Don's Backyard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Don's Backyard clear sky chart from 2005-11-06 to 2025-01-22.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 31%	104812
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	104490
Transparency	Day	51% 6% 11% 20% 11% 1	108501
	Night	51% 6% 11% 20% 11% 1	108940
Seeing	Day	21% 12% 27% 21% 14% 6%	14847
	Night	20% 9% 27% 29% 13% 1	24812

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	18% 21% 7% 16% 5% 4 4 2 4 5% 15%	2926
	11th to 20th	Day	14% 23% 8% 13% 7% 5% 4 2 4 5% 16%	2930
	21st to end	Day	26% 20% 10% 12% 7% 4 3 3 3 3 9%	3133
February	1st to 10th	Day	18% 18% 5% 13% 6% 5% 4 2 4 4 21%	1710
	11th to 20th	Day	15% 14% 6% 13% 7% 4 5% 3 4 5% 22%	1715
	21st to end	Day	17% 16% 4 11% 5% 4 4 4 4 5% 25%	1570
March	1st to 10th	Day	21% 14% 5% 9% 5% 5% 4 3 5% 6% 25%	3372
	11th to 20th	Day	13% 12% 5% 11% 5% 5% 5% 3 4 6% 33%	3480
	21st to end	Day	20% 11% 4 9% 5% 4 4 3 4 6% 30%	3815
April	1st to 10th	Day	14% 13% 3 8% 6% 4 5% 3 5% 8% 31%	2131
	11th to 20th	Day	18% 11% 3 7% 5% 4 5% 4 4 5% 34%	2392
	21st to end	Day	19% 10% 3 7% 5% 4 4 4 5% 6% 33%	2470
May	1st to 10th	Day	21% 10% 3 7% 6% 4 5% 3 5% 7% 29%	4082
	11th to 20th	Day	19% 8% 3 7% 5% 4 5% 4 5% 8% 32%	4081
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	4497
June	1st to 10th	Day	13% 8% 2 6% 5% 5% 5% 4 7% 9% 35%	2660
	11th to 20th	Day	13% 7% 3 8% 6% 4 4 4 6% 9% 36%	2660
	21st to end	Day	12% 6% 2 5% 5% 4 5% 5% 8% 9% 38%	2620
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 5% 7% 10% 38%	4120
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 8% 10% 42%	4120
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 5% 7% 8% 37%	4466
August	1st to 10th	Day	12% 8% 3 8% 5% 6% 6% 4 7% 8% 33%	4104
	11th to 20th	Day	11% 7% 3 7% 4 5% 6% 5% 8% 9% 34%	4044
	21st to end	Day	8% 5% 2 6% 6% 6% 5% 4 8% 9% 41%	4438
September	1st to 10th	Day	13% 7% 2 5% 5% 4 4 3 5% 7% 45%	2185
	11th to 20th	Day	10% 6% 2 7% 4 4 5% 4 6% 8% 43%	2090
	21st to end	Day	20% 8% 2 6% 5% 3 4 3 5% 7% 37%	2090
October	1st to 10th	Day	15% 6% 3 7% 4 5% 5% 3 6% 8% 39%	1957
	11th to 20th	Day	18% 10% 2 7% 4 5% 4 4 4 5% 36%	1900
	21st to end	Day	20% 11% 3 9% 5% 4 4 3 5% 7% 30%	2090
November	1st to 10th	Day	12% 9% 3 8% 4 4 4 3 5% 7% 40%	1778
	11th to 20th	Day	17% 14% 4 8% 5% 4 4 3 5% 6% 30%	1783
	21st to end	Day	16% 18% 6% 10% 6% 5% 4 4 4 5% 22%	1600
December	1st to 10th	Day	19% 14% 8% 15% 6% 4 3 3 3 5% 20%	2600
	11th to 20th	Day	23% 17% 8% 12% 4 4 3 3 4 5% 19%	2990
	21st to end	Day	23% 23% 6% 13% 6% 5% 3 3 3 4 11%	4213
January	1st to 10th	Night	15% 21% 7% 16% 6% 4 4 2 4 5% 16%	4239
	11th to 20th	Night	13% 20% 9% 16% 7% 5% 5% 4 4 4 13%	4250
	21st to end	Night	27% 18% 8% 13% 6% 3 3 2 3 3 15%	4546
February	1st to 10th	Night	21% 15% 5% 10% 6% 5% 4 3 4 5% 22%	2470
	11th to 20th	Night	16% 15% 5% 11% 6% 4 5% 4 4 6% 23%	2349
	21st to end	Night	20% 13% 5% 11% 7% 5% 4 3 4 4 24%	1884
March	1st to 10th	Night	22% 14% 4 7% 4 4 2 3 3 5% 33%	3480
	11th to 20th	Night	17% 9% 4 7% 4 4 5% 4 6% 6% 34%	3480
	21st to end	Night	18% 11% 4 8% 5% 3 4 3 4 6% 36%	3817
April	1st to 10th	Night	16% 12% 4 7% 4 5% 4 3 5% 7% 33%	2047
	11th to 20th	Night	16% 10% 3 8% 5% 4 4 3 6% 6% 35%	1754
	21st to end	Night	15% 9% 3 8% 4 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	25% 10% 3 7% 4 5% 4 3 5% 7% 27%	2813
	11th to 20th	Night	17% 9% 3 7% 4 4 4 2 5% 7% 38%	2812
	21st to end	Night	10% 6% 2 6% 3 4 4 4 7% 12% 43%	3053
June	1st to 10th	Night	8% 7% 2 7% 5% 5% 5% 4 6% 7% 44%	1520
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 4 8% 8% 42%	1520
	21st to end	Night	10% 5% 2 6% 6% 4 5% 4 7% 9% 43%	1551
July	1st to 10th	Night	11% 6% 3 7% 4 3 4 3 6% 9% 44%	2840
	11th to 20th	Night	10% 4 2 4 4 4 4 3 6% 10% 47%	2840
	21st to end	Night	8% 6% 3 8% 5% 4 5% 4 6% 8% 43%	3087
August	1st to 10th	Night	10% 8% 3 8% 5% 5% 5% 4 6% 9% 38%	2825
	11th to 20th	Night	11% 8% 3 6% 4 4 3 3 5% 7% 45%	2785
	21st to end	Night	7% 4 2 7% 5% 5% 6% 5% 7% 8% 43%	3184
September	1st to 10th	Night	13% 6% 2 4 3 4 4 4 5% 8% 48%	1990
	11th to 20th	Night	10% 4 2 4 5% 4 3 3 6% 7% 53%	2090
	21st to end	Night	16% 8% 3 5% 4 3 3 4 6% 6% 42%	2090
October	1st to 10th	Night	13% 7% 2 5% 4 4 4 4 5% 7% 44%	2222
	11th to 20th	Night	16% 9% 4 8% 5% 5% 4 3 5% 7% 35%	2280
	21st to end	Night	21% 11% 2 7% 4 4 4 3 4 6% 33%	2508
November	1st to 10th	Night	13% 9% 2 6% 4 4 4 3 5% 7% 42%	2517
	11th to 20th	Night	17% 14% 4 8% 5% 4 4 3 5% 6% 30%	2614
	21st to end	Night	19% 15% 5% 8% 5% 4 5% 3 4 7% 25%	2800
December	1st to 10th	Night	16% 16% 7% 11% 6% 4 5% 4 5% 5% 22%	4580
	11th to 20th	Night	20% 19% 7% 12% 5% 4 3 3 4 5% 19%	5205
	21st to end	Night	23% 20% 6% 11% 5% 4 4 3 4 5% 14%	6738

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	72% 5% 9% 11% 3	2927
	11th to 20th	Day	74% 3 9% 10% 4	2930
	21st to end	Day	80% 4 7% 6% 2	3133
February	1st to 10th	Day	68% 4 8% 13% 6% 1	1710
	11th to 20th	Day	65% 5% 8% 15% 8%	1724
	21st to end	Day	61% 4 11% 15% 7% 2	1570
March	1st to 10th	Day	60% 5% 9% 21% 5%	3372
	11th to 20th	Day	53% 5% 11% 23% 8% 1	3480
	21st to end	Day	56% 5% 9% 17% 13% 1	3815
April	1st to 10th	Day	52% 6% 11% 19% 11% 1	2131
	11th to 20th	Day	51% 6% 10% 18% 13% 2	2392
	21st to end	Day	52% 5% 11% 21% 10% 2	2470
May	1st to 10th	Day	54% 6% 10% 21% 8% 1	4120
	11th to 20th	Day	49% 6% 13% 21% 9% 1	4085
	21st to end	Day	39% 8% 14% 21% 14% 4	4511
June	1st to 10th	Day	43% 8% 13% 23% 12% 1	2660
	11th to 20th	Day	44% 7% 13% 22% 14% 1	2660
	21st to end	Day	39% 8% 13% 24% 15% 2	2620
July	1st to 10th	Day	40% 7% 12% 24% 15% 2	4055
	11th to 20th	Day	34% 8% 13% 26% 19% 1	4051
	21st to end	Day	42% 8% 13% 17% 18% 2	4324
August	1st to 10th	Day	46% 8% 13% 22% 11%	4091
	11th to 20th	Day	41% 8% 10% 29% 10% 1	4083
	21st to end	Day	37% 8% 13% 30% 10% 2	4438
September	1st to 10th	Day	39% 7% 12% 22% 17% 4	2185
	11th to 20th	Day	37% 8% 14% 24% 16% 2	2090
	21st to end	Day	47% 6% 10% 21% 15% 1	2090
October	1st to 10th	Day	32% 5% 12% 30% 19% 1	3256
	11th to 20th	Day	46% 4 11% 20% 15% 4	3140
	21st to end	Day	41% 4 13% 25% 16% 1	3428
November	1st to 10th	Day	45% 5% 11% 23% 15% 1	1778
	11th to 20th	Day	56% 4 10% 19% 9% 3	1783
	21st to end	Day	63% 5% 10% 15% 6% 1	1600
December	1st to 10th	Day	69% 4 11% 12% 3	2600
	11th to 20th	Day	70% 4 9% 13% 4	2986
	21st to end	Day	78% 4 7% 8% 2	4213
January	1st to 10th	Night	71% 4 9% 11% 4	4239
	11th to 20th	Night	74% 5% 9% 9% 3	4250
	21st to end	Night	77% 3 6% 11% 3	4546
February	1st to 10th	Night	65% 5% 9% 15% 6%	2470
	11th to 20th	Night	62% 4 10% 16% 7%	2351
	21st to end	Night	63% 5% 8% 13% 8% 2	1884
March	1st to 10th	Night	55% 4 9% 22% 9%	3480
	11th to 20th	Night	48% 6% 14% 23% 8% 1	3480
	21st to end	Night	49% 6% 10% 23% 11% 1	3817
April	1st to 10th	Night	51% 6% 10% 17% 14% 2	2047
	11th to 20th	Night	49% 5% 12% 16% 16% 2	1754
	21st to end	Night	46% 5% 12% 19% 16% 2	1710
May	1st to 10th	Night	56% 7% 8% 19% 8% 2	2840
	11th to 20th	Night	45% 7% 12% 24% 11% 2	2813
	21st to end	Night	32% 8% 15% 27% 15% 4	3060
June	1st to 10th	Night	39% 7% 10% 25% 16% 3	1520
	11th to 20th	Night	37% 7% 14% 23% 16% 3	1520
	21st to end	Night	36% 9% 13% 24% 18% 1	1551
July	1st to 10th	Night	37% 6% 12% 23% 18% 4	2795
	11th to 20th	Night	32% 7% 12% 31% 17% 1	2786
	21st to end	Night	38% 7% 12% 19% 23% 2	2988
August	1st to 10th	Night	43% 7% 11% 24% 14% 1	2814
	11th to 20th	Night	39% 6% 11% 28% 16% 1	2812
	21st to end	Night	36% 7% 12% 30% 13% 2	3184
September	1st to 10th	Night	35% 6% 12% 25% 18% 5%	1990
	11th to 20th	Night	30% 7% 11% 26% 23% 2	2090
	21st to end	Night	42% 6% 11% 22% 18% 1	2090
October	1st to 10th	Night	32% 5% 8% 30% 25% 1	3703
	11th to 20th	Night	42% 5% 12% 21% 16% 5%	3779
	21st to end	Night	38% 4 10% 29% 18% 2	4120
November	1st to 10th	Night	42% 5% 11% 24% 16% 3	2517
	11th to 20th	Night	54% 5% 10% 18% 11% 3	2614
	21st to end	Night	59% 5% 12% 17% 6% 1	2800
December	1st to 10th	Night	62% 6% 11% 17% 4	4580
	11th to 20th	Night	68% 4 11% 14% 2	5208
	21st to end	Night	72% 5% 9% 11% 2	6738

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	28% 20% 37% 10% 3 2	300
	11th to 20th	Day	28% 23% 38% 8% 3	300
	21st to end	Day	27% 25% 33% 11% 3 1	300
February	1st to 10th	Day	31% 17% 37% 13% 1 1	270
	11th to 20th	Day	29% 25% 33% 11% 2	270
	21st to end	Day	28% 28% 34% 9%	225
March	1st to 10th	Day	24% 20% 31% 20% 5%	419
	11th to 20th	Day	30% 13% 32% 17% 7% 1	460
	21st to end	Day	32% 13% 29% 21% 5% 1	506
April	1st to 10th	Day	22% 19% 32% 16% 10% 1	460
	11th to 20th	Day	25% 17% 28% 16% 9% 5%	452
	21st to end	Day	26% 12% 32% 19% 8% 3	460
May	1st to 10th	Day	27% 13% 27% 19% 10% 3	463
	11th to 20th	Day	20% 10% 31% 21% 13% 6%	487
	21st to end	Day	14% 6% 23% 27% 23% 7%	529
June	1st to 10th	Day	16% 6% 28% 30% 16% 4	490
	11th to 20th	Day	17% 6% 23% 29% 18% 6%	490
	21st to end	Day	14% 8% 26% 27% 18% 7%	490
July	1st to 10th	Day	12% 3 20% 25% 25% 15%	485
	11th to 20th	Day	11% 5% 22% 27% 23% 11%	484
	21st to end	Day	13% 3 23% 26% 25% 11%	532
August	1st to 10th	Day	15% 3 19% 26% 25% 12%	486
	11th to 20th	Day	16% 6% 18% 26% 22% 12%	479
	21st to end	Day	11% 5% 19% 25% 26% 14%	539
September	1st to 10th	Day	15% 6% 20% 24% 23% 12%	490
	11th to 20th	Day	9% 4 15% 28% 31% 14%	490
	21st to end	Day	26% 10% 21% 22% 14% 8%	490
October	1st to 10th	Day	13% 9% 29% 24% 21% 6%	490
	11th to 20th	Day	20% 15% 32% 21% 9% 3	490
	21st to end	Day	29% 13% 29% 20% 8%	539
November	1st to 10th	Day	17% 20% 34% 18% 9% 2	316
	11th to 20th	Day	27% 21% 31% 14% 6% 1	291
	21st to end	Day	25% 18% 40% 15% 2	200
December	1st to 10th	Day	27% 19% 36% 13% 5% 2	200
	11th to 20th	Day	26% 17% 41% 12% 5%	200
	21st to end	Day	30% 20% 34% 13% 3	275
January	1st to 10th	Night	25% 18% 35% 18% 4	800
	11th to 20th	Night	27% 16% 34% 18% 5%	800
	21st to end	Night	26% 15% 31% 24% 4	841
February	1st to 10th	Night	29% 13% 32% 23% 3	760
	11th to 20th	Night	25% 15% 35% 23% 2	743
	21st to end	Night	27% 15% 36% 19% 3	628
March	1st to 10th	Night	24% 11% 34% 26% 6%	760
	11th to 20th	Night	23% 13% 32% 26% 6% 1	760
	21st to end	Night	24% 10% 33% 26% 7% 1	836
April	1st to 10th	Night	21% 13% 33% 26% 6%	760
	11th to 20th	Night	22% 9% 34% 24% 10% 1	630
	21st to end	Night	18% 8% 34% 32% 8% 1	570
May	1st to 10th	Night	21% 6% 27% 35% 10% 1	570
	11th to 20th	Night	19% 3 23% 39% 14% 2	561
	21st to end	Night	12% 2 18% 42% 22% 3	603
June	1st to 10th	Night	9% 4 26% 44% 17% 1	380
	11th to 20th	Night	12% 4 21% 45% 17% 1	380
	21st to end	Night	11% 3 24% 40% 23%	411
July	1st to 10th	Night	10% 2 14% 41% 29% 5%	555
	11th to 20th	Night	10% 3 17% 40% 27% 4	552
	21st to end	Night	9% 3 13% 43% 30% 2	624
August	1st to 10th	Night	10% 2 13% 42% 30% 2	567
	11th to 20th	Night	13% 1 13% 39% 29% 4	564
	21st to end	Night	10% 2 14% 37% 33% 5%	661
September	1st to 10th	Night	15% 3 16% 34% 28% 5%	760
	11th to 20th	Night	11% 2 12% 38% 32% 5%	760
	21st to end	Night	19% 5% 22% 30% 20% 4	760
October	1st to 10th	Night	17% 5% 23% 32% 20% 3	760
	11th to 20th	Night	21% 8% 31% 27% 11% 2	760
	21st to end	Night	26% 9% 27% 30% 7%	836
November	1st to 10th	Night	18% 9% 30% 30% 12% 1	780
	11th to 20th	Night	24% 13% 33% 22% 8% 1	800
	21st to end	Night	28% 13% 31% 21% 7%	800
December	1st to 10th	Night	30% 12% 32% 21% 5%	800
	11th to 20th	Night	26% 14% 32% 23% 4	800
	21st to end	Night	29% 13% 33% 23% 3	880

Page generated at 2025-01-19 03:06:49. Version ac07

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