Backus Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Backus Mountain Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	14% 10% 4 8% 5% 5% 5% 4 6% 7% 32%	101232
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	97772
Transparency	Day	47% 8% 12% 20% 12% 2	104908
	Night	46% 8% 11% 20% 12% 2	102235
Seeing	Day	23% 15% 24% 21% 13% 4	15169
	Night	21% 13% 22% 28% 15% 1	25237

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% 17% 5% 15% 7% 4 3 3 4 6% 22%	2836
	11th to 20th	Day	12% 17% 9% 19% 7% 6% 4 2 3 2 20%	2840
	21st to end	Day	18% 18% 8% 10% 5% 3 3 2 4 4 24%	3124
February	1st to 10th	Day	16% 12% 5% 11% 6% 4 4 3 4 7% 27%	1710
	11th to 20th	Day	19% 11% 6% 11% 5% 4 5% 3 5% 5% 26%	1752
	21st to end	Day	19% 14% 5% 11% 5% 4 3 3 4 4 29%	1589
March	1st to 10th	Day	20% 13% 6% 8% 4 4 4 3 4 6% 28%	3435
	11th to 20th	Day	13% 9% 4 8% 5% 4 3 3 4 6% 41%	3480
	21st to end	Day	16% 12% 4 8% 4 5% 5% 4 5% 6% 31%	3815
April	1st to 10th	Day	14% 12% 4 7% 5% 4 4 4 4 7% 36%	2120
	11th to 20th	Day	14% 10% 4 7% 5% 5% 4 3 5% 6% 39%	2282
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 6% 35%	2470
May	1st to 10th	Day	20% 10% 3 9% 6% 5% 6% 5% 6% 6% 24%	4082
	11th to 20th	Day	20% 10% 3 8% 6% 5% 6% 4 7% 6% 25%	4081
	21st to end	Day	10% 7% 3 7% 6% 6% 6% 5% 8% 10% 32%	4476
June	1st to 10th	Day	12% 7% 3 8% 6% 5% 5% 4 7% 9% 34%	2470
	11th to 20th	Day	10% 7% 3 8% 6% 6% 6% 6% 8% 8% 33%	2470
	21st to end	Day	10% 7% 2 7% 6% 6% 6% 5% 7% 9% 35%	2470
July	1st to 10th	Day	8% 5% 3 8% 6% 5% 6% 5% 7% 9% 38%	4120
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	4120
	21st to end	Day	18% 10% 4 9% 6% 6% 6% 5% 6% 8% 22%	4466
August	1st to 10th	Day	13% 8% 3 8% 5% 6% 7% 6% 9% 10% 26%	4104
	11th to 20th	Day	11% 6% 3 8% 6% 7% 7% 6% 9% 10% 29%	4044
	21st to end	Day	6% 4 2 6% 5% 5% 5% 5% 7% 9% 46%	4315
September	1st to 10th	Day	12% 6% 2 7% 6% 5% 5% 5% 5% 6% 42%	2176
	11th to 20th	Day	12% 8% 3 8% 6% 5% 5% 5% 4 7% 38%	2090
	21st to end	Day	20% 9% 2 7% 5% 5% 5% 4 5% 7% 32%	2090
October	1st to 10th	Day	16% 6% 2 7% 4 3 4 3 5% 7% 42%	1952
	11th to 20th	Day	15% 10% 2 6% 3 4 4 3 5% 5% 42%	1900
	21st to end	Day	17% 12% 2 6% 4 3 4 3 4 6% 39%	2090
November	1st to 10th	Day	11% 10% 3 4 4 4 3 3 5% 6% 46%	1856
	11th to 20th	Day	14% 12% 4 7% 5% 4 4 3 4 7% 36%	1710
	21st to end	Day	16% 11% 4 7% 4 4 4 3 5% 6% 35%	1710
December	1st to 10th	Day	22% 16% 5% 9% 5% 3 3 3 4 5% 24%	1710
	11th to 20th	Day	18% 14% 5% 9% 3 3 4 4 4 6% 30%	2153
	21st to end	Day	17% 15% 6% 12% 6% 5% 6% 3 5% 6% 19%	3124
January	1st to 10th	Night	11% 18% 9% 16% 5% 5% 4 3 4 4 20%	4109
	11th to 20th	Night	18% 13% 5% 12% 7% 6% 5% 3 3 4 26%	4120
	21st to end	Night	19% 16% 5% 10% 6% 7% 4 3 4 4 22%	4532
February	1st to 10th	Night	15% 13% 5% 11% 5% 5% 4 4 5% 5% 28%	2470
	11th to 20th	Night	19% 11% 6% 12% 7% 4 5% 3 5% 5% 25%	2312
	21st to end	Night	18% 14% 5% 11% 5% 4 4 3 4 5% 26%	1865
March	1st to 10th	Night	21% 14% 4 8% 6% 4 4 2 4 5% 29%	3418
	11th to 20th	Night	11% 10% 5% 10% 5% 3 4 3 5% 5% 37%	3480
	21st to end	Night	14% 11% 3 7% 5% 4 5% 4 5% 7% 37%	3817
April	1st to 10th	Night	13% 12% 3 7% 5% 5% 4 3 6% 8% 34%	2058
	11th to 20th	Night	13% 9% 4 5% 5% 4 4 3 6% 8% 38%	1864
	21st to end	Night	15% 7% 2 8% 6% 5% 4 3 5% 7% 38%	1710
May	1st to 10th	Night	24% 11% 4 9% 4 4 4 3 5% 6% 26%	2813
	11th to 20th	Night	19% 12% 4 8% 4 4 4 3 5% 7% 30%	2812
	21st to end	Night	10% 9% 3 9% 6% 4 5% 5% 6% 8% 35%	3079
June	1st to 10th	Night	9% 7% 2 6% 5% 4 5% 4 7% 10% 41%	1710
	11th to 20th	Night	10% 5% 2 7% 5% 4 6% 6% 7% 10% 39%	1710
	21st to end	Night	10% 7% 3 6% 4 4 5% 4 6% 10% 42%	1710
July	1st to 10th	Night	7% 6% 3 7% 5% 6% 6% 5% 8% 10% 37%	2840
	11th to 20th	Night	8% 6% 2 5% 4 4 4 4 7% 10% 46%	2840
	21st to end	Night	13% 11% 4 9% 6% 5% 5% 4 7% 8% 28%	3087
August	1st to 10th	Night	12% 9% 4 10% 5% 4 5% 4 7% 8% 32%	2825
	11th to 20th	Night	13% 8% 3 7% 4 4 5% 4 7% 7% 38%	2785
	21st to end	Night	5% 5% 2 6% 4 4 4 3 6% 8% 53%	3307
September	1st to 10th	Night	11% 7% 2 6% 4 5% 5% 3 5% 7% 45%	2000
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2090
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 5% 7% 38%	2090
October	1st to 10th	Night	14% 7% 2 5% 4 5% 4 3 5% 6% 45%	2228
	11th to 20th	Night	12% 11% 3 6% 3 3 4 3 4 6% 43%	2280
	21st to end	Night	15% 11% 3 7% 4 4 2 2 5% 5% 43%	2508
November	1st to 10th	Night	12% 7% 3 7% 5% 3 5% 3 4 7% 44%	2319
	11th to 20th	Night	14% 11% 4 8% 5% 4 5% 3 4 5% 37%	2470
	21st to end	Night	14% 11% 4 8% 5% 4 4 3 5% 5% 36%	2470
December	1st to 10th	Night	19% 14% 6% 11% 6% 4 4 3 4 5% 23%	2470
	11th to 20th	Night	15% 13% 7% 13% 6% 3 4 3 4 6% 26%	3042
	21st to end	Night	14% 16% 9% 11% 6% 5% 5% 3 4 6% 21%	4532

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	62% 4 7% 18% 8% 1	2837
	11th to 20th	Day	69% 4 6% 12% 7% 1	2840
	21st to end	Day	61% 6% 9% 15% 9% 1	3124
February	1st to 10th	Day	56% 4 10% 19% 9% 1	1710
	11th to 20th	Day	58% 4 10% 16% 10% 2	1761
	21st to end	Day	58% 5% 8% 14% 12% 2	1589
March	1st to 10th	Day	51% 9% 12% 19% 8% 1	3435
	11th to 20th	Day	41% 7% 10% 21% 19% 2	3480
	21st to end	Day	48% 9% 10% 19% 12% 2	3815
April	1st to 10th	Day	47% 5% 11% 19% 13% 4	2120
	11th to 20th	Day	45% 6% 10% 21% 15% 3	2282
	21st to end	Day	49% 7% 11% 21% 11% 2	2470
May	1st to 10th	Day	54% 11% 10% 17% 8% 1	4120
	11th to 20th	Day	51% 11% 12% 18% 7% 1	4085
	21st to end	Day	39% 13% 18% 18% 11% 3	4489
June	1st to 10th	Day	42% 11% 12% 21% 13%	2470
	11th to 20th	Day	42% 11% 13% 18% 15% 1	2470
	21st to end	Day	42% 11% 14% 21% 12% 1	2470
July	1st to 10th	Day	39% 10% 19% 19% 9% 3	4055
	11th to 20th	Day	38% 10% 14% 30% 8% 1	4051
	21st to end	Day	56% 9% 12% 15% 8% 1	4324
August	1st to 10th	Day	47% 11% 18% 17% 6%	4091
	11th to 20th	Day	44% 11% 13% 23% 10%	4083
	21st to end	Day	31% 9% 17% 29% 13% 1	4315
September	1st to 10th	Day	40% 8% 12% 19% 17% 4	2176
	11th to 20th	Day	44% 7% 11% 19% 17% 1	2090
	21st to end	Day	49% 8% 11% 17% 14% 1	2090
October	1st to 10th	Day	30% 5% 11% 20% 28% 5%	3238
	11th to 20th	Day	33% 6% 10% 23% 15% 13%	3140
	21st to end	Day	32% 4 9% 30% 15% 11%	3428
November	1st to 10th	Day	37% 6% 10% 22% 22% 3	1856
	11th to 20th	Day	48% 4 8% 19% 18% 3	1710
	21st to end	Day	48% 6% 11% 19% 14% 2	1710
December	1st to 10th	Day	61% 5% 9% 16% 7% 1	1710
	11th to 20th	Day	53% 5% 9% 17% 14% 1	2150
	21st to end	Day	60% 9% 10% 16% 5%	3124
January	1st to 10th	Night	64% 7% 7% 13% 9% 1	4109
	11th to 20th	Night	58% 6% 8% 17% 10% 1	4120
	21st to end	Night	63% 6% 8% 15% 7% 1	4532
February	1st to 10th	Night	56% 5% 10% 16% 12% 1	2470
	11th to 20th	Night	60% 5% 8% 14% 10% 2	2314
	21st to end	Night	58% 6% 10% 14% 8% 4	1865
March	1st to 10th	Night	54% 7% 9% 20% 9% 1	3418
	11th to 20th	Night	43% 8% 10% 24% 14% 1	3480
	21st to end	Night	42% 8% 12% 21% 16% 2	3817
April	1st to 10th	Night	47% 8% 11% 19% 13% 3	2058
	11th to 20th	Night	42% 6% 13% 21% 14% 4	1864
	21st to end	Night	44% 7% 14% 23% 11% 1	1710
May	1st to 10th	Night	56% 8% 11% 16% 8% 1	2840
	11th to 20th	Night	49% 11% 15% 20% 5% 1	2813
	21st to end	Night	38% 14% 17% 20% 6% 4	3087
June	1st to 10th	Night	35% 12% 17% 23% 12% 1	1710
	11th to 20th	Night	35% 12% 18% 21% 13% 1	1710
	21st to end	Night	35% 11% 15% 24% 13% 1	1710
July	1st to 10th	Night	38% 11% 20% 18% 11% 3	2795
	11th to 20th	Night	31% 9% 18% 32% 8% 2	2786
	21st to end	Night	48% 11% 12% 20% 8%	2988
August	1st to 10th	Night	46% 10% 16% 21% 6%	2814
	11th to 20th	Night	42% 9% 12% 26% 10%	2812
	21st to end	Night	27% 8% 13% 34% 16% 1	3307
September	1st to 10th	Night	37% 8% 15% 20% 16% 5%	2000
	11th to 20th	Night	37% 9% 11% 22% 18% 2	2090
	21st to end	Night	42% 9% 13% 22% 13% 1	2090
October	1st to 10th	Night	32% 6% 11% 21% 27% 4	3722
	11th to 20th	Night	34% 6% 8% 21% 19% 12%	3779
	21st to end	Night	28% 4 10% 28% 18% 12%	4120
November	1st to 10th	Night	39% 6% 11% 19% 22% 4	2319
	11th to 20th	Night	48% 5% 9% 17% 17% 4	2470
	21st to end	Night	47% 6% 10% 20% 15% 3	2470
December	1st to 10th	Night	62% 5% 7% 16% 7% 2	2470
	11th to 20th	Night	58% 5% 9% 17% 10% 1	3044
	21st to end	Night	60% 10% 9% 13% 8%	4532

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	35% 28% 25% 9% 4	270
	11th to 20th	Day	41% 26% 26% 7%	270
	21st to end	Day	38% 23% 26% 12% 1	297
February	1st to 10th	Day	36% 27% 24% 11% 3	270
	11th to 20th	Day	51% 21% 20% 7% 1	270
	21st to end	Day	43% 22% 27% 7% 1	244
March	1st to 10th	Day	32% 22% 27% 15% 4	460
	11th to 20th	Day	30% 27% 27% 12% 4 1	460
	21st to end	Day	30% 24% 25% 16% 4	506
April	1st to 10th	Day	31% 20% 25% 17% 6%	460
	11th to 20th	Day	26% 17% 29% 17% 8% 2	452
	21st to end	Day	27% 21% 26% 17% 8% 2	460
May	1st to 10th	Day	29% 18% 26% 17% 7% 3	463
	11th to 20th	Day	23% 15% 27% 18% 14% 3	487
	21st to end	Day	16% 10% 26% 24% 17% 7%	529
June	1st to 10th	Day	13% 12% 26% 25% 17% 6%	490
	11th to 20th	Day	13% 10% 27% 26% 17% 7%	490
	21st to end	Day	14% 10% 27% 27% 17% 6%	490
July	1st to 10th	Day	14% 6% 21% 27% 21% 12%	485
	11th to 20th	Day	7% 5% 23% 33% 22% 10%	484
	21st to end	Day	14% 6% 20% 29% 21% 10%	532
August	1st to 10th	Day	13% 8% 23% 25% 20% 10%	486
	11th to 20th	Day	13% 9% 20% 26% 23% 10%	479
	21st to end	Day	10% 5% 20% 28% 26% 10%	539
September	1st to 10th	Day	13% 8% 20% 27% 23% 9%	490
	11th to 20th	Day	16% 6% 19% 27% 26% 7%	490
	21st to end	Day	25% 13% 20% 26% 14% 3	490
October	1st to 10th	Day	18% 9% 23% 27% 19% 4	490
	11th to 20th	Day	22% 15% 23% 25% 13% 3	490
	21st to end	Day	25% 15% 25% 22% 12% 1	539
November	1st to 10th	Day	21% 14% 28% 24% 10% 3	430
	11th to 20th	Day	33% 16% 24% 20% 7%	270
	21st to end	Day	27% 20% 28% 21% 4	270
December	1st to 10th	Day	39% 20% 25% 12% 4 1	270
	11th to 20th	Day	31% 22% 25% 17% 4	270
	21st to end	Day	32% 23% 26% 14% 5%	297
January	1st to 10th	Night	29% 23% 27% 18% 2	760
	11th to 20th	Night	36% 18% 25% 19% 3	760
	21st to end	Night	28% 22% 30% 18% 2	836
February	1st to 10th	Night	27% 19% 28% 24% 2	760
	11th to 20th	Night	34% 22% 26% 17% 1	743
	21st to end	Night	30% 23% 30% 15% 1	628
March	1st to 10th	Night	28% 18% 28% 23% 3	760
	11th to 20th	Night	30% 20% 24% 18% 6%	760
	21st to end	Night	26% 18% 28% 22% 5%	836
April	1st to 10th	Night	24% 20% 25% 27% 5%	760
	11th to 20th	Night	19% 16% 26% 27% 11% 1	740
	21st to end	Night	19% 16% 24% 34% 8% 1	570
May	1st to 10th	Night	23% 12% 25% 28% 12% 1	570
	11th to 20th	Night	19% 11% 22% 32% 16%	561
	21st to end	Night	13% 5% 18% 35% 25% 4	617
June	1st to 10th	Night	9% 6% 20% 39% 24% 3	570
	11th to 20th	Night	11% 5% 16% 34% 31% 3	570
	21st to end	Night	12% 3 16% 39% 28% 2	570
July	1st to 10th	Night	12% 3 13% 35% 34% 5%	555
	11th to 20th	Night	8% 3 12% 33% 39% 5%	552
	21st to end	Night	10% 5% 11% 36% 35% 4	624
August	1st to 10th	Night	14% 3 11% 34% 36% 2	567
	11th to 20th	Night	14% 4 10% 36% 34% 3	564
	21st to end	Night	11% 3 10% 38% 33% 6%	732
September	1st to 10th	Night	13% 5% 13% 34% 31% 5%	760
	11th to 20th	Night	14% 6% 16% 32% 30% 3	760
	21st to end	Night	19% 9% 17% 31% 22% 2	760
October	1st to 10th	Night	16% 10% 20% 30% 22% 3	760
	11th to 20th	Night	21% 13% 20% 32% 12% 1	760
	21st to end	Night	22% 13% 21% 31% 12% 1	836
November	1st to 10th	Night	16% 11% 25% 33% 13% 1	760
	11th to 20th	Night	25% 15% 23% 28% 8%	760
	21st to end	Night	22% 16% 22% 31% 7% 1	760
December	1st to 10th	Night	32% 18% 27% 18% 5%	760
	11th to 20th	Night	25% 20% 26% 25% 4	760
	21st to end	Night	28% 17% 28% 24% 3	836

Page generated at 2024-11-03 02:11:15. Version ac07

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