StarHome Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarHome Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 8% 3 8% 5% 4 4 4 5% 7% 39%	138884
	Night	12% 9% 4 9% 5% 4 4 4 5% 6% 38%	134103
Transparency	Day	44% 5% 13% 26% 12% 1	142435
	Night	46% 5% 12% 24% 12% 1	138631
Seeing	Day	16% 9% 29% 27% 14% 4	18401
	Night	16% 8% 32% 32% 11% 1	28267

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	9% 14% 6% 18% 7% 6% 5% 3 4 5% 23%	4691
	11th to 20th	Day	6% 11% 7% 13% 7% 7% 8% 7% 7% 5% 22%	4710
	21st to end	Day	4 13% 5% 11% 4 5% 6% 4 6% 7% 34%	5752
February	1st to 10th	Day	9% 12% 5% 10% 6% 5% 5% 4 5% 7% 31%	1890
	11th to 20th	Day	7% 12% 4 9% 6% 4 4 4 5% 7% 37%	1844
	21st to end	Day	10% 11% 6% 9% 6% 5% 4 3 5% 7% 34%	1626
March	1st to 10th	Day	12% 9% 3 7% 4 3 3 3 5% 8% 44%	4898
	11th to 20th	Day	9% 10% 3 7% 5% 4 4 3 5% 8% 40%	5090
	21st to end	Day	20% 10% 3 10% 5% 4 5% 4 5% 6% 28%	5674
April	1st to 10th	Day	18% 10% 3 8% 4 4 5% 3 5% 7% 34%	2520
	11th to 20th	Day	19% 12% 4 8% 5% 4 4 3 5% 5% 32%	2500
	21st to end	Day	21% 10% 3 6% 5% 4 4 3 4 6% 32%	2730
May	1st to 10th	Day	11% 5% 2 5% 4 5% 4 3 4 6% 50%	8031
	11th to 20th	Day	16% 7% 2 6% 3 3 4 3 5% 6% 45%	8222
	21st to end	Day	15% 7% 2 7% 4 4 4 3 6% 7% 39%	9047
June	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 4 6% 8% 45%	2940
	11th to 20th	Day	14% 7% 2 7% 5% 4 4 4 7% 8% 37%	2940
	21st to end	Day	12% 4 2 5% 5% 5% 5% 4 7% 10% 41%	3089
July	1st to 10th	Day	9% 4 2 5% 4 5% 5% 4 7% 10% 45%	6500
	11th to 20th	Day	9% 4 1 4 3 3 3 3 5% 8% 56%	6431
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 9% 49%	6924
August	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 7% 9% 43%	4242
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 8% 43%	4171
	21st to end	Day	12% 5% 2 7% 4 4 4 3 5% 7% 46%	4457
September	1st to 10th	Day	11% 5% 2 6% 5% 4 4 4 6% 8% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 4 6% 8% 44%	2390
	21st to end	Day	15% 5% 2 5% 4 4 4 3 6% 7% 45%	2090
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 4 5% 7% 43%	2000
	11th to 20th	Day	15% 11% 3 7% 4 4 5% 3 5% 6% 37%	2000
	21st to end	Day	17% 10% 4 8% 5% 4 4 2 5% 5% 36%	2150
November	1st to 10th	Day	11% 13% 3 9% 6% 3 4 4 5% 6% 35%	1727
	11th to 20th	Day	12% 14% 5% 11% 6% 5% 4 3 5% 6% 29%	1680
	21st to end	Day	14% 11% 5% 9% 4 5% 5% 4 5% 6% 32%	1680
December	1st to 10th	Day	8% 11% 4 16% 12% 7% 6% 4 5% 5% 21%	2690
	11th to 20th	Day	12% 16% 6% 10% 6% 5% 4 3 5% 5% 28%	3088
	21st to end	Day	20% 19% 8% 19% 5% 4 3 2 3 3 14%	4070
January	1st to 10th	Night	9% 13% 6% 14% 8% 4 4 3 5% 6% 30%	8232
	11th to 20th	Night	6% 12% 5% 13% 8% 7% 9% 7% 6% 5% 22%	8280
	21st to end	Night	6% 14% 5% 9% 6% 5% 5% 3 6% 6% 36%	8444
February	1st to 10th	Night	7% 12% 5% 10% 7% 6% 4 3 6% 7% 35%	2730
	11th to 20th	Night	7% 10% 4 8% 5% 6% 4 3 6% 8% 39%	2663
	21st to end	Night	11% 10% 4 8% 6% 6% 5% 3 5% 7% 36%	2178
March	1st to 10th	Night	10% 7% 2 6% 5% 4 4 3 5% 7% 47%	5221
	11th to 20th	Night	11% 8% 3 7% 5% 5% 5% 4 7% 9% 37%	5090
	21st to end	Night	24% 11% 4 10% 4 3 3 2 4 5% 29%	5496
April	1st to 10th	Night	18% 10% 3 8% 5% 3 5% 3 5% 7% 36%	2100
	11th to 20th	Night	20% 10% 3 7% 4 3 3 3 4 7% 37%	2065
	21st to end	Night	19% 10% 3 6% 4 4 4 3 5% 6% 35%	1890
May	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 49%	4850
	11th to 20th	Night	17% 6% 2 4 3 3 3 3 4 6% 49%	4670
	21st to end	Night	15% 8% 2 7% 5% 5% 4 4 5% 7% 39%	5139
June	1st to 10th	Night	12% 5% 2 5% 5% 4 5% 4 6% 8% 46%	1680
	11th to 20th	Night	10% 6% 2 6% 5% 4 5% 3 6% 8% 45%	1680
	21st to end	Night	9% 4 1 6% 4 3 5% 4 7% 7% 49%	1511
July	1st to 10th	Night	9% 3 1 5% 3 4 4 4 7% 9% 52%	3670
	11th to 20th	Night	7% 3 2 5% 3 3 4 3 6% 8% 56%	3657
	21st to end	Night	8% 4 2 5% 4 4 4 4 6% 7% 51%	3947
August	1st to 10th	Night	13% 5% 2 6% 4 3 3 3 5% 7% 48%	2915
	11th to 20th	Night	13% 4 2 4 3 4 4 3 6% 7% 49%	2888
	21st to end	Night	11% 6% 2 6% 4 5% 4 3 5% 8% 47%	3444
September	1st to 10th	Night	9% 5% 2 5% 4 4 5% 3 5% 8% 52%	2000
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 5% 8% 54%	2008
	21st to end	Night	14% 4 2 4 3 3 4 3 4 6% 52%	2289
October	1st to 10th	Night	13% 5% 3 5% 3 3 4 3 4 7% 49%	2400
	11th to 20th	Night	17% 9% 3 5% 5% 4 4 3 4 6% 41%	2400
	21st to end	Night	18% 10% 3 7% 5% 3 3 3 5% 7% 36%	2690
November	1st to 10th	Night	14% 9% 3 7% 4 4 4 4 5% 6% 39%	2787
	11th to 20th	Night	13% 12% 4 11% 6% 4 4 3 4 5% 34%	2940
	21st to end	Night	13% 11% 4 10% 5% 4 4 4 5% 7% 34%	2940
December	1st to 10th	Night	7% 11% 4 15% 8% 7% 5% 4 5% 6% 28%	4720
	11th to 20th	Night	11% 16% 6% 14% 6% 5% 5% 4 5% 4 24%	5339
	21st to end	Night	21% 18% 8% 15% 5% 4 3 3 3 4 18%	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	65% 3 9% 16% 7%	4692
	11th to 20th	Day	60% 4 10% 16% 10%	4710
	21st to end	Day	49% 4 14% 23% 10%	5752
February	1st to 10th	Day	51% 5% 14% 21% 7% 1	1890
	11th to 20th	Day	48% 5% 12% 26% 8%	1853
	21st to end	Day	50% 5% 13% 21% 10% 1	1626
March	1st to 10th	Day	39% 5% 18% 24% 11% 3	4898
	11th to 20th	Day	42% 5% 15% 29% 9% 1	5090
	21st to end	Day	57% 5% 10% 19% 8% 1	5676
April	1st to 10th	Day	50% 5% 11% 20% 13% 1	2520
	11th to 20th	Day	54% 5% 10% 18% 11% 1	2500
	21st to end	Day	53% 6% 11% 20% 10%	2730
May	1st to 10th	Day	35% 4 10% 29% 19% 2	8070
	11th to 20th	Day	40% 5% 9% 24% 18% 4	8226
	21st to end	Day	43% 5% 10% 29% 12% 1	9061
June	1st to 10th	Day	35% 8% 14% 30% 11% 1	2940
	11th to 20th	Day	43% 7% 15% 22% 12% 1	2940
	21st to end	Day	37% 8% 17% 28% 9% 1	3089
July	1st to 10th	Day	34% 5% 18% 34% 10%	6431
	11th to 20th	Day	28% 4 15% 38% 14% 1	6356
	21st to end	Day	33% 4 15% 31% 16%	6728
August	1st to 10th	Day	38% 5% 14% 29% 13% 1	4227
	11th to 20th	Day	37% 5% 13% 34% 9%	4210
	21st to end	Day	38% 4 17% 28% 12% 1	4456
September	1st to 10th	Day	35% 7% 13% 33% 10%	2400
	11th to 20th	Day	37% 7% 18% 31% 7%	2390
	21st to end	Day	38% 7% 12% 32% 11%	2090
October	1st to 10th	Day	29% 4 10% 37% 20% 1	3260
	11th to 20th	Day	41% 4 10% 27% 16% 2	3240
	21st to end	Day	46% 3 15% 27% 9%	3452
November	1st to 10th	Day	50% 5% 13% 19% 12% 1	1727
	11th to 20th	Day	56% 5% 13% 18% 7%	1680
	21st to end	Day	52% 7% 15% 19% 6% 2	1680
December	1st to 10th	Day	65% 5% 13% 13% 4 1	2690
	11th to 20th	Day	59% 4 11% 19% 7%	3085
	21st to end	Day	76% 5% 7% 8% 4	4070
January	1st to 10th	Night	57% 3 15% 16% 8%	8233
	11th to 20th	Night	61% 5% 9% 15% 10%	8280
	21st to end	Night	50% 4 11% 25% 10%	8444
February	1st to 10th	Night	49% 6% 13% 22% 10% 1	2730
	11th to 20th	Night	43% 6% 14% 26% 10% 1	2664
	21st to end	Night	48% 5% 13% 22% 11% 1	2178
March	1st to 10th	Night	37% 5% 14% 24% 17% 3	5221
	11th to 20th	Night	40% 6% 16% 25% 13%	5090
	21st to end	Night	59% 3 10% 16% 11%	5496
April	1st to 10th	Night	49% 5% 8% 22% 16% 1	2100
	11th to 20th	Night	48% 4 9% 22% 14% 2	2065
	21st to end	Night	49% 6% 10% 20% 14% 2	1890
May	1st to 10th	Night	35% 5% 9% 31% 20% 2	4876
	11th to 20th	Night	37% 4 10% 21% 21% 7%	4671
	21st to end	Night	44% 5% 9% 27% 13% 1	5149
June	1st to 10th	Night	33% 8% 14% 28% 15% 1	1680
	11th to 20th	Night	36% 7% 14% 25% 17% 3	1680
	21st to end	Night	30% 8% 16% 29% 14% 3	1511
July	1st to 10th	Night	30% 4 20% 32% 13% 1	3632
	11th to 20th	Night	27% 4 14% 37% 16% 1	3612
	21st to end	Night	33% 4 16% 30% 16% 2	3832
August	1st to 10th	Night	37% 4 12% 29% 14% 4	2906
	11th to 20th	Night	34% 5% 14% 33% 13% 1	2915
	21st to end	Night	37% 4 16% 29% 11% 2	3444
September	1st to 10th	Night	30% 6% 15% 33% 15% 1	2000
	11th to 20th	Night	28% 6% 19% 31% 15%	2008
	21st to end	Night	34% 5% 12% 34% 13% 1	2289
October	1st to 10th	Night	25% 3 7% 36% 26% 4	3920
	11th to 20th	Night	37% 4 11% 24% 22% 2	3899
	21st to end	Night	46% 3 11% 27% 12%	4338
November	1st to 10th	Night	44% 6% 13% 24% 12% 1	2787
	11th to 20th	Night	53% 4 12% 19% 10% 1	2940
	21st to end	Night	50% 5% 16% 21% 6% 1	2940
December	1st to 10th	Night	57% 5% 15% 18% 3	4720
	11th to 20th	Night	63% 4 10% 16% 5% 1	5341
	21st to end	Night	71% 3 8% 12% 4	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	13% 9% 33% 32% 10% 3	330
	11th to 20th	Day	14% 10% 39% 30% 7% 1	330
	21st to end	Day	15% 7% 40% 27% 10% 1	363
February	1st to 10th	Day	14% 12% 45% 22% 5% 1	330
	11th to 20th	Day	13% 13% 41% 24% 7% 2	330
	21st to end	Day	15% 16% 38% 25% 5%	273
March	1st to 10th	Day	18% 11% 41% 23% 6% 1	330
	11th to 20th	Day	19% 10% 36% 26% 8% 2	330
	21st to end	Day	24% 14% 30% 22% 8% 3	403
April	1st to 10th	Day	22% 16% 32% 24% 5% 1	540
	11th to 20th	Day	27% 15% 25% 23% 9% 1	538
	21st to end	Day	27% 9% 31% 21% 9% 3	750
May	1st to 10th	Day	20% 11% 29% 27% 11% 2	753
	11th to 20th	Day	19% 12% 27% 25% 13% 3	775
	21st to end	Day	18% 10% 25% 27% 17% 4	844
June	1st to 10th	Day	11% 7% 27% 33% 18% 5%	780
	11th to 20th	Day	15% 9% 33% 31% 10% 2	780
	21st to end	Day	15% 6% 30% 28% 16% 5%	780
July	1st to 10th	Day	9% 4 19% 32% 25% 12%	771
	11th to 20th	Day	8% 5% 24% 33% 20% 9%	744
	21st to end	Day	6% 4 24% 34% 23% 8%	794
August	1st to 10th	Day	10% 5% 20% 32% 25% 7%	726
	11th to 20th	Day	13% 5% 19% 30% 26% 7%	715
	21st to end	Day	11% 7% 21% 33% 21% 6%	676
September	1st to 10th	Day	14% 6% 26% 30% 18% 6%	530
	11th to 20th	Day	13% 4 27% 33% 20% 4	520
	21st to end	Day	18% 8% 26% 22% 18% 8%	330
October	1st to 10th	Day	18% 8% 31% 22% 15% 6%	330
	11th to 20th	Day	18% 16% 32% 24% 8% 1	330
	21st to end	Day	22% 13% 38% 17% 7% 2	363
November	1st to 10th	Day	21% 13% 32% 24% 8% 1	330
	11th to 20th	Day	17% 17% 41% 19% 5% 1	330
	21st to end	Day	22% 13% 37% 23% 5%	330
December	1st to 10th	Day	16% 9% 39% 26% 10% 1	330
	11th to 20th	Day	20% 15% 38% 23% 4	330
	21st to end	Day	29% 10% 36% 21% 3 1	363
January	1st to 10th	Night	13% 10% 35% 33% 8% 1	1050
	11th to 20th	Night	16% 9% 38% 29% 8% 1	1050
	21st to end	Night	17% 9% 37% 29% 7% 1	1155
February	1st to 10th	Night	12% 11% 39% 30% 8% 1	1050
	11th to 20th	Night	11% 9% 43% 28% 8% 1	1029
	21st to end	Night	17% 10% 36% 29% 8%	794
March	1st to 10th	Night	15% 10% 36% 30% 8% 1	724
	11th to 20th	Night	19% 8% 39% 27% 6%	630
	21st to end	Night	23% 8% 33% 25% 9% 2	693
April	1st to 10th	Night	21% 12% 34% 27% 5%	630
	11th to 20th	Night	24% 9% 31% 27% 7%	623
	21st to end	Night	24% 8% 30% 29% 8% 1	630
May	1st to 10th	Night	17% 10% 34% 32% 7%	630
	11th to 20th	Night	16% 9% 30% 33% 11% 1	621
	21st to end	Night	14% 7% 26% 39% 14% 1	683
June	1st to 10th	Night	12% 4 28% 43% 13%	630
	11th to 20th	Night	11% 5% 32% 43% 9%	630
	21st to end	Night	11% 4 23% 44% 15% 2	630
July	1st to 10th	Night	6% 2 20% 44% 26% 2	615
	11th to 20th	Night	8% 3 21% 44% 22% 2	597
	21st to end	Night	5% 2 21% 47% 23% 2	657
August	1st to 10th	Night	10% 3 16% 49% 21% 1	597
	11th to 20th	Night	14% 3 18% 37% 26% 3	594
	21st to end	Night	10% 4 22% 38% 23% 2	660
September	1st to 10th	Night	10% 6% 24% 39% 18% 3	600
	11th to 20th	Night	10% 3 27% 38% 20% 2	600
	21st to end	Night	14% 6% 30% 32% 16% 1	689
October	1st to 10th	Night	17% 7% 29% 32% 13% 1	800
	11th to 20th	Night	21% 9% 34% 29% 7% 1	800
	21st to end	Night	25% 7% 30% 29% 8%	880
November	1st to 10th	Night	19% 9% 33% 32% 7% 1	941
	11th to 20th	Night	19% 12% 34% 27% 8% 1	1050
	21st to end	Night	18% 13% 37% 25% 7%	1050
December	1st to 10th	Night	16% 9% 37% 30% 8% 1	1050
	11th to 20th	Night	20% 11% 37% 25% 7% 1	1050
	21st to end	Night	26% 10% 31% 27% 5%	1155

Page generated at 2026-07-13 02:14:50. Version ac07

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