Rusty's Backyard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rusty's Backyard clear sky chart from 2005-11-06 to 2026-08-30.

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 6% 4 4 4 4 5% 7% 44%	143631
	Night	12% 8% 3 6% 4 4 4 3 5% 7% 46%	138872
Transparency	Day	39% 4 11% 26% 17% 3	147237
	Night	38% 3 10% 26% 18% 4	143344
Seeing	Day	18% 11% 28% 26% 14% 3	18099
	Night	17% 12% 31% 27% 12% 1	27814

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	13% 10% 3 7% 5% 4 3 3 4 6% 41%	5208
	11th to 20th	Day	7% 10% 3 8% 4 4 4 3 5% 6% 45%	5310
	21st to end	Day	16% 10% 3 7% 4 3 3 2 3 5% 42%	5812
February	1st to 10th	Day	13% 12% 3 9% 7% 4 4 2 4 6% 36%	1890
	11th to 20th	Day	14% 10% 3 7% 6% 5% 4 3 4 5% 39%	1902
	21st to end	Day	14% 10% 3 5% 4 4 4 3 6% 7% 39%	1747
March	1st to 10th	Day	13% 9% 4 6% 3 3 3 3 4 5% 46%	5046
	11th to 20th	Day	10% 7% 2 5% 3 3 3 3 5% 7% 53%	5090
	21st to end	Day	12% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	5600
April	1st to 10th	Day	14% 7% 2 5% 4 5% 4 3 5% 7% 44%	2520
	11th to 20th	Day	16% 8% 3 7% 4 5% 4 3 5% 5% 40%	2494
	21st to end	Day	19% 8% 3 7% 5% 3 4 4 4 6% 35%	2520
May	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 4 6% 7% 38%	7380
	11th to 20th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	7627
	21st to end	Day	22% 8% 3 7% 4 4 4 4 5% 7% 33%	9047
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	2940
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 4 7% 9% 47%	2940
	21st to end	Day	6% 4 2 4 4 4 4 5% 7% 9% 51%	2940
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 45%	8127
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	7680
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 7% 11% 43%	8337
August	1st to 10th	Day	4 3 2 6% 4 5% 4 3 6% 10% 54%	4364
	11th to 20th	Day	10% 5% 3 6% 4 4 5% 4 6% 8% 47%	4142
	21st to end	Day	7% 4 2 5% 4 3 4 4 5% 9% 53%	4420
September	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 5% 8% 50%	2400
	11th to 20th	Day	10% 6% 2 6% 4 5% 4 4 5% 6% 47%	2395
	21st to end	Day	8% 5% 2 6% 4 3 4 3 5% 7% 51%	2086
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 6% 56%	2000
	11th to 20th	Day	13% 6% 2 4 3 3 4 2 4 5% 54%	2000
	21st to end	Day	18% 8% 2 5% 3 3 3 3 4 5% 46%	2200
November	1st to 10th	Day	12% 6% 3 5% 4 3 2 3 4 4 55%	2047
	11th to 20th	Day	13% 7% 2 6% 3 4 4 3 5% 6% 48%	1890
	21st to end	Day	16% 9% 3 5% 4 2 3 3 5% 6% 44%	1682
December	1st to 10th	Day	10% 10% 3 6% 3 3 3 3 4 5% 51%	2690
	11th to 20th	Day	11% 10% 4 7% 4 3 3 2 4 6% 46%	3088
	21st to end	Day	18% 21% 5% 8% 4 3 3 3 3 4 27%	4070
January	1st to 10th	Night	12% 10% 5% 11% 6% 4 4 3 3 4 38%	7711
	11th to 20th	Night	8% 7% 3 9% 5% 4 5% 3 3 5% 47%	7680
	21st to end	Night	15% 10% 3 8% 5% 5% 3 2 3 5% 40%	8402
February	1st to 10th	Night	15% 10% 3 7% 4 3 3 3 5% 6% 40%	2730
	11th to 20th	Night	15% 9% 3 7% 4 3 3 3 4 6% 42%	2604
	21st to end	Night	13% 9% 3 6% 4 5% 4 3 5% 5% 43%	2076
March	1st to 10th	Night	15% 12% 3 7% 3 3 3 2 4 4 43%	5029
	11th to 20th	Night	10% 9% 2 5% 3 3 3 3 5% 6% 51%	5090
	21st to end	Night	12% 8% 3 7% 5% 5% 4 4 6% 7% 39%	5556
April	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 8% 49%	2100
	11th to 20th	Night	14% 8% 3 5% 3 3 3 3 4 8% 46%	2075
	21st to end	Night	18% 7% 3 7% 4 4 3 3 5% 7% 40%	2100
May	1st to 10th	Night	15% 9% 2 6% 4 3 4 3 5% 8% 41%	5476
	11th to 20th	Night	12% 7% 2 5% 3 4 4 3 6% 9% 44%	5248
	21st to end	Night	21% 10% 3 8% 4 4 4 3 5% 6% 33%	5139
June	1st to 10th	Night	9% 4 2 5% 5% 3 5% 2 6% 9% 50%	1680
	11th to 20th	Night	6% 4 1 4 3 3 4 5% 7% 10% 53%	1680
	21st to end	Night	4 3 1 3 3 3 4 4 6% 10% 62%	1680
July	1st to 10th	Night	9% 4 2 5% 4 4 4 4 6% 9% 49%	4842
	11th to 20th	Night	4 3 2 5% 3 4 5% 4 6% 10% 53%	5307
	21st to end	Night	5% 4 2 5% 4 4 4 4 7% 11% 51%	5770
August	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 9% 62%	3016
	11th to 20th	Night	9% 4 2 5% 4 5% 4 4 5% 6% 51%	3131
	21st to end	Night	4 3 1 4 3 3 3 3 5% 9% 61%	3676
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 56%	2000
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 4 7% 53%	2000
	21st to end	Night	9% 5% 2 5% 3 3 3 3 4 7% 56%	2292
October	1st to 10th	Night	8% 3 1 3 3 3 3 3 4 7% 61%	2400
	11th to 20th	Night	14% 4 2 4 3 3 3 3 4 7% 54%	2400
	21st to end	Night	18% 7% 2 4 3 3 3 2 4 4 50%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 2 3 2 4 6% 57%	2468
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 6% 51%	2730
	21st to end	Night	15% 9% 3 5% 3 4 3 3 4 5% 46%	2935
December	1st to 10th	Night	11% 9% 3 7% 4 3 3 3 4 6% 48%	4720
	11th to 20th	Night	15% 9% 3 6% 3 3 3 3 4 6% 45%	5339
	21st to end	Night	22% 17% 5% 8% 4 4 3 3 3 4 27%	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	46% 2 9% 27% 15% 2	5209
	11th to 20th	Day	40% 2 6% 29% 17% 5%	5310
	21st to end	Day	46% 2 8% 19% 16% 9%	5812
February	1st to 10th	Day	51% 2 10% 22% 12% 1	1890
	11th to 20th	Day	47% 3 10% 21% 16% 4	1911
	21st to end	Day	44% 3 7% 23% 17% 5%	1747
March	1st to 10th	Day	41% 2 7% 22% 17% 10%	5046
	11th to 20th	Day	32% 2 8% 24% 24% 10%	5090
	21st to end	Day	48% 3 13% 27% 9% 1	5602
April	1st to 10th	Day	41% 3 12% 23% 18% 2	2520
	11th to 20th	Day	46% 3 12% 20% 16% 3	2494
	21st to end	Day	49% 4 11% 23% 11% 2	2520
May	1st to 10th	Day	46% 4 11% 23% 15% 1	7415
	11th to 20th	Day	43% 4 12% 23% 16% 1	7631
	21st to end	Day	51% 5% 9% 22% 13% 1	9061
June	1st to 10th	Day	36% 6% 15% 26% 16% 2	2940
	11th to 20th	Day	32% 5% 13% 32% 16% 2	2940
	21st to end	Day	27% 5% 16% 31% 16% 4	2940
July	1st to 10th	Day	36% 4 15% 25% 19% 2	8060
	11th to 20th	Day	35% 6% 16% 27% 14% 3	7611
	21st to end	Day	35% 4 19% 33% 9% 1	8155
August	1st to 10th	Day	27% 5% 18% 30% 20% 1	4350
	11th to 20th	Day	36% 4 14% 30% 14% 1	4181
	21st to end	Day	29% 4 13% 36% 14% 4	4419
September	1st to 10th	Day	32% 5% 12% 28% 16% 6%	2400
	11th to 20th	Day	36% 5% 8% 26% 24% 1	2395
	21st to end	Day	32% 4 8% 27% 25% 4	2086
October	1st to 10th	Day	19% 2 5% 20% 38% 16%	3260
	11th to 20th	Day	23% 2 4 24% 34% 13%	3240
	21st to end	Day	31% 2 7% 32% 25% 4	3538
November	1st to 10th	Day	33% 3 9% 27% 23% 5%	2047
	11th to 20th	Day	38% 2 7% 25% 22% 6%	1890
	21st to end	Day	42% 3 8% 27% 19% 2	1682
December	1st to 10th	Day	37% 2 8% 36% 15% 1	2690
	11th to 20th	Day	42% 2 7% 27% 20% 1	3085
	21st to end	Day	62% 3 8% 17% 9% 1	4070
January	1st to 10th	Night	50% 2 9% 26% 12% 1	7712
	11th to 20th	Night	41% 3 5% 25% 23% 3	7680
	21st to end	Night	49% 2 8% 20% 17% 4	8402
February	1st to 10th	Night	46% 3 11% 24% 14% 2	2730
	11th to 20th	Night	44% 3 10% 23% 16% 3	2605
	21st to end	Night	43% 3 7% 22% 19% 5%	2076
March	1st to 10th	Night	46% 2 7% 19% 21% 5%	5029
	11th to 20th	Night	36% 3 8% 24% 21% 9%	5090
	21st to end	Night	45% 4 13% 25% 13% 1	5556
April	1st to 10th	Night	34% 3 14% 25% 21% 4	2100
	11th to 20th	Night	38% 4 12% 24% 18% 5%	2075
	21st to end	Night	44% 4 10% 27% 14% 1	2100
May	1st to 10th	Night	43% 4 13% 26% 13% 2	5505
	11th to 20th	Night	39% 4 12% 25% 17% 4	5249
	21st to end	Night	53% 5% 9% 21% 11% 2	5149
June	1st to 10th	Night	32% 4 16% 27% 19% 3	1680
	11th to 20th	Night	25% 6% 14% 30% 21% 4	1680
	21st to end	Night	19% 4 18% 34% 19% 6%	1680
July	1st to 10th	Night	32% 4 15% 25% 22% 2	4802
	11th to 20th	Night	27% 6% 15% 33% 18% 2	5256
	21st to end	Night	28% 4 19% 38% 10%	5641
August	1st to 10th	Night	21% 4 17% 34% 23% 1	3006
	11th to 20th	Night	33% 5% 16% 29% 15% 2	3158
	21st to end	Night	22% 4 15% 39% 17% 4	3676
September	1st to 10th	Night	31% 3 10% 28% 21% 6%	2000
	11th to 20th	Night	32% 3 10% 26% 25% 4	2000
	21st to end	Night	29% 3 9% 25% 30% 4	2292
October	1st to 10th	Night	14% 2 6% 19% 45% 13%	3920
	11th to 20th	Night	23% 2 6% 23% 29% 18%	3899
	21st to end	Night	32% 2 6% 31% 25% 4	4252
November	1st to 10th	Night	31% 3 9% 28% 23% 6%	2468
	11th to 20th	Night	35% 3 7% 25% 24% 6%	2730
	21st to end	Night	42% 3 7% 26% 19% 3	2935
December	1st to 10th	Night	40% 2 8% 33% 16% 1	4720
	11th to 20th	Night	43% 2 9% 25% 20% 2	5341
	21st to end	Night	63% 3 9% 17% 6% 2	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	22% 21% 42% 13% 1	330
	11th to 20th	Day	29% 24% 38% 8% 1	330
	21st to end	Day	23% 25% 38% 12% 2	363
February	1st to 10th	Day	26% 23% 37% 12% 2	330
	11th to 20th	Day	32% 22% 33% 11% 2	330
	21st to end	Day	30% 21% 36% 10% 2	273
March	1st to 10th	Day	25% 20% 40% 12% 2 1	330
	11th to 20th	Day	33% 15% 36% 14% 1 1	330
	21st to end	Day	30% 22% 38% 10% 1	368
April	1st to 10th	Day	27% 16% 35% 18% 4 1	540
	11th to 20th	Day	29% 19% 32% 15% 5%	532
	21st to end	Day	27% 16% 33% 19% 5%	540
May	1st to 10th	Day	22% 15% 39% 18% 6%	669
	11th to 20th	Day	21% 12% 30% 23% 12% 2	775
	21st to end	Day	17% 9% 33% 24% 14% 4	844
June	1st to 10th	Day	12% 5% 26% 30% 22% 6%	780
	11th to 20th	Day	9% 4 26% 39% 19% 3	780
	21st to end	Day	9% 5% 32% 36% 13% 4	780
July	1st to 10th	Day	13% 3 14% 34% 27% 9%	771
	11th to 20th	Day	6% 3 22% 39% 25% 5%	768
	21st to end	Day	10% 2 14% 38% 28% 7%	849
August	1st to 10th	Day	9% 3 19% 41% 24% 5%	776
	11th to 20th	Day	11% 3 17% 35% 27% 6%	671
	21st to end	Day	11% 2 12% 36% 30% 9%	619
September	1st to 10th	Day	12% 4 14% 36% 28% 6%	530
	11th to 20th	Day	10% 2 20% 34% 25% 10%	525
	21st to end	Day	11% 8% 25% 33% 20% 5%	330
October	1st to 10th	Day	14% 9% 28% 34% 14% 2	330
	11th to 20th	Day	18% 15% 36% 22% 8% 1	330
	21st to end	Day	32% 17% 31% 18% 2	363
November	1st to 10th	Day	16% 15% 40% 21% 7% 1	330
	11th to 20th	Day	22% 18% 36% 16% 6% 1	330
	21st to end	Day	30% 16% 32% 16% 5%	330
December	1st to 10th	Day	22% 21% 36% 15% 5% 2	330
	11th to 20th	Day	25% 18% 36% 16% 3 1	330
	21st to end	Day	26% 21% 38% 12% 2	363
January	1st to 10th	Night	19% 20% 37% 20% 4	1050
	11th to 20th	Night	20% 25% 35% 15% 4	1050
	21st to end	Night	21% 21% 37% 20% 1	1155
February	1st to 10th	Night	24% 18% 39% 16% 2	1050
	11th to 20th	Night	23% 19% 39% 17% 2	970
	21st to end	Night	23% 22% 37% 16% 2	692
March	1st to 10th	Night	25% 13% 39% 20% 2	630
	11th to 20th	Night	26% 14% 36% 21% 4	630
	21st to end	Night	24% 14% 40% 18% 4	693
April	1st to 10th	Night	16% 16% 41% 23% 3	630
	11th to 20th	Night	21% 15% 35% 24% 5%	623
	21st to end	Night	23% 12% 38% 20% 7%	630
May	1st to 10th	Night	16% 7% 37% 32% 7%	630
	11th to 20th	Night	17% 7% 30% 31% 14%	621
	21st to end	Night	17% 8% 26% 29% 17% 3	683
June	1st to 10th	Night	12% 3 21% 37% 25% 2	630
	11th to 20th	Night	7% 2 27% 40% 22% 2	630
	21st to end	Night	5% 2 29% 45% 19% 1	630
July	1st to 10th	Night	11% 2 14% 37% 31% 5%	615
	11th to 20th	Night	4 1 17% 45% 30% 3	612
	21st to end	Night	7% 1 12% 45% 30% 4	690
August	1st to 10th	Night	7% 1 18% 44% 29% 2	627
	11th to 20th	Night	8% 2 15% 44% 29% 2	624
	21st to end	Night	6% 1 13% 41% 35% 4	687
September	1st to 10th	Night	13% 2 14% 39% 30% 3	600
	11th to 20th	Night	13% 2 14% 36% 30% 4	600
	21st to end	Night	10% 5% 24% 35% 22% 3	692
October	1st to 10th	Night	10% 6% 26% 33% 22% 3	800
	11th to 20th	Night	16% 11% 36% 28% 10%	800
	21st to end	Night	25% 11% 33% 26% 6%	880
November	1st to 10th	Night	16% 12% 37% 25% 9%	820
	11th to 20th	Night	19% 13% 36% 24% 7%	840
	21st to end	Night	24% 16% 34% 21% 5%	1045
December	1st to 10th	Night	21% 17% 37% 21% 4	1050
	11th to 20th	Night	23% 15% 36% 22% 4	1050
	21st to end	Night	25% 18% 36% 19% 2	1155

Page generated at 2026-08-27 03:07:11. Version ac07

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