% 13%	135929
	Night	25% 4 9% 22% 25% 15%	134995
Seeing	Day	10% 6% 22% 30% 24% 8%	16882
	Night	10% 5% 28% 40% 16% 1	26925

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% 6% 2 4 2 3 3 2 4 6% 61%	5289
	11th to 20th	Day	7% 6% 2 4 2 2 2 2 5% 6% 63%	5310
	21st to end	Day	18% 7% 2 5% 4 3 3 2 3 4 50%	6385
February	1st to 10th	Day	12% 5% 2 4 3 2 3 2 4 7% 55%	2100
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 5% 54%	2049
	21st to end	Day	12% 6% 1 4 3 3 2 2 4 6% 58%	1730
March	1st to 10th	Day	9% 6% 2 5% 3 4 4 3 4 5% 56%	4769
	11th to 20th	Day	12% 5% 1 4 3 3 2 3 4 6% 58%	5090
	21st to end	Day	6% 4 2 5% 3 3 4 4 6% 7% 55%	5583
April	1st to 10th	Day	10% 5% 2 4 3 3 4 4 4 7% 54%	2310
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 7% 50%	2489
	21st to end	Day	10% 5% 2 4 4 3 3 3 4 6% 55%	2616
May	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 5% 5% 50%	5876
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 4 5% 51%	5862
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 8% 46%	6434
June	1st to 10th	Day	5% 2 1 4 4 4 4 3 5% 7% 60%	2600
	11th to 20th	Day	3 2 1 4 3 4 4 4 6% 8% 61%	2600
	21st to end	Day	5% 4 1 4 4 4 4 4 6% 8% 57%	2600
July	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 5% 7% 43%	5900
	11th to 20th	Day	4 2 1 3 3 4 4 4 7% 9% 60%	5900
	21st to end	Day	3 3 1 3 3 3 4 4 6% 9% 61%	6422
August	1st to 10th	Day	1 1 1 3 2 3 2 3 4 7% 74%	4081
	11th to 20th	Day	4 2 1 3 3 3 3 3 5% 9% 65%	3846
	21st to end	Day	3 2 1 5% 4 5% 5% 4 6% 8% 58%	4312
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 4 6% 53%	2200
	11th to 20th	Day	10% 4 2 5% 4 4 5% 3 5% 6% 51%	2200
	21st to end	Day	8% 3 1 4 3 4 3 3 5% 7% 60%	2200
October	1st to 10th	Day	4 3 2 3 3 3 3 3 5% 6% 66%	2200
	11th to 20th	Day	9% 3 1 4 2 2 2 3 4 5% 66%	2133
	21st to end	Day	11% 4 2 4 2 2 3 1 3 4 64%	2200
November	1st to 10th	Day	11% 4 1 5% 3 2 2 2 3 6% 60%	1947
	11th to 20th	Day	7% 3 1 4 3 3 3 3 4 7% 62%	1890
	21st to end	Day	11% 6% 2 4 4 3 3 2 4 5% 56%	1890
December	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 4 4 49%	3030
	11th to 20th	Day	11% 5% 1 5% 4 4 4 3 4 5% 53%	3481
	21st to end	Day	11% 5% 1 4 2 2 2 2 3 4 65%	4587
January	1st to 10th	Night	10% 5% 2 4 2 3 3 3 4 5% 60%	7634
	11th to 20th	Night	5% 3 1 3 2 2 2 2 3 4 72%	7680
	21st to end	Night	20% 6% 2 5% 3 3 3 2 4 5% 48%	7885
February	1st to 10th	Night	10% 5% 3 4 3 2 2 3 4 5% 58%	2520
	11th to 20th	Night	12% 5% 2 4 2 2 2 3 4 6% 56%	2458
	21st to end	Night	11% 5% 1 4 3 3 3 3 5% 5% 58%	2076
March	1st to 10th	Night	7% 4 1 3 2 3 3 3 5% 7% 61%	5315
	11th to 20th	Night	12% 4 1 3 2 3 3 3 5% 7% 57%	5090
	21st to end	Night	7% 5% 1 5% 3 3 4 3 5% 6% 58%	5588
April	1st to 10th	Night	7% 4 1 3 3 3 4 4 6% 7% 57%	2310
	11th to 20th	Night	12% 6% 1 5% 3 4 3 2 4 7% 52%	2077
	21st to end	Night	10% 3 1 3 3 3 2 3 4 6% 61%	2000
May	1st to 10th	Night	15% 4 2 5% 3 3 3 3 4 7% 51%	4062
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 6% 6% 54%	4053
	21st to end	Night	10% 5% 3 5% 3 4 4 4 6% 10% 48%	4443
June	1st to 10th	Night	3 3 1 2 3 3 4 3 6% 11% 62%	1800
	11th to 20th	Night	3 2 2 2 3 4 4 5% 10% 66%	1800
	21st to end	Night	4 3 1 3 3 3 3 3 5% 9% 64%	1800
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 9% 49%	4078
	11th to 20th	Night	2 1 3 2 2 3 2 5% 8% 71%	4077
	21st to end	Night	2 2 1 4 2 3 3 4 5% 9% 64%	4449
August	1st to 10th	Night	2 1 1 2 2 2 2 2 4 7% 75%	3074
	11th to 20th	Night	3 2 1 2 2 2 3 3 5% 9% 66%	3213
	21st to end	Night	3 2 1 4 3 3 4 4 7% 10% 59%	3597
September	1st to 10th	Night	7% 4 2 4 4 4 4 4 5% 8% 56%	2199
	11th to 20th	Night	8% 5% 1 5% 3 3 4 4 6% 7% 53%	2200
	21st to end	Night	6% 4 1 4 3 3 3 4 4 8% 59%	2200
October	1st to 10th	Night	4 2 1 3 2 3 3 3 4 6% 69%	2200
	11th to 20th	Night	9% 4 1 3 3 2 3 2 4 4 65%	2245
	21st to end	Night	11% 4 1 3 2 2 2 2 3 4 66%	2640
November	1st to 10th	Night	9% 4 1 4 2 2 4 3 4 5% 62%	2567
	11th to 20th	Night	6% 3 1 3 3 2 3 3 4 5% 66%	2730
	21st to end	Night	15% 4 2 4 3 3 3 2 4 6% 54%	2730
December	1st to 10th	Night	15% 5% 2 4 3 2 3 3 5% 5% 53%	4380
	11th to 20th	Night	11% 6% 2 5% 4 3 3 2 4 6% 54%	4946
	21st to end	Night	11% 5% 1 3 2 2 3 2 4 5% 62%	6633

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	27% 2 4 24% 30% 12%	5290
	11th to 20th	Day	24% 2 4 20% 33% 17%	5310
	21st to end	Day	39% 3 4 15% 25% 13%	6385
February	1st to 10th	Day	32% 4 4 18% 21% 21%	2100
	11th to 20th	Day	34% 3 6% 21% 20% 15%	2059
	21st to end	Day	30% 4 4 14% 27% 22%	1730
March	1st to 10th	Day	31% 4 6% 11% 21% 27%	4769
	11th to 20th	Day	28% 4 7% 11% 16% 33%	5090
	21st to end	Day	26% 7% 8% 20% 23% 16%	5585
April	1st to 10th	Day	30% 5% 10% 18% 20% 17%	2310
	11th to 20th	Day	33% 6% 8% 15% 22% 16%	2489
	21st to end	Day	31% 4 8% 19% 25% 13%	2616
May	1st to 10th	Day	35% 6% 9% 15% 22% 13%	5914
	11th to 20th	Day	35% 5% 6% 14% 26% 14%	5866
	21st to end	Day	34% 7% 11% 24% 16% 8%	6447
June	1st to 10th	Day	23% 6% 13% 28% 24% 5%	2600
	11th to 20th	Day	20% 6% 13% 28% 24% 8%	2600
	21st to end	Day	25% 7% 12% 27% 22% 8%	2600
July	1st to 10th	Day	40% 6% 13% 29% 12% 1	5836
	11th to 20th	Day	19% 5% 18% 34% 20% 4	5831
	21st to end	Day	20% 6% 14% 34% 24% 2	6240
August	1st to 10th	Day	12% 4 10% 34% 35% 5%	4067
	11th to 20th	Day	19% 4 15% 35% 23% 3	3882
	21st to end	Day	23% 8% 17% 26% 22% 4	4311
September	1st to 10th	Day	32% 6% 9% 21% 22% 11%	2200
	11th to 20th	Day	33% 7% 10% 20% 24% 7%	2200
	21st to end	Day	25% 5% 10% 22% 25% 15%	2200
October	1st to 10th	Day	11% 3 6% 15% 30% 35%	3590
	11th to 20th	Day	19% 3 5% 24% 25% 24%	3451
	21st to end	Day	19% 2 4 20% 39% 16%	3538
November	1st to 10th	Day	28% 4 6% 21% 27% 14%	1947
	11th to 20th	Day	22% 4 8% 21% 25% 20%	1890
	21st to end	Day	32% 4 5% 19% 25% 14%	1890
December	1st to 10th	Day	40% 3 5% 16% 24% 11%	3030
	11th to 20th	Day	34% 5% 7% 13% 27% 15%	3479
	21st to end	Day	25% 3 4 21% 30% 18%	4587
January	1st to 10th	Night	28% 3 5% 21% 30% 14%	7635
	11th to 20th	Night	18% 2 3 19% 33% 25%	7680
	21st to end	Night	41% 3 4 13% 23% 15%	7885
February	1st to 10th	Night	29% 4 6% 16% 24% 21%	2520
	11th to 20th	Night	31% 3 7% 19% 22% 18%	2458
	21st to end	Night	28% 4 6% 14% 25% 23%	2076
March	1st to 10th	Night	23% 5% 6% 17% 28% 22%	5315
	11th to 20th	Night	28% 5% 8% 12% 15% 31%	5090
	21st to end	Night	26% 5% 9% 18% 26% 16%	5588
April	1st to 10th	Night	25% 5% 11% 20% 18% 21%	2310
	11th to 20th	Night	33% 6% 9% 17% 20% 16%	2077
	21st to end	Night	23% 5% 13% 23% 25% 11%	2000
May	1st to 10th	Night	35% 5% 8% 21% 19% 12%	4089
	11th to 20th	Night	29% 7% 11% 17% 19% 18%	4054
	21st to end	Night	31% 7% 13% 26% 16% 8%	4452
June	1st to 10th	Night	17% 7% 16% 32% 23% 4	1800
	11th to 20th	Night	15% 5% 14% 34% 24% 9%	1800
	21st to end	Night	18% 5% 13% 30% 26% 8%	1800
July	1st to 10th	Night	33% 5% 14% 30% 15% 2	4035
	11th to 20th	Night	13% 5% 14% 38% 27% 4	4026
	21st to end	Night	17% 6% 16% 34% 25% 2	4320
August	1st to 10th	Night	12% 3 10% 31% 39% 5%	3064
	11th to 20th	Night	15% 4 17% 39% 21% 3	3243
	21st to end	Night	19% 7% 22% 27% 22% 4	3597
September	1st to 10th	Night	26% 7% 10% 24% 22% 11%	2199
	11th to 20th	Night	29% 5% 14% 24% 19% 8%	2200
	21st to end	Night	23% 4 10% 24% 23% 17%	2200
October	1st to 10th	Night	10% 3 7% 16% 26% 39%	3590
	11th to 20th	Night	18% 3 7% 18% 29% 25%	3653
	21st to end	Night	19% 2 4 23% 37% 15%	4252
November	1st to 10th	Night	25% 4 8% 19% 25% 20%	2567
	11th to 20th	Night	20% 4 7% 23% 25% 20%	2730
	21st to end	Night	33% 3 7% 17% 26% 15%	2730
December	1st to 10th	Night	35% 3 5% 20% 24% 13%	4380
	11th to 20th	Night	33% 3 6% 14% 27% 16%	4947
	21st to end	Night	27% 3 5% 21% 26% 19%	6633

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	11% 12% 37% 31% 8% 1	330
	11th to 20th	Day	14% 13% 28% 29% 14% 2	330
	21st to end	Day	13% 11% 38% 29% 8% 1	363
February	1st to 10th	Day	15% 13% 32% 28% 11% 1	330
	11th to 20th	Day	18% 12% 38% 27% 4 1	330
	21st to end	Day	16% 11% 39% 24% 8% 1	273
March	1st to 10th	Day	16% 16% 36% 25% 7%	330
	11th to 20th	Day	14% 13% 36% 26% 9% 1	330
	21st to end	Day	9% 12% 41% 27% 10% 1	363
April	1st to 10th	Day	14% 13% 41% 24% 8% 1	330
	11th to 20th	Day	14% 15% 46% 21% 4	527
	21st to end	Day	12% 12% 41% 27% 7% 1	636
May	1st to 10th	Day	11% 10% 34% 36% 8%	709
	11th to 20th	Day	13% 6% 32% 33% 13% 3	725
	21st to end	Day	11% 4 29% 38% 16% 2	789
June	1st to 10th	Day	6% 2 16% 44% 28% 5%	730
	11th to 20th	Day	3 1 16% 48% 27% 5%	730
	21st to end	Day	4 1 10% 36% 38% 12%	730
July	1st to 10th	Day	8% 1 6% 32% 40% 14%	721
	11th to 20th	Day	4 1 3 28% 48% 17%	718
	21st to end	Day	2 4 26% 46% 21%	794
August	1st to 10th	Day	2 3 25% 46% 24%	638
	11th to 20th	Day	6% 1 3 26% 43% 21%	517
	21st to end	Day	4 1 5% 26% 39% 25%	583
September	1st to 10th	Day	10% 5% 19% 39% 26%	330
	11th to 20th	Day	5% 2 4 21% 41% 27%	330
	21st to end	Day	11% 2 4 30% 34% 20%	330
October	1st to 10th	Day	2 2 10% 35% 35% 16%	330
	11th to 20th	Day	8% 2 25% 34% 25% 5%	330
	21st to end	Day	16% 7% 27% 24% 21% 6%	363
November	1st to 10th	Day	16% 5% 23% 30% 18% 7%	330
	11th to 20th	Day	12% 8% 23% 31% 19% 7%	330
	21st to end	Day	16% 14% 28% 25% 15% 2	330
December	1st to 10th	Day	16% 9% 29% 31% 14% 2	330
	11th to 20th	Day	10% 10% 35% 28% 14% 2	330
	21st to end	Day	19% 9% 35% 20% 16% 1	363
January	1st to 10th	Night	13% 6% 35% 37% 9%	1050
	11th to 20th	Night	13% 8% 33% 38% 9%	1050
	21st to end	Night	15% 8% 35% 36% 6%	972
February	1st to 10th	Night	13% 7% 38% 35% 6%	840
	11th to 20th	Night	16% 8% 41% 31% 4	824
	21st to end	Night	14% 11% 41% 30% 3	692
March	1st to 10th	Night	14% 8% 39% 35% 4	765
	11th to 20th	Night	14% 9% 40% 32% 5%	630
	21st to end	Night	9% 9% 45% 34% 3	693
April	1st to 10th	Night	10% 9% 48% 30% 4	630
	11th to 20th	Night	13% 10% 49% 26% 2	623
	21st to end	Night	10% 7% 50% 26% 7%	630
May	1st to 10th	Night	10% 7% 44% 35% 4	603
	11th to 20th	Night	10% 4 39% 37% 10% 1	591
	21st to end	Night	9% 3 37% 40% 10% 1	650
June	1st to 10th	Night	4 2 25% 53% 16% 1	600
	11th to 20th	Night	4 23% 61% 12%	600
	21st to end	Night	5% 1 18% 51% 26% 1	600
July	1st to 10th	Night	8% 1 8% 47% 35% 2	585
	11th to 20th	Night	3 7% 53% 36% 2	582
	21st to end	Night	2 4 52% 39% 2	657
August	1st to 10th	Night	1 5% 52% 41% 1	597
	11th to 20th	Night	6% 5% 42% 43% 3	594
	21st to end	Night	3 1 5% 40% 47% 4	660
September	1st to 10th	Night	8% 8% 45% 36% 3	600
	11th to 20th	Night	10% 1 6% 45% 37% 1	600
	21st to end	Night	8% 1 11% 49% 31% 1	600
October	1st to 10th	Night	5% 2 14% 50% 29% 1	600
	11th to 20th	Night	9% 4 22% 45% 19% 1	645
	21st to end	Night	13% 6% 24% 39% 17% 1	880
November	1st to 10th	Night	12% 3 24% 45% 16%	927
	11th to 20th	Night	6% 6% 29% 44% 13% 1	1050
	21st to end	Night	17% 6% 27% 38% 11% 1	1050
December	1st to 10th	Night	13% 5% 30% 43% 9%	1050
	11th to 20th	Night	14% 7% 32% 39% 8%	1050
	21st to end	Night	14% 6% 33% 37% 10%	1155

Page generated at 2026-04-29 01:21:48. Version ac07

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