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 2024-11-02.

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 8% 5% 5% 5% 4 6% 8% 34%	101739
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	97202
Transparency	Day	47% 6% 12% 21% 13% 1	101545
	Night	46% 6% 11% 22% 13% 2	97066
Seeing	Day	17% 9% 26% 24% 16% 8%	15261
	Night	17% 8% 27% 31% 15% 2	24790

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% 12% 6% 5% 5% 4 5% 4 21%	2836
	11th to 20th	Day	15% 15% 6% 15% 5% 9% 7% 3 4 4 17%	2840
	21st to end	Day	27% 17% 11% 10% 5% 4 4 3 3 4 13%	3124
February	1st to 10th	Day	19% 14% 5% 10% 6% 5% 5% 3 5% 5% 24%	1756
	11th to 20th	Day	14% 13% 5% 10% 8% 5% 4 4 4 6% 26%	1848
	21st to end	Day	18% 15% 5% 9% 5% 4 4 3 4 4 29%	1570
March	1st to 10th	Day	20% 15% 5% 11% 5% 5% 4 3 4 5% 24%	3202
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 4 6% 8% 38%	3480
	21st to end	Day	17% 10% 4 8% 4 4 4 4 5% 8% 32%	3815
April	1st to 10th	Day	13% 11% 4 8% 4 4 4 4 6% 7% 34%	2194
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 4 5% 7% 38%	2347
	21st to end	Day	17% 8% 3 7% 6% 5% 5% 4 5% 7% 33%	2470
May	1st to 10th	Day	20% 11% 3 7% 4 5% 4 3 5% 8% 30%	4082
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 6% 8% 32%	4081
	21st to end	Day	9% 7% 3 8% 5% 5% 6% 5% 8% 10% 34%	4476
June	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 7% 10% 37%	2470
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 10% 41%	2586
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	10% 7% 3 7% 6% 5% 5% 4 7% 10% 37%	4440
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 7% 11% 44%	4162
	21st to end	Day	10% 7% 2 7% 6% 5% 6% 5% 8% 10% 34%	4466
August	1st to 10th	Day	8% 6% 2 6% 4 4 6% 5% 7% 10% 41%	4104
	11th to 20th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 39%	4044
	21st to end	Day	6% 4 2 5% 4 5% 4 4 6% 9% 52%	4318
September	1st to 10th	Day	11% 5% 1 5% 4 4 5% 4 6% 6% 49%	2133
	11th to 20th	Day	9% 6% 2 5% 4 4 4 4 5% 8% 49%	2090
	21st to end	Day	15% 7% 2 6% 5% 4 3 3 5% 7% 45%	2090
October	1st to 10th	Day	10% 5% 3 6% 5% 5% 4 3 6% 7% 47%	2081
	11th to 20th	Day	13% 8% 2 6% 4 3 4 3 5% 6% 45%	1900
	21st to end	Day	18% 10% 3 7% 4 4 3 3 5% 7% 36%	1986
November	1st to 10th	Day	9% 8% 2 6% 4 3 4 2 5% 7% 50%	1681
	11th to 20th	Day	13% 11% 5% 8% 5% 4 4 4 4 6% 36%	1710
	21st to end	Day	18% 10% 4 8% 6% 4 4 3 4 6% 32%	1710
December	1st to 10th	Day	18% 14% 5% 11% 7% 5% 4 4 4 6% 21%	1710
	11th to 20th	Day	16% 15% 4 12% 7% 5% 4 3 5% 5% 25%	2153
	21st to end	Day	20% 19% 8% 11% 6% 5% 4 4 4 5% 14%	3124
January	1st to 10th	Night	14% 17% 8% 14% 6% 5% 5% 3 4 6% 19%	4109
	11th to 20th	Night	13% 15% 6% 15% 8% 5% 4 3 5% 6% 20%	4120
	21st to end	Night	24% 18% 7% 9% 4 5% 4 2 3 4 18%	4532
February	1st to 10th	Night	19% 13% 4 10% 5% 4 4 2 4 5% 29%	2423
	11th to 20th	Night	16% 11% 4 9% 5% 5% 5% 3 5% 6% 31%	2218
	21st to end	Night	19% 13% 5% 9% 5% 4 4 3 4 4 29%	1884
March	1st to 10th	Night	21% 12% 3 6% 5% 4 3 3 5% 5% 33%	3649
	11th to 20th	Night	12% 7% 3 7% 4 5% 4 4 6% 7% 42%	3480
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 4 5% 39%	3817
April	1st to 10th	Night	13% 8% 3 7% 4 4 5% 4 6% 7% 37%	2003
	11th to 20th	Night	14% 8% 3 5% 4 4 5% 3 6% 8% 40%	1782
	21st to end	Night	16% 8% 2 7% 4 4 4 4 5% 8% 38%	1710
May	1st to 10th	Night	22% 11% 3 8% 6% 4 4 4 5% 6% 27%	2813
	11th to 20th	Night	19% 7% 2 6% 3 4 4 3 6% 8% 37%	2812
	21st to end	Night	8% 5% 2 6% 4 5% 4 5% 7% 9% 45%	3079
June	1st to 10th	Night	8% 6% 3 5% 4 4 5% 4 5% 9% 48%	1710
	11th to 20th	Night	7% 4 2 5% 5% 3 4 3 8% 11% 47%	1587
	21st to end	Night	5% 4 2 5% 5% 5% 5% 5% 8% 11% 45%	1520
July	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 6% 7% 48%	2530
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 4 6% 10% 49%	2795
	21st to end	Night	10% 6% 2 7% 6% 5% 5% 5% 7% 9% 39%	3087
August	1st to 10th	Night	6% 7% 3 8% 5% 4 5% 5% 6% 9% 41%	2825
	11th to 20th	Night	7% 8% 3 8% 5% 4 4 4 6% 9% 44%	2785
	21st to end	Night	4 2 2 4 4 5% 5% 4 6% 10% 54%	3334
September	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 4 7% 51%	2027
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 5% 7% 54%	2090
	21st to end	Night	13% 6% 2 6% 4 3 3 3 5% 7% 48%	2090
October	1st to 10th	Night	8% 6% 2 6% 4 3 4 3 6% 7% 52%	2097
	11th to 20th	Night	14% 8% 3 5% 4 4 4 3 5% 6% 45%	2280
	21st to end	Night	17% 9% 3 6% 4 4 3 3 5% 6% 40%	2610
November	1st to 10th	Night	13% 7% 2 6% 4 4 3 3 4 6% 47%	2420
	11th to 20th	Night	16% 8% 3 7% 5% 3 4 4 6% 7% 38%	2470
	21st to end	Night	17% 12% 5% 7% 6% 4 5% 3 5% 6% 30%	2470
December	1st to 10th	Night	20% 14% 5% 10% 5% 5% 4 3 4 5% 26%	2470
	11th to 20th	Night	17% 12% 5% 10% 5% 4 4 4 5% 6% 28%	3042
	21st to end	Night	20% 19% 7% 11% 6% 5% 4 3 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	61% 6% 8% 18% 7% 1	3202
	11th to 20th	Day	43% 6% 10% 25% 15% 1	3480
	21st to end	Day	49% 6% 12% 22% 10% 1	3815
April	1st to 10th	Day	49% 5% 12% 22% 11% 1	2194
	11th to 20th	Day	46% 4 12% 22% 14% 3	2347
	21st to end	Day	49% 7% 11% 20% 12% 2	2470
May	1st to 10th	Day	52% 6% 12% 22% 7% 1	4120
	11th to 20th	Day	47% 7% 13% 23% 9% 1	4085
	21st to end	Day	39% 10% 15% 20% 14% 2	4489
June	1st to 10th	Day	41% 7% 11% 24% 15% 2	2470
	11th to 20th	Day	37% 8% 15% 23% 16% 1	2586
	21st to end	Day	37% 9% 15% 21% 14% 3	2660
July	1st to 10th	Day	42% 6% 12% 22% 17% 2	4371
	11th to 20th	Day	32% 7% 12% 31% 18% 1	4091
	21st to end	Day	42% 9% 14% 18% 14% 4	4324
August	1st to 10th	Day	35% 8% 17% 29% 11% 1	4091
	11th to 20th	Day	35% 7% 12% 29% 16% 1	4083
	21st to end	Day	29% 6% 14% 29% 18% 3	4317
September	1st to 10th	Day	34% 6% 10% 24% 21% 5%	2133
	11th to 20th	Day	32% 6% 11% 25% 22% 3	2090
	21st to end	Day	40% 5% 10% 22% 22% 1	2090
October	1st to 10th	Day	36% 6% 11% 25% 21% 1	2081
	11th to 20th	Day	39% 4 10% 26% 18% 2	1900
	21st to end	Day	47% 4 13% 20% 14% 1	1986
November	1st to 10th	Day	36% 3 11% 27% 20% 3	1681
	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	52% 5% 9% 23% 11% 1	3649
	11th to 20th	Night	39% 6% 13% 28% 13% 2	3480
	21st to end	Night	47% 5% 9% 28% 11% 1	3817
April	1st to 10th	Night	43% 6% 13% 23% 13% 2	2003
	11th to 20th	Night	42% 5% 12% 17% 19% 4	1782
	21st to end	Night	45% 5% 12% 23% 13% 3	1710
May	1st to 10th	Night	56% 6% 9% 20% 7% 2	2840
	11th to 20th	Night	44% 7% 13% 26% 9% 1	2813
	21st to end	Night	31% 10% 14% 28% 13% 5%	3087
June	1st to 10th	Night	33% 8% 11% 26% 21% 2	1710
	11th to 20th	Night	30% 9% 16% 23% 19% 4	1587
	21st to end	Night	29% 10% 15% 24% 17% 4	1520
July	1st to 10th	Night	35% 6% 12% 24% 18% 6%	2490
	11th to 20th	Night	29% 7% 12% 32% 18% 2	2743
	21st to end	Night	39% 6% 13% 23% 15% 4	2988
August	1st to 10th	Night	38% 7% 14% 27% 12% 1	2814
	11th to 20th	Night	36% 7% 12% 26% 19%	2812
	21st to end	Night	24% 7% 16% 29% 21% 3	3334
September	1st to 10th	Night	33% 6% 12% 24% 19% 6%	2027
	11th to 20th	Night	28% 6% 9% 26% 27% 3	2090
	21st to end	Night	36% 5% 10% 21% 26% 2	2090
October	1st to 10th	Night	30% 7% 10% 26% 24% 3	2097
	11th to 20th	Night	40% 5% 8% 26% 19% 2	2280
	21st to end	Night	46% 4 8% 23% 17% 3	2610
November	1st to 10th	Night	38% 4 11% 22% 22% 3	2420
	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	22% 19% 35% 19% 4	313
	11th to 20th	Day	22% 16% 34% 20% 7% 1	460
	21st to end	Day	27% 14% 34% 20% 5%	506
April	1st to 10th	Day	20% 13% 32% 23% 8% 2	460
	11th to 20th	Day	18% 18% 27% 23% 11% 3	452
	21st to end	Day	22% 11% 36% 19% 9% 3	460
May	1st to 10th	Day	23% 12% 30% 21% 10% 4	463
	11th to 20th	Day	18% 8% 26% 28% 13% 8%	487
	21st to end	Day	11% 5% 25% 28% 19% 12%	529
June	1st to 10th	Day	13% 4 22% 28% 24% 9%	490
	11th to 20th	Day	12% 4 21% 31% 24% 9%	606
	21st to end	Day	11% 4 26% 33% 19% 9%	680
July	1st to 10th	Day	12% 3 17% 26% 23% 20%	671
	11th to 20th	Day	9% 3 21% 24% 27% 16%	511
	21st to end	Day	10% 2 15% 29% 28% 15%	532
August	1st to 10th	Day	10% 3 11% 28% 27% 21%	486
	11th to 20th	Day	12% 3 15% 30% 27% 14%	479
	21st to end	Day	8% 3 12% 26% 33% 18%	539
September	1st to 10th	Day	13% 3 13% 24% 32% 15%	490
	11th to 20th	Day	9% 2 12% 27% 32% 17%	490
	21st to end	Day	18% 9% 18% 24% 23% 8%	490
October	1st to 10th	Day	9% 4 23% 34% 20% 9%	490
	11th to 20th	Day	14% 12% 31% 23% 15% 5%	490
	21st to end	Day	22% 10% 35% 25% 8% 1	435
November	1st to 10th	Day	13% 10% 32% 27% 13% 5%	273
	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	20% 12% 34% 27% 6% 1	760
	11th to 20th	Night	20% 13% 34% 26% 7%	760
	21st to end	Night	24% 10% 33% 25% 8%	836
April	1st to 10th	Night	16% 13% 34% 30% 6% 1	673
	11th to 20th	Night	17% 9% 33% 29% 10% 1	563
	21st to end	Night	17% 6% 38% 30% 7% 1	570
May	1st to 10th	Night	20% 8% 31% 28% 11% 1	570
	11th to 20th	Night	18% 4 24% 35% 16% 2	561
	21st to end	Night	8% 3 21% 36% 26% 5%	617
June	1st to 10th	Night	8% 3 21% 43% 23% 2	570
	11th to 20th	Night	8% 3 17% 45% 25% 2	570
	21st to end	Night	6% 3 22% 40% 26% 3	570
July	1st to 10th	Night	10% 2 11% 36% 32% 8%	555
	11th to 20th	Night	6% 3 14% 38% 34% 6%	552
	21st to end	Night	11% 2 11% 40% 33% 3	624
August	1st to 10th	Night	7% 1 11% 40% 36% 5%	567
	11th to 20th	Night	9% 2 9% 45% 31% 4	564
	21st to end	Night	8% 1 12% 34% 37% 8%	627
September	1st to 10th	Night	13% 3 14% 31% 33% 5%	697
	11th to 20th	Night	8% 3 11% 37% 36% 5%	760
	21st to end	Night	16% 4 19% 36% 23% 2	760
October	1st to 10th	Night	11% 4 23% 34% 23% 4	760
	11th to 20th	Night	18% 9% 27% 32% 14% 1	760
	21st to end	Night	21% 9% 31% 27% 11%	836
November	1st to 10th	Night	17% 8% 29% 29% 17% 1	745
	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 2024-10-30 01:22:54. Version ac07

Copyright © 2024 Attilla Danko.