Jefferson Observatory
Clear Sky Chart
History

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

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% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	79380
	Night	14% 9% 4 7% 6% 3 5% 4 5% 7% 35%	80144
Transparency	Day	49% 6% 12% 20% 12% 2	79097
	Night	48% 6% 11% 20% 13% 2	79947
Seeing	Day	17% 9% 26% 24% 16% 8%	14278
	Night	17% 8% 27% 30% 15% 2	23979

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	16% 16% 7% 11% 7% 3 5% 5% 4 5% 21%	2836
	11th to 20th	Day	15% 14% 7% 14% 6% 6% 9% 4 4 4 18%	2840
	21st to end	Day	27% 16% 11% 9% 5% 3 5% 4 3 4 13%	3124
February	1st to 10th	Day	19% 13% 6% 10% 6% 3 5% 5% 4 5% 24%	1756
	11th to 20th	Day	15% 12% 6% 8% 10% 4 5% 4 4 6% 26%	1848
	21st to end	Day	18% 14% 5% 9% 5% 3 4 4 4 4 29%	1570
March	1st to 10th	Day	16% 9% 4 8% 7% 3 5% 4 5% 6% 31%	1890
	11th to 20th	Day	18% 10% 6% 8% 6% 3 4 5% 6% 8% 27%	1980
	21st to end	Day	20% 11% 5% 8% 6% 3 5% 4 5% 8% 27%	2178
April	1st to 10th	Day	13% 10% 4 7% 5% 3 5% 4 5% 7% 35%	2078
	11th to 20th	Day	16% 8% 3 6% 6% 3 4 4 5% 7% 38%	2222
	21st to end	Day	18% 8% 3 7% 6% 3 6% 5% 4 7% 32%	2340
May	1st to 10th	Day	20% 9% 3 6% 5% 3 5% 4 5% 7% 32%	2340
	11th to 20th	Day	18% 8% 4 6% 6% 3 6% 5% 6% 8% 31%	2301
	21st to end	Day	8% 7% 3 6% 5% 4 7% 7% 7% 10% 36%	2531
June	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 5% 6% 10% 38%	2340
	11th to 20th	Day	8% 6% 2 6% 6% 4 5% 5% 6% 10% 41%	2449
	21st to end	Day	9% 5% 3 6% 5% 3 6% 7% 6% 10% 39%	2520
July	1st to 10th	Day	11% 6% 2 5% 6% 4 6% 6% 6% 9% 38%	2520
	11th to 20th	Day	8% 5% 3 6% 6% 4 7% 5% 7% 9% 41%	2368
	21st to end	Day	8% 5% 3 5% 5% 3 6% 7% 7% 11% 40%	2547
August	1st to 10th	Day	8% 5% 2 5% 6% 4 6% 6% 7% 10% 42%	2327
	11th to 20th	Day	9% 5% 2 6% 7% 4 6% 6% 7% 10% 37%	2303
	21st to end	Day	8% 5% 3 5% 5% 3 5% 5% 7% 9% 46%	2458
September	1st to 10th	Day	12% 5% 2 4 5% 3 5% 4 6% 7% 47%	2021
	11th to 20th	Day	9% 6% 2 4 5% 3 3 6% 6% 8% 48%	1980
	21st to end	Day	14% 5% 3 5% 5% 3 3 3 5% 7% 47%	1980
October	1st to 10th	Day	11% 5% 3 5% 6% 4 5% 4 6% 7% 45%	1972
	11th to 20th	Day	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	1800
	21st to end	Day	20% 10% 3 6% 4 3 4 4 4 7% 35%	1882
November	1st to 10th	Day	9% 7% 3 5% 5% 2 4 3 5% 7% 50%	1672
	11th to 20th	Day	14% 10% 4 8% 6% 3 4 5% 4 6% 36%	1710
	21st to end	Day	18% 9% 5% 7% 7% 3 4 4 4 6% 33%	1710
December	1st to 10th	Day	19% 13% 6% 10% 8% 4 5% 4 4 6% 21%	1710
	11th to 20th	Day	16% 14% 5% 10% 8% 3 4 4 5% 5% 26%	2153
	21st to end	Day	20% 18% 8% 10% 8% 3 5% 4 4 5% 14%	3124
January	1st to 10th	Night	14% 16% 8% 12% 8% 4 5% 4 4 6% 20%	4109
	11th to 20th	Night	13% 14% 7% 13% 10% 3 5% 4 5% 6% 20%	4120
	21st to end	Night	24% 17% 8% 8% 5% 4 5% 3 3 4 19%	4532
February	1st to 10th	Night	20% 12% 5% 9% 6% 3 5% 3 4 5% 30%	2423
	11th to 20th	Night	16% 10% 4 8% 6% 4 5% 4 5% 6% 31%	2218
	21st to end	Night	20% 13% 6% 8% 6% 3 4 4 4 4 29%	1884
March	1st to 10th	Night	17% 7% 3 8% 7% 3 4 4 5% 5% 37%	2185
	11th to 20th	Night	16% 8% 4 6% 5% 3 5% 5% 5% 7% 36%	1980
	21st to end	Night	19% 9% 4 7% 6% 3 4 4 4 7% 33%	2178
April	1st to 10th	Night	13% 7% 3 6% 5% 3 5% 5% 6% 8% 39%	1898
	11th to 20th	Night	14% 7% 4 5% 5% 2 5% 5% 6% 8% 40%	1687
	21st to end	Night	16% 8% 3 6% 5% 3 4 4 5% 8% 37%	1620
May	1st to 10th	Night	18% 8% 3 6% 7% 3 5% 5% 6% 7% 33%	1620
	11th to 20th	Night	16% 8% 3 6% 4 3 4 5% 6% 9% 37%	1592
	21st to end	Night	7% 4 2 5% 5% 3 6% 7% 8% 11% 43%	1744
June	1st to 10th	Night	7% 5% 3 5% 4 3 5% 5% 5% 8% 49%	1620
	11th to 20th	Night	7% 3 3 5% 5% 3 5% 4 8% 11% 47%	1504
	21st to end	Night	5% 3 3 4 5% 3 5% 6% 8% 11% 46%	1440
July	1st to 10th	Night	8% 5% 3 6% 5% 3 4 4 6% 8% 48%	1440
	11th to 20th	Night	6% 4 2 5% 5% 4 6% 6% 6% 10% 47%	1588
	21st to end	Night	8% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	1771
August	1st to 10th	Night	5% 5% 2 5% 6% 3 6% 7% 7% 10% 45%	1611
	11th to 20th	Night	7% 6% 2 5% 5% 3 4 3 7% 10% 47%	1598
	21st to end	Night	7% 3 2 4 4 4 4 5% 6% 9% 52%	1894
September	1st to 10th	Night	12% 5% 2 4 4 3 4 4 4 8% 50%	1920
	11th to 20th	Night	8% 4 3 4 5% 2 4 4 5% 7% 54%	1980
	21st to end	Night	12% 5% 2 5% 4 2 3 4 4 7% 51%	1980
October	1st to 10th	Night	8% 6% 2 5% 5% 2 4 4 6% 8% 51%	1986
	11th to 20th	Night	15% 8% 3 5% 5% 3 4 4 5% 6% 43%	2160
	21st to end	Night	18% 9% 3 5% 5% 3 4 4 5% 6% 38%	2472
November	1st to 10th	Night	13% 6% 3 5% 4 3 4 4 4 7% 47%	2406
	11th to 20th	Night	16% 7% 4 6% 6% 3 4 5% 5% 8% 38%	2470
	21st to end	Night	17% 11% 5% 6% 6% 3 5% 4 5% 6% 31%	2470
December	1st to 10th	Night	20% 13% 5% 9% 6% 3 5% 4 4 5% 26%	2470
	11th to 20th	Night	17% 11% 5% 9% 6% 3 4 5% 4 7% 28%	3042
	21st to end	Night	20% 18% 7% 10% 8% 4 4 4 4 5% 16%	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	64% 5% 9% 14% 8%	2837
	11th to 20th	Day	70% 3 12% 10% 5%	2840
	21st to end	Day	76% 4 9% 9% 3	3124
February	1st to 10th	Day	62% 4 10% 14% 9% 1	1756
	11th to 20th	Day	60% 4 10% 16% 9% 1	1858
	21st to end	Day	58% 5% 10% 18% 7% 1	1570
March	1st to 10th	Day	52% 6% 10% 23% 8% 1	1890
	11th to 20th	Day	55% 5% 12% 18% 9% 1	1980
	21st to end	Day	57% 5% 11% 19% 7% 1	2178
April	1st to 10th	Day	48% 5% 12% 22% 11% 1	2078
	11th to 20th	Day	47% 4 12% 20% 14% 3	2222
	21st to end	Day	50% 7% 10% 20% 11% 2	2340
May	1st to 10th	Day	51% 6% 11% 21% 9% 1	2340
	11th to 20th	Day	50% 8% 13% 18% 11% 1	2305
	21st to end	Day	40% 10% 15% 23% 11% 1	2531
June	1st to 10th	Day	41% 8% 11% 24% 15% 2	2340
	11th to 20th	Day	37% 8% 15% 22% 16% 1	2449
	21st to end	Day	38% 9% 15% 21% 14% 3	2520
July	1st to 10th	Day	41% 8% 12% 20% 15% 3	2451
	11th to 20th	Day	37% 8% 12% 27% 15% 1	2297
	21st to end	Day	35% 10% 16% 23% 13% 3	2405
August	1st to 10th	Day	36% 8% 15% 26% 13% 2	2314
	11th to 20th	Day	38% 9% 13% 25% 13% 1	2303
	21st to end	Day	34% 7% 14% 22% 17% 5%	2458
September	1st to 10th	Day	35% 7% 11% 24% 19% 4	2021
	11th to 20th	Day	33% 6% 12% 25% 21% 3	1980
	21st to end	Day	37% 5% 10% 23% 23% 1	1980
October	1st to 10th	Day	38% 7% 11% 24% 20% 1	1972
	11th to 20th	Day	40% 4 10% 26% 18% 3	1800
	21st to end	Day	49% 4 13% 20% 13% 1	1882
November	1st to 10th	Day	36% 3 11% 28% 19% 3	1672
	11th to 20th	Day	49% 4 10% 20% 14% 2	1710
	21st to end	Day	53% 4 10% 20% 11% 1	1710
December	1st to 10th	Day	64% 5% 11% 16% 4	1710
	11th to 20th	Day	60% 5% 9% 17% 9%	2150
	21st to end	Day	73% 5% 9% 8% 5% 1	3124
January	1st to 10th	Night	64% 5% 10% 13% 8%	4109
	11th to 20th	Night	65% 4 11% 15% 5% 1	4120
	21st to end	Night	69% 4 9% 13% 4 1	4532
February	1st to 10th	Night	59% 4 9% 18% 9% 1	2423
	11th to 20th	Night	54% 5% 11% 18% 12% 1	2219
	21st to end	Night	59% 4 10% 16% 10% 2	1884
March	1st to 10th	Night	48% 5% 9% 22% 13% 2	2185
	11th to 20th	Night	48% 4 13% 22% 10% 3	1980
	21st to end	Night	52% 5% 9% 22% 10% 2	2178
April	1st to 10th	Night	42% 6% 13% 24% 13% 2	1898
	11th to 20th	Night	42% 5% 12% 17% 19% 5%	1687
	21st to end	Night	45% 5% 12% 23% 12% 3	1620
May	1st to 10th	Night	50% 6% 12% 20% 10% 3	1620
	11th to 20th	Night	45% 6% 13% 20% 13% 2	1593
	21st to end	Night	32% 10% 15% 26% 13% 3	1745
June	1st to 10th	Night	32% 8% 11% 26% 20% 2	1620
	11th to 20th	Night	30% 9% 16% 23% 18% 4	1504
	21st to end	Night	29% 10% 16% 24% 17% 4	1440
July	1st to 10th	Night	34% 7% 14% 23% 18% 4	1400
	11th to 20th	Night	30% 9% 14% 28% 17% 3	1536
	21st to end	Night	32% 7% 14% 30% 16% 2	1672
August	1st to 10th	Night	31% 9% 15% 28% 16% 2	1600
	11th to 20th	Night	31% 8% 15% 26% 19% 1	1598
	21st to end	Night	27% 8% 14% 25% 21% 5%	1894
September	1st to 10th	Night	34% 6% 12% 24% 18% 6%	1920
	11th to 20th	Night	29% 7% 9% 27% 26% 3	1980
	21st to end	Night	34% 5% 10% 22% 27% 2	1980
October	1st to 10th	Night	31% 7% 10% 26% 22% 3	1986
	11th to 20th	Night	42% 5% 8% 25% 18% 2	2160
	21st to end	Night	48% 4 8% 22% 15% 2	2472
November	1st to 10th	Night	38% 4 11% 22% 21% 3	2406
	11th to 20th	Night	44% 5% 9% 21% 17% 3	2470
	21st to end	Night	53% 5% 11% 20% 10% 1	2470
December	1st to 10th	Night	60% 5% 9% 18% 7% 1	2470
	11th to 20th	Night	55% 5% 10% 21% 8% 1	3044
	21st to end	Night	70% 5% 8% 13% 4	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	24% 15% 38% 17% 6%	270
	11th to 20th	Day	22% 21% 38% 13% 4 1	270
	21st to end	Day	30% 13% 35% 17% 5%	297
February	1st to 10th	Day	27% 15% 36% 17% 4	270
	11th to 20th	Day	27% 20% 33% 16% 4	270
	21st to end	Day	27% 14% 42% 16% 2	225
March	1st to 10th	Day	21% 18% 37% 20% 5%	295
	11th to 20th	Day	22% 15% 34% 20% 8% 1	420
	21st to end	Day	29% 13% 34% 18% 6%	462
April	1st to 10th	Day	20% 13% 31% 25% 9% 2	420
	11th to 20th	Day	18% 17% 26% 23% 11% 3	412
	21st to end	Day	24% 11% 35% 18% 9% 3	420
May	1st to 10th	Day	23% 13% 30% 21% 10% 4	423
	11th to 20th	Day	18% 7% 27% 28% 13% 6%	447
	21st to end	Day	11% 5% 24% 27% 19% 14%	485
June	1st to 10th	Day	13% 4 20% 29% 25% 9%	450
	11th to 20th	Day	12% 4 22% 29% 24% 9%	559
	21st to end	Day	11% 4 25% 32% 19% 9%	630
July	1st to 10th	Day	12% 2 15% 26% 24% 21%	621
	11th to 20th	Day	9% 3 20% 25% 25% 18%	470
	21st to end	Day	10% 2 15% 28% 28% 16%	488
August	1st to 10th	Day	10% 4 10% 28% 28% 21%	446
	11th to 20th	Day	12% 3 13% 29% 28% 15%	439
	21st to end	Day	8% 3 13% 27% 31% 17%	495
September	1st to 10th	Day	14% 3 13% 24% 31% 16%	450
	11th to 20th	Day	10% 2 13% 26% 32% 17%	450
	21st to end	Day	15% 9% 18% 25% 24% 8%	450
October	1st to 10th	Day	10% 5% 24% 33% 20% 9%	450
	11th to 20th	Day	15% 12% 32% 24% 14% 3	450
	21st to end	Day	23% 10% 35% 24% 7%	397
November	1st to 10th	Day	13% 10% 32% 27% 13% 5%	270
	11th to 20th	Day	18% 20% 27% 22% 11% 2	270
	21st to end	Day	24% 9% 39% 23% 6%	270
December	1st to 10th	Day	20% 19% 37% 16% 8% 1	270
	11th to 20th	Day	19% 13% 41% 22% 5%	270
	21st to end	Day	29% 18% 32% 15% 6%	297
January	1st to 10th	Night	20% 18% 35% 20% 6%	760
	11th to 20th	Night	24% 15% 36% 21% 4	760
	21st to end	Night	20% 14% 37% 25% 4	836
February	1st to 10th	Night	27% 11% 33% 28% 2	760
	11th to 20th	Night	24% 13% 36% 24% 3	743
	21st to end	Night	25% 11% 39% 22% 2	628
March	1st to 10th	Night	19% 12% 35% 27% 6% 1	743
	11th to 20th	Night	21% 12% 34% 26% 7%	720
	21st to end	Night	24% 9% 32% 25% 9%	792
April	1st to 10th	Night	15% 12% 35% 30% 7% 1	638
	11th to 20th	Night	17% 9% 33% 29% 11% 1	533
	21st to end	Night	17% 7% 38% 30% 8% 1	540
May	1st to 10th	Night	20% 8% 32% 28% 11% 1	540
	11th to 20th	Night	18% 5% 24% 35% 16% 2	531
	21st to end	Night	8% 3 21% 36% 27% 5%	584
June	1st to 10th	Night	8% 2 21% 44% 23% 2	540
	11th to 20th	Night	8% 3 18% 44% 25% 2	540
	21st to end	Night	6% 3 21% 42% 26% 3	540
July	1st to 10th	Night	10% 2 11% 35% 34% 9%	525
	11th to 20th	Night	6% 3 13% 38% 35% 6%	522
	21st to end	Night	10% 2 12% 40% 34% 3	591
August	1st to 10th	Night	7% 1 10% 40% 36% 5%	537
	11th to 20th	Night	9% 1 10% 43% 33% 4	534
	21st to end	Night	8% 1 13% 34% 37% 7%	594
September	1st to 10th	Night	13% 3 14% 30% 33% 6%	660
	11th to 20th	Night	9% 3 12% 36% 36% 6%	720
	21st to end	Night	14% 4 19% 36% 24% 2	720
October	1st to 10th	Night	12% 4 23% 34% 23% 4	720
	11th to 20th	Night	18% 9% 27% 32% 13% 1	720
	21st to end	Night	23% 9% 30% 27% 10%	792
November	1st to 10th	Night	16% 7% 29% 29% 17% 1	740
	11th to 20th	Night	19% 14% 31% 26% 9% 1	760
	21st to end	Night	21% 12% 30% 30% 7% 1	760
December	1st to 10th	Night	25% 11% 35% 24% 5% 1	760
	11th to 20th	Night	24% 10% 34% 26% 6% 1	760
	21st to end	Night	29% 13% 29% 26% 3	836

Page generated at 2024-03-03 06:28:56. Version ac07

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