Howe Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Howe Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	17% 8% 3 6% 4 4 4 4 5% 7% 38%	149273
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	142806
Transparency	Day	44% 6% 13% 22% 14% 2	152795
	Night	43% 6% 13% 21% 15% 2	147373
Seeing	Day	24% 18% 31% 18% 7% 2	17539
	Night	22% 17% 33% 21% 6%	26430

By Month

Percentage Cloud Cover NE (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	10% 9% 4 10% 6% 5% 5% 4 5% 6% 36%	4661
	11th to 20th	Day	14% 9% 3 7% 4 4 4 3 4 5% 42%	4650
	21st to end	Day	13% 11% 2 6% 4 3 4 3 4 7% 43%	5124
February	1st to 10th	Day	16% 8% 3 7% 5% 5% 6% 5% 5% 7% 34%	1959
	11th to 20th	Day	14% 8% 3 8% 6% 5% 4 4 5% 7% 36%	2048
	21st to end	Day	18% 11% 3 7% 4 4 3 3 4 5% 37%	1730
March	1st to 10th	Day	20% 12% 3 7% 4 4 4 3 4 5% 34%	4576
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 6% 36%	4797
	21st to end	Day	23% 9% 3 5% 4 3 5% 4 6% 6% 32%	6040
April	1st to 10th	Day	19% 10% 3 6% 5% 4 4 3 4 6% 36%	2520
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 5% 7% 39%	2494
	21st to end	Day	20% 7% 2 7% 4 5% 5% 3 4 7% 36%	2698
May	1st to 10th	Day	27% 10% 3 7% 4 4 4 3 5% 7% 27%	7639
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 4 6% 35%	7922
	21st to end	Day	21% 8% 3 6% 5% 4 5% 4 7% 7% 30%	9060
June	1st to 10th	Day	17% 7% 2 6% 5% 5% 4 4 7% 8% 35%	2940
	11th to 20th	Day	16% 6% 3 6% 4 5% 4 4 7% 9% 36%	2940
	21st to end	Day	17% 7% 2 6% 5% 4 5% 4 6% 8% 35%	2940
July	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 5% 7% 9% 34%	8280
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 5% 7% 11% 41%	8280
	21st to end	Day	17% 7% 2 5% 3 4 4 3 5% 7% 43%	8993
August	1st to 10th	Day	14% 7% 3 8% 5% 6% 6% 5% 7% 9% 31%	6203
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 43%	5723
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 7% 9% 41%	6080
September	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 3 6% 6% 44%	2520
	11th to 20th	Day	14% 6% 3 4 3 3 3 2 5% 6% 50%	2342
	21st to end	Day	18% 7% 2 5% 4 3 5% 4 6% 7% 41%	2310
October	1st to 10th	Day	20% 7% 3 5% 4 3 4 3 4 6% 42%	2310
	11th to 20th	Day	16% 7% 2 5% 3 3 4 3 5% 7% 45%	2038
	21st to end	Day	19% 8% 3 6% 4 3 4 3 5% 6% 40%	2052
November	1st to 10th	Day	12% 8% 2 7% 4 3 4 3 4 5% 47%	1851
	11th to 20th	Day	17% 7% 3 4 3 3 3 3 4 6% 45%	1890
	21st to end	Day	18% 8% 2 5% 4 5% 3 4 5% 5% 41%	1869
December	1st to 10th	Day	15% 8% 2 6% 4 4 4 2 5% 6% 43%	2680
	11th to 20th	Day	19% 8% 2 6% 4 3 4 3 4 5% 43%	3066
	21st to end	Day	19% 11% 3 6% 4 4 4 3 4 6% 36%	4048
January	1st to 10th	Night	14% 8% 3 8% 4 4 4 4 5% 7% 40%	8234
	11th to 20th	Night	15% 11% 3 9% 5% 5% 4 3 6% 6% 34%	8235
	21st to end	Night	13% 10% 3 6% 5% 4 4 3 5% 5% 41%	9057
February	1st to 10th	Night	15% 9% 3 6% 4 4 5% 4 5% 7% 38%	2657
	11th to 20th	Night	13% 9% 2 8% 5% 5% 5% 4 5% 7% 36%	2458
	21st to end	Night	17% 10% 2 7% 5% 5% 4 4 4 5% 37%	2076
March	1st to 10th	Night	21% 10% 2 5% 4 3 4 3 5% 6% 36%	5484
	11th to 20th	Night	16% 7% 3 6% 5% 5% 4 4 5% 7% 38%	5382
	21st to end	Night	23% 11% 3 6% 4 4 4 3 5% 6% 33%	5130
April	1st to 10th	Night	20% 8% 2 7% 6% 5% 5% 3 5% 6% 34%	2100
	11th to 20th	Night	20% 7% 2 6% 5% 4 4 4 4 6% 38%	2075
	21st to end	Night	20% 8% 2 7% 4 3 3 4 4 6% 40%	1920
May	1st to 10th	Night	27% 10% 2 6% 4 4 4 4 5% 7% 28%	5259
	11th to 20th	Night	16% 10% 3 9% 5% 5% 5% 4 6% 7% 31%	4979
	21st to end	Night	22% 8% 3 7% 5% 4 4 3 5% 7% 30%	5114
June	1st to 10th	Night	16% 8% 3 7% 5% 4 4 4 5% 6% 39%	1680
	11th to 20th	Night	18% 9% 2 6% 4 3 4 4 6% 8% 37%	1680
	21st to end	Night	14% 6% 3 6% 4 4 4 4 5% 8% 42%	1680
July	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 6% 9% 36%	4680
	11th to 20th	Night	14% 6% 2 5% 3 4 4 4 6% 9% 43%	4680
	21st to end	Night	14% 6% 2 6% 4 3 4 3 5% 6% 46%	5084
August	1st to 10th	Night	17% 6% 3 7% 5% 6% 5% 4 6% 8% 33%	3898
	11th to 20th	Night	13% 7% 3 8% 5% 4 5% 4 5% 8% 39%	4317
	21st to end	Night	11% 5% 2 6% 4 3 4 3 5% 7% 51%	5045
September	1st to 10th	Night	13% 8% 2 6% 4 3 4 3 5% 7% 45%	2100
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 6% 51%	2277
	21st to end	Night	18% 8% 2 5% 4 3 4 3 5% 6% 43%	2310
October	1st to 10th	Night	18% 6% 2 7% 4 3 4 3 4 6% 44%	2310
	11th to 20th	Night	15% 6% 2 5% 4 4 4 3 5% 6% 46%	2578
	21st to end	Night	19% 7% 2 6% 4 4 3 3 4 5% 41%	2965
November	1st to 10th	Night	14% 7% 2 6% 3 3 4 3 4 6% 47%	2667
	11th to 20th	Night	17% 6% 3 5% 5% 4 4 3 4 5% 44%	2730
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 7% 42%	2747
December	1st to 10th	Night	17% 9% 2 6% 4 3 4 3 5% 6% 40%	4720
	11th to 20th	Night	23% 8% 3 6% 3 3 4 3 4 4 39%	5348
	21st to end	Night	18% 10% 4 6% 4 3 3 3 4 5% 38%	7150

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	45% 6% 11% 24% 12% 2	4662
	11th to 20th	Day	43% 5% 10% 21% 17% 5%	4680
	21st to end	Day	41% 4 11% 23% 20% 1	5124
February	1st to 10th	Day	46% 6% 15% 17% 14% 2	1959
	11th to 20th	Day	45% 7% 11% 21% 13% 3	2057
	21st to end	Day	50% 5% 10% 19% 14% 2	1730
March	1st to 10th	Day	51% 5% 11% 14% 11% 8%	4579
	11th to 20th	Day	42% 7% 16% 24% 8% 2	4797
	21st to end	Day	49% 5% 7% 18% 17% 5%	6040
April	1st to 10th	Day	49% 4 10% 20% 14% 3	2520
	11th to 20th	Day	44% 6% 12% 19% 17% 2	2494
	21st to end	Day	48% 5% 9% 19% 16% 3	2698
May	1st to 10th	Day	55% 7% 12% 15% 10%	7651
	11th to 20th	Day	49% 6% 14% 19% 11% 1	7926
	21st to end	Day	50% 6% 11% 21% 11% 1	9061
June	1st to 10th	Day	44% 8% 13% 24% 11% 1	2940
	11th to 20th	Day	42% 7% 15% 21% 13% 1	2940
	21st to end	Day	45% 8% 12% 21% 13% 1	2940
July	1st to 10th	Day	46% 7% 13% 21% 12% 1	8210
	11th to 20th	Day	34% 7% 18% 27% 13% 1	8205
	21st to end	Day	41% 5% 11% 26% 16% 1	8795
August	1st to 10th	Day	47% 8% 17% 17% 9% 1	6189
	11th to 20th	Day	37% 7% 18% 30% 7% 1	5762
	21st to end	Day	36% 7% 15% 27% 13% 2	6080
September	1st to 10th	Day	39% 7% 10% 25% 18% 1	2520
	11th to 20th	Day	34% 5% 9% 26% 22% 3	2342
	21st to end	Day	42% 6% 10% 22% 18% 2	2310
October	1st to 10th	Day	38% 6% 10% 23% 20% 3	3700
	11th to 20th	Day	33% 5% 9% 25% 25% 3	3228
	21st to end	Day	33% 4 13% 28% 18% 3	3252
November	1st to 10th	Day	39% 5% 12% 25% 17% 2	1851
	11th to 20th	Day	39% 5% 11% 23% 19% 3	1890
	21st to end	Day	44% 5% 13% 26% 10% 2	1869
December	1st to 10th	Day	42% 4 16% 26% 11% 1	2680
	11th to 20th	Day	43% 4 15% 18% 19% 2	3066
	21st to end	Day	49% 4 9% 19% 13% 5%	4048
January	1st to 10th	Night	41% 5% 13% 26% 13% 2	8234
	11th to 20th	Night	47% 5% 13% 19% 13% 4	8280
	21st to end	Night	43% 5% 11% 18% 20% 3	9057
February	1st to 10th	Night	45% 7% 13% 22% 12% 2	2657
	11th to 20th	Night	45% 5% 11% 20% 14% 3	2460
	21st to end	Night	49% 5% 9% 22% 12% 4	2076
March	1st to 10th	Night	46% 5% 11% 18% 15% 5%	5494
	11th to 20th	Night	44% 7% 18% 18% 11% 3	5383
	21st to end	Night	50% 4 7% 14% 21% 4	5130
April	1st to 10th	Night	50% 5% 10% 18% 15% 2	2100
	11th to 20th	Night	46% 7% 9% 19% 17% 1	2075
	21st to end	Night	45% 5% 11% 19% 16% 4	1920
May	1st to 10th	Night	53% 8% 14% 13% 10% 1	5265
	11th to 20th	Night	51% 8% 13% 18% 9% 2	4980
	21st to end	Night	51% 7% 12% 20% 9%	5115
June	1st to 10th	Night	45% 7% 11% 24% 13% 1	1680
	11th to 20th	Night	45% 8% 13% 22% 12% 1	1680
	21st to end	Night	38% 10% 14% 24% 14%	1680
July	1st to 10th	Night	44% 7% 15% 21% 13%	4640
	11th to 20th	Night	37% 6% 17% 23% 15% 1	4632
	21st to end	Night	39% 6% 12% 27% 16% 1	4967
August	1st to 10th	Night	47% 10% 15% 17% 10% 1	3888
	11th to 20th	Night	42% 8% 17% 25% 8%	4344
	21st to end	Night	32% 6% 16% 30% 15% 1	5045
September	1st to 10th	Night	39% 7% 11% 25% 18% 1	2100
	11th to 20th	Night	34% 6% 10% 23% 25% 2	2277
	21st to end	Night	42% 6% 8% 24% 17% 3	2310
October	1st to 10th	Night	37% 5% 14% 25% 16% 2	3700
	11th to 20th	Night	28% 5% 12% 25% 27% 3	4127
	21st to end	Night	34% 4 14% 30% 17% 2	4715
November	1st to 10th	Night	37% 7% 10% 23% 19% 3	2667
	11th to 20th	Night	41% 5% 12% 21% 18% 3	2730
	21st to end	Night	40% 6% 16% 26% 11% 1	2747
December	1st to 10th	Night	43% 5% 13% 29% 9%	4720
	11th to 20th	Night	47% 5% 14% 14% 19% 2	5348
	21st to end	Night	47% 5% 9% 21% 15% 4	7150

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	29% 26% 27% 15% 3	220
	11th to 20th	Day	32% 31% 28% 7% 2	220
	21st to end	Day	27% 31% 31% 9% 3	242
February	1st to 10th	Day	33% 28% 29% 10% 1	411
	11th to 20th	Day	32% 29% 31% 8% 1	536
	21st to end	Day	32% 23% 32% 9% 3	446
March	1st to 10th	Day	31% 24% 32% 10% 3	540
	11th to 20th	Day	32% 26% 28% 11% 2	540
	21st to end	Day	32% 27% 29% 8% 3 2	594
April	1st to 10th	Day	35% 23% 27% 9% 5% 1	540
	11th to 20th	Day	33% 22% 30% 10% 5% 1	532
	21st to end	Day	30% 21% 31% 12% 4 1	540
May	1st to 10th	Day	34% 22% 27% 12% 4 1	543
	11th to 20th	Day	24% 18% 38% 16% 4	567
	21st to end	Day	22% 15% 35% 19% 8% 1	617
June	1st to 10th	Day	16% 15% 36% 23% 8% 2	570
	11th to 20th	Day	20% 14% 38% 19% 7% 1	570
	21st to end	Day	17% 16% 35% 22% 9% 2	570
July	1st to 10th	Day	18% 10% 31% 29% 11% 2	565
	11th to 20th	Day	14% 9% 31% 32% 12% 2	564
	21st to end	Day	15% 10% 35% 30% 9% 2	620
August	1st to 10th	Day	12% 8% 31% 34% 11% 3	566
	11th to 20th	Day	14% 10% 31% 27% 13% 4	559
	21st to end	Day	12% 14% 32% 24% 14% 4	627
September	1st to 10th	Day	18% 10% 32% 21% 12% 5%	570
	11th to 20th	Day	18% 10% 30% 21% 14% 8%	570
	21st to end	Day	22% 15% 30% 19% 10% 4	570
October	1st to 10th	Day	25% 15% 25% 20% 10% 4	570
	11th to 20th	Day	24% 19% 28% 20% 6% 2	528
	21st to end	Day	25% 19% 30% 17% 6% 2	486
November	1st to 10th	Day	23% 20% 32% 16% 8% 1	425
	11th to 20th	Day	28% 25% 28% 15% 3 1	430
	21st to end	Day	31% 24% 27% 14% 4	409
December	1st to 10th	Day	25% 30% 28% 13% 4	220
	11th to 20th	Day	40% 23% 29% 6% 2	220
	21st to end	Day	31% 26% 31% 8% 3 1	242
January	1st to 10th	Night	24% 23% 39% 13% 2	840
	11th to 20th	Night	31% 21% 32% 14% 2	840
	21st to end	Night	22% 25% 34% 17% 2	924
February	1st to 10th	Night	23% 23% 36% 17% 2	840
	11th to 20th	Night	24% 26% 38% 11% 1	823
	21st to end	Night	28% 23% 35% 13% 2	692
March	1st to 10th	Night	26% 23% 35% 15% 2	840
	11th to 20th	Night	23% 25% 34% 15% 2	840
	21st to end	Night	28% 22% 32% 16% 3	924
April	1st to 10th	Night	27% 20% 35% 15% 3	840
	11th to 20th	Night	25% 22% 32% 17% 3	831
	21st to end	Night	23% 21% 35% 18% 3	660
May	1st to 10th	Night	30% 16% 33% 17% 3	630
	11th to 20th	Night	17% 17% 41% 20% 4	535
	21st to end	Night	20% 9% 39% 27% 5%	455
June	1st to 10th	Night	17% 11% 35% 31% 7%	420
	11th to 20th	Night	20% 9% 39% 25% 6%	420
	21st to end	Night	14% 7% 37% 36% 7%	420
July	1st to 10th	Night	16% 6% 31% 39% 8%	410
	11th to 20th	Night	16% 3 29% 39% 12%	408
	21st to end	Night	12% 6% 33% 38% 11%	460
August	1st to 10th	Night	12% 4 32% 41% 11% 1	520
	11th to 20th	Night	13% 7% 28% 37% 14% 1	717
	21st to end	Night	14% 6% 29% 34% 15% 2	924
September	1st to 10th	Night	17% 6% 34% 31% 11% 1	840
	11th to 20th	Night	14% 10% 27% 29% 18% 2	840
	21st to end	Night	20% 10% 30% 27% 10% 2	840
October	1st to 10th	Night	23% 12% 28% 25% 10% 2	840
	11th to 20th	Night	21% 14% 33% 22% 9% 1	840
	21st to end	Night	26% 17% 32% 20% 6%	913
November	1st to 10th	Night	19% 19% 35% 20% 7% 1	820
	11th to 20th	Night	22% 24% 32% 16% 6% 1	840
	21st to end	Night	23% 21% 35% 17% 4	840
December	1st to 10th	Night	28% 20% 33% 15% 4	840
	11th to 20th	Night	27% 22% 34% 14% 2	840
	21st to end	Night	27% 22% 35% 13% 2	924

Page generated at 2026-10-26 04:37:49. Version ac07

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