Howell Woods
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Howell Woods clear sky chart from 2005-11-06 to 2025-08-05.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	124744
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	123312
Transparency	Day	39% 6% 12% 20% 18% 5%	128346
	Night	38% 6% 11% 19% 20% 6%	127779
Seeing	Day	15% 9% 23% 25% 19% 9%	16387
	Night	15% 8% 25% 33% 17% 2	26756

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% 5% 2 5% 3 3 3 2 3 4 56%	4056
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 3 57%	4060
	21st to end	Day	12% 8% 3 7% 4 5% 4 3 4 4 45%	4466
February	1st to 10th	Day	15% 6% 3 6% 4 4 3 3 6% 7% 44%	1800
	11th to 20th	Day	21% 7% 2 5% 4 3 2 3 4 5% 45%	1815
	21st to end	Day	16% 7% 3 6% 4 5% 4 3 5% 6% 43%	1762
March	1st to 10th	Day	20% 8% 2 6% 4 4 4 3 5% 6% 39%	4935
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 4 6% 54%	4980
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 6% 49%	5465
April	1st to 10th	Day	10% 4 2 4 3 4 4 3 4 8% 53%	2353
	11th to 20th	Day	9% 5% 2 5% 3 3 4 4 6% 7% 50%	2374
	21st to end	Day	11% 5% 2 5% 4 3 5% 4 6% 6% 49%	2400
May	1st to 10th	Day	8% 7% 3 7% 5% 5% 6% 5% 7% 10% 36%	5630
	11th to 20th	Day	17% 8% 3 7% 5% 5% 6% 5% 7% 9% 29%	5861
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 6% 9% 42%	6433
June	1st to 10th	Day	9% 6% 2 8% 6% 6% 5% 5% 7% 9% 38%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	2600
	21st to end	Day	8% 4 2 6% 5% 6% 6% 5% 8% 11% 38%	2600
July	1st to 10th	Day	8% 5% 2 8% 6% 6% 7% 6% 9% 12% 31%	5900
	11th to 20th	Day	8% 7% 2 8% 7% 6% 7% 6% 9% 13% 27%	5900
	21st to end	Day	8% 7% 3 7% 6% 6% 6% 5% 9% 12% 31%	6424
August	1st to 10th	Day	19% 10% 3 10% 6% 6% 6% 5% 6% 9% 20%	4156
	11th to 20th	Day	9% 7% 3 9% 5% 5% 6% 5% 7% 9% 34%	3831
	21st to end	Day	7% 4 2 6% 4 4 5% 4 6% 12% 47%	4180
September	1st to 10th	Day	8% 5% 2 6% 5% 6% 5% 5% 7% 8% 43%	2280
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 3 5% 7% 41%	2206
	21st to end	Day	17% 7% 4 7% 5% 4 5% 4 5% 5% 37%	2019
October	1st to 10th	Day	15% 7% 2 5% 3 3 3 3 5% 5% 48%	1900
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 5% 55%	1900
	21st to end	Day	12% 5% 2 6% 4 3 3 2 3 5% 55%	2090
November	1st to 10th	Day	13% 6% 2 4 4 3 3 2 4 6% 52%	1957
	11th to 20th	Day	15% 5% 2 5% 4 3 3 3 5% 5% 50%	2000
	21st to end	Day	14% 5% 2 4 4 3 3 2 6% 6% 51%	1681
December	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 5% 52%	2600
	11th to 20th	Day	14% 7% 3 6% 4 4 3 3 4 6% 46%	3064
	21st to end	Day	15% 6% 3 6% 4 4 3 3 5% 7% 45%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 55%	5889
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 4 4 52%	5900
	21st to end	Night	13% 8% 2 6% 4 4 4 3 4 7% 46%	6490
February	1st to 10th	Night	17% 6% 2 6% 3 4 4 3 5% 5% 45%	2600
	11th to 20th	Night	19% 7% 3 5% 4 4 4 4 5% 5% 41%	2468
	21st to end	Night	15% 6% 2 4 3 3 4 3 5% 7% 47%	1865
March	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 4 5% 41%	4918
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 4 5% 50%	5467
April	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	2045
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 3 5% 7% 50%	1975
	21st to end	Night	10% 4 2 5% 4 4 4 4 6% 8% 49%	2000
May	1st to 10th	Night	9% 8% 3 8% 5% 5% 6% 4 6% 9% 36%	4306
	11th to 20th	Night	16% 8% 3 8% 5% 5% 4 4 6% 9% 33%	4032
	21st to end	Night	14% 6% 3 6% 4 4 4 4 6% 9% 41%	4421
June	1st to 10th	Night	10% 6% 2 6% 5% 5% 6% 5% 8% 10% 38%	1800
	11th to 20th	Night	10% 5% 2 7% 6% 5% 6% 4 7% 11% 38%	1800
	21st to end	Night	6% 5% 2 7% 5% 6% 5% 6% 9% 11% 38%	1800
July	1st to 10th	Night	10% 7% 3 8% 5% 5% 6% 5% 8% 12% 29%	4058
	11th to 20th	Night	11% 7% 3 9% 6% 6% 7% 6% 10% 13% 23%	4057
	21st to end	Night	12% 7% 3 9% 6% 5% 6% 6% 8% 10% 29%	4427
August	1st to 10th	Night	18% 10% 4 9% 6% 5% 5% 5% 6% 9% 24%	2861
	11th to 20th	Night	8% 6% 2 10% 6% 6% 6% 5% 8% 10% 32%	2965
	21st to end	Night	6% 4 2 5% 4 5% 5% 6% 7% 10% 46%	3476
September	1st to 10th	Night	11% 5% 2 6% 4 5% 5% 4 5% 8% 44%	1900
	11th to 20th	Night	14% 7% 3 6% 5% 4 4 3 7% 8% 40%	1969
	21st to end	Night	18% 7% 4 7% 5% 5% 4 4 4 6% 37%	2160
October	1st to 10th	Night	13% 6% 2 6% 3 3 3 3 5% 7% 49%	2280
	11th to 20th	Night	12% 5% 2 4 4 2 3 3 4 5% 55%	2280
	21st to end	Night	11% 5% 2 4 3 3 2 2 5% 6% 57%	2508
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 2 4 6% 52%	2341
	11th to 20th	Night	14% 6% 3 5% 4 3 3 3 5% 5% 49%	2400
	21st to end	Night	12% 4 2 5% 3 3 3 4 5% 7% 51%	2707
December	1st to 10th	Night	11% 6% 3 5% 3 4 2 3 4 5% 54%	4580
	11th to 20th	Night	15% 8% 3 8% 4 4 4 3 5% 5% 40%	5097
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 7% 48%	6490

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	35% 2 5% 18% 32% 9%	4057
	11th to 20th	Day	32% 3 4 16% 35% 10%	4060
	21st to end	Day	43% 4 5% 17% 21% 10%	4466
February	1st to 10th	Day	40% 4 10% 21% 20% 6%	1800
	11th to 20th	Day	43% 4 6% 17% 23% 6%	1824
	21st to end	Day	43% 4 8% 18% 20% 7%	1762
March	1st to 10th	Day	46% 4 7% 15% 19% 10%	4935
	11th to 20th	Day	32% 4 7% 16% 25% 15%	4980
	21st to end	Day	37% 4 9% 22% 21% 7%	5465
April	1st to 10th	Day	30% 5% 11% 23% 24% 7%	2353
	11th to 20th	Day	32% 5% 11% 24% 22% 7%	2374
	21st to end	Day	34% 7% 13% 23% 20% 4	2400
May	1st to 10th	Day	40% 9% 17% 23% 10% 1	5662
	11th to 20th	Day	49% 8% 13% 21% 9% 1	5865
	21st to end	Day	36% 8% 13% 22% 18% 2	6447
June	1st to 10th	Day	40% 8% 14% 21% 16% 1	2600
	11th to 20th	Day	34% 9% 17% 24% 14% 2	2600
	21st to end	Day	36% 9% 17% 22% 13% 2	2600
July	1st to 10th	Day	41% 10% 20% 20% 8% 1	5835
	11th to 20th	Day	45% 12% 28% 11% 3 1	5831
	21st to end	Day	42% 10% 18% 20% 9%	6240
August	1st to 10th	Day	59% 8% 16% 11% 5%	4143
	11th to 20th	Day	43% 9% 14% 23% 10%	3870
	21st to end	Day	31% 7% 16% 30% 15% 2	4180
September	1st to 10th	Day	37% 9% 15% 19% 17% 4	2280
	11th to 20th	Day	41% 6% 12% 18% 20% 3	2206
	21st to end	Day	47% 5% 10% 18% 15% 4	2019
October	1st to 10th	Day	34% 4 9% 23% 24% 6%	3160
	11th to 20th	Day	21% 2 6% 20% 25% 25%	3140
	21st to end	Day	26% 3 5% 24% 26% 16%	3428
November	1st to 10th	Day	35% 3 7% 23% 25% 7%	1957
	11th to 20th	Day	36% 4 8% 20% 25% 7%	2000
	21st to end	Day	34% 3 8% 24% 25% 7%	1681
December	1st to 10th	Day	35% 4 8% 20% 24% 9%	2600
	11th to 20th	Day	41% 3 12% 19% 20% 6%	3060
	21st to end	Day	40% 4 11% 24% 20% 2	4466
January	1st to 10th	Night	32% 3 6% 16% 36% 8%	5889
	11th to 20th	Night	36% 3 6% 14% 33% 8%	5900
	21st to end	Night	40% 3 6% 17% 26% 7%	6490
February	1st to 10th	Night	41% 3 9% 21% 20% 6%	2600
	11th to 20th	Night	44% 4 9% 15% 21% 7%	2470
	21st to end	Night	37% 4 9% 18% 23% 9%	1865
March	1st to 10th	Night	46% 4 6% 12% 22% 9%	4918
	11th to 20th	Night	33% 4 8% 12% 25% 17%	4980
	21st to end	Night	36% 4 9% 23% 21% 7%	5467
April	1st to 10th	Night	36% 4 10% 18% 23% 9%	2045
	11th to 20th	Night	33% 6% 10% 23% 20% 7%	1975
	21st to end	Night	32% 7% 12% 26% 18% 4	2000
May	1st to 10th	Night	43% 8% 16% 19% 12% 2	4336
	11th to 20th	Night	48% 7% 13% 19% 12% 1	4033
	21st to end	Night	38% 6% 15% 23% 16% 2	4429
June	1st to 10th	Night	37% 10% 19% 20% 13% 2	1800
	11th to 20th	Night	39% 10% 17% 21% 13% 1	1800
	21st to end	Night	37% 10% 18% 23% 11% 2	1800
July	1st to 10th	Night	44% 12% 21% 16% 7% 1	4015
	11th to 20th	Night	48% 15% 24% 10% 3 1	4006
	21st to end	Night	47% 10% 16% 18% 9%	4297
August	1st to 10th	Night	56% 9% 18% 14% 3	2850
	11th to 20th	Night	43% 11% 19% 20% 7% 1	2992
	21st to end	Night	31% 8% 14% 30% 15% 2	3476
September	1st to 10th	Night	37% 9% 15% 19% 16% 5%	1900
	11th to 20th	Night	41% 7% 12% 18% 19% 3	1969
	21st to end	Night	48% 6% 11% 16% 16% 3	2160
October	1st to 10th	Night	38% 4 8% 20% 25% 6%	3800
	11th to 20th	Night	23% 3 6% 17% 25% 26%	3779
	21st to end	Night	22% 3 8% 27% 23% 16%	4120
November	1st to 10th	Night	35% 4 6% 19% 27% 8%	2341
	11th to 20th	Night	36% 4 9% 15% 26% 10%	2400
	21st to end	Night	32% 4 7% 22% 26% 8%	2707
December	1st to 10th	Night	33% 3 7% 16% 31% 10%	4580
	11th to 20th	Night	46% 4 10% 16% 17% 7%	5100
	21st to end	Night	37% 4 11% 25% 22% 2	6490

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	18% 21% 40% 17% 4	300
	11th to 20th	Day	19% 23% 35% 17% 6%	300
	21st to end	Day	19% 19% 40% 16% 5%	330
February	1st to 10th	Day	20% 14% 41% 20% 4 1	300
	11th to 20th	Day	33% 14% 33% 17% 2	362
	21st to end	Day	21% 17% 32% 23% 6% 1	414
March	1st to 10th	Day	19% 18% 32% 24% 7% 1	500
	11th to 20th	Day	19% 16% 38% 20% 6% 1	500
	21st to end	Day	19% 14% 35% 23% 9% 1	550
April	1st to 10th	Day	16% 17% 31% 23% 12% 1	500
	11th to 20th	Day	12% 13% 30% 24% 17% 4	492
	21st to end	Day	14% 10% 28% 25% 21% 2	500
May	1st to 10th	Day	12% 11% 33% 24% 14% 6%	503
	11th to 20th	Day	16% 7% 20% 27% 21% 8%	527
	21st to end	Day	16% 7% 15% 26% 22% 14%	573
June	1st to 10th	Day	11% 4 19% 31% 24% 11%	530
	11th to 20th	Day	11% 2 16% 28% 26% 17%	530
	21st to end	Day	9% 3 11% 29% 34% 14%	530
July	1st to 10th	Day	8% 3 10% 24% 35% 20%	525
	11th to 20th	Day	9% 3 7% 24% 35% 22%	524
	21st to end	Day	8% 2 9% 31% 34% 16%	576
August	1st to 10th	Day	15% 3 14% 22% 27% 19%	502
	11th to 20th	Day	13% 3 9% 23% 28% 23%	479
	21st to end	Day	11% 2 9% 29% 26% 23%	539
September	1st to 10th	Day	11% 3 15% 24% 26% 21%	490
	11th to 20th	Day	19% 3 13% 25% 28% 13%	490
	21st to end	Day	21% 4 19% 26% 23% 7%	490
October	1st to 10th	Day	18% 5% 19% 23% 24% 11%	490
	11th to 20th	Day	12% 7% 24% 29% 21% 7%	490
	21st to end	Day	12% 7% 24% 31% 20% 6%	539
November	1st to 10th	Day	15% 7% 27% 31% 14% 5%	495
	11th to 20th	Day	17% 13% 30% 29% 11% 2	500
	21st to end	Day	15% 12% 27% 27% 16% 3	270
December	1st to 10th	Day	19% 12% 42% 20% 8% 1	200
	11th to 20th	Day	20% 10% 34% 22% 11% 3	217
	21st to end	Day	22% 14% 34% 19% 11% 2	330
January	1st to 10th	Night	18% 15% 36% 26% 6%	800
	11th to 20th	Night	18% 13% 33% 30% 6%	800
	21st to end	Night	20% 9% 40% 27% 3	880
February	1st to 10th	Night	20% 10% 38% 28% 4	800
	11th to 20th	Night	22% 16% 39% 22% 2	783
	21st to end	Night	18% 14% 35% 28% 5%	660
March	1st to 10th	Night	16% 14% 35% 30% 5%	800
	11th to 20th	Night	19% 11% 39% 24% 6% 1	800
	21st to end	Night	16% 11% 39% 29% 5%	880
April	1st to 10th	Night	16% 14% 36% 27% 7%	800
	11th to 20th	Night	13% 11% 34% 31% 11% 1	791
	21st to end	Night	14% 6% 32% 38% 11% 1	800
May	1st to 10th	Night	12% 9% 32% 37% 10%	720
	11th to 20th	Night	15% 5% 23% 40% 17% 1	591
	21st to end	Night	12% 3 14% 41% 27% 2	650
June	1st to 10th	Night	13% 3 16% 42% 25% 2	600
	11th to 20th	Night	11% 3 15% 38% 29% 4	600
	21st to end	Night	8% 2 14% 42% 32% 3	600
July	1st to 10th	Night	10% 3 9% 37% 37% 4	585
	11th to 20th	Night	9% 2 5% 37% 41% 6%	582
	21st to end	Night	9% 2 9% 37% 39% 4	657
August	1st to 10th	Night	13% 3 9% 37% 34% 4	579
	11th to 20th	Night	13% 3 11% 32% 35% 6%	666
	21st to end	Night	12% 2 8% 34% 38% 6%	836
September	1st to 10th	Night	12% 3 11% 31% 36% 7%	760
	11th to 20th	Night	16% 3 14% 32% 31% 3	760
	21st to end	Night	22% 4 17% 30% 24% 2	760
October	1st to 10th	Night	14% 3 18% 33% 27% 4	760
	11th to 20th	Night	15% 6% 23% 33% 20% 2	760
	21st to end	Night	13% 6% 24% 33% 21% 2	836
November	1st to 10th	Night	16% 6% 24% 37% 16% 3	780
	11th to 20th	Night	16% 11% 27% 33% 13% 1	800
	21st to end	Night	14% 8% 30% 32% 15% 1	800
December	1st to 10th	Night	19% 11% 32% 32% 6%	800
	11th to 20th	Night	19% 8% 30% 34% 8% 1	800
	21st to end	Night	19% 11% 30% 32% 9%	880

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