The Backyard Country Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Backyard Country Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 7% 3 7% 4 4 4 3 5% 7% 44%	143529
	Night	11% 7% 3 7% 4 4 4 3 5% 7% 47%	138691
Transparency	Day	39% 3 12% 26% 17% 2	147140
	Night	38% 3 11% 26% 19% 3	143160
Seeing	Day	15% 8% 29% 29% 15% 4	18207
	Night	14% 8% 33% 33% 11% 1	27972

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	12% 10% 3 9% 5% 3 3 3 5% 7% 39%	5289
	11th to 20th	Day	6% 8% 3 8% 7% 6% 6% 4 5% 6% 41%	5310
	21st to end	Day	11% 12% 5% 11% 5% 5% 4 2 4 6% 34%	5812
February	1st to 10th	Day	13% 10% 4 8% 5% 6% 4 3 5% 7% 35%	1890
	11th to 20th	Day	12% 10% 4 7% 5% 4 4 4 4 6% 41%	1844
	21st to end	Day	11% 10% 3 6% 4 4 3 4 4 7% 46%	1752
March	1st to 10th	Day	15% 8% 2 6% 4 3 3 2 4 4 49%	5046
	11th to 20th	Day	10% 7% 2 5% 3 3 4 3 4 6% 51%	5090
	21st to end	Day	15% 11% 5% 8% 5% 4 5% 4 5% 7% 32%	5583
April	1st to 10th	Day	15% 8% 3 6% 4 3 4 3 5% 6% 43%	2440
	11th to 20th	Day	15% 9% 2 7% 4 4 4 3 5% 7% 41%	2494
	21st to end	Day	22% 7% 3 7% 5% 4 4 3 4 7% 35%	2689
May	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 5% 7% 44%	7617
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 6% 47%	7627
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 6% 39%	8883
June	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 4 7% 8% 47%	2940
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 8% 48%	2940
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 7% 9% 47%	2940
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 6% 10% 45%	8006
	11th to 20th	Day	7% 4 2 6% 3 4 4 4 5% 9% 53%	7680
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 7% 9% 43%	8337
August	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 7% 10% 49%	4364
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 45%	4224
	21st to end	Day	6% 5% 2 5% 4 5% 4 4 6% 8% 52%	4421
September	1st to 10th	Day	12% 4 2 4 4 4 3 3 5% 7% 53%	2400
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 5% 8% 48%	2236
	21st to end	Day	10% 4 1 5% 3 4 3 3 4 6% 56%	2177
October	1st to 10th	Day	13% 5% 1 6% 4 2 3 2 5% 6% 51%	2000
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 4 5% 55%	2000
	21st to end	Day	15% 7% 3 5% 3 3 5% 3 5% 6% 46%	2200
November	1st to 10th	Day	9% 6% 2 4 4 4 4 3 4 6% 54%	2029
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 7% 49%	1741
	21st to end	Day	16% 10% 2 6% 3 3 3 3 4 7% 43%	1680
December	1st to 10th	Day	6% 5% 2 7% 4 4 4 3 5% 6% 53%	2690
	11th to 20th	Day	11% 8% 3 9% 4 4 3 3 4 6% 45%	3088
	21st to end	Day	18% 15% 7% 13% 4 4 3 3 5% 5% 24%	4070
January	1st to 10th	Night	11% 9% 4 15% 6% 4 4 3 5% 5% 34%	7634
	11th to 20th	Night	6% 7% 4 8% 4 5% 5% 4 5% 6% 46%	7680
	21st to end	Night	12% 12% 4 11% 4 4 4 2 3 4 38%	8402
February	1st to 10th	Night	11% 12% 4 8% 4 4 4 3 4 6% 39%	2730
	11th to 20th	Night	10% 10% 3 7% 5% 4 4 3 6% 6% 41%	2663
	21st to end	Night	11% 8% 2 6% 3 4 4 2 5% 5% 49%	2070
March	1st to 10th	Night	16% 7% 3 5% 3 3 3 3 3 5% 50%	5029
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 5% 6% 56%	5090
	21st to end	Night	15% 8% 4 8% 4 4 4 4 6% 6% 36%	5588
April	1st to 10th	Night	12% 7% 2 5% 3 4 3 3 6% 7% 48%	2179
	11th to 20th	Night	13% 7% 3 6% 4 3 4 3 5% 6% 47%	2075
	21st to end	Night	16% 8% 3 6% 5% 4 4 3 4 6% 41%	1927
May	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 47%	5266
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 5% 7% 51%	5265
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 44%	5298
June	1st to 10th	Night	6% 4 1 4 4 4 4 3 7% 8% 55%	1680
	11th to 20th	Night	5% 3 2 4 4 4 4 4 8% 10% 51%	1680
	21st to end	Night	6% 3 2 4 3 3 4 3 5% 10% 59%	1680
July	1st to 10th	Night	8% 4 2 5% 3 3 4 3 7% 9% 51%	4963
	11th to 20th	Night	4 3 2 5% 3 4 4 3 5% 8% 57%	5307
	21st to end	Night	9% 5% 3 6% 4 4 5% 4 6% 11% 42%	5770
August	1st to 10th	Night	4 2 1 3 3 3 4 3 7% 9% 61%	3016
	11th to 20th	Night	11% 6% 1 5% 3 3 3 3 4 6% 54%	2924
	21st to end	Night	5% 3 2 6% 3 4 4 3 4 7% 59%	3480
September	1st to 10th	Night	10% 3 1 5% 3 3 3 3 4 6% 57%	2000
	11th to 20th	Night	9% 5% 1 4 3 4 4 5% 5% 7% 53%	2160
	21st to end	Night	8% 4 2 4 3 3 4 3 4 7% 59%	2201
October	1st to 10th	Night	12% 5% 2 5% 3 2 3 2 4 6% 55%	2400
	11th to 20th	Night	13% 5% 2 5% 3 4 3 2 4 5% 55%	2400
	21st to end	Night	16% 7% 2 5% 3 3 3 2 3 4 51%	2640
November	1st to 10th	Night	9% 4 2 4 2 2 3 3 4 7% 60%	2486
	11th to 20th	Night	11% 6% 2 4 3 4 4 3 4 6% 53%	2877
	21st to end	Night	14% 9% 2 5% 4 3 3 3 4 6% 49%	2940
December	1st to 10th	Night	7% 7% 2 5% 4 4 4 3 5% 6% 54%	4720
	11th to 20th	Night	11% 8% 3 7% 4 3 3 3 5% 7% 45%	5339
	21st to end	Night	20% 15% 6% 10% 4 4 3 2 3 4 29%	7132

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	46% 3 12% 25% 13% 1	5290
	11th to 20th	Day	46% 2 10% 22% 15% 5%	5310
	21st to end	Day	54% 3 11% 19% 13% 1	5812
February	1st to 10th	Day	49% 5% 12% 22% 9% 2	1890
	11th to 20th	Day	44% 5% 12% 23% 13% 2	1853
	21st to end	Day	39% 3 11% 27% 18% 3	1752
March	1st to 10th	Day	40% 3 9% 23% 22% 4	5046
	11th to 20th	Day	34% 4 7% 24% 27% 4	5090
	21st to end	Day	51% 5% 17% 20% 5% 1	5585
April	1st to 10th	Day	42% 4 12% 25% 15% 2	2440
	11th to 20th	Day	44% 4 12% 23% 15% 1	2494
	21st to end	Day	49% 5% 11% 24% 10% 1	2689
May	1st to 10th	Day	41% 4 9% 28% 17% 1	7655
	11th to 20th	Day	38% 3 10% 28% 19% 2	7631
	21st to end	Day	47% 3 9% 25% 15% 1	8897
June	1st to 10th	Day	33% 7% 14% 29% 14% 2	2940
	11th to 20th	Day	33% 6% 13% 28% 19% 1	2940
	21st to end	Day	33% 6% 16% 28% 15% 3	2940
July	1st to 10th	Day	35% 2 15% 32% 14% 2	7941
	11th to 20th	Day	31% 2 13% 29% 20% 5%	7611
	21st to end	Day	38% 3 16% 32% 9% 2	8155
August	1st to 10th	Day	29% 5% 16% 30% 19% 1	4350
	11th to 20th	Day	36% 4 15% 28% 16% 1	4263
	21st to end	Day	31% 4 16% 30% 14% 5%	4420
September	1st to 10th	Day	31% 5% 12% 27% 20% 5%	2400
	11th to 20th	Day	35% 5% 11% 27% 20% 2	2236
	21st to end	Day	30% 4 8% 31% 24% 3	2177
October	1st to 10th	Day	21% 2 7% 21% 46% 3	3260
	11th to 20th	Day	22% 2 5% 27% 35% 9%	3240
	21st to end	Day	35% 2 10% 27% 23% 3	3538
November	1st to 10th	Day	32% 3 12% 29% 20% 4	2029
	11th to 20th	Day	36% 3 11% 30% 19% 2	1741
	21st to end	Day	42% 4 12% 25% 16%	1680
December	1st to 10th	Day	33% 3 18% 30% 16% 1	2690
	11th to 20th	Day	42% 3 14% 30% 11% 1	3085
	21st to end	Day	64% 2 13% 14% 6% 1	4070
January	1st to 10th	Night	53% 4 11% 21% 10% 1	7635
	11th to 20th	Night	40% 2 9% 27% 19% 4	7680
	21st to end	Night	51% 2 11% 17% 15% 3	8402
February	1st to 10th	Night	47% 4 12% 22% 13% 3	2730
	11th to 20th	Night	43% 4 12% 22% 16% 3	2664
	21st to end	Night	37% 4 9% 24% 20% 6%	2070
March	1st to 10th	Night	39% 3 8% 23% 23% 3	5029
	11th to 20th	Night	31% 3 6% 28% 25% 8%	5090
	21st to end	Night	48% 5% 14% 24% 8% 2	5588
April	1st to 10th	Night	36% 4 12% 26% 19% 4	2179
	11th to 20th	Night	39% 4 12% 26% 16% 4	2075
	21st to end	Night	44% 5% 10% 24% 17% 1	1927
May	1st to 10th	Night	38% 4 9% 27% 20% 2	5293
	11th to 20th	Night	35% 3 10% 25% 23% 5%	5266
	21st to end	Night	43% 4 9% 25% 18% 1	5309
June	1st to 10th	Night	25% 7% 18% 31% 16% 3	1680
	11th to 20th	Night	25% 7% 15% 30% 19% 4	1680
	21st to end	Night	22% 4 17% 33% 19% 4	1680
July	1st to 10th	Night	30% 2 16% 28% 21% 3	4921
	11th to 20th	Night	27% 3 15% 29% 21% 5%	5256
	21st to end	Night	37% 2 18% 33% 10% 1	5641
August	1st to 10th	Night	20% 4 20% 29% 26% 2	3006
	11th to 20th	Night	34% 3 12% 30% 21% 1	2951
	21st to end	Night	27% 3 14% 32% 18% 5%	3480
September	1st to 10th	Night	29% 4 10% 27% 23% 7%	2000
	11th to 20th	Night	29% 5% 10% 30% 23% 3	2160
	21st to end	Night	27% 3 8% 30% 28% 4	2201
October	1st to 10th	Night	20% 2 6% 19% 49% 4	3920
	11th to 20th	Night	23% 2 6% 27% 32% 11%	3899
	21st to end	Night	37% 2 6% 29% 24% 2	4252
November	1st to 10th	Night	25% 3 14% 30% 22% 5%	2486
	11th to 20th	Night	33% 3 9% 31% 20% 4	2877
	21st to end	Night	39% 3 11% 28% 18% 2	2940
December	1st to 10th	Night	32% 3 13% 37% 15% 1	4720
	11th to 20th	Night	40% 3 14% 29% 14% 1	5341
	21st to end	Night	61% 2 9% 20% 6% 1	7132

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	22% 12% 36% 25% 6%	330
	11th to 20th	Day	21% 15% 42% 20% 2	330
	21st to end	Day	16% 12% 40% 28% 4	363
February	1st to 10th	Day	17% 17% 44% 19% 2 1	330
	11th to 20th	Day	19% 16% 42% 17% 5% 1	330
	21st to end	Day	15% 18% 43% 19% 4	273
March	1st to 10th	Day	20% 14% 44% 19% 2	330
	11th to 20th	Day	28% 10% 36% 21% 4 1	330
	21st to end	Day	32% 15% 34% 17% 2 1	363
April	1st to 10th	Day	23% 13% 37% 20% 7%	460
	11th to 20th	Day	20% 16% 37% 20% 6% 1	532
	21st to end	Day	25% 13% 35% 19% 7% 1	709
May	1st to 10th	Day	18% 10% 34% 27% 9% 1	753
	11th to 20th	Day	20% 7% 31% 26% 13% 3	775
	21st to end	Day	19% 8% 28% 28% 14% 3	844
June	1st to 10th	Day	8% 4 22% 35% 24% 7%	780
	11th to 20th	Day	8% 4 28% 36% 19% 5%	780
	21st to end	Day	10% 5% 33% 35% 14% 2	780
July	1st to 10th	Day	12% 2 15% 30% 30% 10%	771
	11th to 20th	Day	9% 3 17% 37% 26% 9%	768
	21st to end	Day	8% 2 16% 40% 28% 7%	849
August	1st to 10th	Day	9% 2 19% 39% 25% 6%	776
	11th to 20th	Day	9% 3 16% 39% 27% 7%	736
	21st to end	Day	10% 1 16% 32% 31% 10%	653
September	1st to 10th	Day	14% 3 17% 32% 28% 5%	530
	11th to 20th	Day	9% 2 18% 41% 22% 8%	366
	21st to end	Day	14% 5% 27% 29% 19% 6%	330
October	1st to 10th	Day	16% 6% 27% 29% 14% 8%	330
	11th to 20th	Day	11% 19% 39% 22% 8% 1	330
	21st to end	Day	20% 13% 37% 22% 7% 2	363
November	1st to 10th	Day	16% 9% 41% 25% 7% 2	330
	11th to 20th	Day	16% 16% 36% 24% 7% 1	330
	21st to end	Day	24% 11% 35% 23% 7% 1	330
December	1st to 10th	Day	8% 16% 42% 26% 6% 2	330
	11th to 20th	Day	19% 15% 35% 24% 6% 1	330
	21st to end	Day	22% 18% 39% 20% 2	363
January	1st to 10th	Night	13% 12% 40% 29% 6%	1050
	11th to 20th	Night	14% 13% 42% 25% 6%	1050
	21st to end	Night	15% 10% 42% 29% 3	1155
February	1st to 10th	Night	17% 11% 41% 26% 4	1050
	11th to 20th	Night	16% 14% 38% 28% 4	1029
	21st to end	Night	14% 14% 46% 19% 6% 1	686
March	1st to 10th	Night	16% 12% 41% 26% 5%	630
	11th to 20th	Night	20% 10% 37% 29% 4 1	630
	21st to end	Night	26% 10% 39% 21% 4	693
April	1st to 10th	Night	17% 11% 43% 26% 4	630
	11th to 20th	Night	18% 11% 38% 28% 5%	623
	21st to end	Night	19% 10% 38% 25% 7% 1	630
May	1st to 10th	Night	17% 7% 36% 32% 7% 1	630
	11th to 20th	Night	18% 6% 30% 35% 10% 1	621
	21st to end	Night	13% 6% 29% 37% 13% 1	683
June	1st to 10th	Night	7% 3 24% 45% 19% 1	630
	11th to 20th	Night	8% 3 26% 43% 19% 1	630
	21st to end	Night	7% 2 30% 46% 14%	630
July	1st to 10th	Night	9% 2 13% 42% 32% 2	615
	11th to 20th	Night	6% 1 16% 46% 31% 1	612
	21st to end	Night	8% 2 13% 47% 29% 1	690
August	1st to 10th	Night	6% 1 18% 47% 27%	627
	11th to 20th	Night	7% 2 13% 45% 30% 3	606
	21st to end	Night	9% 1 12% 43% 31% 3	660
September	1st to 10th	Night	11% 2 18% 41% 25% 3	600
	11th to 20th	Night	11% 2 19% 43% 24% 2	600
	21st to end	Night	10% 3 24% 42% 20% 1	601
October	1st to 10th	Night	13% 5% 29% 34% 17% 2	800
	11th to 20th	Night	16% 7% 38% 31% 8%	800
	21st to end	Night	19% 9% 36% 29% 7%	880
November	1st to 10th	Night	10% 10% 36% 34% 10%	846
	11th to 20th	Night	14% 13% 37% 28% 7%	1050
	21st to end	Night	20% 9% 35% 29% 7%	1050
December	1st to 10th	Night	15% 12% 38% 30% 6% 1	1050
	11th to 20th	Night	19% 9% 38% 28% 5%	1050
	21st to end	Night	21% 12% 35% 27% 3	1155

Page generated at 2026-08-12 02:37:18. Version ac07

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