Heilwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heilwood clear sky chart from 2005-11-06 to 2026-02-01.

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	16% 11% 4 10% 5% 5% 4 4 5% 7% 29%	127448
	Night	16% 13% 5% 11% 6% 4 4 3 5% 6% 29%	124560
Transparency	Day	54% 5% 11% 20% 10% 1	131046
	Night	55% 5% 10% 18% 10% 1	129032
Seeing	Day	26% 16% 27% 19% 9% 3	16253
	Night	25% 16% 26% 23% 9% 1	25920

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% 19% 8% 26% 8% 3 3 2 2 4 12%	3889
	11th to 20th	Day	11% 17% 10% 24% 11% 7% 3 2 2 2 11%	4150
	21st to end	Day	19% 16% 10% 11% 9% 4 3 3 3 4 18%	4565
February	1st to 10th	Day	17% 20% 6% 14% 8% 4 4 2 3 4 19%	1800
	11th to 20th	Day	16% 15% 8% 15% 6% 5% 4 3 5% 5% 18%	1753
	21st to end	Day	17% 18% 4 14% 5% 4 4 3 4 5% 21%	1632
March	1st to 10th	Day	15% 12% 10% 9% 4 3 3 3 4 5% 31%	4935
	11th to 20th	Day	13% 12% 6% 10% 5% 4 4 3 5% 6% 31%	4980
	21st to end	Day	14% 12% 4 9% 6% 5% 4 3 5% 6% 31%	5465
April	1st to 10th	Day	16% 16% 3 9% 5% 4 4 4 5% 5% 28%	2363
	11th to 20th	Day	15% 14% 4 7% 5% 4 5% 3 5% 6% 33%	2374
	21st to end	Day	20% 11% 3 7% 4 4 5% 3 4 7% 31%	2550
May	1st to 10th	Day	22% 10% 3 8% 6% 5% 5% 5% 6% 6% 25%	5862
	11th to 20th	Day	19% 9% 3 8% 5% 5% 6% 4 6% 8% 26%	6012
	21st to end	Day	20% 9% 3 7% 4 5% 6% 4 6% 7% 30%	6934
June	1st to 10th	Day	16% 8% 3 6% 6% 5% 5% 4 7% 8% 32%	2800
	11th to 20th	Day	17% 7% 3 8% 5% 5% 5% 5% 7% 9% 31%	2800
	21st to end	Day	14% 6% 2 6% 6% 5% 5% 4 7% 10% 34%	2800
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	6360
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 10% 32%	6023
	21st to end	Day	14% 7% 3 7% 4 5% 5% 4 6% 7% 40%	6424
August	1st to 10th	Day	18% 8% 3 8% 5% 6% 6% 4 7% 8% 28%	4234
	11th to 20th	Day	14% 7% 3 7% 5% 6% 6% 5% 6% 8% 34%	4174
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 5% 7% 11% 38%	4355
September	1st to 10th	Day	17% 6% 3 4 4 4 4 4 5% 7% 42%	2400
	11th to 20th	Day	14% 5% 2 6% 4 4 4 4 5% 8% 44%	2300
	21st to end	Day	21% 9% 4 7% 5% 4 4 3 5% 7% 32%	2144
October	1st to 10th	Day	17% 8% 2 7% 4 4 5% 3 5% 7% 38%	2000
	11th to 20th	Day	18% 12% 3 7% 5% 4 4 3 5% 5% 34%	2000
	21st to end	Day	21% 15% 3 6% 5% 3 3 3 4 6% 31%	2200
November	1st to 10th	Day	12% 12% 3 6% 5% 4 5% 4 5% 6% 40%	2012
	11th to 20th	Day	15% 18% 5% 8% 4 4 5% 4 6% 5% 26%	1680
	21st to end	Day	18% 19% 8% 9% 5% 4 3 3 4 5% 21%	1680
December	1st to 10th	Day	15% 16% 8% 27% 6% 3 3 2 3 3 14%	2680
	11th to 20th	Day	19% 20% 6% 12% 5% 3 3 2 4 6% 21%	3070
	21st to end	Day	19% 23% 7% 12% 5% 4 3 3 4 4 16%	4048
January	1st to 10th	Night	11% 19% 8% 25% 10% 4 3 2 3 4 12%	6272
	11th to 20th	Night	12% 16% 8% 22% 11% 6% 4 2 3 3 13%	6030
	21st to end	Night	17% 16% 7% 13% 10% 6% 4 3 3 3 17%	6633
February	1st to 10th	Night	16% 16% 6% 14% 7% 4 3 3 4 4 23%	2601
	11th to 20th	Night	16% 16% 5% 14% 8% 5% 4 3 4 5% 20%	2533
	21st to end	Night	17% 15% 6% 13% 7% 3 3 3 4 4 25%	1993
March	1st to 10th	Night	17% 13% 5% 10% 6% 4 3 3 4 5% 29%	4918
	11th to 20th	Night	12% 11% 7% 7% 4 4 3 3 4 6% 39%	4980
	21st to end	Night	13% 12% 5% 10% 5% 4 4 3 4 5% 34%	5467
April	1st to 10th	Night	17% 16% 5% 8% 4 4 4 3 5% 6% 29%	2036
	11th to 20th	Night	16% 12% 2 6% 4 4 5% 4 6% 7% 34%	1975
	21st to end	Night	15% 11% 3 8% 5% 5% 5% 3 4 6% 35%	1866
May	1st to 10th	Night	26% 11% 3 8% 4 5% 4 3 5% 5% 25%	4033
	11th to 20th	Night	17% 11% 3 7% 5% 4 4 4 5% 7% 32%	3877
	21st to end	Night	21% 11% 3 6% 4 5% 4 3 5% 8% 30%	3920
June	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 4 5% 7% 42%	1600
	11th to 20th	Night	13% 8% 2 6% 5% 4 5% 4 7% 9% 38%	1600
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 11% 40%	1600
July	1st to 10th	Night	13% 9% 3 8% 4 4 5% 4 6% 7% 38%	3598
	11th to 20th	Night	14% 7% 3 6% 5% 5% 5% 5% 7% 10% 35%	3916
	21st to end	Night	12% 7% 2 7% 4 4 4 3 5% 8% 42%	4427
August	1st to 10th	Night	17% 9% 3 6% 4 4 5% 4 6% 7% 35%	2915
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 7% 40%	2875
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 7% 9% 45%	3510
September	1st to 10th	Night	14% 6% 2 6% 3 4 4 4 5% 6% 47%	2000
	11th to 20th	Night	10% 5% 2 5% 3 3 4 4 6% 9% 50%	2095
	21st to end	Night	18% 9% 3 7% 4 4 4 4 5% 6% 37%	2255
October	1st to 10th	Night	16% 8% 1 5% 4 3 4 3 5% 6% 44%	2400
	11th to 20th	Night	18% 10% 2 6% 4 4 3 3 4 6% 39%	2400
	21st to end	Night	23% 13% 3 5% 4 3 3 3 4 5% 34%	2640
November	1st to 10th	Night	11% 13% 4 6% 4 3 3 3 4 6% 42%	2500
	11th to 20th	Night	17% 15% 5% 9% 5% 4 4 3 5% 5% 28%	2940
	21st to end	Night	18% 17% 7% 9% 4 4 4 3 4 5% 26%	2940
December	1st to 10th	Night	16% 16% 8% 22% 4 3 3 3 4 4 17%	4720
	11th to 20th	Night	20% 16% 7% 14% 6% 4 3 3 3 4 21%	5345
	21st to end	Night	16% 21% 7% 13% 6% 5% 4 3 4 4 17%	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	79% 4 5% 8% 3	3890
	11th to 20th	Day	82% 2 4 7% 3 1	4150
	21st to end	Day	71% 3 8% 11% 6%	4565
February	1st to 10th	Day	71% 4 8% 12% 6%	1800
	11th to 20th	Day	67% 6% 8% 13% 6%	1762
	21st to end	Day	65% 4 9% 14% 6% 1	1632
March	1st to 10th	Day	56% 3 11% 21% 8% 1	4935
	11th to 20th	Day	52% 5% 14% 20% 7% 2	4980
	21st to end	Day	52% 5% 9% 21% 12% 1	5465
April	1st to 10th	Day	57% 5% 10% 16% 10% 2	2363
	11th to 20th	Day	52% 5% 11% 17% 14% 2	2374
	21st to end	Day	53% 4 9% 21% 11% 1	2550
May	1st to 10th	Day	58% 5% 10% 17% 8% 2	5897
	11th to 20th	Day	54% 6% 10% 20% 9% 1	6016
	21st to end	Day	50% 7% 13% 19% 10% 2	6949
June	1st to 10th	Day	47% 6% 12% 23% 11%	2800
	11th to 20th	Day	48% 6% 15% 19% 11% 1	2800
	21st to end	Day	44% 7% 13% 25% 12%	2800
July	1st to 10th	Day	43% 5% 11% 27% 13% 2	6290
	11th to 20th	Day	45% 5% 16% 23% 10%	5951
	21st to end	Day	43% 4 9% 25% 19% 1	6240
August	1st to 10th	Day	52% 6% 14% 21% 7%	4221
	11th to 20th	Day	47% 4 12% 27% 9%	4213
	21st to end	Day	39% 5% 14% 28% 12% 2	4355
September	1st to 10th	Day	41% 7% 8% 24% 17% 3	2400
	11th to 20th	Day	37% 6% 12% 27% 17% 1	2300
	21st to end	Day	52% 4 12% 19% 12% 1	2144
October	1st to 10th	Day	37% 4 10% 28% 19% 1	3260
	11th to 20th	Day	46% 5% 10% 22% 11% 7%	3240
	21st to end	Day	40% 3 11% 27% 18% 1	3538
November	1st to 10th	Day	45% 5% 12% 19% 16% 3	2012
	11th to 20th	Day	58% 4 10% 16% 9% 2	1680
	21st to end	Day	66% 3 11% 15% 6%	1680
December	1st to 10th	Day	77% 4 9% 7% 4	2680
	11th to 20th	Day	67% 3 10% 17% 3	3066
	21st to end	Day	73% 3 7% 11% 5%	4048
January	1st to 10th	Night	78% 3 6% 9% 4	6272
	11th to 20th	Night	78% 4 6% 9% 2 1	6030
	21st to end	Night	73% 3 7% 12% 5% 1	6633
February	1st to 10th	Night	65% 4 8% 16% 7%	2601
	11th to 20th	Night	67% 4 8% 15% 7%	2535
	21st to end	Night	63% 4 8% 16% 9% 1	1993
March	1st to 10th	Night	58% 4 12% 19% 8%	4918
	11th to 20th	Night	44% 7% 16% 22% 7% 4	4980
	21st to end	Night	51% 5% 10% 17% 16% 1	5467
April	1st to 10th	Night	57% 5% 10% 16% 12% 1	2036
	11th to 20th	Night	47% 6% 12% 18% 14% 2	1975
	21st to end	Night	49% 6% 8% 23% 12% 2	1866
May	1st to 10th	Night	61% 5% 8% 13% 9% 3	4060
	11th to 20th	Night	52% 7% 10% 22% 8% 1	3878
	21st to end	Night	51% 6% 10% 21% 9% 3	3927
June	1st to 10th	Night	40% 7% 12% 26% 14% 2	1600
	11th to 20th	Night	41% 8% 14% 24% 12% 2	1600
	21st to end	Night	39% 6% 15% 24% 15%	1600
July	1st to 10th	Night	45% 5% 12% 23% 14% 2	3560
	11th to 20th	Night	44% 5% 18% 21% 12% 1	3869
	21st to end	Night	40% 5% 10% 22% 22% 1	4297
August	1st to 10th	Night	47% 6% 15% 22% 9%	2904
	11th to 20th	Night	42% 5% 11% 29% 12% 1	2902
	21st to end	Night	34% 6% 15% 31% 13% 2	3510
September	1st to 10th	Night	37% 6% 10% 25% 19% 3	2000
	11th to 20th	Night	31% 6% 12% 30% 20% 1	2095
	21st to end	Night	47% 4 12% 20% 15% 1	2255
October	1st to 10th	Night	38% 5% 9% 26% 21% 1	3920
	11th to 20th	Night	40% 4 11% 22% 15% 9%	3899
	21st to end	Night	41% 3 10% 26% 20% 2	4252
November	1st to 10th	Night	44% 4 11% 20% 17% 3	2500
	11th to 20th	Night	58% 4 8% 18% 10% 3	2940
	21st to end	Night	61% 4 10% 18% 6% 1	2940
December	1st to 10th	Night	72% 3 11% 10% 3	4720
	11th to 20th	Night	69% 2 11% 15% 3	5348
	21st to end	Night	71% 4 9% 12% 5%	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	39% 26% 23% 9% 4	270
	11th to 20th	Day	42% 33% 19% 4 2	330
	21st to end	Day	36% 28% 27% 7% 1	363
February	1st to 10th	Day	34% 29% 30% 5% 2	300
	11th to 20th	Day	45% 25% 26% 4 1	300
	21st to end	Day	36% 27% 29% 6% 1	339
March	1st to 10th	Day	35% 22% 28% 12% 3	500
	11th to 20th	Day	34% 23% 27% 11% 4 1	500
	21st to end	Day	34% 25% 25% 14% 3 1	550
April	1st to 10th	Day	34% 23% 27% 12% 4	500
	11th to 20th	Day	28% 22% 29% 11% 7% 3	492
	21st to end	Day	31% 18% 31% 14% 4 1	500
May	1st to 10th	Day	33% 15% 29% 16% 6% 2	503
	11th to 20th	Day	27% 14% 30% 18% 8% 3	527
	21st to end	Day	21% 12% 28% 24% 12% 4	573
June	1st to 10th	Day	21% 15% 28% 23% 12% 2	530
	11th to 20th	Day	24% 12% 27% 21% 13% 3	530
	21st to end	Day	18% 11% 30% 27% 12% 2	530
July	1st to 10th	Day	14% 9% 26% 25% 18% 8%	525
	11th to 20th	Day	13% 10% 28% 31% 13% 6%	524
	21st to end	Day	16% 7% 27% 30% 14% 5%	576
August	1st to 10th	Day	15% 6% 26% 28% 18% 6%	526
	11th to 20th	Day	18% 6% 24% 26% 16% 8%	519
	21st to end	Day	13% 9% 25% 28% 16% 9%	583
September	1st to 10th	Day	19% 10% 20% 26% 18% 8%	530
	11th to 20th	Day	14% 3 22% 34% 19% 8%	530
	21st to end	Day	28% 12% 23% 23% 11% 4	530
October	1st to 10th	Day	20% 13% 23% 26% 15% 3	530
	11th to 20th	Day	26% 17% 28% 21% 7% 1	530
	21st to end	Day	33% 16% 30% 16% 4	583
November	1st to 10th	Day	24% 18% 28% 20% 8% 1	508
	11th to 20th	Day	34% 19% 33% 10% 3	220
	21st to end	Day	34% 23% 34% 8% 1	220
December	1st to 10th	Day	36% 29% 26% 6% 2 1	220
	11th to 20th	Day	40% 20% 32% 7% 1	220
	21st to end	Day	42% 22% 25% 8% 2	242
January	1st to 10th	Night	33% 28% 26% 10% 3	840
	11th to 20th	Night	34% 26% 27% 11% 2	840
	21st to end	Night	31% 26% 29% 12% 2	924
February	1st to 10th	Night	33% 22% 28% 15% 2	801
	11th to 20th	Night	32% 28% 29% 9% 1	783
	21st to end	Night	32% 24% 29% 14% 1	660
March	1st to 10th	Night	30% 20% 31% 17% 2	800
	11th to 20th	Night	29% 24% 27% 15% 6%	800
	21st to end	Night	27% 21% 32% 16% 4 1	880
April	1st to 10th	Night	30% 18% 30% 18% 4	800
	11th to 20th	Night	25% 18% 31% 19% 7%	791
	21st to end	Night	21% 16% 32% 27% 5%	666
May	1st to 10th	Night	25% 11% 26% 30% 6% 1	600
	11th to 20th	Night	22% 10% 31% 30% 7%	527
	21st to end	Night	15% 7% 27% 30% 20% 2	433
June	1st to 10th	Night	13% 6% 31% 36% 14% 1	400
	11th to 20th	Night	16% 7% 27% 35% 15% 1	400
	21st to end	Night	14% 5% 29% 36% 15% 1	400
July	1st to 10th	Night	16% 3 19% 42% 17% 3	390
	11th to 20th	Night	13% 5% 19% 40% 21% 3	523
	21st to end	Night	14% 4 18% 41% 22% 1	657
August	1st to 10th	Night	11% 4 18% 43% 22% 2	597
	11th to 20th	Night	14% 5% 15% 38% 25% 3	594
	21st to end	Night	12% 5% 19% 39% 21% 4	830
September	1st to 10th	Night	17% 6% 20% 34% 20% 4	800
	11th to 20th	Night	12% 5% 18% 36% 25% 5%	800
	21st to end	Night	21% 11% 22% 27% 16% 3	800
October	1st to 10th	Night	20% 8% 24% 29% 17% 2	800
	11th to 20th	Night	24% 15% 31% 21% 8% 1	800
	21st to end	Night	32% 13% 28% 22% 4	880
November	1st to 10th	Night	21% 15% 30% 27% 8%	820
	11th to 20th	Night	32% 18% 26% 18% 5% 1	840
	21st to end	Night	33% 20% 28% 15% 4	840
December	1st to 10th	Night	36% 22% 25% 13% 3	840
	11th to 20th	Night	35% 25% 25% 13% 2	840
	21st to end	Night	31% 25% 28% 15% 1	924

Page generated at 2026-01-29 01:21:44. Version ac07

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