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 2025-07-06.

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% 37%	119595
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 38%	119189
Transparency	Day	45% 6% 12% 21% 14% 2	123160
	Night	44% 6% 12% 21% 15% 2	123785
Seeing	Day	24% 18% 31% 18% 7% 2	15806
	Night	22% 17% 34% 21% 6%	24778

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	11% 9% 4 10% 6% 5% 5% 3 5% 5% 37%	3597
	11th to 20th	Day	13% 9% 3 7% 4 4 4 3 5% 5% 43%	3570
	21st to end	Day	16% 10% 3 6% 4 3 4 3 5% 6% 40%	3960
February	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 5% 6% 7% 33%	1865
	11th to 20th	Day	14% 7% 3 8% 7% 5% 4 4 5% 7% 36%	1948
	21st to end	Day	18% 11% 3 7% 4 4 4 3 4 5% 37%	1650
March	1st to 10th	Day	20% 12% 3 8% 4 4 4 3 4 5% 34%	4476
	11th to 20th	Day	15% 8% 2 7% 5% 4 5% 4 6% 6% 37%	4694
	21st to end	Day	23% 9% 3 5% 4 3 5% 4 6% 6% 32%	5910
April	1st to 10th	Day	19% 10% 3 7% 5% 4 4 3 4 6% 36%	2400
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 5% 7% 39%	2374
	21st to end	Day	20% 7% 2 7% 4 5% 5% 3 4 7% 35%	2570
May	1st to 10th	Day	30% 10% 3 7% 4 4 4 3 5% 7% 24%	5886
	11th to 20th	Day	20% 10% 3 9% 5% 5% 4 3 4 6% 30%	6088
	21st to end	Day	19% 8% 3 6% 5% 5% 5% 4 6% 7% 34%	6949
June	1st to 10th	Day	18% 7% 2 6% 5% 4 4 4 7% 9% 34%	2800
	11th to 20th	Day	17% 7% 3 6% 4 5% 4 4 7% 9% 36%	2800
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 6% 8% 35%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 9% 35%	4510
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 5% 7% 10% 38%	4440
	21st to end	Day	19% 8% 2 7% 4 5% 4 4 6% 8% 33%	4814
August	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 32%	4233
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 38%	3890
	21st to end	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 44%	4180
September	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 3 6% 6% 44%	2280
	11th to 20th	Day	14% 7% 3 4 3 2 3 2 5% 6% 50%	2119
	21st to end	Day	17% 6% 2 5% 4 3 5% 3 5% 7% 42%	2090
October	1st to 10th	Day	21% 8% 3 5% 4 3 4 3 4 6% 38%	2090
	11th to 20th	Day	15% 7% 2 5% 3 3 4 3 5% 7% 46%	1844
	21st to end	Day	20% 8% 2 6% 5% 3 4 3 5% 6% 38%	1881
November	1st to 10th	Day	13% 8% 2 7% 4 3 3 3 4 5% 48%	1761
	11th to 20th	Day	18% 7% 2 4 3 4 3 3 4 6% 45%	1800
	21st to end	Day	18% 8% 2 5% 4 5% 3 4 5% 6% 41%	1780
December	1st to 10th	Day	15% 9% 2 6% 5% 4 4 2 5% 6% 43%	2600
	11th to 20th	Day	19% 8% 2 6% 3 3 4 3 5% 5% 43%	2986
	21st to end	Day	19% 12% 3 6% 4 4 4 3 4 6% 36%	3960
January	1st to 10th	Night	13% 7% 3 8% 4 5% 4 4 5% 7% 40%	6349
	11th to 20th	Night	14% 9% 4 9% 5% 4 4 3 6% 6% 37%	6315
	21st to end	Night	15% 10% 3 6% 5% 4 4 3 5% 5% 38%	6996
February	1st to 10th	Night	16% 9% 3 6% 4 4 5% 4 5% 6% 37%	2531
	11th to 20th	Night	14% 9% 2 8% 5% 5% 5% 4 5% 7% 37%	2338
	21st to end	Night	17% 10% 2 7% 5% 5% 4 4 4 5% 37%	1980
March	1st to 10th	Night	20% 10% 2 5% 4 4 4 3 5% 6% 36%	5364
	11th to 20th	Night	16% 7% 3 7% 5% 5% 4 4 5% 7% 38%	5265
	21st to end	Night	23% 11% 3 6% 4 4 4 3 5% 6% 33%	5018
April	1st to 10th	Night	19% 8% 3 7% 6% 5% 5% 4 5% 6% 33%	2000
	11th to 20th	Night	20% 7% 2 6% 4 4 4 4 4 6% 37%	1975
	21st to end	Night	20% 8% 2 7% 4 3 3 4 4 6% 39%	1829
May	1st to 10th	Night	31% 10% 2 6% 4 3 4 4 5% 7% 24%	4054
	11th to 20th	Night	17% 11% 4 10% 5% 5% 5% 4 5% 7% 28%	3833
	21st to end	Night	21% 8% 3 7% 5% 5% 4 3 5% 7% 32%	3926
June	1st to 10th	Night	17% 8% 3 7% 6% 4 4 4 5% 6% 37%	1600
	11th to 20th	Night	18% 9% 2 6% 4 3 4 4 6% 8% 37%	1600
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 8% 42%	1600
July	1st to 10th	Night	14% 6% 2 6% 6% 5% 5% 5% 7% 9% 35%	2560
	11th to 20th	Night	17% 7% 2 6% 3 4 4 4 6% 9% 38%	2520
	21st to end	Night	16% 7% 3 8% 5% 4 4 4 5% 6% 37%	2739
August	1st to 10th	Night	22% 6% 3 7% 5% 5% 4 4 5% 7% 32%	2663
	11th to 20th	Night	13% 8% 3 8% 5% 4 5% 4 5% 7% 38%	2932
	21st to end	Night	8% 4 1 5% 4 3 3 3 5% 7% 58%	3476
September	1st to 10th	Night	13% 8% 2 5% 4 3 4 3 5% 7% 46%	1900
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 6% 52%	2060
	21st to end	Night	16% 8% 2 5% 4 3 4 3 5% 6% 44%	2090
October	1st to 10th	Night	20% 7% 2 7% 4 3 4 3 4 6% 40%	2090
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 5% 6% 46%	2332
	21st to end	Night	21% 8% 3 6% 4 4 3 3 4 6% 39%	2717
November	1st to 10th	Night	14% 6% 2 6% 3 3 4 3 4 6% 48%	2537
	11th to 20th	Night	17% 6% 3 5% 5% 4 4 3 4 5% 44%	2600
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 7% 42%	2616
December	1st to 10th	Night	18% 9% 2 6% 4 4 4 3 5% 6% 40%	4580
	11th to 20th	Night	23% 8% 3 6% 3 3 3 3 4 4 38%	5208
	21st to end	Night	19% 10% 4 6% 4 3 3 3 4 5% 38%	6996

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	44% 6% 9% 27% 12% 2	3598
	11th to 20th	Day	42% 5% 11% 21% 16% 5%	3600
	21st to end	Day	44% 5% 10% 22% 18% 1	3960
February	1st to 10th	Day	47% 7% 15% 17% 12% 2	1865
	11th to 20th	Day	45% 7% 11% 20% 14% 3	1957
	21st to end	Day	50% 5% 10% 19% 14% 2	1650
March	1st to 10th	Day	51% 5% 11% 14% 11% 8%	4479
	11th to 20th	Day	42% 7% 16% 25% 8% 2	4694
	21st to end	Day	49% 5% 7% 17% 17% 5%	5910
April	1st to 10th	Day	49% 4 10% 20% 14% 3	2400
	11th to 20th	Day	44% 6% 12% 19% 17% 3	2374
	21st to end	Day	48% 5% 9% 19% 16% 3	2570
May	1st to 10th	Day	59% 7% 13% 12% 8% 1	5897
	11th to 20th	Day	53% 7% 12% 17% 10% 1	6092
	21st to end	Day	46% 7% 12% 21% 12% 1	6949
June	1st to 10th	Day	45% 8% 13% 23% 10% 1	2800
	11th to 20th	Day	43% 7% 15% 20% 13% 1	2800
	21st to end	Day	45% 8% 12% 21% 13% 1	2800
July	1st to 10th	Day	44% 8% 11% 21% 14% 1	4440
	11th to 20th	Day	39% 8% 17% 23% 12% 1	4365
	21st to end	Day	48% 7% 13% 19% 13% 2	4661
August	1st to 10th	Day	48% 9% 15% 20% 8% 1	4219
	11th to 20th	Day	41% 8% 16% 26% 8% 1	3929
	21st to end	Day	33% 7% 13% 30% 15% 1	4180
September	1st to 10th	Day	39% 7% 11% 24% 18% 1	2280
	11th to 20th	Day	35% 5% 9% 26% 22% 3	2119
	21st to end	Day	41% 6% 10% 22% 19% 2	2090
October	1st to 10th	Day	40% 6% 10% 23% 19% 3	3480
	11th to 20th	Day	32% 5% 9% 26% 25% 3	3034
	21st to end	Day	34% 5% 13% 28% 17% 3	3081
November	1st to 10th	Day	39% 5% 12% 25% 17% 2	1761
	11th to 20th	Day	39% 5% 11% 22% 20% 3	1800
	21st to end	Day	43% 5% 14% 25% 11% 2	1780
December	1st to 10th	Day	42% 4 16% 27% 10% 1	2600
	11th to 20th	Day	43% 4 15% 18% 19% 2	2986
	21st to end	Day	49% 4 9% 19% 14% 5%	3960
January	1st to 10th	Night	40% 6% 11% 28% 13% 2	6349
	11th to 20th	Night	45% 5% 15% 20% 12% 3	6360
	21st to end	Night	46% 5% 11% 16% 17% 4	6996
February	1st to 10th	Night	45% 7% 13% 21% 11% 2	2531
	11th to 20th	Night	46% 6% 11% 20% 15% 3	2340
	21st to end	Night	48% 5% 9% 22% 13% 4	1980
March	1st to 10th	Night	46% 5% 11% 17% 15% 5%	5374
	11th to 20th	Night	44% 7% 18% 18% 10% 3	5266
	21st to end	Night	50% 4 7% 14% 21% 4	5018
April	1st to 10th	Night	50% 5% 10% 19% 14% 3	2000
	11th to 20th	Night	46% 7% 9% 19% 18% 1	1975
	21st to end	Night	45% 5% 10% 19% 15% 5%	1829
May	1st to 10th	Night	57% 9% 13% 12% 8% 1	4060
	11th to 20th	Night	53% 9% 12% 15% 9% 2	3834
	21st to end	Night	50% 8% 13% 18% 10%	3927
June	1st to 10th	Night	46% 7% 12% 22% 12% 1	1600
	11th to 20th	Night	45% 8% 13% 22% 12% 1	1600
	21st to end	Night	38% 10% 14% 23% 15%	1600
July	1st to 10th	Night	42% 9% 14% 21% 13% 1	2520
	11th to 20th	Night	42% 7% 16% 22% 12%	2472
	21st to end	Night	47% 6% 13% 18% 15% 1	2651
August	1st to 10th	Night	51% 10% 11% 19% 8% 1	2653
	11th to 20th	Night	44% 8% 15% 24% 8%	2959
	21st to end	Night	26% 6% 16% 32% 19% 1	3476
September	1st to 10th	Night	38% 8% 11% 24% 18% 1	1900
	11th to 20th	Night	34% 6% 10% 24% 25% 2	2060
	21st to end	Night	40% 6% 8% 24% 18% 3	2090
October	1st to 10th	Night	39% 5% 14% 24% 15% 2	3480
	11th to 20th	Night	28% 5% 12% 25% 27% 3	3881
	21st to end	Night	34% 4 14% 30% 16% 2	4467
November	1st to 10th	Night	37% 7% 10% 22% 20% 3	2537
	11th to 20th	Night	41% 5% 12% 20% 19% 3	2600
	21st to end	Night	40% 6% 16% 26% 11% 1	2616
December	1st to 10th	Night	43% 5% 13% 29% 9%	4580
	11th to 20th	Night	47% 5% 14% 14% 19% 1	5208
	21st to end	Night	47% 5% 9% 21% 15% 4	6996

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% 16% 3	200
	11th to 20th	Day	31% 30% 30% 8% 2	200
	21st to end	Day	25% 30% 33% 10% 3	220
February	1st to 10th	Day	35% 28% 27% 10% 1	377
	11th to 20th	Day	32% 30% 29% 8% 1	496
	21st to end	Day	32% 22% 33% 10% 3	414
March	1st to 10th	Day	32% 24% 33% 9% 3	500
	11th to 20th	Day	30% 27% 29% 11% 3	500
	21st to end	Day	31% 25% 30% 9% 4 2	550
April	1st to 10th	Day	34% 23% 28% 10% 4 1	500
	11th to 20th	Day	34% 21% 30% 10% 5% 1	492
	21st to end	Day	30% 22% 30% 13% 4 1	500
May	1st to 10th	Day	33% 22% 27% 13% 5% 1	503
	11th to 20th	Day	24% 17% 38% 16% 5%	527
	21st to end	Day	20% 14% 35% 20% 9% 1	573
June	1st to 10th	Day	17% 15% 36% 22% 8% 2	530
	11th to 20th	Day	19% 14% 38% 20% 7% 2	530
	21st to end	Day	17% 17% 34% 21% 9% 2	530
July	1st to 10th	Day	17% 10% 31% 28% 11% 3	505
	11th to 20th	Day	14% 8% 31% 32% 13% 2	484
	21st to end	Day	14% 10% 36% 30% 9% 2	532
August	1st to 10th	Day	12% 9% 32% 34% 10% 2	486
	11th to 20th	Day	15% 10% 31% 27% 13% 4	479
	21st to end	Day	12% 13% 32% 25% 15% 4	539
September	1st to 10th	Day	18% 11% 30% 22% 13% 6%	490
	11th to 20th	Day	19% 10% 30% 19% 13% 9%	490
	21st to end	Day	20% 15% 32% 20% 10% 4	490
October	1st to 10th	Day	27% 15% 25% 20% 10% 4	490
	11th to 20th	Day	21% 19% 29% 22% 6% 2	455
	21st to end	Day	26% 19% 30% 18% 6% 1	429
November	1st to 10th	Day	22% 18% 33% 17% 8% 1	395
	11th to 20th	Day	29% 25% 27% 16% 3 1	400
	21st to end	Day	29% 24% 28% 15% 4	380
December	1st to 10th	Day	23% 32% 29% 13% 4	200
	11th to 20th	Day	40% 22% 30% 7% 2	200
	21st to end	Day	28% 26% 32% 9% 4 1	220
January	1st to 10th	Night	24% 23% 39% 13% 2	800
	11th to 20th	Night	30% 21% 32% 15% 2	800
	21st to end	Night	21% 24% 34% 17% 2	880
February	1st to 10th	Night	24% 23% 36% 16% 1	800
	11th to 20th	Night	25% 27% 38% 10% 1	783
	21st to end	Night	27% 23% 35% 13% 2	660
March	1st to 10th	Night	26% 24% 34% 15% 2	800
	11th to 20th	Night	22% 26% 35% 15% 2	800
	21st to end	Night	28% 21% 31% 16% 3	880
April	1st to 10th	Night	26% 20% 35% 16% 3	800
	11th to 20th	Night	25% 22% 32% 18% 4	791
	21st to end	Night	24% 21% 34% 18% 3	629
May	1st to 10th	Night	30% 16% 33% 18% 3	600
	11th to 20th	Night	17% 17% 41% 20% 4	509
	21st to end	Night	19% 9% 39% 27% 6%	433
June	1st to 10th	Night	18% 11% 35% 29% 7%	400
	11th to 20th	Night	20% 8% 39% 26% 7%	400
	21st to end	Night	14% 7% 38% 34% 8%	400
July	1st to 10th	Night	15% 6% 31% 39% 9%	380
	11th to 20th	Night	16% 3 28% 40% 13% 1	368
	21st to end	Night	12% 6% 32% 40% 10%	416
August	1st to 10th	Night	11% 4 34% 39% 10% 1	470
	11th to 20th	Night	13% 7% 28% 37% 13% 1	647
	21st to end	Night	14% 6% 27% 36% 16% 2	836
September	1st to 10th	Night	17% 6% 34% 31% 10% 1	760
	11th to 20th	Night	14% 10% 28% 29% 18% 2	760
	21st to end	Night	19% 10% 32% 27% 11% 2	760
October	1st to 10th	Night	24% 12% 27% 25% 10% 2	760
	11th to 20th	Night	20% 15% 34% 21% 10% 1	760
	21st to end	Night	27% 18% 32% 19% 5%	836
November	1st to 10th	Night	17% 19% 35% 21% 7% 1	780
	11th to 20th	Night	22% 24% 32% 16% 6% 1	800
	21st to end	Night	23% 22% 35% 17% 4	800
December	1st to 10th	Night	28% 20% 33% 15% 4	800
	11th to 20th	Night	26% 22% 34% 15% 3	800
	21st to end	Night	25% 23% 36% 14% 3	880

Page generated at 2025-07-03 01:30:15. Version ac07

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