Hart Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hart Island clear sky chart from 2005-11-06 to 2026-03-18.

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% 7% 2 6% 4 4 4 4 6% 7% 40%	131034
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	130454
Transparency	Day	41% 6% 12% 22% 16% 2	134561
	Night	42% 5% 11% 22% 17% 3	135014
Seeing	Day	19% 12% 31% 24% 11% 3	16692
	Night	20% 11% 31% 27% 9% 1	25933

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	12% 7% 2 6% 4 4 4 3 5% 6% 48%	4661
	11th to 20th	Day	15% 9% 2 5% 4 4 4 3 4 6% 45%	4650
	21st to end	Day	16% 7% 3 6% 4 4 4 4 5% 6% 41%	5518
February	1st to 10th	Day	17% 8% 2 6% 5% 3 5% 4 4 6% 41%	1920
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 5% 44%	2048
	21st to end	Day	19% 8% 2 5% 4 3 4 3 5% 7% 40%	1730
March	1st to 10th	Day	21% 7% 2 5% 3 3 3 3 5% 5% 42%	4576
	11th to 20th	Day	11% 8% 2 6% 4 6% 4 4 5% 7% 42%	4793
	21st to end	Day	19% 7% 2 5% 4 4 4 3 5% 6% 42%	5962
April	1st to 10th	Day	19% 8% 2 6% 4 3 3 4 5% 6% 38%	2400
	11th to 20th	Day	17% 6% 2 6% 4 4 4 3 6% 7% 41%	2374
	21st to end	Day	19% 6% 2 4 5% 4 4 3 6% 8% 40%	2441
May	1st to 10th	Day	29% 8% 2 5% 3 3 4 3 5% 8% 28%	5886
	11th to 20th	Day	22% 9% 3 6% 4 3 3 3 5% 7% 35%	6221
	21st to end	Day	21% 6% 2 5% 4 4 4 4 6% 8% 36%	6949
June	1st to 10th	Day	16% 6% 2 6% 4 3 4 5% 7% 9% 37%	2800
	11th to 20th	Day	15% 8% 2 6% 4 4 5% 4 6% 8% 37%	2800
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 5% 7% 9% 39%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 36%	6360
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 5% 8% 11% 33%	6360
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 7% 9% 37%	6926
August	1st to 10th	Day	19% 7% 2 6% 4 5% 6% 4 7% 8% 33%	4301
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 41%	3850
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 10% 45%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 4 4 5% 7% 51%	2300
	21st to end	Day	18% 6% 2 6% 5% 4 5% 4 5% 7% 39%	2200
October	1st to 10th	Day	19% 6% 2 5% 3 3 4 3 5% 7% 43%	2110
	11th to 20th	Day	16% 5% 2 6% 3 3 4 4 4 7% 47%	2000
	21st to end	Day	20% 5% 2 5% 4 4 3 3 4 5% 45%	2024
November	1st to 10th	Day	14% 6% 3 5% 3 2 3 3 5% 5% 51%	1851
	11th to 20th	Day	17% 5% 2 4 3 3 4 3 4 7% 47%	1890
	21st to end	Day	16% 6% 3 4 4 3 4 4 5% 7% 44%	1827
December	1st to 10th	Day	16% 7% 2 5% 4 3 4 3 5% 7% 44%	2680
	11th to 20th	Day	22% 6% 2 5% 2 2 2 3 3 6% 45%	3066
	21st to end	Day	18% 8% 3 7% 4 4 4 3 5% 6% 39%	4048
January	1st to 10th	Night	13% 7% 3 6% 3 4 3 3 5% 6% 46%	8234
	11th to 20th	Night	16% 8% 3 6% 3 3 3 3 4 5% 45%	8235
	21st to end	Night	16% 8% 2 5% 4 4 4 3 5% 6% 43%	8658
February	1st to 10th	Night	17% 9% 3 6% 4 4 3 3 5% 6% 41%	2698
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 5% 40%	2458
	21st to end	Night	18% 8% 3 5% 4 4 4 4 5% 5% 41%	2076
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 5% 43%	5484
	11th to 20th	Night	15% 9% 2 5% 4 4 3 3 5% 6% 44%	5304
	21st to end	Night	19% 9% 3 6% 3 4 4 3 5% 5% 39%	4968
April	1st to 10th	Night	19% 7% 3 7% 5% 3 4 3 6% 6% 37%	2000
	11th to 20th	Night	19% 7% 2 5% 3 3 5% 5% 4 7% 39%	1975
	21st to end	Night	19% 6% 2 5% 4 3 3 4 5% 5% 43%	1956
May	1st to 10th	Night	27% 9% 4 7% 4 4 4 3 5% 7% 27%	4054
	11th to 20th	Night	22% 10% 3 6% 4 4 4 3 4 6% 36%	3713
	21st to end	Night	22% 8% 2 6% 4 3 4 3 5% 7% 36%	3926
June	1st to 10th	Night	18% 6% 1 4 4 5% 5% 4 5% 8% 41%	1600
	11th to 20th	Night	14% 7% 3 6% 5% 4 4 4 6% 9% 39%	1600
	21st to end	Night	11% 5% 2 6% 5% 4 5% 3 8% 9% 42%	1600
July	1st to 10th	Night	10% 7% 3 8% 5% 5% 6% 4 6% 9% 35%	3600
	11th to 20th	Night	14% 9% 3 8% 4 5% 5% 4 7% 8% 32%	3600
	21st to end	Night	14% 6% 3 6% 4 4 4 4 6% 8% 42%	3926
August	1st to 10th	Night	23% 7% 2 7% 4 4 5% 4 5% 7% 32%	2800
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 5% 8% 39%	3197
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 5% 7% 44%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 6% 52%	2100
	21st to end	Night	17% 7% 3 6% 3 4 3 3 4 6% 44%	2200
October	1st to 10th	Night	17% 8% 2 7% 4 4 3 3 4 7% 43%	2285
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 50%	2400
	21st to end	Night	21% 6% 2 6% 3 3 3 3 5% 5% 43%	2815
November	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 6% 6% 50%	2667
	11th to 20th	Night	18% 7% 2 4 3 3 2 3 5% 5% 49%	2730
	21st to end	Night	15% 6% 2 5% 4 4 3 3 6% 7% 46%	2791
December	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 6% 47%	4720
	11th to 20th	Night	25% 7% 2 5% 3 3 3 3 4 4 43%	5348
	21st to end	Night	19% 8% 3 5% 4 4 4 3 4 5% 41%	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	35% 3 10% 31% 18% 2	4662
	11th to 20th	Day	41% 4 10% 22% 18% 6%	4680
	21st to end	Day	43% 4 11% 25% 15% 1	5518
February	1st to 10th	Day	44% 5% 12% 23% 13% 2	1920
	11th to 20th	Day	42% 4 9% 26% 16% 2	2057
	21st to end	Day	45% 4 11% 22% 15% 3	1730
March	1st to 10th	Day	43% 4 13% 19% 14% 8%	4579
	11th to 20th	Day	37% 7% 14% 27% 11% 3	4793
	21st to end	Day	42% 5% 10% 13% 24% 5%	5962
April	1st to 10th	Day	44% 6% 10% 20% 16% 4	2400
	11th to 20th	Day	42% 5% 13% 21% 17% 3	2374
	21st to end	Day	41% 6% 11% 21% 17% 4	2441
May	1st to 10th	Day	53% 6% 16% 14% 10% 1	5897
	11th to 20th	Day	48% 7% 10% 20% 14% 2	6225
	21st to end	Day	43% 7% 12% 23% 15% 1	6949
June	1st to 10th	Day	40% 8% 13% 22% 15% 1	2800
	11th to 20th	Day	42% 8% 13% 24% 12% 2	2800
	21st to end	Day	37% 9% 15% 22% 15% 1	2800
July	1st to 10th	Day	42% 10% 12% 21% 15% 1	6290
	11th to 20th	Day	40% 9% 19% 22% 10% 1	6285
	21st to end	Day	40% 7% 12% 21% 20% 1	6728
August	1st to 10th	Day	45% 8% 15% 22% 9% 1	4287
	11th to 20th	Day	37% 7% 14% 31% 11% 1	3886
	21st to end	Day	32% 7% 12% 28% 20% 2	4312
September	1st to 10th	Day	37% 6% 13% 23% 19% 2	2400
	11th to 20th	Day	31% 6% 13% 24% 24% 2	2300
	21st to end	Day	45% 6% 10% 19% 18% 3	2200
October	1st to 10th	Day	35% 6% 11% 20% 23% 6%	3435
	11th to 20th	Day	26% 5% 11% 23% 30% 5%	3240
	21st to end	Day	28% 4 14% 26% 24% 3	3249
November	1st to 10th	Day	35% 4 13% 22% 23% 3	1851
	11th to 20th	Day	37% 4 10% 23% 22% 4	1890
	21st to end	Day	40% 4 14% 29% 11% 2	1827
December	1st to 10th	Day	40% 4 16% 27% 12% 1	2680
	11th to 20th	Day	41% 3 14% 22% 17% 3	3066
	21st to end	Day	46% 6% 8% 22% 15% 2	4048
January	1st to 10th	Night	38% 4 12% 24% 20% 3	8234
	11th to 20th	Night	42% 3 12% 21% 17% 6%	8280
	21st to end	Night	43% 4 9% 21% 20% 4	8658
February	1st to 10th	Night	44% 5% 12% 24% 13% 2	2698
	11th to 20th	Night	45% 5% 9% 20% 17% 4	2460
	21st to end	Night	44% 4 9% 24% 16% 3	2076
March	1st to 10th	Night	44% 4 10% 20% 13% 9%	5494
	11th to 20th	Night	40% 5% 15% 24% 12% 4	5305
	21st to end	Night	47% 4 6% 14% 23% 5%	4968
April	1st to 10th	Night	47% 5% 10% 18% 16% 4	2000
	11th to 20th	Night	44% 6% 10% 19% 20% 2	1975
	21st to end	Night	42% 6% 9% 20% 19% 4	1956
May	1st to 10th	Night	57% 5% 13% 14% 10% 1	4060
	11th to 20th	Night	50% 6% 13% 14% 14% 2	3714
	21st to end	Night	48% 5% 13% 21% 11% 2	3927
June	1st to 10th	Night	42% 7% 10% 21% 20% 1	1600
	11th to 20th	Night	40% 8% 14% 27% 11% 1	1600
	21st to end	Night	36% 8% 16% 23% 14% 2	1600
July	1st to 10th	Night	46% 8% 12% 21% 12% 2	3560
	11th to 20th	Night	47% 8% 15% 21% 9%	3552
	21st to end	Night	38% 6% 14% 22% 18% 1	3809
August	1st to 10th	Night	51% 7% 15% 17% 10% 1	2790
	11th to 20th	Night	42% 7% 13% 28% 10% 1	3227
	21st to end	Night	31% 6% 12% 32% 18% 1	3586
September	1st to 10th	Night	39% 7% 11% 23% 18% 1	2000
	11th to 20th	Night	34% 5% 11% 24% 23% 3	2100
	21st to end	Night	42% 5% 10% 21% 20% 2	2200
October	1st to 10th	Night	38% 5% 12% 23% 20% 3	3740
	11th to 20th	Night	25% 3 10% 28% 30% 4	3899
	21st to end	Night	31% 4 12% 29% 22% 2	4540
November	1st to 10th	Night	35% 5% 11% 24% 21% 4	2667
	11th to 20th	Night	39% 3 8% 23% 21% 5%	2730
	21st to end	Night	37% 5% 12% 29% 15% 2	2791
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4720
	11th to 20th	Night	46% 3 10% 22% 16% 3	5348
	21st to end	Night	45% 5% 11% 19% 17% 4	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	21% 28% 31% 15% 5%	220
	11th to 20th	Day	21% 31% 33% 12% 2	220
	21st to end	Day	18% 23% 40% 13% 4 2	314
February	1st to 10th	Day	23% 16% 41% 18% 2	360
	11th to 20th	Day	23% 19% 39% 17% 2	536
	21st to end	Day	24% 19% 36% 17% 4	446
March	1st to 10th	Day	20% 20% 35% 20% 4	540
	11th to 20th	Day	20% 18% 38% 18% 5% 1	528
	21st to end	Day	24% 20% 33% 18% 5% 1	550
April	1st to 10th	Day	24% 21% 34% 14% 7% 1	500
	11th to 20th	Day	22% 16% 33% 20% 8% 2	492
	21st to end	Day	25% 14% 33% 20% 7% 1	500
May	1st to 10th	Day	29% 14% 31% 18% 7% 2	503
	11th to 20th	Day	20% 12% 32% 25% 9% 1	527
	21st to end	Day	19% 7% 26% 28% 16% 4	573
June	1st to 10th	Day	16% 8% 29% 27% 16% 3	530
	11th to 20th	Day	16% 9% 30% 31% 12% 2	530
	21st to end	Day	13% 6% 29% 30% 17% 4	530
July	1st to 10th	Day	12% 5% 23% 36% 17% 7%	525
	11th to 20th	Day	10% 4 24% 34% 24% 5%	524
	21st to end	Day	13% 5% 25% 35% 20% 3	576
August	1st to 10th	Day	13% 4 23% 34% 20% 7%	526
	11th to 20th	Day	14% 6% 18% 37% 20% 6%	519
	21st to end	Day	11% 3 28% 32% 18% 8%	583
September	1st to 10th	Day	17% 6% 27% 26% 18% 6%	530
	11th to 20th	Day	13% 4 27% 30% 16% 10%	530
	21st to end	Day	22% 7% 28% 26% 13% 4	530
October	1st to 10th	Day	22% 9% 25% 23% 15% 6%	530
	11th to 20th	Day	18% 12% 31% 24% 12% 3	530
	21st to end	Day	22% 13% 33% 22% 9% 1	486
November	1st to 10th	Day	15% 15% 32% 26% 10% 2	425
	11th to 20th	Day	20% 20% 33% 21% 5% 1	430
	21st to end	Day	18% 19% 34% 22% 6% 1	367
December	1st to 10th	Day	20% 22% 37% 15% 5% 1	220
	11th to 20th	Day	28% 25% 32% 12% 3	220
	21st to end	Day	25% 17% 36% 12% 9%	242
January	1st to 10th	Night	19% 19% 40% 18% 4	840
	11th to 20th	Night	24% 17% 34% 22% 3	840
	21st to end	Night	19% 18% 41% 20% 2	924
February	1st to 10th	Night	21% 14% 38% 24% 2	840
	11th to 20th	Night	22% 19% 38% 19% 1	823
	21st to end	Night	23% 17% 37% 21% 3	692
March	1st to 10th	Night	22% 14% 38% 21% 5%	840
	11th to 20th	Night	22% 14% 38% 21% 4 1	829
	21st to end	Night	21% 14% 38% 22% 4	880
April	1st to 10th	Night	23% 14% 36% 22% 5%	800
	11th to 20th	Night	21% 13% 34% 27% 6%	791
	21st to end	Night	21% 12% 33% 28% 6% 1	756
May	1st to 10th	Night	24% 10% 34% 25% 7% 1	600
	11th to 20th	Night	22% 10% 34% 27% 7%	454
	21st to end	Night	22% 4 20% 39% 14% 2	433
June	1st to 10th	Night	19% 5% 27% 35% 13% 1	400
	11th to 20th	Night	15% 4 27% 39% 16%	400
	21st to end	Night	11% 3 23% 49% 14% 1	400
July	1st to 10th	Night	11% 2 21% 47% 17% 1	390
	11th to 20th	Night	14% 3 18% 42% 21% 3	388
	21st to end	Night	13% 4 18% 46% 17% 1	438
August	1st to 10th	Night	13% 2 19% 43% 21% 1	521
	11th to 20th	Night	14% 4 18% 38% 24% 2	790
	21st to end	Night	14% 4 20% 36% 24% 2	880
September	1st to 10th	Night	16% 4 24% 34% 19% 2	800
	11th to 20th	Night	15% 5% 22% 35% 20% 5%	800
	21st to end	Night	21% 5% 23% 31% 17% 3	800
October	1st to 10th	Night	21% 9% 27% 26% 15% 3	800
	11th to 20th	Night	17% 13% 29% 30% 11% 1	800
	21st to end	Night	23% 12% 32% 25% 8% 1	880
November	1st to 10th	Night	15% 12% 34% 26% 11% 1	820
	11th to 20th	Night	21% 16% 33% 23% 8%	840
	21st to end	Night	18% 17% 33% 25% 6% 1	840
December	1st to 10th	Night	25% 15% 35% 19% 5% 1	840
	11th to 20th	Night	23% 17% 33% 22% 4	840
	21st to end	Night	23% 16% 36% 21% 3	924

Page generated at 2026-03-15 02:23:54. Version ac07

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