Boeing Observatory
Clear Sky Chart
History

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

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	101238
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	97888
Transparency	Day	37% 8% 12% 19% 18% 6%	104941
	Night	37% 8% 11% 18% 19% 8%	102324
Seeing	Day	14% 8% 22% 24% 21% 10%	15365
	Night	15% 6% 23% 33% 21% 3	25425

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	16% 5% 2 4 3 3 3 2 6% 8% 48%	2836
	11th to 20th	Day	13% 6% 1 5% 3 4 3 2 4 5% 54%	2840
	21st to end	Day	18% 8% 3 6% 4 4 3 2 3 4 45%	3124
February	1st to 10th	Day	15% 7% 2 6% 4 5% 4 4 5% 7% 41%	1724
	11th to 20th	Day	19% 6% 2 5% 4 3 3 3 4 5% 45%	1848
	21st to end	Day	19% 5% 3 6% 4 4 3 3 5% 6% 43%	1630
March	1st to 10th	Day	23% 11% 3 5% 3 3 4 3 4 6% 38%	3435
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 6% 54%	3480
	21st to end	Day	19% 7% 2 6% 4 3 4 3 5% 6% 42%	3815
April	1st to 10th	Day	10% 4 2 5% 4 4 5% 3 6% 6% 52%	2090
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 7% 7% 49%	2227
	21st to end	Day	8% 5% 2 4 4 4 4 4 6% 8% 52%	2391
May	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 10% 42%	4082
	11th to 20th	Day	16% 7% 3 7% 4 4 4 4 6% 7% 39%	4081
	21st to end	Day	7% 4 1 5% 4 5% 4 4 7% 10% 50%	4476
June	1st to 10th	Day	9% 6% 2 7% 5% 4 6% 5% 8% 10% 38%	2470
	11th to 20th	Day	8% 5% 1 5% 4 6% 6% 5% 7% 10% 44%	2470
	21st to end	Day	5% 5% 2 6% 6% 6% 6% 6% 8% 11% 39%	2470
July	1st to 10th	Day	13% 6% 3 7% 5% 6% 7% 6% 8% 11% 29%	4120
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 6% 10% 14% 32%	4120
	21st to end	Day	9% 7% 3 8% 7% 6% 8% 6% 10% 13% 23%	4466
August	1st to 10th	Day	22% 6% 2 7% 5% 5% 6% 5% 8% 10% 25%	4104
	11th to 20th	Day	11% 6% 2 7% 5% 6% 5% 5% 8% 10% 35%	4044
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 7% 11% 46%	4195
September	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 8% 42%	2090
	11th to 20th	Day	14% 6% 3 7% 5% 5% 4 4 5% 8% 40%	2090
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 7% 43%	2090
October	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 4 6% 50%	1986
	11th to 20th	Day	10% 4 2 5% 4 3 3 2 3 5% 59%	1900
	21st to end	Day	11% 5% 1 5% 3 3 4 3 3 4 58%	2090
November	1st to 10th	Day	13% 6% 2 4 4 4 3 3 4 5% 53%	1957
	11th to 20th	Day	15% 5% 2 4 3 3 3 2 5% 5% 53%	1800
	21st to end	Day	14% 6% 2 5% 3 4 3 3 4 5% 49%	1710
December	1st to 10th	Day	19% 6% 2 6% 4 5% 4 3 5% 7% 39%	1710
	11th to 20th	Day	16% 4 2 5% 3 3 3 3 4 5% 51%	2153
	21st to end	Day	19% 7% 3 5% 3 3 3 3 5% 6% 44%	3124
January	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 6% 47%	4109
	11th to 20th	Night	11% 6% 2 7% 5% 5% 4 4 5% 7% 44%	4120
	21st to end	Night	18% 7% 2 6% 3 3 3 3 4 5% 45%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 5% 6% 45%	2451
	11th to 20th	Night	19% 6% 3 6% 4 3 3 3 4 4 46%	2218
	21st to end	Night	16% 5% 2 6% 5% 4 4 3 5% 6% 45%	1822
March	1st to 10th	Night	23% 9% 3 6% 4 3 3 3 4 6% 36%	3418
	11th to 20th	Night	12% 4 1 4 4 4 4 3 6% 7% 51%	3480
	21st to end	Night	18% 5% 2 6% 3 3 4 3 5% 7% 43%	3817
April	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 7% 52%	2090
	11th to 20th	Night	12% 4 2 5% 3 4 3 4 5% 5% 53%	1900
	21st to end	Night	9% 4 2 4 4 3 4 3 5% 7% 54%	1803
May	1st to 10th	Night	11% 5% 2 6% 4 4 5% 3 7% 9% 43%	2813
	11th to 20th	Night	15% 5% 2 6% 4 5% 5% 4 5% 9% 41%	2812
	21st to end	Night	7% 3 2 5% 5% 5% 5% 4 6% 9% 48%	3079
June	1st to 10th	Night	12% 5% 2 7% 6% 5% 5% 5% 9% 10% 34%	1710
	11th to 20th	Night	7% 6% 2 6% 6% 6% 5% 5% 8% 11% 39%	1710
	21st to end	Night	6% 4 3 6% 7% 6% 6% 7% 10% 13% 33%	1710
July	1st to 10th	Night	13% 8% 3 7% 5% 6% 5% 5% 7% 10% 30%	2840
	11th to 20th	Night	9% 6% 3 8% 5% 6% 6% 6% 10% 12% 29%	2840
	21st to end	Night	13% 7% 3 10% 7% 6% 6% 5% 8% 10% 24%	3087
August	1st to 10th	Night	22% 7% 3 7% 5% 5% 6% 6% 7% 9% 23%	2825
	11th to 20th	Night	11% 7% 3 7% 5% 6% 6% 6% 6% 9% 35%	2785
	21st to end	Night	6% 4 1 6% 3 4 3 4 7% 12% 51%	3425
September	1st to 10th	Night	9% 5% 2 7% 5% 5% 5% 4 9% 9% 41%	2090
	11th to 20th	Night	12% 6% 2 9% 5% 5% 5% 3 5% 6% 41%	2090
	21st to end	Night	16% 6% 3 6% 4 4 5% 4 5% 6% 42%	2090
October	1st to 10th	Night	13% 6% 1 6% 3 3 3 2 4 6% 53%	2194
	11th to 20th	Night	13% 5% 2 3 3 3 3 3 4 5% 56%	2280
	21st to end	Night	10% 5% 1 4 3 2 3 4 5% 5% 57%	2508
November	1st to 10th	Night	14% 5% 2 4 3 3 3 3 4 6% 54%	2341
	11th to 20th	Night	14% 6% 2 4 4 3 3 3 4 5% 53%	2385
	21st to end	Night	14% 5% 2 5% 4 4 3 3 5% 6% 48%	2470
December	1st to 10th	Night	18% 6% 2 5% 4 5% 4 3 5% 6% 43%	2470
	11th to 20th	Night	17% 5% 2 5% 4 2 3 3 3 5% 52%	3042
	21st to end	Night	20% 7% 2 5% 4 4 4 3 5% 6% 40%	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	33% 6% 9% 23% 22% 7%	2837
	11th to 20th	Day	33% 5% 6% 17% 30% 10%	2840
	21st to end	Day	44% 4 4 16% 24% 8%	3124
February	1st to 10th	Day	41% 5% 10% 19% 19% 6%	1724
	11th to 20th	Day	42% 5% 5% 18% 24% 6%	1858
	21st to end	Day	43% 5% 8% 15% 20% 9%	1630
March	1st to 10th	Day	49% 6% 8% 15% 13% 10%	3435
	11th to 20th	Day	29% 7% 9% 15% 29% 11%	3480
	21st to end	Day	42% 6% 7% 18% 22% 5%	3815
April	1st to 10th	Day	31% 6% 11% 19% 26% 7%	2090
	11th to 20th	Day	32% 7% 11% 18% 21% 10%	2227
	21st to end	Day	29% 8% 11% 26% 22% 4	2391
May	1st to 10th	Day	35% 10% 16% 23% 14% 2	4120
	11th to 20th	Day	43% 8% 12% 20% 16% 1	4085
	21st to end	Day	27% 10% 18% 21% 22% 2	4489
June	1st to 10th	Day	38% 11% 16% 22% 11% 2	2470
	11th to 20th	Day	32% 10% 19% 25% 13% 1	2470
	21st to end	Day	34% 12% 19% 21% 12% 2	2470
July	1st to 10th	Day	45% 12% 19% 16% 7% 2	4055
	11th to 20th	Day	36% 16% 28% 16% 3 1	4051
	21st to end	Day	46% 14% 19% 16% 4	4324
August	1st to 10th	Day	52% 11% 20% 13% 4	4091
	11th to 20th	Day	40% 11% 16% 19% 13%	4083
	21st to end	Day	29% 9% 15% 31% 15% 2	4195
September	1st to 10th	Day	36% 11% 14% 17% 17% 5%	2090
	11th to 20th	Day	42% 8% 11% 16% 19% 3	2090
	21st to end	Day	41% 7% 9% 20% 17% 7%	2090
October	1st to 10th	Day	33% 4 7% 19% 27% 9%	3298
	11th to 20th	Day	17% 3 5% 17% 28% 29%	3140
	21st to end	Day	25% 4 5% 20% 31% 15%	3428
November	1st to 10th	Day	34% 4 7% 17% 25% 12%	1957
	11th to 20th	Day	34% 4 8% 20% 24% 11%	1800
	21st to end	Day	37% 5% 8% 19% 22% 9%	1710
December	1st to 10th	Day	44% 7% 9% 18% 16% 6%	1710
	11th to 20th	Day	34% 5% 7% 20% 24% 10%	2150
	21st to end	Day	41% 6% 8% 20% 23% 3	3124
January	1st to 10th	Night	37% 5% 8% 18% 24% 8%	4109
	11th to 20th	Night	36% 7% 8% 19% 23% 7%	4120
	21st to end	Night	42% 6% 5% 14% 26% 8%	4532
February	1st to 10th	Night	40% 5% 8% 19% 20% 9%	2451
	11th to 20th	Night	42% 5% 7% 17% 21% 8%	2219
	21st to end	Night	39% 6% 9% 17% 18% 11%	1822
March	1st to 10th	Night	48% 8% 9% 13% 15% 7%	3418
	11th to 20th	Night	31% 6% 8% 14% 26% 15%	3480
	21st to end	Night	39% 6% 8% 20% 18% 8%	3817
April	1st to 10th	Night	31% 6% 12% 18% 23% 9%	2090
	11th to 20th	Night	31% 6% 10% 20% 21% 12%	1900
	21st to end	Night	29% 6% 11% 25% 23% 5%	1803
May	1st to 10th	Night	33% 11% 16% 24% 14% 3	2840
	11th to 20th	Night	39% 9% 16% 19% 16% 1	2813
	21st to end	Night	27% 12% 13% 25% 20% 2	3087
June	1st to 10th	Night	39% 12% 18% 18% 11% 1	1710
	11th to 20th	Night	35% 11% 18% 23% 10% 2	1710
	21st to end	Night	36% 15% 19% 18% 10% 2	1710
July	1st to 10th	Night	47% 11% 19% 14% 6% 2	2795
	11th to 20th	Night	42% 16% 25% 13% 4 1	2786
	21st to end	Night	53% 14% 17% 12% 4	2988
August	1st to 10th	Night	54% 12% 18% 12% 3 1	2814
	11th to 20th	Night	41% 11% 21% 20% 7% 1	2812
	21st to end	Night	25% 9% 17% 31% 16% 2	3425
September	1st to 10th	Night	37% 10% 17% 18% 13% 5%	2090
	11th to 20th	Night	43% 9% 10% 16% 18% 3	2090
	21st to end	Night	41% 8% 12% 17% 16% 5%	2090
October	1st to 10th	Night	33% 5% 9% 17% 29% 7%	3662
	11th to 20th	Night	19% 3 6% 15% 25% 32%	3779
	21st to end	Night	24% 5% 6% 19% 28% 19%	4120
November	1st to 10th	Night	33% 5% 8% 15% 25% 14%	2341
	11th to 20th	Night	34% 5% 7% 16% 25% 13%	2385
	21st to end	Night	35% 5% 9% 18% 24% 10%	2470
December	1st to 10th	Night	42% 7% 9% 17% 19% 7%	2470
	11th to 20th	Night	36% 3 7% 18% 25% 10%	3044
	21st to end	Night	43% 5% 8% 20% 19% 4	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	17% 16% 45% 17% 4	270
	11th to 20th	Day	20% 17% 36% 19% 7%	270
	21st to end	Day	22% 21% 38% 16% 3	297
February	1st to 10th	Day	19% 19% 37% 20% 6%	270
	11th to 20th	Day	29% 16% 37% 16% 2	270
	21st to end	Day	22% 15% 39% 18% 5%	285
March	1st to 10th	Day	19% 15% 28% 27% 11%	460
	11th to 20th	Day	17% 13% 42% 21% 6% 1	460
	21st to end	Day	19% 14% 33% 23% 10% 1	506
April	1st to 10th	Day	15% 17% 31% 25% 12% 1	460
	11th to 20th	Day	11% 14% 30% 23% 17% 4	452
	21st to end	Day	12% 8% 28% 28% 17% 7%	460
May	1st to 10th	Day	8% 12% 31% 24% 20% 6%	463
	11th to 20th	Day	16% 5% 18% 26% 22% 12%	487
	21st to end	Day	12% 4 13% 27% 25% 18%	529
June	1st to 10th	Day	9% 3 14% 28% 31% 14%	490
	11th to 20th	Day	10% 3 14% 27% 27% 19%	490
	21st to end	Day	8% 2 12% 30% 30% 18%	490
July	1st to 10th	Day	9% 2 7% 21% 31% 30%	485
	11th to 20th	Day	10% 2 6% 23% 36% 23%	484
	21st to end	Day	6% 2 9% 29% 35% 20%	532
August	1st to 10th	Day	14% 3 11% 24% 30% 18%	486
	11th to 20th	Day	13% 3 11% 20% 31% 21%	479
	21st to end	Day	12% 2 10% 21% 32% 22%	539
September	1st to 10th	Day	9% 3 10% 26% 32% 19%	490
	11th to 20th	Day	18% 3 12% 23% 30% 14%	490
	21st to end	Day	19% 2 15% 25% 27% 12%	490
October	1st to 10th	Day	16% 3 18% 32% 21% 9%	490
	11th to 20th	Day	10% 5% 22% 30% 23% 10%	490
	21st to end	Day	13% 6% 24% 29% 22% 7%	539
November	1st to 10th	Day	15% 6% 28% 27% 19% 5%	495
	11th to 20th	Day	19% 9% 32% 28% 10% 2	360
	21st to end	Day	13% 12% 31% 28% 14% 1	270
December	1st to 10th	Day	24% 14% 34% 21% 6% 1	270
	11th to 20th	Day	21% 10% 39% 20% 10% 1	270
	21st to end	Day	24% 15% 32% 19% 7% 3	297
January	1st to 10th	Night	21% 11% 31% 29% 8%	760
	11th to 20th	Night	18% 11% 34% 31% 7%	760
	21st to end	Night	18% 10% 36% 32% 3	836
February	1st to 10th	Night	19% 11% 30% 35% 5%	760
	11th to 20th	Night	22% 11% 34% 30% 2	743
	21st to end	Night	18% 14% 34% 29% 5%	628
March	1st to 10th	Night	17% 10% 32% 35% 6%	760
	11th to 20th	Night	16% 9% 37% 29% 8%	760
	21st to end	Night	16% 10% 36% 30% 9%	836
April	1st to 10th	Night	13% 14% 32% 31% 10%	760
	11th to 20th	Night	13% 10% 28% 32% 16%	751
	21st to end	Night	12% 6% 27% 40% 15%	663
May	1st to 10th	Night	9% 7% 28% 38% 17% 1	570
	11th to 20th	Night	11% 4 24% 38% 20% 3	561
	21st to end	Night	10% 1 13% 39% 33% 4	617
June	1st to 10th	Night	13% 3 13% 40% 27% 5%	570
	11th to 20th	Night	10% 1 13% 34% 31% 11%	570
	21st to end	Night	8% 2 11% 37% 35% 6%	570
July	1st to 10th	Night	10% 2 8% 32% 40% 7%	555
	11th to 20th	Night	10% 1 6% 28% 48% 7%	552
	21st to end	Night	7% 2 9% 34% 43% 5%	624
August	1st to 10th	Night	11% 1 8% 27% 44% 8%	567
	11th to 20th	Night	13% 3 6% 31% 41% 7%	564
	21st to end	Night	12% 1 10% 27% 43% 7%	793
September	1st to 10th	Night	10% 2 10% 29% 43% 6%	760
	11th to 20th	Night	14% 2 12% 29% 38% 4	760
	21st to end	Night	19% 2 13% 31% 31% 3	760
October	1st to 10th	Night	14% 3 17% 32% 29% 5%	760
	11th to 20th	Night	14% 5% 20% 34% 24% 3	760
	21st to end	Night	12% 5% 23% 33% 26% 2	836
November	1st to 10th	Night	15% 6% 23% 34% 19% 3	780
	11th to 20th	Night	17% 9% 27% 33% 14% 1	763
	21st to end	Night	14% 7% 25% 35% 17% 1	760
December	1st to 10th	Night	18% 8% 28% 34% 10% 1	760
	11th to 20th	Night	19% 7% 26% 35% 12% 1	760
	21st to end	Night	24% 11% 25% 30% 10%	836

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