Howell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Howell Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	126421
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	124038
Transparency	Day	37% 7% 11% 21% 18% 6%	130174
	Night	36% 6% 10% 21% 20% 8%	128360
Seeing	Day	13% 8% 20% 24% 22% 13%	16406
	Night	14% 6% 21% 31% 23% 5%	26065

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	14% 9% 4 7% 3 3 3 3 4 5% 44%	3777
	11th to 20th	Day	12% 5% 3 7% 3 3 3 3 4 6% 51%	4080
	21st to end	Day	20% 11% 2 6% 3 3 2 2 3 5% 42%	4924
February	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 5% 7% 40%	2000
	11th to 20th	Day	21% 9% 3 6% 4 4 3 3 4 4 40%	1948
	21st to end	Day	19% 8% 3 6% 5% 3 3 2 4 5% 41%	1650
March	1st to 10th	Day	19% 9% 3 9% 4 4 4 3 5% 6% 34%	4480
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 5% 6% 45%	4790
	21st to end	Day	12% 7% 3 8% 5% 4 4 3 5% 6% 41%	5954
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 7% 46%	2400
	11th to 20th	Day	14% 6% 2 5% 3 4 4 3 5% 7% 46%	2374
	21st to end	Day	9% 5% 2 7% 5% 4 5% 3 6% 7% 47%	2400
May	1st to 10th	Day	14% 6% 3 7% 5% 4 5% 4 6% 8% 38%	5405
	11th to 20th	Day	12% 6% 2 8% 6% 5% 6% 4 7% 9% 34%	5803
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 6% 9% 40%	6433
June	1st to 10th	Day	8% 5% 2 6% 6% 6% 6% 4 8% 10% 39%	2643
	11th to 20th	Day	5% 4 1 6% 5% 6% 6% 5% 8% 9% 45%	2800
	21st to end	Day	6% 3 2 4 6% 5% 7% 6% 7% 12% 43%	2800
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 7% 6% 8% 10% 37%	6360
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 10% 13% 35%	6133
	21st to end	Day	8% 6% 3 8% 6% 6% 7% 5% 9% 12% 31%	5927
August	1st to 10th	Day	3 3 1 5% 4 4 5% 4 6% 7% 59%	3905
	11th to 20th	Day	4 4 2 6% 5% 6% 5% 5% 8% 9% 46%	3849
	21st to end	Day	7% 3 2 5% 5% 5% 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 3 5% 6% 46%	2268
	21st to end	Day	10% 5% 2 5% 3 4 4 4 6% 8% 51%	2200
October	1st to 10th	Day	7% 4 2 6% 3 3 3 3 5% 5% 58%	2200
	11th to 20th	Day	9% 4 1 4 4 3 4 2 4 6% 59%	2020
	21st to end	Day	13% 4 2 5% 3 3 4 3 4 5% 54%	2200
November	1st to 10th	Day	9% 6% 2 4 3 3 3 3 4 5% 56%	1862
	11th to 20th	Day	11% 5% 2 5% 3 4 3 3 3 5% 56%	1863
	21st to end	Day	15% 6% 2 5% 5% 3 5% 3 4 7% 46%	1800
December	1st to 10th	Day	15% 8% 2 7% 4 3 4 3 4 5% 44%	2930
	11th to 20th	Day	14% 9% 4 8% 4 4 4 3 4 5% 41%	3373
	21st to end	Day	16% 7% 3 6% 4 4 4 3 4 5% 45%	4158
January	1st to 10th	Night	17% 8% 3 7% 3 3 3 2 4 5% 45%	6185
	11th to 20th	Night	16% 7% 4 7% 3 3 2 2 3 4 47%	5900
	21st to end	Night	22% 8% 2 5% 3 3 3 2 4 4 44%	6015
February	1st to 10th	Night	17% 8% 3 6% 3 4 4 3 5% 6% 42%	2400
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 3 4 42%	2338
	21st to end	Night	19% 7% 3 6% 4 3 4 3 5% 5% 41%	1980
March	1st to 10th	Night	18% 10% 3 7% 4 4 4 3 4 6% 37%	5373
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 5% 7% 52%	5211
	21st to end	Night	18% 7% 2 6% 4 4 4 2 4 5% 44%	4973
April	1st to 10th	Night	12% 6% 2 6% 4 4 5% 4 5% 7% 47%	2000
	11th to 20th	Night	11% 6% 2 5% 3 3 4 4 5% 7% 51%	1975
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 7% 49%	2000
May	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 6% 9% 41%	4490
	11th to 20th	Night	18% 7% 2 8% 5% 4 5% 4 5% 7% 36%	4088
	21st to end	Night	9% 5% 2 7% 5% 5% 6% 5% 8% 9% 40%	4421
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 4 6% 10% 45%	1750
	11th to 20th	Night	4 3 1 3 5% 4 4 5% 7% 10% 52%	1600
	21st to end	Night	4 4 2 5% 4 4 5% 5% 7% 11% 49%	1600
July	1st to 10th	Night	4 4 2 7% 5% 6% 6% 6% 9% 12% 38%	3618
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 10% 13% 41%	3838
	21st to end	Night	6% 8% 3 9% 6% 5% 6% 6% 9% 14% 30%	4919
August	1st to 10th	Night	3 2 1 3 3 3 4 4 6% 9% 64%	3244
	11th to 20th	Night	3 2 1 4 3 4 4 4 7% 12% 53%	3200
	21st to end	Night	5% 3 2 6% 4 4 6% 5% 7% 9% 50%	3586
September	1st to 10th	Night	9% 4 2 6% 5% 4 5% 4 6% 7% 49%	2000
	11th to 20th	Night	10% 5% 2 5% 4 4 3 4 7% 9% 46%	2110
	21st to end	Night	8% 5% 2 4 3 3 4 4 5% 7% 56%	2200
October	1st to 10th	Night	7% 3 1 4 3 3 3 3 5% 6% 62%	2200
	11th to 20th	Night	10% 5% 1 4 3 2 4 3 4 7% 57%	2375
	21st to end	Night	15% 6% 2 4 3 2 2 2 4 5% 56%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 5% 56%	2651
	11th to 20th	Night	12% 5% 2 4 4 3 3 2 4 5% 55%	2692
	21st to end	Night	14% 5% 2 5% 4 4 3 3 4 5% 50%	2600
December	1st to 10th	Night	19% 6% 2 5% 3 2 3 3 4 5% 47%	4250
	11th to 20th	Night	17% 8% 3 6% 4 3 3 2 4 4 46%	4822
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 6% 42%	6794

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	42% 4 5% 28% 18% 3	3778
	11th to 20th	Day	35% 4 5% 25% 27% 4	4080
	21st to end	Day	47% 3 6% 16% 20% 8%	4924
February	1st to 10th	Day	43% 5% 9% 19% 18% 6%	2000
	11th to 20th	Day	48% 4 4 19% 17% 7%	1958
	21st to end	Day	46% 4 6% 14% 21% 8%	1650
March	1st to 10th	Day	51% 4 6% 13% 18% 7%	4480
	11th to 20th	Day	38% 5% 7% 16% 23% 12%	4790
	21st to end	Day	43% 5% 8% 21% 18% 5%	5956
April	1st to 10th	Day	36% 5% 11% 21% 20% 7%	2400
	11th to 20th	Day	36% 5% 7% 20% 24% 7%	2374
	21st to end	Day	34% 6% 10% 25% 22% 3	2400
May	1st to 10th	Day	42% 7% 11% 26% 11% 2	5440
	11th to 20th	Day	43% 8% 16% 22% 10%	5807
	21st to end	Day	38% 8% 13% 26% 14% 1	6447
June	1st to 10th	Day	36% 10% 13% 22% 17% 2	2643
	11th to 20th	Day	31% 9% 15% 24% 18% 3	2800
	21st to end	Day	32% 10% 17% 23% 15% 3	2800
July	1st to 10th	Day	37% 11% 17% 25% 8% 1	6291
	11th to 20th	Day	34% 12% 27% 23% 4 1	6058
	21st to end	Day	41% 11% 22% 22% 5%	5757
August	1st to 10th	Day	21% 8% 12% 31% 27% 1	3893
	11th to 20th	Day	30% 9% 16% 28% 15% 2	3885
	21st to end	Day	31% 8% 14% 20% 22% 4	4311
September	1st to 10th	Day	34% 10% 12% 20% 18% 5%	2400
	11th to 20th	Day	39% 6% 10% 17% 22% 6%	2268
	21st to end	Day	31% 6% 11% 23% 21% 9%	2200
October	1st to 10th	Day	19% 3 6% 15% 26% 30%	3590
	11th to 20th	Day	17% 2 4 15% 37% 24%	3273
	21st to end	Day	24% 3 5% 19% 35% 13%	3538
November	1st to 10th	Day	31% 4 8% 21% 27% 10%	1862
	11th to 20th	Day	32% 3 7% 19% 27% 12%	1863
	21st to end	Day	39% 5% 6% 17% 25% 8%	1800
December	1st to 10th	Day	43% 3 7% 23% 21% 4	2930
	11th to 20th	Day	46% 4 8% 18% 19% 6%	3370
	21st to end	Day	40% 4 8% 22% 24% 1	4158
January	1st to 10th	Night	42% 3 5% 24% 23% 2	6185
	11th to 20th	Night	42% 3 3 19% 25% 7%	5900
	21st to end	Night	45% 3 4 17% 24% 7%	6015
February	1st to 10th	Night	44% 3 7% 17% 20% 8%	2400
	11th to 20th	Night	46% 4 5% 17% 18% 10%	2339
	21st to end	Night	45% 5% 6% 15% 19% 10%	1980
March	1st to 10th	Night	49% 4 7% 13% 17% 10%	5373
	11th to 20th	Night	32% 5% 6% 18% 21% 18%	5211
	21st to end	Night	44% 3 7% 21% 19% 7%	4973
April	1st to 10th	Night	35% 6% 9% 20% 20% 9%	2000
	11th to 20th	Night	33% 4 7% 22% 23% 10%	1975
	21st to end	Night	33% 6% 11% 22% 23% 5%	2000
May	1st to 10th	Night	40% 7% 15% 21% 15% 2	4520
	11th to 20th	Night	47% 7% 11% 24% 10% 1	4089
	21st to end	Night	36% 9% 13% 22% 18% 2	4429
June	1st to 10th	Night	33% 9% 15% 21% 20% 3	1750
	11th to 20th	Night	24% 9% 15% 28% 18% 6%	1600
	21st to end	Night	28% 8% 19% 22% 19% 4	1600
July	1st to 10th	Night	32% 13% 23% 22% 10% 2	3580
	11th to 20th	Night	28% 13% 30% 24% 4 1	3793
	21st to end	Night	41% 12% 21% 20% 5% 1	4776
August	1st to 10th	Night	16% 7% 14% 33% 27% 3	3232
	11th to 20th	Night	21% 9% 22% 30% 15% 3	3230
	21st to end	Night	29% 8% 14% 21% 21% 6%	3586
September	1st to 10th	Night	32% 6% 14% 24% 18% 6%	2000
	11th to 20th	Night	32% 8% 13% 17% 23% 7%	2110
	21st to end	Night	28% 6% 8% 25% 22% 10%	2200
October	1st to 10th	Night	17% 3 7% 16% 27% 31%	3590
	11th to 20th	Night	19% 3 4 17% 29% 29%	3861
	21st to end	Night	22% 2 6% 19% 31% 20%	4252
November	1st to 10th	Night	30% 4 8% 18% 27% 12%	2651
	11th to 20th	Night	31% 4 7% 19% 27% 12%	2692
	21st to end	Night	36% 4 6% 21% 23% 10%	2600
December	1st to 10th	Night	40% 3 5% 22% 26% 4	4250
	11th to 20th	Night	42% 4 8% 18% 23% 4	4824
	21st to end	Night	42% 5% 7% 21% 22% 2	6794

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% 21% 32% 20% 4 1	300
	11th to 20th	Day	22% 21% 33% 15% 7% 2	300
	21st to end	Day	22% 20% 35% 18% 4 1	330
February	1st to 10th	Day	21% 15% 34% 19% 10% 1	300
	11th to 20th	Day	34% 16% 31% 15% 3 1	300
	21st to end	Day	28% 19% 33% 16% 4	249
March	1st to 10th	Day	20% 18% 33% 20% 8%	300
	11th to 20th	Day	21% 16% 37% 21% 4 1	300
	21st to end	Day	18% 14% 33% 23% 11%	537
April	1st to 10th	Day	17% 18% 27% 25% 11% 2	500
	11th to 20th	Day	18% 12% 30% 23% 13% 2	492
	21st to end	Day	10% 14% 25% 31% 17% 4	500
May	1st to 10th	Day	16% 9% 20% 32% 17% 5%	503
	11th to 20th	Day	10% 6% 23% 30% 20% 12%	527
	21st to end	Day	10% 4 19% 30% 21% 15%	573
June	1st to 10th	Day	9% 3 15% 29% 27% 17%	573
	11th to 20th	Day	6% 2 11% 27% 32% 21%	730
	21st to end	Day	7% 2 10% 24% 35% 22%	730
July	1st to 10th	Day	7% 2 7% 21% 38% 25%	721
	11th to 20th	Day	7% 2 6% 20% 32% 34%	718
	21st to end	Day	6% 1 6% 20% 37% 30%	581
August	1st to 10th	Day	5% 2 6% 22% 36% 28%	526
	11th to 20th	Day	8% 1 8% 21% 33% 30%	519
	21st to end	Day	12% 2 7% 21% 30% 27%	583
September	1st to 10th	Day	9% 3 8% 25% 37% 18%	530
	11th to 20th	Day	11% 2 11% 20% 34% 23%	530
	21st to end	Day	10% 3 11% 30% 32% 15%	530
October	1st to 10th	Day	9% 3 17% 30% 29% 12%	530
	11th to 20th	Day	9% 7% 27% 27% 22% 7%	350
	21st to end	Day	17% 10% 23% 28% 16% 5%	363
November	1st to 10th	Day	15% 10% 23% 30% 17% 5%	330
	11th to 20th	Day	12% 11% 33% 32% 9% 2	321
	21st to end	Day	13% 16% 33% 21% 13% 4	300
December	1st to 10th	Day	22% 12% 37% 20% 8% 1	300
	11th to 20th	Day	18% 14% 35% 19% 12% 2	300
	21st to end	Day	22% 13% 36% 21% 8% 1	330
January	1st to 10th	Night	20% 12% 33% 28% 7%	1000
	11th to 20th	Night	21% 12% 32% 28% 7%	1000
	21st to end	Night	20% 11% 36% 29% 3	898
February	1st to 10th	Night	21% 10% 31% 32% 7%	800
	11th to 20th	Night	26% 12% 28% 29% 4	783
	21st to end	Night	21% 14% 34% 29% 3	660
March	1st to 10th	Night	18% 12% 30% 33% 7%	800
	11th to 20th	Night	16% 9% 36% 30% 9%	701
	21st to end	Night	16% 10% 32% 30% 13% 1	660
April	1st to 10th	Night	13% 11% 31% 32% 13%	600
	11th to 20th	Night	12% 7% 32% 33% 14% 2	593
	21st to end	Night	11% 7% 31% 35% 14% 2	600
May	1st to 10th	Night	11% 4 24% 39% 20% 2	600
	11th to 20th	Night	13% 4 17% 35% 27% 4	591
	21st to end	Night	9% 1 12% 38% 32% 7%	650
June	1st to 10th	Night	8% 3 8% 38% 37% 7%	600
	11th to 20th	Night	5% 1 8% 29% 46% 12%	600
	21st to end	Night	5% 1 8% 27% 47% 12%	600
July	1st to 10th	Night	4 1 5% 23% 51% 16%	585
	11th to 20th	Night	4 1 4 24% 46% 21%	582
	21st to end	Night	5% 1 5% 23% 49% 16%	657
August	1st to 10th	Night	5% 3 21% 51% 19%	597
	11th to 20th	Night	6% 1 3 24% 50% 16%	594
	21st to end	Night	8% 1 7% 22% 47% 14%	660
September	1st to 10th	Night	11% 2 7% 31% 43% 7%	600
	11th to 20th	Night	12% 2 10% 26% 43% 7%	710
	21st to end	Night	9% 3 11% 33% 39% 5%	800
October	1st to 10th	Night	8% 3 14% 34% 36% 6%	800
	11th to 20th	Night	13% 4 18% 40% 24% 1	800
	21st to end	Night	17% 5% 21% 32% 23% 2	880
November	1st to 10th	Night	13% 5% 17% 39% 22% 3	820
	11th to 20th	Night	13% 6% 29% 36% 15% 1	829
	21st to end	Night	18% 8% 25% 34% 15% 1	800
December	1st to 10th	Night	21% 9% 27% 33% 10%	800
	11th to 20th	Night	21% 8% 28% 32% 11%	800
	21st to end	Night	23% 9% 31% 29% 7%	1015

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