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 2025-04-26.

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 6% 4 4 4 4 5% 7% 44%	113699
	Night	12% 8% 3 6% 4 4 4 3 4 6% 46%	115074
Transparency	Day	39% 4 11% 26% 17% 4	117346
	Night	39% 3 10% 25% 19% 4	119568
Seeing	Day	18% 11% 28% 26% 14% 3	16121
	Night	17% 12% 31% 27% 12% 1	26098

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% 4 7% 5% 4 3 3 4 6% 39%	4009
	11th to 20th	Day	9% 11% 4 8% 4 4 4 3 5% 6% 42%	4080
	21st to end	Day	17% 10% 3 7% 4 3 4 3 3 5% 40%	4488
February	1st to 10th	Day	14% 13% 4 9% 7% 4 4 2 4 6% 35%	1800
	11th to 20th	Day	14% 10% 3 7% 6% 5% 4 3 4 5% 39%	1809
	21st to end	Day	15% 10% 3 5% 3 4 5% 3 6% 7% 39%	1666
March	1st to 10th	Day	13% 9% 4 5% 3 3 3 3 4 5% 47%	4936
	11th to 20th	Day	10% 7% 2 5% 3 3 3 3 5% 7% 53%	4980
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	5479
April	1st to 10th	Day	14% 7% 3 5% 4 4 4 3 5% 7% 44%	2400
	11th to 20th	Day	17% 8% 3 6% 4 5% 4 3 5% 5% 40%	2374
	21st to end	Day	20% 9% 3 7% 5% 3 4 4 4 5% 36%	2340
May	1st to 10th	Day	16% 8% 2 8% 5% 5% 5% 4 6% 7% 34%	3957
	11th to 20th	Day	16% 7% 2 6% 4 5% 4 4 6% 7% 40%	4082
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	4824
June	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	2660
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 4 7% 8% 48%	2660
	21st to end	Day	5% 3 1 4 4 4 4 5% 7% 9% 52%	2660
July	1st to 10th	Day	12% 6% 2 6% 4 4 5% 5% 7% 9% 40%	4357
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 4 7% 9% 47%	4120
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 11% 37%	4465
August	1st to 10th	Day	4 4 2 6% 4 5% 4 3 6% 10% 53%	4104
	11th to 20th	Day	10% 5% 3 6% 4 4 5% 4 6% 8% 45%	3892
	21st to end	Day	7% 4 2 5% 3 3 4 4 5% 9% 53%	4180
September	1st to 10th	Day	11% 5% 1 5% 5% 3 4 4 5% 8% 50%	2280
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 5% 6% 47%	2275
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 7% 51%	1982
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 6% 56%	1900
	11th to 20th	Day	13% 6% 2 4 2 3 3 2 4 5% 54%	1900
	21st to end	Day	17% 8% 2 5% 3 3 3 3 4 5% 47%	2090
November	1st to 10th	Day	12% 6% 3 5% 4 3 2 3 4 4 55%	1948
	11th to 20th	Day	14% 7% 2 6% 3 4 4 3 5% 6% 48%	1800
	21st to end	Day	15% 9% 3 5% 4 2 3 3 5% 6% 45%	1602
December	1st to 10th	Day	10% 9% 3 6% 3 3 3 3 4 5% 51%	2610
	11th to 20th	Day	11% 10% 4 7% 5% 3 3 2 4 6% 46%	3008
	21st to end	Day	18% 21% 5% 8% 4 3 3 3 3 4 27%	3982
January	1st to 10th	Night	14% 10% 6% 13% 6% 4 4 3 4 4 33%	5952
	11th to 20th	Night	10% 8% 3 10% 6% 4 6% 3 3 5% 41%	5900
	21st to end	Night	16% 12% 4 8% 4 3 3 2 3 5% 41%	6490
February	1st to 10th	Night	16% 10% 3 7% 4 3 3 3 5% 6% 38%	2600
	11th to 20th	Night	15% 9% 4 7% 4 3 3 3 4 6% 42%	2477
	21st to end	Night	14% 9% 3 6% 4 4 4 3 5% 5% 42%	1980
March	1st to 10th	Night	15% 12% 3 7% 3 3 3 2 4 4 43%	4919
	11th to 20th	Night	11% 9% 2 5% 3 3 3 3 5% 6% 51%	4980
	21st to end	Night	12% 9% 3 7% 5% 5% 4 4 6% 7% 38%	5435
April	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 8% 50%	2000
	11th to 20th	Night	14% 8% 3 5% 3 3 3 3 4 8% 46%	1975
	21st to end	Night	18% 7% 3 6% 5% 4 3 3 5% 7% 39%	1956
May	1st to 10th	Night	15% 9% 2 5% 4 3 4 3 6% 8% 43%	2929
	11th to 20th	Night	13% 9% 3 6% 3 4 3 3 4 9% 43%	2806
	21st to end	Night	12% 6% 2 7% 4 4 4 4 5% 8% 41%	2743
June	1st to 10th	Night	8% 3 2 4 5% 3 5% 3 6% 9% 51%	1520
	11th to 20th	Night	5% 4 1 4 3 4 4 4 7% 10% 55%	1520
	21st to end	Night	3 2 1 3 2 3 3 4 6% 10% 62%	1520
July	1st to 10th	Night	11% 6% 3 5% 4 4 4 3 6% 7% 46%	2608
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 10% 58%	2850
	21st to end	Night	6% 5% 1 5% 4 4 5% 4 7% 10% 50%	3098
August	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 9% 62%	2836
	11th to 20th	Night	9% 4 2 5% 4 5% 4 4 5% 6% 50%	2942
	21st to end	Night	4 3 1 4 3 3 3 3 6% 9% 61%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 57%	1900
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 4 7% 53%	1900
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 7% 56%	2177
October	1st to 10th	Night	8% 3 1 3 3 3 3 3 4 8% 61%	2280
	11th to 20th	Night	14% 4 2 4 3 3 3 3 4 6% 54%	2280
	21st to end	Night	17% 7% 2 4 3 3 3 2 4 5% 50%	2508
November	1st to 10th	Night	12% 6% 2 4 3 2 3 2 3 6% 57%	2347
	11th to 20th	Night	13% 7% 2 5% 3 3 4 3 4 6% 50%	2600
	21st to end	Night	14% 9% 3 5% 3 3 3 3 4 5% 47%	2795
December	1st to 10th	Night	11% 9% 3 7% 4 3 3 2 4 6% 49%	4580
	11th to 20th	Night	15% 10% 3 6% 3 3 3 3 4 6% 44%	5199
	21st to end	Night	22% 17% 5% 9% 4 4 3 3 3 4 27%	6996

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	49% 2 9% 23% 16% 1	4010
	11th to 20th	Day	44% 3 6% 26% 16% 5%	4080
	21st to end	Day	47% 2 8% 17% 16% 10%	4488
February	1st to 10th	Day	53% 2 10% 22% 12% 1	1800
	11th to 20th	Day	48% 3 10% 20% 16% 4	1818
	21st to end	Day	44% 4 7% 24% 16% 5%	1666
March	1st to 10th	Day	41% 2 7% 23% 17% 11%	4936
	11th to 20th	Day	32% 3 8% 24% 24% 10%	4980
	21st to end	Day	48% 3 13% 26% 8% 1	5481
April	1st to 10th	Day	40% 4 12% 24% 19% 2	2400
	11th to 20th	Day	45% 3 12% 21% 15% 3	2374
	21st to end	Day	49% 4 11% 23% 11% 2	2340
May	1st to 10th	Day	49% 6% 11% 20% 13% 1	3992
	11th to 20th	Day	43% 5% 14% 24% 14% 1	4086
	21st to end	Day	41% 6% 10% 26% 17% 1	4837
June	1st to 10th	Day	35% 6% 15% 27% 15% 2	2660
	11th to 20th	Day	31% 5% 14% 32% 16% 2	2660
	21st to end	Day	26% 6% 16% 31% 17% 5%	2660
July	1st to 10th	Day	40% 5% 14% 25% 13% 2	4290
	11th to 20th	Day	31% 5% 14% 32% 17% 2	4051
	21st to end	Day	38% 6% 20% 26% 9% 1	4325
August	1st to 10th	Day	27% 5% 18% 29% 21% 1	4090
	11th to 20th	Day	37% 4 14% 29% 15% 1	3931
	21st to end	Day	29% 4 13% 37% 13% 4	4179
September	1st to 10th	Day	33% 5% 12% 27% 16% 7%	2280
	11th to 20th	Day	36% 5% 8% 25% 25% 1	2275
	21st to end	Day	31% 5% 9% 27% 24% 4	1982
October	1st to 10th	Day	18% 2 5% 20% 38% 16%	3160
	11th to 20th	Day	22% 2 5% 23% 35% 13%	3140
	21st to end	Day	30% 2 7% 32% 25% 4	3428
November	1st to 10th	Day	34% 3 10% 25% 23% 5%	1948
	11th to 20th	Day	38% 3 7% 24% 23% 6%	1800
	21st to end	Day	41% 3 8% 27% 19% 2	1602
December	1st to 10th	Day	37% 2 8% 36% 16% 1	2610
	11th to 20th	Day	42% 2 7% 27% 20% 1	3005
	21st to end	Day	63% 3 8% 16% 9% 1	3982
January	1st to 10th	Night	55% 3 8% 20% 13% 1	5952
	11th to 20th	Night	46% 3 7% 23% 18% 2	5900
	21st to end	Night	48% 2 7% 20% 18% 5%	6490
February	1st to 10th	Night	47% 3 11% 24% 13% 3	2600
	11th to 20th	Night	45% 3 10% 22% 17% 3	2478
	21st to end	Night	44% 3 7% 22% 18% 5%	1980
March	1st to 10th	Night	46% 2 6% 19% 21% 5%	4919
	11th to 20th	Night	36% 3 8% 23% 21% 9%	4980
	21st to end	Night	45% 4 13% 24% 13% 1	5435
April	1st to 10th	Night	33% 3 14% 25% 21% 4	2000
	11th to 20th	Night	37% 4 12% 25% 17% 5%	1975
	21st to end	Night	44% 5% 10% 26% 14% 1	1956
May	1st to 10th	Night	40% 5% 14% 24% 14% 3	2958
	11th to 20th	Night	41% 4 14% 27% 13% 2	2807
	21st to end	Night	40% 6% 12% 28% 13% 1	2751
June	1st to 10th	Night	30% 4 17% 27% 19% 3	1520
	11th to 20th	Night	22% 6% 14% 31% 22% 4	1520
	21st to end	Night	17% 5% 18% 35% 19% 7%	1520
July	1st to 10th	Night	37% 5% 12% 26% 17% 3	2566
	11th to 20th	Night	22% 5% 10% 41% 20% 2	2796
	21st to end	Night	29% 5% 19% 35% 11% 1	2999
August	1st to 10th	Night	21% 4 17% 33% 23% 1	2826
	11th to 20th	Night	34% 5% 15% 28% 16% 2	2969
	21st to end	Night	22% 4 15% 40% 16% 4	3476
September	1st to 10th	Night	30% 3 10% 28% 22% 7%	1900
	11th to 20th	Night	32% 3 9% 25% 27% 4	1900
	21st to end	Night	29% 4 9% 25% 30% 3	2177
October	1st to 10th	Night	14% 2 6% 19% 45% 13%	3800
	11th to 20th	Night	22% 2 6% 22% 29% 18%	3779
	21st to end	Night	31% 3 6% 31% 26% 4	4120
November	1st to 10th	Night	32% 3 10% 27% 23% 6%	2347
	11th to 20th	Night	36% 3 6% 24% 25% 6%	2600
	21st to end	Night	41% 3 8% 26% 20% 3	2795
December	1st to 10th	Night	39% 2 8% 34% 16% 1	4580
	11th to 20th	Night	43% 2 9% 25% 20% 2	5201
	21st to end	Night	64% 3 9% 17% 6% 2	6996

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% 14% 1	300
	11th to 20th	Day	30% 22% 39% 8% 1	300
	21st to end	Day	22% 24% 38% 13% 2	330
February	1st to 10th	Day	28% 22% 37% 11% 2	300
	11th to 20th	Day	32% 23% 31% 11% 2	300
	21st to end	Day	30% 22% 36% 10% 2	249
March	1st to 10th	Day	23% 20% 41% 13% 2 1	300
	11th to 20th	Day	33% 16% 35% 15% 1 1	300
	21st to end	Day	32% 21% 36% 10% 1	335
April	1st to 10th	Day	26% 16% 35% 18% 4 1	500
	11th to 20th	Day	29% 17% 32% 16% 5%	492
	21st to end	Day	26% 17% 33% 19% 5%	480
May	1st to 10th	Day	23% 14% 38% 19% 6%	577
	11th to 20th	Day	20% 10% 30% 24% 13% 2	675
	21st to end	Day	13% 9% 34% 24% 14% 4	734
June	1st to 10th	Day	10% 4 25% 32% 23% 6%	680
	11th to 20th	Day	8% 2 26% 41% 21% 3	680
	21st to end	Day	8% 6% 32% 36% 14% 4	680
July	1st to 10th	Day	15% 3 14% 32% 28% 8%	671
	11th to 20th	Day	6% 3 22% 39% 26% 5%	668
	21st to end	Day	10% 2 14% 36% 30% 8%	739
August	1st to 10th	Day	9% 3 18% 41% 25% 5%	676
	11th to 20th	Day	13% 4 17% 35% 25% 6%	581
	21st to end	Day	11% 2 10% 36% 31% 10%	539
September	1st to 10th	Day	12% 3 13% 36% 29% 6%	490
	11th to 20th	Day	9% 2 21% 34% 23% 10%	485
	21st to end	Day	10% 8% 25% 33% 19% 5%	300
October	1st to 10th	Day	15% 9% 28% 32% 14% 2	300
	11th to 20th	Day	18% 15% 38% 22% 7%	300
	21st to end	Day	30% 17% 31% 19% 3	330
November	1st to 10th	Day	18% 13% 39% 21% 8% 1	300
	11th to 20th	Day	23% 17% 35% 16% 7% 1	300
	21st to end	Day	29% 14% 33% 18% 5%	300
December	1st to 10th	Day	21% 21% 35% 15% 5% 2	300
	11th to 20th	Day	26% 18% 34% 17% 4 1	300
	21st to end	Day	28% 22% 35% 12% 2	330
January	1st to 10th	Night	20% 20% 37% 20% 4	1000
	11th to 20th	Night	20% 24% 36% 16% 4	1000
	21st to end	Night	21% 21% 37% 20% 1	1100
February	1st to 10th	Night	25% 19% 38% 16% 2	1000
	11th to 20th	Night	23% 20% 38% 17% 2	923
	21st to end	Night	24% 22% 37% 15% 2	660
March	1st to 10th	Night	25% 13% 39% 21% 2	600
	11th to 20th	Night	25% 14% 36% 22% 4	600
	21st to end	Night	25% 14% 39% 18% 4	660
April	1st to 10th	Night	17% 17% 40% 23% 4	600
	11th to 20th	Night	20% 15% 35% 24% 5%	593
	21st to end	Night	22% 12% 38% 20% 8%	587
May	1st to 10th	Night	16% 7% 37% 33% 7% 1	570
	11th to 20th	Night	18% 6% 30% 32% 14% 1	561
	21st to end	Night	14% 8% 27% 29% 18% 3	617
June	1st to 10th	Night	11% 3 20% 38% 26% 2	570
	11th to 20th	Night	5% 1 27% 41% 24% 2	570
	21st to end	Night	3 2 29% 45% 20% 1	570
July	1st to 10th	Night	12% 1 14% 36% 31% 5%	555
	11th to 20th	Night	4 1 17% 45% 31% 3	552
	21st to end	Night	7% 1 13% 44% 32% 3	624
August	1st to 10th	Night	7% 1 15% 44% 31% 2	567
	11th to 20th	Night	9% 2 14% 44% 29% 2	564
	21st to end	Night	6% 1 13% 39% 37% 4	627
September	1st to 10th	Night	13% 2 13% 38% 31% 3	570
	11th to 20th	Night	14% 2 15% 36% 29% 4	570
	21st to end	Night	10% 5% 25% 36% 21% 3	657
October	1st to 10th	Night	10% 6% 26% 32% 22% 4	760
	11th to 20th	Night	16% 11% 36% 28% 9%	760
	21st to end	Night	24% 11% 33% 26% 6%	836
November	1st to 10th	Night	16% 13% 37% 25% 9%	780
	11th to 20th	Night	20% 13% 35% 24% 7% 1	800
	21st to end	Night	23% 16% 34% 21% 6%	995
December	1st to 10th	Night	21% 17% 36% 22% 4 1	1000
	11th to 20th	Night	24% 15% 36% 22% 4	1000
	21st to end	Night	25% 18% 36% 20% 2	1100

Page generated at 2025-04-23 08:25:01. Version ac07

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