Hampton-Harmon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hampton-Harmon Observatory clear sky chart from 2005-11-06 to 2024-06-03.

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	14% 6% 2 6% 4 4 4 4 6% 8% 44%	89311
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 45%	89169
Transparency	Day	38% 7% 12% 21% 18% 4	89071
	Night	38% 6% 11% 20% 19% 5%	88993
Seeing	Day	16% 7% 22% 25% 20% 11%	14706
	Night	16% 6% 24% 32% 19% 2	24860

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	19% 4 2 4 3 2 3 2 3 6% 52%	2836
	11th to 20th	Day	14% 6% 2 5% 4 3 3 2 4 5% 51%	2840
	21st to end	Day	23% 6% 3 6% 3 3 3 3 3 5% 42%	3124
February	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 6% 7% 43%	1710
	11th to 20th	Day	20% 7% 1 4 3 3 4 3 5% 5% 44%	1760
	21st to end	Day	19% 7% 3 6% 5% 3 3 3 5% 7% 40%	1643
March	1st to 10th	Day	24% 6% 2 5% 3 3 4 3 5% 7% 38%	3435
	11th to 20th	Day	11% 4 2 5% 3 3 4 3 5% 7% 54%	3480
	21st to end	Day	20% 7% 2 6% 3 3 3 3 4 6% 43%	3815
April	1st to 10th	Day	11% 4 2 4 3 4 3 3 6% 8% 53%	2128
	11th to 20th	Day	13% 5% 1 4 5% 4 4 3 5% 7% 48%	2254
	21st to end	Day	13% 5% 2 6% 4 4 4 4 5% 7% 46%	2395
May	1st to 10th	Day	12% 7% 3 6% 6% 5% 6% 5% 8% 9% 34%	4082
	11th to 20th	Day	18% 7% 3 7% 5% 4 4 4 6% 8% 35%	4081
	21st to end	Day	8% 4 2 5% 5% 4 4 4 7% 10% 47%	4451
June	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 7% 10% 41%	2366
	11th to 20th	Day	8% 6% 1 6% 4 4 6% 4 9% 11% 39%	2340
	21st to end	Day	7% 4 2 6% 4 5% 6% 6% 8% 11% 39%	2340
July	1st to 10th	Day	9% 5% 2 8% 6% 6% 6% 5% 7% 11% 35%	2340
	11th to 20th	Day	6% 5% 3 7% 6% 7% 7% 6% 9% 12% 32%	2340
	21st to end	Day	7% 6% 3 7% 6% 6% 6% 5% 7% 10% 36%	2547
August	1st to 10th	Day	11% 5% 2 7% 5% 5% 6% 5% 9% 11% 33%	2327
	11th to 20th	Day	10% 6% 3 9% 6% 5% 7% 6% 8% 10% 31%	2303
	21st to end	Day	10% 5% 2 7% 4 5% 5% 5% 7% 10% 40%	2394
September	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 7% 9% 47%	2079
	11th to 20th	Day	12% 6% 2 5% 3 5% 4 4 6% 9% 44%	1980
	21st to end	Day	19% 6% 3 6% 4 5% 4 3 5% 7% 39%	1980
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 4 6% 53%	1835
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 5% 55%	1800
	21st to end	Day	14% 5% 2 5% 3 3 3 2 3 5% 55%	1980
November	1st to 10th	Day	13% 5% 1 4 3 3 3 3 3 5% 56%	1857
	11th to 20th	Day	12% 6% 2 4 3 3 2 3 5% 6% 55%	1772
	21st to end	Day	15% 5% 2 4 3 4 4 2 5% 5% 50%	1710
December	1st to 10th	Day	19% 7% 3 4 4 5% 3 4 4 7% 41%	1710
	11th to 20th	Day	16% 5% 1 4 2 2 2 3 4 6% 54%	2153
	21st to end	Day	19% 6% 2 5% 3 3 5% 4 5% 7% 41%	3124
January	1st to 10th	Night	15% 6% 2 5% 3 4 4 3 5% 5% 49%	4109
	11th to 20th	Night	14% 6% 3 6% 4 3 3 3 4 5% 50%	4120
	21st to end	Night	23% 7% 2 5% 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	16% 5% 3 5% 4 5% 4 3 5% 6% 44%	2470
	11th to 20th	Night	20% 7% 2 4 3 3 4 3 4 5% 45%	2304
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 44%	1809
March	1st to 10th	Night	21% 9% 2 6% 4 4 4 3 4 5% 37%	3418
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 54%	3480
	21st to end	Night	19% 5% 1 4 4 3 3 3 5% 6% 46%	3817
April	1st to 10th	Night	14% 4 2 5% 4 3 3 3 4 8% 49%	2052
	11th to 20th	Night	12% 5% 3 5% 3 4 4 2 5% 7% 49%	1875
	21st to end	Night	14% 6% 2 5% 4 3 4 3 5% 7% 47%	1800
May	1st to 10th	Night	14% 8% 2 7% 4 5% 4 3 5% 8% 41%	2813
	11th to 20th	Night	15% 8% 4 9% 4 4 4 3 6% 7% 37%	2812
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 7% 11% 45%	3078
June	1st to 10th	Night	9% 5% 2 6% 6% 5% 5% 5% 8% 13% 36%	1644
	11th to 20th	Night	11% 5% 2 5% 4 5% 5% 5% 7% 10% 41%	1620
	21st to end	Night	7% 5% 2 6% 5% 6% 6% 5% 8% 11% 39%	1620
July	1st to 10th	Night	8% 6% 2 8% 6% 5% 6% 6% 9% 10% 35%	1620
	11th to 20th	Night	7% 6% 2 7% 6% 6% 7% 5% 9% 11% 35%	1620
	21st to end	Night	8% 4 2 6% 6% 5% 6% 6% 9% 13% 35%	1771
August	1st to 10th	Night	12% 5% 3 7% 6% 6% 6% 5% 8% 11% 32%	1611
	11th to 20th	Night	11% 7% 3 8% 4 6% 6% 6% 8% 9% 33%	1598
	21st to end	Night	9% 5% 2 7% 5% 5% 6% 4 8% 10% 38%	1941
September	1st to 10th	Night	9% 6% 2 5% 4 3 5% 5% 7% 8% 46%	1877
	11th to 20th	Night	13% 6% 2 7% 5% 4 5% 4 6% 7% 41%	1980
	21st to end	Night	16% 7% 3 7% 4 4 6% 3 6% 7% 38%	1980
October	1st to 10th	Night	13% 7% 2 6% 4 3 4 2 5% 6% 49%	2124
	11th to 20th	Night	14% 4 2 4 3 3 3 3 4 5% 54%	2160
	21st to end	Night	13% 5% 2 5% 3 3 3 3 3 5% 56%	2376
November	1st to 10th	Night	15% 6% 2 5% 3 3 3 2 3 4 55%	2221
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 4 5% 53%	2403
	21st to end	Night	14% 5% 1 4 3 3 4 4 5% 7% 50%	2470
December	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 4 5% 43%	2470
	11th to 20th	Night	17% 5% 2 5% 3 3 4 3 4 6% 49%	3042
	21st to end	Night	21% 5% 2 5% 4 3 4 3 5% 7% 42%	4532

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	36% 2 7% 27% 20% 7%	2837
	11th to 20th	Day	37% 3 6% 18% 31% 5%	2840
	21st to end	Day	46% 3 7% 18% 22% 5%	3124
February	1st to 10th	Day	40% 5% 11% 20% 19% 5%	1710
	11th to 20th	Day	41% 4 9% 21% 19% 6%	1769
	21st to end	Day	46% 5% 10% 15% 18% 7%	1643
March	1st to 10th	Day	46% 5% 8% 19% 16% 8%	3435
	11th to 20th	Day	30% 6% 9% 21% 23% 12%	3480
	21st to end	Day	44% 4 8% 21% 20% 3	3815
April	1st to 10th	Day	29% 5% 12% 23% 25% 6%	2128
	11th to 20th	Day	35% 6% 10% 20% 21% 9%	2254
	21st to end	Day	36% 7% 12% 24% 17% 4	2395
May	1st to 10th	Day	42% 10% 15% 19% 11% 3	4120
	11th to 20th	Day	46% 8% 11% 22% 11% 2	4085
	21st to end	Day	30% 10% 16% 23% 17% 4	4451
June	1st to 10th	Day	36% 9% 15% 25% 14% 1	2366
	11th to 20th	Day	34% 11% 18% 20% 14% 2	2340
	21st to end	Day	34% 11% 18% 21% 14% 1	2340
July	1st to 10th	Day	40% 13% 15% 16% 13% 3	2275
	11th to 20th	Day	38% 13% 22% 17% 9% 1	2271
	21st to end	Day	39% 11% 19% 21% 9% 1	2405
August	1st to 10th	Day	40% 12% 17% 22% 8%	2314
	11th to 20th	Day	43% 14% 15% 18% 10% 1	2303
	21st to end	Day	37% 10% 18% 21% 10% 3	2394
September	1st to 10th	Day	34% 9% 14% 21% 17% 4	2079
	11th to 20th	Day	35% 9% 13% 19% 21% 3	1980
	21st to end	Day	46% 6% 8% 19% 18% 3	1980
October	1st to 10th	Day	33% 5% 10% 23% 25% 5%	1835
	11th to 20th	Day	32% 4 8% 26% 22% 7%	1800
	21st to end	Day	33% 4 7% 23% 24% 9%	1980
November	1st to 10th	Day	32% 4 7% 21% 28% 8%	1857
	11th to 20th	Day	31% 5% 8% 22% 27% 7%	1772
	21st to end	Day	35% 4 9% 22% 25% 5%	1710
December	1st to 10th	Day	44% 6% 8% 20% 18% 4	1710
	11th to 20th	Day	32% 3 9% 23% 25% 9%	2150
	21st to end	Day	41% 4 11% 23% 19% 2	3124
January	1st to 10th	Night	37% 3 8% 20% 25% 6%	4109
	11th to 20th	Night	37% 5% 7% 18% 27% 6%	4120
	21st to end	Night	45% 4 6% 18% 21% 7%	4532
February	1st to 10th	Night	40% 4 10% 21% 20% 4	2470
	11th to 20th	Night	43% 4 8% 17% 21% 7%	2306
	21st to end	Night	42% 4 10% 18% 20% 6%	1809
March	1st to 10th	Night	49% 5% 8% 16% 17% 6%	3418
	11th to 20th	Night	32% 4 7% 19% 23% 15%	3480
	21st to end	Night	40% 4 7% 23% 20% 6%	3817
April	1st to 10th	Night	35% 5% 10% 20% 24% 6%	2052
	11th to 20th	Night	34% 5% 10% 20% 22% 8%	1875
	21st to end	Night	37% 5% 11% 24% 18% 4	1800
May	1st to 10th	Night	41% 7% 15% 23% 13% 1	2840
	11th to 20th	Night	45% 9% 11% 20% 15% 1	2813
	21st to end	Night	31% 9% 16% 28% 15% 2	3079
June	1st to 10th	Night	37% 10% 15% 23% 13% 2	1644
	11th to 20th	Night	36% 10% 17% 21% 14% 2	1620
	21st to end	Night	37% 10% 18% 19% 13% 4	1620
July	1st to 10th	Night	38% 13% 16% 18% 12% 3	1575
	11th to 20th	Night	36% 13% 20% 19% 8% 3	1566
	21st to end	Night	35% 12% 22% 22% 9% 1	1672
August	1st to 10th	Night	43% 12% 18% 18% 9% 1	1600
	11th to 20th	Night	42% 14% 16% 18% 9% 2	1598
	21st to end	Night	38% 10% 19% 22% 9% 3	1941
September	1st to 10th	Night	34% 9% 15% 21% 16% 6%	1877
	11th to 20th	Night	41% 8% 12% 18% 20% 2	1980
	21st to end	Night	45% 7% 12% 16% 17% 3	1980
October	1st to 10th	Night	37% 5% 9% 22% 23% 4	2124
	11th to 20th	Night	33% 5% 8% 25% 23% 6%	2160
	21st to end	Night	33% 3 7% 21% 26% 11%	2376
November	1st to 10th	Night	35% 4 7% 19% 25% 10%	2221
	11th to 20th	Night	34% 4 7% 18% 26% 11%	2403
	21st to end	Night	34% 4 7% 23% 26% 6%	2470
December	1st to 10th	Night	44% 5% 9% 19% 19% 5%	2470
	11th to 20th	Night	37% 4 8% 18% 23% 9%	3044
	21st to end	Night	40% 5% 11% 21% 21% 2	4532

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% 16% 35% 21% 6%	270
	11th to 20th	Day	23% 17% 34% 19% 7%	270
	21st to end	Day	20% 17% 37% 20% 5%	297
February	1st to 10th	Day	18% 16% 34% 21% 11%	270
	11th to 20th	Day	31% 14% 38% 12% 5%	270
	21st to end	Day	28% 15% 33% 20% 4	298
March	1st to 10th	Day	20% 12% 33% 24% 10% 1	460
	11th to 20th	Day	20% 11% 39% 25% 5% 1	460
	21st to end	Day	20% 13% 34% 23% 9%	506
April	1st to 10th	Day	18% 14% 29% 25% 11% 2	460
	11th to 20th	Day	14% 12% 28% 24% 17% 4	452
	21st to end	Day	16% 10% 27% 25% 18% 4	460
May	1st to 10th	Day	14% 11% 26% 27% 16% 6%	463
	11th to 20th	Day	15% 6% 18% 28% 20% 14%	487
	21st to end	Day	17% 2 14% 23% 27% 16%	529
June	1st to 10th	Day	11% 3 15% 29% 28% 14%	458
	11th to 20th	Day	10% 4 16% 22% 25% 22%	450
	21st to end	Day	8% 2 11% 34% 29% 16%	450
July	1st to 10th	Day	11% 2 7% 23% 31% 26%	445
	11th to 20th	Day	5% 1 6% 24% 34% 30%	444
	21st to end	Day	7% 2 9% 24% 33% 25%	488
August	1st to 10th	Day	14% 1 12% 22% 29% 22%	446
	11th to 20th	Day	11% 2 8% 26% 29% 25%	439
	21st to end	Day	12% 1 9% 24% 33% 22%	495
September	1st to 10th	Day	12% 2 9% 20% 33% 24%	450
	11th to 20th	Day	16% 2 11% 21% 30% 20%	450
	21st to end	Day	23% 4 17% 26% 23% 7%	450
October	1st to 10th	Day	18% 5% 14% 28% 23% 12%	450
	11th to 20th	Day	12% 6% 22% 29% 24% 8%	450
	21st to end	Day	17% 6% 21% 31% 19% 6%	495
November	1st to 10th	Day	19% 5% 25% 29% 16% 6%	455
	11th to 20th	Day	15% 8% 36% 23% 14% 4	332
	21st to end	Day	13% 9% 33% 26% 14% 5%	270
December	1st to 10th	Day	23% 9% 30% 28% 8% 1	270
	11th to 20th	Day	17% 11% 29% 30% 11% 2	270
	21st to end	Day	24% 13% 26% 26% 8% 2	297
January	1st to 10th	Night	19% 10% 36% 28% 7%	760
	11th to 20th	Night	20% 10% 35% 30% 4	760
	21st to end	Night	18% 9% 40% 29% 3	836
February	1st to 10th	Night	18% 9% 34% 35% 4	760
	11th to 20th	Night	24% 13% 38% 24% 2	743
	21st to end	Night	19% 12% 38% 29% 3	628
March	1st to 10th	Night	17% 10% 34% 33% 6%	760
	11th to 20th	Night	19% 10% 35% 27% 8%	760
	21st to end	Night	18% 8% 34% 34% 6%	836
April	1st to 10th	Night	16% 11% 33% 30% 9%	760
	11th to 20th	Night	15% 10% 30% 32% 13%	751
	21st to end	Night	16% 6% 29% 36% 13%	660
May	1st to 10th	Night	10% 7% 28% 36% 18% 1	570
	11th to 20th	Night	14% 5% 22% 33% 24% 2	561
	21st to end	Night	13% 2 12% 37% 33% 3	617
June	1st to 10th	Night	10% 2 14% 38% 33% 3	548
	11th to 20th	Night	11% 3 12% 31% 39% 6%	540
	21st to end	Night	8% 1 11% 34% 43% 2	540
July	1st to 10th	Night	9% 2 8% 32% 43% 7%	525
	11th to 20th	Night	7% 2 5% 30% 48% 8%	522
	21st to end	Night	9% 1 7% 33% 42% 8%	591
August	1st to 10th	Night	14% 2 8% 28% 40% 8%	537
	11th to 20th	Night	12% 2 7% 31% 40% 7%	534
	21st to end	Night	11% 1 8% 28% 44% 7%	753
September	1st to 10th	Night	10% 2 9% 31% 41% 7%	720
	11th to 20th	Night	15% 3 11% 28% 37% 6%	720
	21st to end	Night	19% 4 18% 31% 27% 2	720
October	1st to 10th	Night	15% 6% 16% 29% 29% 5%	720
	11th to 20th	Night	17% 5% 23% 35% 18% 2	720
	21st to end	Night	15% 6% 24% 36% 17% 2	792
November	1st to 10th	Night	16% 6% 22% 37% 16% 2	740
	11th to 20th	Night	16% 6% 28% 37% 13%	760
	21st to end	Night	16% 6% 28% 37% 13% 1	760
December	1st to 10th	Night	22% 7% 30% 33% 7%	760
	11th to 20th	Night	19% 5% 31% 36% 9%	760
	21st to end	Night	25% 6% 27% 35% 8%	836

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