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 2024-06-30.

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% 12% 5% 9% 5% 4 4 3 5% 6% 28%	89549
	Night	16% 13% 5% 9% 5% 4 4 3 5% 6% 30%	89533
Transparency	Day	55% 6% 11% 19% 9% 1	89314
	Night	55% 6% 10% 18% 9% 1	89373
Seeing	Day	26% 16% 27% 19% 9% 3	14234
	Night	25% 16% 26% 23% 9% 1	23841

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	17% 21% 10% 18% 5% 3 3 2 3 4 15%	2654
	11th to 20th	Day	11% 19% 10% 21% 9% 5% 3 2 3 3 14%	2840
	21st to end	Day	24% 19% 10% 13% 8% 4 3 2 3 3 11%	3124
February	1st to 10th	Day	16% 20% 6% 13% 8% 5% 4 2 3 4 19%	1710
	11th to 20th	Day	16% 15% 8% 15% 6% 5% 4 3 5% 5% 19%	1663
	21st to end	Day	18% 18% 4 14% 5% 4 3 3 3 5% 22%	1552
March	1st to 10th	Day	18% 12% 8% 9% 4 3 4 3 5% 6% 28%	3435
	11th to 20th	Day	13% 15% 6% 12% 5% 4 4 3 5% 5% 28%	3480
	21st to end	Day	16% 12% 3 8% 5% 4 4 3 6% 6% 32%	3815
April	1st to 10th	Day	16% 16% 3 9% 5% 4 4 4 5% 5% 29%	2245
	11th to 20th	Day	16% 14% 4 7% 4 4 4 3 5% 6% 33%	2254
	21st to end	Day	20% 11% 3 7% 4 4 5% 4 4 7% 30%	2422
May	1st to 10th	Day	25% 10% 3 8% 5% 4 4 4 5% 6% 26%	4082
	11th to 20th	Day	21% 10% 3 7% 5% 6% 5% 4 6% 7% 26%	4190
	21st to end	Day	13% 7% 3 6% 4 5% 5% 4 7% 9% 37%	4823
June	1st to 10th	Day	17% 8% 3 6% 6% 5% 5% 4 7% 8% 32%	2660
	11th to 20th	Day	16% 6% 3 8% 5% 5% 5% 5% 7% 9% 32%	2660
	21st to end	Day	15% 7% 2 7% 6% 5% 5% 4 7% 10% 34%	2646
July	1st to 10th	Day	14% 7% 3 8% 6% 5% 6% 4 5% 7% 36%	2520
	11th to 20th	Day	12% 8% 2 6% 5% 5% 4 5% 7% 9% 36%	2393
	21st to end	Day	14% 8% 3 8% 6% 6% 5% 5% 6% 8% 33%	2547
August	1st to 10th	Day	12% 7% 2 7% 6% 7% 7% 6% 8% 10% 29%	2327
	11th to 20th	Day	14% 8% 3 7% 5% 6% 5% 5% 6% 8% 32%	2303
	21st to end	Day	15% 6% 2 6% 5% 4 6% 4 7% 10% 33%	2404
September	1st to 10th	Day	18% 7% 3 5% 4 4 5% 4 5% 7% 39%	2160
	11th to 20th	Day	15% 5% 2 6% 4 3 4 4 5% 8% 42%	2070
	21st to end	Day	19% 9% 3 6% 5% 4 5% 3 5% 7% 34%	1930
October	1st to 10th	Day	17% 8% 2 8% 5% 4 5% 4 5% 7% 35%	1800
	11th to 20th	Day	20% 12% 3 7% 5% 4 4 3 5% 5% 32%	1800
	21st to end	Day	23% 15% 3 6% 5% 4 3 3 4 5% 29%	1980
November	1st to 10th	Day	13% 12% 3 6% 5% 3 5% 4 5% 6% 39%	1818
	11th to 20th	Day	16% 17% 5% 8% 4 4 5% 4 6% 5% 27%	1520
	21st to end	Day	19% 17% 8% 9% 5% 4 3 3 4 5% 23%	1520
December	1st to 10th	Day	19% 20% 10% 12% 6% 3 3 3 4 4 16%	1520
	11th to 20th	Day	17% 22% 7% 12% 5% 3 3 2 4 6% 19%	1910
	21st to end	Day	23% 24% 9% 11% 5% 3 3 2 3 4 14%	2772
January	1st to 10th	Night	13% 22% 9% 16% 5% 4 4 3 4 4 16%	4287
	11th to 20th	Night	14% 18% 9% 17% 7% 6% 4 2 3 4 14%	4120
	21st to end	Night	23% 18% 8% 12% 10% 3 3 2 3 3 15%	4532
February	1st to 10th	Night	16% 16% 6% 14% 7% 4 3 3 4 4 23%	2470
	11th to 20th	Night	16% 16% 5% 13% 8% 5% 4 3 4 6% 21%	2403
	21st to end	Night	17% 15% 6% 13% 7% 4 3 3 5% 4 25%	1897
March	1st to 10th	Night	21% 13% 4 8% 4 4 4 3 4 6% 30%	3418
	11th to 20th	Night	14% 12% 9% 9% 5% 4 3 3 4 6% 30%	3480
	21st to end	Night	14% 12% 4 9% 4 3 3 3 4 5% 37%	3817
April	1st to 10th	Night	15% 16% 4 8% 4 4 4 3 5% 6% 30%	1934
	11th to 20th	Night	16% 11% 2 6% 4 3 5% 3 6% 7% 35%	1875
	21st to end	Night	15% 11% 3 8% 5% 5% 5% 3 4 6% 35%	1773
May	1st to 10th	Night	27% 11% 3 7% 4 5% 4 3 4 6% 27%	2813
	11th to 20th	Night	18% 12% 3 7% 5% 4 4 5% 5% 6% 31%	2699
	21st to end	Night	14% 10% 2 6% 4 5% 4 3 6% 9% 35%	2732
June	1st to 10th	Night	13% 5% 2 5% 4 5% 5% 5% 6% 8% 42%	1520
	11th to 20th	Night	13% 7% 2 6% 5% 4 5% 4 7% 10% 38%	1520
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 11% 40%	1512
July	1st to 10th	Night	12% 6% 2 6% 5% 4 6% 4 6% 6% 44%	1440
	11th to 20th	Night	10% 5% 2 4 5% 3 3 4 6% 10% 47%	1563
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 5% 8% 8% 38%	1771
August	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 4 7% 9% 42%	1611
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 6% 8% 43%	1598
	21st to end	Night	11% 6% 2 5% 4 5% 5% 5% 7% 8% 42%	1934
September	1st to 10th	Night	14% 6% 2 6% 3 4 4 4 5% 6% 45%	1800
	11th to 20th	Night	11% 4 2 4 3 3 4 4 6% 8% 50%	1885
	21st to end	Night	16% 8% 3 7% 4 4 4 4 5% 6% 40%	2029
October	1st to 10th	Night	16% 8% 1 6% 4 3 4 3 5% 7% 42%	2160
	11th to 20th	Night	19% 10% 2 6% 4 4 4 3 5% 6% 38%	2160
	21st to end	Night	25% 14% 2 5% 3 3 3 3 5% 5% 31%	2376
November	1st to 10th	Night	11% 13% 4 6% 4 4 3 3 4 6% 42%	2255
	11th to 20th	Night	17% 15% 5% 8% 5% 4 4 3 5% 5% 29%	2660
	21st to end	Night	18% 17% 6% 8% 4 4 4 3 5% 5% 26%	2660
December	1st to 10th	Night	19% 21% 8% 12% 5% 4 4 3 4 4 16%	2660
	11th to 20th	Night	18% 18% 7% 14% 6% 4 3 3 3 4 20%	3285
	21st to end	Night	19% 22% 8% 11% 6% 5% 4 3 4 4 13%	4884

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	73% 6% 6% 11% 4 1	2655
	11th to 20th	Day	76% 3 6% 10% 3 1	2840
	21st to end	Day	79% 4 6% 7% 3	3124
February	1st to 10th	Day	71% 4 8% 12% 6%	1710
	11th to 20th	Day	67% 6% 8% 13% 6%	1672
	21st to end	Day	65% 4 9% 14% 6% 1	1552
March	1st to 10th	Day	57% 4 11% 22% 6%	3435
	11th to 20th	Day	56% 6% 13% 18% 6% 1	3480
	21st to end	Day	51% 6% 11% 20% 11% 1	3815
April	1st to 10th	Day	55% 5% 10% 17% 10% 2	2245
	11th to 20th	Day	52% 5% 10% 17% 15% 2	2254
	21st to end	Day	54% 4 9% 21% 10% 1	2422
May	1st to 10th	Day	58% 5% 9% 20% 8% 1	4120
	11th to 20th	Day	54% 8% 11% 19% 7% 1	4194
	21st to end	Day	40% 8% 15% 23% 12% 2	4837
June	1st to 10th	Day	48% 7% 12% 23% 10%	2660
	11th to 20th	Day	46% 6% 15% 20% 12% 1	2660
	21st to end	Day	44% 7% 12% 26% 11%	2646
July	1st to 10th	Day	46% 8% 10% 21% 12% 3	2450
	11th to 20th	Day	42% 7% 14% 24% 13% 1	2321
	21st to end	Day	46% 7% 11% 22% 13% 1	2405
August	1st to 10th	Day	45% 9% 15% 24% 7%	2314
	11th to 20th	Day	48% 6% 13% 23% 10% 1	2303
	21st to end	Day	44% 6% 14% 22% 12% 3	2404
September	1st to 10th	Day	44% 7% 9% 22% 15% 3	2160
	11th to 20th	Day	39% 6% 11% 27% 15% 1	2070
	21st to end	Day	49% 4 13% 20% 13% 1	1930
October	1st to 10th	Day	47% 7% 13% 20% 12% 1	1800
	11th to 20th	Day	52% 5% 10% 23% 9%	1800
	21st to end	Day	57% 4 10% 17% 11% 1	1980
November	1st to 10th	Day	46% 5% 11% 19% 16% 3	1818
	11th to 20th	Day	57% 4 11% 16% 9% 2	1520
	21st to end	Day	64% 3 11% 15% 6% 1	1520
December	1st to 10th	Day	73% 4 9% 10% 4	1520
	11th to 20th	Day	68% 3 8% 17% 3	1906
	21st to end	Day	76% 3 7% 11% 2	2772
January	1st to 10th	Night	72% 4 7% 12% 5%	4287
	11th to 20th	Night	74% 6% 9% 10% 2	4120
	21st to end	Night	76% 4 6% 12% 3	4532
February	1st to 10th	Night	65% 4 8% 16% 7%	2470
	11th to 20th	Night	66% 4 8% 15% 7%	2405
	21st to end	Night	63% 4 8% 16% 9% 1	1897
March	1st to 10th	Night	55% 5% 12% 20% 7%	3418
	11th to 20th	Night	51% 10% 14% 19% 6% 1	3480
	21st to end	Night	48% 5% 10% 21% 15% 2	3817
April	1st to 10th	Night	55% 5% 10% 17% 12% 1	1934
	11th to 20th	Night	47% 6% 12% 18% 14% 2	1875
	21st to end	Night	50% 6% 8% 22% 12% 2	1773
May	1st to 10th	Night	58% 5% 8% 17% 10% 2	2840
	11th to 20th	Night	52% 8% 12% 19% 8% 1	2700
	21st to end	Night	42% 8% 13% 22% 11% 4	2739
June	1st to 10th	Night	39% 7% 12% 27% 13% 1	1520
	11th to 20th	Night	39% 8% 14% 24% 12% 2	1520
	21st to end	Night	38% 7% 15% 25% 15%	1512
July	1st to 10th	Night	40% 8% 13% 22% 14% 4	1400
	11th to 20th	Night	32% 7% 14% 26% 19% 2	1513
	21st to end	Night	39% 8% 15% 22% 14% 2	1672
August	1st to 10th	Night	36% 8% 16% 27% 13%	1600
	11th to 20th	Night	38% 7% 15% 25% 13% 2	1598
	21st to end	Night	36% 8% 15% 25% 13% 3	1934
September	1st to 10th	Night	39% 6% 11% 24% 17% 3	1800
	11th to 20th	Night	31% 6% 13% 29% 20% 1	1885
	21st to end	Night	44% 4 13% 22% 16% 1	2029
October	1st to 10th	Night	42% 7% 13% 23% 13% 2	2160
	11th to 20th	Night	47% 5% 12% 23% 12% 1	2160
	21st to end	Night	55% 4 9% 20% 10% 2	2376
November	1st to 10th	Night	44% 5% 11% 20% 16% 4	2255
	11th to 20th	Night	57% 4 8% 18% 10% 2	2660
	21st to end	Night	60% 4 10% 18% 6% 1	2660
December	1st to 10th	Night	72% 4 9% 10% 4	2660
	11th to 20th	Night	69% 3 8% 16% 4	3288
	21st to end	Night	74% 4 9% 10% 2	4884

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	37% 25% 24% 9% 5%	220
	11th to 20th	Day	41% 33% 20% 4 2	270
	21st to end	Day	34% 29% 29% 8% 1	297
February	1st to 10th	Day	31% 30% 31% 6% 2	270
	11th to 20th	Day	42% 26% 27% 4 1	270
	21st to end	Day	37% 28% 28% 7% 1	310
March	1st to 10th	Day	34% 22% 27% 12% 3	460
	11th to 20th	Day	35% 22% 27% 11% 4 1	460
	21st to end	Day	34% 24% 24% 15% 3 1	506
April	1st to 10th	Day	32% 24% 28% 13% 4	460
	11th to 20th	Day	26% 22% 30% 12% 7% 3	452
	21st to end	Day	32% 18% 31% 14% 4 2	460
May	1st to 10th	Day	34% 15% 27% 16% 6% 2	463
	11th to 20th	Day	26% 15% 30% 18% 8% 3	487
	21st to end	Day	17% 11% 29% 25% 13% 4	529
June	1st to 10th	Day	22% 14% 27% 23% 12% 2	490
	11th to 20th	Day	22% 12% 27% 22% 14% 4	490
	21st to end	Day	19% 12% 29% 27% 11% 2	486
July	1st to 10th	Day	16% 9% 24% 24% 19% 9%	445
	11th to 20th	Day	13% 9% 27% 33% 12% 6%	444
	21st to end	Day	17% 7% 27% 29% 14% 6%	488
August	1st to 10th	Day	15% 6% 24% 27% 20% 7%	446
	11th to 20th	Day	19% 7% 22% 27% 16% 9%	439
	21st to end	Day	15% 9% 24% 28% 15% 9%	495
September	1st to 10th	Day	21% 8% 20% 26% 18% 8%	450
	11th to 20th	Day	16% 4 22% 33% 18% 8%	450
	21st to end	Day	25% 12% 23% 23% 12% 5%	450
October	1st to 10th	Day	22% 14% 23% 24% 16% 2	450
	11th to 20th	Day	27% 16% 28% 21% 7% 1	450
	21st to end	Day	35% 15% 29% 17% 4	495
November	1st to 10th	Day	24% 17% 26% 23% 9% 1	434
	11th to 20th	Day	32% 21% 31% 12% 3 1	180
	21st to end	Day	29% 27% 35% 8% 1	180
December	1st to 10th	Day	37% 26% 27% 7% 2 1	180
	11th to 20th	Day	38% 21% 31% 8% 2	180
	21st to end	Day	41% 22% 25% 9% 3	198
January	1st to 10th	Night	33% 26% 27% 11% 3	760
	11th to 20th	Night	33% 25% 28% 12% 2	760
	21st to end	Night	30% 26% 30% 13% 2	836
February	1st to 10th	Night	32% 22% 29% 15% 2	760
	11th to 20th	Night	31% 29% 30% 9% 1	743
	21st to end	Night	32% 24% 28% 14% 1	628
March	1st to 10th	Night	29% 20% 31% 18% 2	760
	11th to 20th	Night	29% 24% 26% 15% 6%	760
	21st to end	Night	26% 21% 32% 16% 4 1	836
April	1st to 10th	Night	29% 18% 31% 18% 4	760
	11th to 20th	Night	24% 18% 31% 19% 7%	751
	21st to end	Night	21% 16% 31% 27% 5%	633
May	1st to 10th	Night	25% 12% 26% 30% 6% 1	570
	11th to 20th	Night	22% 9% 30% 31% 7%	500
	21st to end	Night	12% 6% 27% 31% 21% 2	411
June	1st to 10th	Night	13% 6% 31% 36% 14% 1	380
	11th to 20th	Night	15% 7% 26% 36% 15% 1	380
	21st to end	Night	14% 5% 30% 36% 14% 1	378
July	1st to 10th	Night	15% 3 19% 42% 18% 3	350
	11th to 20th	Night	12% 5% 19% 39% 22% 3	469
	21st to end	Night	14% 5% 19% 39% 23% 1	591
August	1st to 10th	Night	11% 4 18% 42% 23% 2	537
	11th to 20th	Night	14% 5% 15% 36% 27% 3	534
	21st to end	Night	13% 5% 18% 37% 21% 4	746
September	1st to 10th	Night	17% 6% 19% 33% 21% 4	720
	11th to 20th	Night	13% 5% 20% 35% 23% 4	720
	21st to end	Night	20% 11% 22% 27% 17% 3	720
October	1st to 10th	Night	20% 9% 25% 28% 16% 2	720
	11th to 20th	Night	26% 15% 30% 21% 7% 1	720
	21st to end	Night	35% 13% 27% 21% 4 1	792
November	1st to 10th	Night	20% 15% 28% 27% 8% 1	740
	11th to 20th	Night	30% 19% 27% 18% 5% 1	760
	21st to end	Night	32% 19% 29% 16% 4	760
December	1st to 10th	Night	37% 22% 24% 13% 3	760
	11th to 20th	Night	34% 25% 25% 14% 3	760
	21st to end	Night	30% 25% 29% 15% 1	836

Page generated at 2024-06-26 22:18:41. Version ac07

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