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 2025-04-11.

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%	113606
	Night	11% 7% 3 7% 4 3 4 3 5% 6% 47%	114876
Transparency	Day	39% 4 12% 26% 17% 2	117258
	Night	37% 4 11% 26% 19% 3	119366
Seeing	Day	15% 8% 29% 29% 15% 4	16209
	Night	14% 8% 32% 33% 11% 1	26247

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% 9% 3 10% 5% 3 4 4 6% 8% 35%	4077
	11th to 20th	Day	8% 9% 3 9% 6% 5% 6% 3 4 5% 41%	4080
	21st to end	Day	12% 12% 6% 12% 5% 5% 4 2 4 6% 32%	4488
February	1st to 10th	Day	13% 11% 4 8% 5% 5% 5% 3 5% 7% 33%	1800
	11th to 20th	Day	12% 11% 4 7% 5% 4 4 4 4 6% 41%	1754
	21st to end	Day	11% 10% 3 6% 4 4 3 3 4 7% 45%	1671
March	1st to 10th	Day	15% 8% 2 6% 4 3 3 2 4 5% 49%	4936
	11th to 20th	Day	10% 7% 2 5% 3 3 4 3 4 6% 51%	4980
	21st to end	Day	15% 11% 5% 8% 5% 4 5% 4 5% 7% 31%	5462
April	1st to 10th	Day	15% 8% 3 6% 4 3 4 3 5% 6% 44%	2324
	11th to 20th	Day	15% 9% 2 7% 4 4 4 3 5% 7% 41%	2254
	21st to end	Day	22% 7% 2 6% 5% 3 4 3 4 7% 36%	2433
May	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 5% 7% 39%	4082
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 3 5% 7% 40%	4082
	21st to end	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	4744
June	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 4 7% 8% 48%	2660
	11th to 20th	Day	8% 5% 2 5% 5% 4 5% 4 7% 8% 48%	2660
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 7% 9% 47%	2660
July	1st to 10th	Day	11% 6% 3 7% 4 4 4 5% 6% 9% 41%	4292
	11th to 20th	Day	7% 4 1 5% 4 4 4 4 5% 9% 54%	4120
	21st to end	Day	5% 3 2 5% 4 4 5% 4 8% 9% 52%	4465
August	1st to 10th	Day	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4104
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 9% 45%	4041
	21st to end	Day	6% 5% 2 5% 4 5% 4 4 6% 8% 52%	4286
September	1st to 10th	Day	12% 3 2 4 3 3 3 3 5% 7% 54%	2280
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 5% 7% 49%	2124
	21st to end	Day	10% 4 1 5% 3 3 3 4 4 6% 56%	2068
October	1st to 10th	Day	14% 5% 2 6% 4 2 3 2 5% 7% 50%	1900
	11th to 20th	Day	13% 6% 2 5% 2 3 2 3 4 5% 55%	1900
	21st to end	Day	14% 7% 3 5% 3 3 5% 3 6% 6% 46%	2090
November	1st to 10th	Day	10% 6% 2 4 4 4 4 3 4 7% 52%	1931
	11th to 20th	Day	13% 7% 2 5% 4 4 3 2 5% 7% 49%	1658
	21st to end	Day	16% 10% 2 6% 3 2 3 3 4 8% 44%	1600
December	1st to 10th	Day	5% 5% 2 6% 4 4 4 3 5% 6% 54%	2610
	11th to 20th	Day	11% 8% 3 9% 4 4 4 3 4 6% 45%	3008
	21st to end	Day	18% 15% 7% 13% 4 4 3 3 5% 5% 23%	3982
January	1st to 10th	Night	11% 9% 4 17% 6% 4 5% 4 5% 5% 30%	5888
	11th to 20th	Night	8% 8% 4 10% 4 5% 6% 4 6% 5% 41%	5900
	21st to end	Night	13% 11% 4 11% 4 3 4 2 3 4 38%	6490
February	1st to 10th	Night	12% 12% 4 8% 4 4 4 3 4 6% 38%	2600
	11th to 20th	Night	11% 10% 3 7% 5% 4 4 3 5% 6% 42%	2533
	21st to end	Night	11% 8% 2 6% 3 4 4 2 5% 5% 48%	1974
March	1st to 10th	Night	15% 7% 3 5% 3 3 3 3 3 5% 50%	4919
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 5% 6% 56%	4980
	21st to end	Night	15% 9% 4 9% 5% 4 4 4 6% 6% 35%	5467
April	1st to 10th	Night	11% 7% 2 5% 3 4 3 3 6% 7% 49%	2075
	11th to 20th	Night	13% 8% 3 7% 4 3 4 3 4 6% 46%	1881
	21st to end	Night	16% 7% 3 6% 4 4 3 3 4 7% 43%	1743
May	1st to 10th	Night	16% 6% 2 5% 5% 4 4 3 5% 7% 43%	2823
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 5% 7% 45%	2823
	21st to end	Night	13% 5% 2 5% 3 4 3 3 4 6% 52%	2819
June	1st to 10th	Night	6% 4 1 3 4 4 4 3 7% 8% 55%	1520
	11th to 20th	Night	5% 3 2 4 4 4 4 4 8% 10% 52%	1520
	21st to end	Night	5% 2 2 4 3 3 4 3 5% 10% 59%	1520
July	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 6% 8% 47%	2673
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 8% 63%	2850
	21st to end	Night	6% 3 2 5% 4 3 5% 4 6% 12% 50%	3098
August	1st to 10th	Night	3 2 1 3 3 3 4 3 7% 9% 62%	2836
	11th to 20th	Night	12% 7% 1 5% 3 3 3 3 4 6% 54%	2798
	21st to end	Night	5% 4 2 6% 3 4 4 3 4 7% 59%	3373
September	1st to 10th	Night	9% 3 1 5% 3 4 3 3 4 6% 58%	1900
	11th to 20th	Night	9% 5% 1 4 3 4 4 4 4 7% 55%	2052
	21st to end	Night	8% 4 1 4 3 3 4 3 4 7% 59%	2091
October	1st to 10th	Night	12% 5% 2 5% 3 2 3 2 4 6% 56%	2280
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 5% 56%	2280
	21st to end	Night	16% 7% 2 5% 3 3 3 2 3 4 52%	2508
November	1st to 10th	Night	9% 4 1 4 2 3 3 3 4 7% 60%	2365
	11th to 20th	Night	12% 6% 2 4 3 4 4 3 4 6% 53%	2740
	21st to end	Night	14% 9% 2 4 4 3 3 3 4 6% 49%	2800
December	1st to 10th	Night	7% 6% 2 4 4 3 4 3 5% 6% 55%	4580
	11th to 20th	Night	12% 8% 3 7% 4 3 3 3 5% 7% 45%	5199
	21st to end	Night	20% 15% 6% 10% 4 3 3 2 3 4 28%	6978

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	48% 4 12% 24% 11% 1	4078
	11th to 20th	Day	48% 3 10% 19% 15% 5%	4080
	21st to end	Day	56% 4 10% 17% 13% 1	4488
February	1st to 10th	Day	51% 5% 11% 21% 10% 2	1800
	11th to 20th	Day	45% 5% 12% 23% 13% 2	1763
	21st to end	Day	40% 3 10% 26% 17% 3	1671
March	1st to 10th	Day	40% 3 9% 23% 22% 4	4936
	11th to 20th	Day	34% 4 7% 24% 27% 4	4980
	21st to end	Day	52% 5% 17% 20% 5% 1	5464
April	1st to 10th	Day	41% 5% 12% 25% 15% 2	2324
	11th to 20th	Day	44% 4 12% 24% 15% 1	2254
	21st to end	Day	48% 5% 11% 24% 11% 1	2433
May	1st to 10th	Day	43% 7% 11% 23% 14% 1	4120
	11th to 20th	Day	44% 6% 11% 26% 13%	4086
	21st to end	Day	37% 5% 11% 29% 16% 2	4757
June	1st to 10th	Day	32% 8% 15% 30% 14% 2	2660
	11th to 20th	Day	32% 7% 14% 28% 19% 1	2660
	21st to end	Day	32% 6% 16% 28% 16% 3	2660
July	1st to 10th	Day	40% 4 15% 26% 12% 3	4227
	11th to 20th	Day	29% 4 10% 35% 20% 3	4051
	21st to end	Day	27% 4 18% 34% 14% 3	4325
August	1st to 10th	Day	29% 5% 16% 30% 20% 1	4090
	11th to 20th	Day	37% 4 15% 27% 16% 1	4080
	21st to end	Day	31% 4 17% 30% 14% 5%	4285
September	1st to 10th	Day	30% 5% 12% 27% 21% 5%	2280
	11th to 20th	Day	34% 5% 11% 26% 21% 2	2124
	21st to end	Day	30% 4 9% 32% 23% 3	2068
October	1st to 10th	Day	21% 3 7% 21% 46% 3	3160
	11th to 20th	Day	21% 2 5% 28% 35% 9%	3140
	21st to end	Day	34% 3 10% 27% 23% 3	3428
November	1st to 10th	Day	33% 4 12% 28% 19% 4	1931
	11th to 20th	Day	36% 3 11% 28% 20% 3	1658
	21st to end	Day	41% 4 12% 26% 17%	1600
December	1st to 10th	Day	32% 3 18% 31% 16% 1	2610
	11th to 20th	Day	42% 3 15% 30% 10% 1	3005
	21st to end	Day	64% 3 13% 14% 6% 1	3982
January	1st to 10th	Night	55% 5% 9% 20% 10% 1	5888
	11th to 20th	Night	45% 3 10% 23% 16% 4	5900
	21st to end	Night	51% 3 11% 16% 16% 4	6490
February	1st to 10th	Night	47% 4 12% 20% 13% 3	2600
	11th to 20th	Night	43% 4 12% 21% 16% 3	2534
	21st to end	Night	37% 4 9% 24% 20% 6%	1974
March	1st to 10th	Night	38% 3 8% 24% 24% 3	4919
	11th to 20th	Night	30% 3 6% 28% 26% 8%	4980
	21st to end	Night	48% 5% 14% 23% 7% 2	5467
April	1st to 10th	Night	34% 4 12% 26% 20% 4	2075
	11th to 20th	Night	39% 4 11% 27% 15% 3	1881
	21st to end	Night	42% 6% 9% 25% 17% 1	1743
May	1st to 10th	Night	39% 7% 13% 21% 19% 2	2850
	11th to 20th	Night	39% 6% 10% 27% 18% 1	2824
	21st to end	Night	34% 6% 12% 26% 20% 2	2827
June	1st to 10th	Night	24% 7% 18% 31% 17% 3	1520
	11th to 20th	Night	24% 8% 15% 31% 19% 3	1520
	21st to end	Night	20% 5% 17% 34% 20% 4	1520
July	1st to 10th	Night	36% 3 13% 24% 19% 5%	2629
	11th to 20th	Night	21% 3 15% 36% 23% 2	2796
	21st to end	Night	27% 3 19% 34% 16% 1	2999
August	1st to 10th	Night	19% 4 20% 29% 26% 2	2826
	11th to 20th	Night	34% 3 12% 29% 21% 1	2825
	21st to end	Night	27% 3 14% 32% 18% 5%	3373
September	1st to 10th	Night	28% 4 10% 27% 23% 8%	1900
	11th to 20th	Night	28% 5% 9% 30% 24% 3	2052
	21st to end	Night	27% 4 8% 31% 27% 4	2091
October	1st to 10th	Night	20% 2 6% 19% 49% 4	3800
	11th to 20th	Night	22% 2 5% 27% 33% 11%	3779
	21st to end	Night	36% 2 7% 29% 24% 2	4120
November	1st to 10th	Night	25% 4 14% 28% 23% 6%	2365
	11th to 20th	Night	34% 3 9% 30% 21% 4	2740
	21st to end	Night	38% 3 11% 28% 18% 2	2800
December	1st to 10th	Night	31% 3 13% 38% 15% 1	4580
	11th to 20th	Night	40% 3 14% 30% 13% 1	5201
	21st to end	Night	61% 3 9% 20% 6% 1	6978

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	23% 11% 35% 25% 6%	300
	11th to 20th	Day	23% 13% 42% 20% 2	300
	21st to end	Day	16% 12% 40% 28% 4	330
February	1st to 10th	Day	19% 17% 43% 19% 2 1	300
	11th to 20th	Day	20% 15% 43% 16% 5% 1	300
	21st to end	Day	16% 17% 43% 20% 4	249
March	1st to 10th	Day	20% 14% 45% 20% 2	300
	11th to 20th	Day	27% 10% 37% 21% 4 1	300
	21st to end	Day	34% 15% 32% 16% 2 1	330
April	1st to 10th	Day	22% 13% 36% 21% 8%	424
	11th to 20th	Day	20% 13% 38% 21% 7% 1	452
	21st to end	Day	25% 12% 35% 19% 8% 1	613
May	1st to 10th	Day	20% 10% 34% 27% 8% 1	653
	11th to 20th	Day	21% 6% 30% 27% 13% 3	675
	21st to end	Day	17% 8% 28% 29% 15% 3	734
June	1st to 10th	Day	7% 4 21% 35% 26% 8%	680
	11th to 20th	Day	7% 3 28% 38% 19% 5%	680
	21st to end	Day	9% 5% 34% 36% 14% 3	680
July	1st to 10th	Day	12% 2 15% 31% 30% 11%	671
	11th to 20th	Day	9% 3 18% 38% 26% 7%	668
	21st to end	Day	7% 1 15% 40% 29% 7%	739
August	1st to 10th	Day	8% 1 18% 39% 26% 7%	676
	11th to 20th	Day	9% 3 16% 39% 27% 6%	665
	21st to end	Day	10% 1 16% 32% 31% 10%	606
September	1st to 10th	Day	14% 3 17% 31% 29% 6%	490
	11th to 20th	Day	9% 2 19% 40% 22% 8%	334
	21st to end	Day	13% 5% 29% 30% 18% 4	300
October	1st to 10th	Day	17% 5% 26% 28% 14% 9%	300
	11th to 20th	Day	11% 19% 40% 21% 8% 1	300
	21st to end	Day	17% 14% 37% 24% 6% 2	330
November	1st to 10th	Day	17% 8% 40% 26% 8% 2	300
	11th to 20th	Day	18% 17% 35% 23% 7% 1	300
	21st to end	Day	22% 10% 36% 24% 7% 1	300
December	1st to 10th	Day	8% 16% 41% 27% 7% 2	300
	11th to 20th	Day	20% 14% 34% 25% 6% 1	300
	21st to end	Day	22% 19% 37% 19% 2	330
January	1st to 10th	Night	13% 12% 40% 29% 7%	1000
	11th to 20th	Night	15% 13% 41% 25% 6% 1	1000
	21st to end	Night	16% 9% 42% 30% 3	1100
February	1st to 10th	Night	18% 11% 41% 26% 4	1000
	11th to 20th	Night	16% 14% 38% 28% 4	979
	21st to end	Night	15% 14% 45% 19% 6% 1	654
March	1st to 10th	Night	15% 12% 41% 27% 5%	600
	11th to 20th	Night	21% 9% 36% 30% 4 1	600
	21st to end	Night	27% 10% 39% 20% 4	660
April	1st to 10th	Night	15% 11% 44% 26% 4	600
	11th to 20th	Night	19% 10% 37% 30% 5%	565
	21st to end	Night	19% 10% 38% 26% 7% 1	570
May	1st to 10th	Night	18% 7% 35% 32% 7% 1	570
	11th to 20th	Night	18% 6% 29% 36% 10% 1	561
	21st to end	Night	13% 6% 30% 36% 14% 1	617
June	1st to 10th	Night	7% 3 23% 45% 20% 2	570
	11th to 20th	Night	7% 3 26% 43% 20% 1	570
	21st to end	Night	6% 2 29% 47% 15%	570
July	1st to 10th	Night	9% 2 13% 42% 32% 3	555
	11th to 20th	Night	6% 1 16% 48% 29% 1	552
	21st to end	Night	7% 2 13% 46% 30% 2	624
August	1st to 10th	Night	6% 1 17% 47% 29% 1	567
	11th to 20th	Night	7% 2 13% 45% 29% 4	564
	21st to end	Night	9% 1 13% 43% 31% 3	627
September	1st to 10th	Night	11% 2 17% 42% 25% 3	570
	11th to 20th	Night	11% 2 20% 41% 24% 2	570
	21st to end	Night	10% 3 25% 43% 19% 1	571
October	1st to 10th	Night	13% 6% 28% 34% 17% 2	760
	11th to 20th	Night	16% 7% 37% 31% 8%	760
	21st to end	Night	19% 9% 35% 29% 7%	836
November	1st to 10th	Night	10% 10% 34% 35% 10%	805
	11th to 20th	Night	15% 14% 37% 28% 7%	1000
	21st to end	Night	19% 9% 35% 29% 7%	1000
December	1st to 10th	Night	15% 11% 37% 30% 6% 1	1000
	11th to 20th	Night	19% 8% 38% 29% 6%	1000
	21st to end	Night	22% 13% 34% 27% 4	1100

Page generated at 2025-04-08 08:17:29. Version ac07

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