Harding Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harding Hill Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	18% 8% 2 6% 4 4 4 4 5% 7% 37%	137210
	Night	17% 8% 3 6% 4 4 4 4 5% 7% 39%	134828
Transparency	Day	45% 5% 11% 22% 15% 3	140719
	Night	45% 5% 10% 22% 15% 3	139395
Seeing	Day	22% 14% 30% 22% 10% 3	16943
	Night	21% 10% 30% 29% 9% 1	25807

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% 7% 3 7% 5% 6% 6% 4 7% 7% 37%	4661
	11th to 20th	Day	13% 11% 3 7% 4 4 3 3 4 6% 43%	4650
	21st to end	Day	10% 9% 2 6% 4 4 4 3 5% 6% 47%	5124
February	1st to 10th	Day	14% 11% 2 7% 4 5% 5% 4 6% 6% 37%	1861
	11th to 20th	Day	12% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	2048
	21st to end	Day	15% 11% 3 8% 4 5% 5% 4 4 5% 37%	1730
March	1st to 10th	Day	21% 12% 2 8% 4 4 4 4 4 5% 32%	4576
	11th to 20th	Day	16% 10% 3 6% 4 4 4 3 5% 5% 38%	4936
	21st to end	Day	26% 10% 2 5% 4 3 3 2 5% 6% 34%	5820
April	1st to 10th	Day	20% 11% 3 6% 4 3 3 3 4 6% 37%	2520
	11th to 20th	Day	18% 7% 2 5% 5% 4 5% 4 5% 6% 38%	2565
	21st to end	Day	22% 8% 3 6% 5% 5% 4 3 4 5% 35%	2730
May	1st to 10th	Day	30% 9% 2 6% 4 4 4 4 5% 6% 25%	7639
	11th to 20th	Day	19% 7% 3 8% 5% 5% 5% 4 5% 8% 32%	7859
	21st to end	Day	19% 7% 3 7% 5% 4 6% 5% 6% 8% 31%	9060
June	1st to 10th	Day	21% 8% 3 6% 4 4 5% 4 5% 8% 32%	2940
	11th to 20th	Day	17% 6% 2 7% 5% 5% 4 4 6% 8% 38%	2870
	21st to end	Day	21% 7% 3 6% 5% 4 5% 4 6% 7% 32%	2800
July	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 5% 7% 9% 32%	6360
	11th to 20th	Day	12% 6% 3 6% 4 5% 5% 5% 8% 10% 37%	6360
	21st to end	Day	16% 5% 2 5% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 5% 4 6% 8% 36%	4312
	11th to 20th	Day	17% 8% 3 7% 5% 4 5% 4 6% 7% 35%	4175
	21st to end	Day	10% 5% 2 5% 4 4 4 5% 7% 9% 45%	4392
September	1st to 10th	Day	13% 4 2 4 3 4 4 4 6% 8% 48%	2380
	11th to 20th	Day	13% 6% 1 4 3 3 3 3 4 5% 55%	2200
	21st to end	Day	17% 4 2 5% 4 4 5% 3 5% 7% 46%	2200
October	1st to 10th	Day	20% 7% 2 6% 4 4 3 3 4 5% 42%	2200
	11th to 20th	Day	18% 6% 2 5% 4 3 4 3 4 7% 45%	1919
	21st to end	Day	23% 7% 2 5% 4 4 4 3 5% 4 39%	1980
November	1st to 10th	Day	16% 6% 2 5% 3 2 4 3 5% 6% 47%	1851
	11th to 20th	Day	18% 5% 2 4 3 3 4 4 6% 6% 45%	1890
	21st to end	Day	22% 8% 3 4 3 2 3 3 5% 5% 41%	1882
December	1st to 10th	Day	17% 9% 2 6% 5% 4 4 4 5% 7% 36%	2680
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 5% 6% 44%	3066
	21st to end	Day	20% 10% 2 5% 4 4 4 4 5% 5% 36%	4048
January	1st to 10th	Night	12% 8% 3 7% 4 4 4 4 6% 7% 42%	8234
	11th to 20th	Night	13% 11% 4 9% 5% 4 5% 4 5% 6% 34%	8235
	21st to end	Night	13% 7% 2 6% 4 5% 4 4 5% 6% 42%	9057
February	1st to 10th	Night	14% 10% 3 7% 5% 4 5% 3 5% 7% 38%	2753
	11th to 20th	Night	12% 9% 2 7% 4 5% 5% 4 5% 5% 42%	2458
	21st to end	Night	13% 9% 3 7% 5% 4 4 4 6% 6% 40%	2076
March	1st to 10th	Night	24% 10% 2 6% 4 4 4 3 4 5% 33%	5484
	11th to 20th	Night	14% 9% 3 8% 4 5% 5% 4 5% 6% 36%	5242
	21st to end	Night	24% 9% 3 6% 3 4 4 3 6% 7% 33%	5337
April	1st to 10th	Night	20% 10% 2 7% 5% 3 4 3 5% 7% 35%	2100
	11th to 20th	Night	18% 8% 3 7% 4 4 4 4 5% 6% 37%	2004
	21st to end	Night	21% 7% 2 6% 4 4 4 4 5% 6% 37%	1890
May	1st to 10th	Night	30% 9% 3 6% 4 4 3 4 5% 6% 26%	5259
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 4 6% 8% 31%	5056
	21st to end	Night	19% 7% 3 7% 4 4 4 4 5% 8% 34%	5114
June	1st to 10th	Night	20% 7% 3 7% 4 4 4 3 6% 7% 35%	1680
	11th to 20th	Night	18% 7% 3 5% 4 4 5% 4 5% 8% 38%	1640
	21st to end	Night	20% 6% 2 6% 5% 4 3 3 5% 9% 37%	1600
July	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 4 7% 10% 38%	3600
	11th to 20th	Night	13% 7% 2 7% 5% 4 4 4 6% 9% 39%	3600
	21st to end	Night	12% 4 1 5% 4 3 4 3 6% 8% 48%	3926
August	1st to 10th	Night	22% 7% 3 7% 5% 4 4 4 4 7% 34%	2803
	11th to 20th	Night	16% 8% 3 7% 4 4 5% 4 5% 7% 38%	2872
	21st to end	Night	10% 4 2 6% 4 4 4 3 6% 8% 48%	3503
September	1st to 10th	Night	13% 5% 2 4 3 3 3 4 5% 6% 51%	2020
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 5% 6% 53%	2200
	21st to end	Night	15% 6% 3 5% 3 4 3 4 5% 7% 47%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	2200
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 5% 7% 42%	2475
	21st to end	Night	23% 8% 2 4 4 3 4 4 5% 6% 37%	2860
November	1st to 10th	Night	15% 7% 2 6% 4 3 4 4 4 7% 45%	2667
	11th to 20th	Night	16% 7% 2 5% 3 4 4 3 5% 6% 45%	2730
	21st to end	Night	17% 8% 2 5% 4 4 4 3 5% 6% 42%	2735
December	1st to 10th	Night	16% 11% 3 7% 4 4 3 3 5% 6% 38%	4720
	11th to 20th	Night	24% 7% 2 5% 4 3 3 3 4 5% 41%	5348
	21st to end	Night	20% 11% 3 7% 4 4 4 2 4 5% 37%	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	44% 5% 10% 25% 15% 1	4662
	11th to 20th	Day	44% 4 10% 24% 15% 3	4680
	21st to end	Day	39% 4 8% 25% 20% 5%	5124
February	1st to 10th	Day	47% 4 11% 20% 14% 3	1861
	11th to 20th	Day	43% 5% 10% 21% 16% 4	2057
	21st to end	Day	50% 4 8% 19% 15% 3	1730
March	1st to 10th	Day	54% 4 7% 12% 13% 9%	4579
	11th to 20th	Day	46% 5% 14% 18% 13% 4	4936
	21st to end	Day	52% 3 4 17% 19% 6%	5820
April	1st to 10th	Day	49% 4 8% 19% 16% 4	2520
	11th to 20th	Day	44% 6% 9% 21% 16% 3	2565
	21st to end	Day	51% 4 7% 17% 16% 4	2730
May	1st to 10th	Day	58% 7% 10% 15% 9% 1	7651
	11th to 20th	Day	49% 6% 14% 18% 10% 2	7863
	21st to end	Day	49% 6% 11% 20% 13% 1	9061
June	1st to 10th	Day	50% 5% 11% 22% 12%	2940
	11th to 20th	Day	43% 5% 11% 20% 17% 3	2870
	21st to end	Day	50% 6% 11% 20% 13% 1	2800
July	1st to 10th	Day	47% 6% 12% 22% 12% 1	6290
	11th to 20th	Day	40% 7% 14% 24% 14% 1	6285
	21st to end	Day	39% 5% 12% 26% 18% 1	6728
August	1st to 10th	Day	45% 6% 14% 23% 11%	4298
	11th to 20th	Day	47% 5% 14% 25% 9% 1	4214
	21st to end	Day	34% 7% 13% 25% 20% 2	4392
September	1st to 10th	Day	33% 6% 12% 29% 19% 1	2380
	11th to 20th	Day	33% 3 9% 30% 23% 3	2200
	21st to end	Day	37% 4 9% 28% 19% 2	2200
October	1st to 10th	Day	45% 5% 7% 24% 18% 2	3590
	11th to 20th	Day	33% 3 9% 31% 22% 2	3096
	21st to end	Day	37% 3 9% 27% 21% 4	3180
November	1st to 10th	Day	37% 4 13% 25% 18% 3	1851
	11th to 20th	Day	38% 4 10% 22% 22% 4	1890
	21st to end	Day	46% 4 14% 22% 13% 1	1882
December	1st to 10th	Day	46% 4 11% 27% 11% 1	2680
	11th to 20th	Day	41% 4 10% 19% 19% 7%	3066
	21st to end	Day	50% 4 8% 19% 13% 6%	4048
January	1st to 10th	Night	41% 4 11% 28% 14% 2	8234
	11th to 20th	Night	50% 5% 10% 18% 14% 4	8280
	21st to end	Night	42% 4 7% 22% 19% 7%	9057
February	1st to 10th	Night	47% 4 11% 18% 16% 3	2753
	11th to 20th	Night	44% 5% 9% 22% 17% 4	2460
	21st to end	Night	44% 4 9% 23% 15% 5%	2076
March	1st to 10th	Night	54% 3 7% 14% 15% 7%	5494
	11th to 20th	Night	46% 7% 15% 18% 12% 3	5243
	21st to end	Night	51% 3 4 14% 22% 5%	5337
April	1st to 10th	Night	50% 4 8% 19% 17% 3	2100
	11th to 20th	Night	47% 5% 7% 22% 18% 1	2004
	21st to end	Night	47% 6% 9% 17% 17% 4	1890
May	1st to 10th	Night	58% 6% 11% 14% 11% 1	5265
	11th to 20th	Night	49% 6% 11% 24% 8% 1	5057
	21st to end	Night	47% 6% 13% 23% 10% 1	5115
June	1st to 10th	Night	48% 6% 10% 23% 12% 1	1680
	11th to 20th	Night	45% 6% 13% 20% 14% 2	1640
	21st to end	Night	46% 6% 13% 24% 10% 2	1600
July	1st to 10th	Night	42% 6% 17% 24% 12% 1	3560
	11th to 20th	Night	42% 7% 13% 22% 17%	3552
	21st to end	Night	34% 5% 13% 34% 13% 1	3809
August	1st to 10th	Night	50% 7% 13% 20% 9%	2793
	11th to 20th	Night	45% 6% 15% 23% 11%	2899
	21st to end	Night	32% 6% 14% 29% 18% 1	3503
September	1st to 10th	Night	34% 6% 12% 30% 18% 1	2020
	11th to 20th	Night	32% 5% 10% 29% 22% 2	2200
	21st to end	Night	37% 4 10% 30% 17% 2	2200
October	1st to 10th	Night	44% 5% 11% 23% 15% 1	3590
	11th to 20th	Night	33% 4 9% 27% 24% 3	4024
	21st to end	Night	39% 4 8% 29% 17% 3	4610
November	1st to 10th	Night	40% 4 10% 25% 18% 2	2667
	11th to 20th	Night	40% 4 9% 25% 19% 3	2730
	21st to end	Night	43% 4 13% 24% 14% 1	2735
December	1st to 10th	Night	47% 4 8% 30% 11% 1	4720
	11th to 20th	Night	47% 4 9% 17% 20% 3	5348
	21st to end	Night	51% 4 7% 20% 16% 2	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	24% 18% 29% 23% 5%	220
	11th to 20th	Day	24% 21% 25% 23% 5% 1	220
	21st to end	Day	21% 19% 34% 20% 6%	242
February	1st to 10th	Day	25% 19% 35% 18% 3	379
	11th to 20th	Day	21% 20% 36% 19% 4	536
	21st to end	Day	24% 18% 31% 23% 4	446
March	1st to 10th	Day	27% 16% 32% 21% 4	540
	11th to 20th	Day	25% 21% 32% 16% 6% 1	540
	21st to end	Day	29% 18% 30% 14% 7% 2	594
April	1st to 10th	Day	30% 21% 26% 16% 5% 2	540
	11th to 20th	Day	28% 19% 30% 14% 7% 2	532
	21st to end	Day	29% 18% 30% 15% 7% 2	540
May	1st to 10th	Day	34% 17% 26% 17% 5% 1	543
	11th to 20th	Day	21% 17% 34% 17% 7% 3	567
	21st to end	Day	22% 14% 31% 22% 9% 2	617
June	1st to 10th	Day	21% 11% 35% 22% 10% 2	570
	11th to 20th	Day	18% 13% 33% 24% 9% 3	550
	21st to end	Day	22% 12% 32% 21% 11% 2	530
July	1st to 10th	Day	18% 9% 30% 26% 13% 4	525
	11th to 20th	Day	14% 6% 32% 29% 14% 5%	524
	21st to end	Day	16% 6% 28% 31% 15% 4	576
August	1st to 10th	Day	13% 7% 26% 30% 19% 5%	526
	11th to 20th	Day	15% 7% 26% 30% 14% 8%	519
	21st to end	Day	12% 9% 26% 31% 16% 6%	583
September	1st to 10th	Day	15% 8% 28% 25% 17% 7%	530
	11th to 20th	Day	15% 10% 25% 22% 16% 11%	530
	21st to end	Day	17% 11% 28% 24% 14% 6%	530
October	1st to 10th	Day	24% 11% 23% 23% 14% 5%	530
	11th to 20th	Day	20% 14% 27% 22% 15% 2	443
	21st to end	Day	30% 11% 27% 23% 8%	462
November	1st to 10th	Day	23% 15% 30% 22% 8% 1	425
	11th to 20th	Day	26% 15% 30% 20% 7% 1	430
	21st to end	Day	31% 15% 27% 21% 6% 1	422
December	1st to 10th	Day	28% 15% 30% 18% 10%	220
	11th to 20th	Day	28% 20% 34% 15% 4	220
	21st to end	Day	29% 18% 26% 19% 7%	242
January	1st to 10th	Night	19% 13% 39% 25% 3	840
	11th to 20th	Night	25% 15% 32% 24% 4	840
	21st to end	Night	17% 15% 38% 25% 5% 1	924
February	1st to 10th	Night	21% 13% 37% 26% 3	840
	11th to 20th	Night	18% 15% 39% 25% 3	823
	21st to end	Night	21% 11% 37% 28% 4	692
March	1st to 10th	Night	25% 13% 36% 24% 3	840
	11th to 20th	Night	23% 13% 33% 24% 6%	840
	21st to end	Night	26% 11% 33% 22% 7% 1	924
April	1st to 10th	Night	26% 14% 31% 26% 4	840
	11th to 20th	Night	22% 12% 35% 26% 5%	760
	21st to end	Night	24% 15% 29% 27% 5%	630
May	1st to 10th	Night	25% 10% 29% 30% 5% 1	630
	11th to 20th	Night	20% 13% 29% 29% 9% 1	560
	21st to end	Night	22% 7% 25% 35% 10% 1	455
June	1st to 10th	Night	24% 5% 26% 32% 12%	420
	11th to 20th	Night	20% 6% 24% 37% 12%	410
	21st to end	Night	23% 2 23% 40% 11% 1	400
July	1st to 10th	Night	15% 3 21% 44% 17%	390
	11th to 20th	Night	14% 4 20% 43% 19% 1	388
	21st to end	Night	13% 3 17% 49% 16% 1	438
August	1st to 10th	Night	16% 3 21% 43% 18%	524
	11th to 20th	Night	15% 5% 26% 34% 19% 2	592
	21st to end	Night	15% 4 23% 36% 19% 2	823
September	1st to 10th	Night	15% 3 28% 35% 17% 1	800
	11th to 20th	Night	17% 6% 22% 31% 20% 4	800
	21st to end	Night	17% 7% 27% 29% 18% 2	800
October	1st to 10th	Night	21% 9% 23% 31% 16% 1	800
	11th to 20th	Night	21% 8% 31% 28% 12% 1	800
	21st to end	Night	28% 9% 29% 28% 7%	880
November	1st to 10th	Night	21% 11% 31% 29% 7% 1	820
	11th to 20th	Night	21% 14% 32% 25% 7% 1	840
	21st to end	Night	21% 15% 34% 24% 5%	840
December	1st to 10th	Night	24% 9% 36% 25% 5%	840
	11th to 20th	Night	24% 14% 34% 23% 4	840
	21st to end	Night	26% 13% 33% 25% 4	924

Page generated at 2026-06-13 02:31:45. Version ac07

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