Johnson East Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Johnson East Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	17% 11% 4 9% 5% 4 4 4 5% 7% 30%	143630
	Night	17% 13% 5% 9% 5% 4 4 3 5% 6% 30%	138576
Transparency	Day	52% 6% 10% 20% 11% 1	147234
	Night	54% 5% 9% 19% 10% 2	143042
Seeing	Day	26% 17% 25% 19% 10% 3	17119
	Night	25% 15% 24% 25% 11% 1	26754

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	17% 19% 7% 22% 5% 4 3 2 3 4 14%	5232
	11th to 20th	Day	12% 22% 9% 17% 11% 5% 3 2 2 3 14%	5280
	21st to end	Day	17% 15% 7% 11% 9% 6% 4 3 4 5% 19%	5780
February	1st to 10th	Day	18% 18% 6% 11% 6% 4 3 3 4 6% 21%	1890
	11th to 20th	Day	18% 17% 8% 12% 5% 4 3 3 5% 5% 21%	1843
	21st to end	Day	20% 17% 6% 10% 4 3 3 3 4 4 24%	1745
March	1st to 10th	Day	15% 17% 6% 8% 5% 3 4 3 4 5% 30%	5045
	11th to 20th	Day	14% 12% 4 8% 5% 4 5% 3 4 6% 35%	5090
	21st to end	Day	18% 13% 4 10% 5% 4 4 3 4 6% 29%	5586
April	1st to 10th	Day	17% 15% 4 7% 4 4 5% 4 5% 7% 28%	2441
	11th to 20th	Day	16% 14% 3 6% 5% 4 5% 4 5% 6% 33%	2494
	21st to end	Day	22% 10% 2 6% 4 4 4 3 5% 7% 32%	2714
May	1st to 10th	Day	20% 9% 3 9% 5% 6% 5% 4 6% 7% 27%	7616
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	7626
	21st to end	Day	24% 8% 3 7% 4 4 5% 4 5% 7% 30%	8923
June	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 4 8% 8% 34%	2940
	11th to 20th	Day	15% 7% 3 7% 6% 5% 6% 4 6% 9% 33%	2940
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 9% 35%	2940
July	1st to 10th	Day	12% 7% 2 8% 5% 5% 5% 4 6% 8% 38%	8064
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 10% 34%	7680
	21st to end	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 35%	8340
August	1st to 10th	Day	16% 7% 3 8% 5% 5% 6% 5% 7% 9% 29%	4364
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	4304
	21st to end	Day	9% 4 2 6% 4 5% 5% 4 7% 10% 43%	4502
September	1st to 10th	Day	16% 6% 2 5% 4 3 4 4 4 7% 45%	2400
	11th to 20th	Day	13% 6% 2 4 4 4 4 3 5% 7% 46%	2240
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	2176
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	2000
	11th to 20th	Day	21% 9% 3 7% 4 4 4 3 4 5% 36%	2000
	21st to end	Day	24% 13% 2 6% 3 3 3 3 3 5% 35%	2200
November	1st to 10th	Day	13% 12% 3 6% 4 4 4 3 3 6% 42%	2025
	11th to 20th	Day	17% 16% 4 8% 4 3 4 3 5% 5% 30%	1732
	21st to end	Day	20% 19% 6% 8% 3 4 2 3 4 5% 25%	1680
December	1st to 10th	Day	17% 15% 9% 21% 5% 4 3 3 3 4 16%	2680
	11th to 20th	Day	21% 20% 7% 10% 3 2 3 2 4 5% 23%	3070
	21st to end	Day	18% 21% 7% 12% 5% 4 4 3 5% 6% 16%	4048
January	1st to 10th	Night	17% 19% 7% 21% 6% 4 3 3 3 3 13%	7657
	11th to 20th	Night	14% 19% 9% 18% 8% 4 3 2 3 3 16%	7680
	21st to end	Night	17% 16% 6% 10% 9% 7% 4 3 3 4 20%	8401
February	1st to 10th	Night	19% 17% 5% 11% 5% 4 3 3 4 4 25%	2730
	11th to 20th	Night	19% 17% 6% 13% 4 4 4 3 4 5% 21%	2663
	21st to end	Night	19% 16% 6% 10% 5% 4 4 3 4 5% 24%	2058
March	1st to 10th	Night	20% 13% 6% 9% 6% 3 3 3 3 5% 31%	5028
	11th to 20th	Night	14% 14% 5% 7% 3 3 3 2 4 6% 39%	5090
	21st to end	Night	16% 14% 3 7% 5% 4 5% 3 5% 5% 33%	5588
April	1st to 10th	Night	19% 16% 4 6% 5% 4 4 3 4 5% 32%	2179
	11th to 20th	Night	17% 11% 2 7% 4 4 4 3 5% 7% 35%	2075
	21st to end	Night	16% 11% 3 7% 3 4 4 4 5% 7% 36%	1924
May	1st to 10th	Night	23% 12% 4 9% 5% 4 4 3 5% 6% 25%	5237
	11th to 20th	Night	17% 10% 3 7% 4 4 5% 4 6% 8% 31%	5234
	21st to end	Night	25% 10% 3 8% 4 4 3 3 4 6% 29%	5226
June	1st to 10th	Night	14% 6% 2 7% 3 4 5% 4 5% 7% 43%	1680
	11th to 20th	Night	14% 6% 3 7% 5% 4 4 4 7% 9% 36%	1680
	21st to end	Night	14% 6% 2 5% 4 4 4 5% 6% 10% 40%	1680
July	1st to 10th	Night	12% 9% 3 8% 5% 4 4 4 6% 7% 37%	4875
	11th to 20th	Night	15% 7% 3 7% 5% 4 5% 4 6% 9% 35%	5277
	21st to end	Night	16% 7% 2 7% 4 4 5% 4 6% 7% 37%	5736
August	1st to 10th	Night	14% 8% 4 7% 5% 5% 6% 4 7% 8% 33%	3005
	11th to 20th	Night	18% 7% 3 7% 4 3 4 3 5% 6% 40%	2965
	21st to end	Night	9% 4 2 6% 4 5% 4 4 6% 9% 45%	3559
September	1st to 10th	Night	14% 5% 2 6% 3 5% 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 4 2 4 3 3 4 3 6% 7% 52%	2155
	21st to end	Night	18% 8% 3 6% 5% 4 4 3 5% 6% 39%	2222
October	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 5% 45%	2400
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 6% 6% 39%	2400
	21st to end	Night	26% 9% 2 5% 3 3 3 3 4 5% 37%	2640
November	1st to 10th	Night	14% 10% 2 6% 5% 3 3 3 4 6% 44%	2491
	11th to 20th	Night	19% 16% 4 7% 5% 3 3 3 4 5% 30%	2886
	21st to end	Night	19% 16% 6% 9% 5% 4 3 3 4 5% 27%	2940
December	1st to 10th	Night	18% 16% 9% 18% 4 3 3 2 4 5% 18%	4720
	11th to 20th	Night	20% 17% 9% 11% 5% 3 3 2 3 4 22%	5345
	21st to end	Night	19% 21% 7% 12% 5% 4 4 3 4 5% 17%	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	76% 3 8% 10% 3 1	5233
	11th to 20th	Day	78% 2 6% 7% 5% 1	5280
	21st to end	Day	70% 3 7% 14% 6%	5780
February	1st to 10th	Day	66% 4 8% 14% 7%	1890
	11th to 20th	Day	64% 6% 8% 14% 8%	1852
	21st to end	Day	62% 5% 10% 13% 8% 2	1745
March	1st to 10th	Day	56% 4 9% 22% 8% 1	5045
	11th to 20th	Day	49% 5% 10% 25% 9% 2	5090
	21st to end	Day	56% 4 8% 18% 12% 1	5586
April	1st to 10th	Day	55% 5% 10% 17% 11% 2	2441
	11th to 20th	Day	50% 7% 11% 15% 15% 2	2494
	21st to end	Day	51% 6% 9% 23% 9% 2	2714
May	1st to 10th	Day	54% 7% 9% 21% 6% 2	7651
	11th to 20th	Day	50% 8% 10% 23% 9%	7630
	21st to end	Day	52% 7% 10% 16% 14% 2	8938
June	1st to 10th	Day	45% 8% 12% 23% 12% 1	2940
	11th to 20th	Day	46% 8% 12% 22% 11% 1	2940
	21st to end	Day	44% 8% 11% 25% 11% 1	2940
July	1st to 10th	Day	43% 8% 11% 23% 14% 1	7997
	11th to 20th	Day	42% 8% 11% 29% 9%	7611
	21st to end	Day	45% 6% 9% 23% 17% 1	8156
August	1st to 10th	Day	48% 9% 13% 22% 8%	4351
	11th to 20th	Day	43% 7% 11% 28% 11%	4343
	21st to end	Day	34% 8% 14% 28% 14% 1	4502
September	1st to 10th	Day	38% 7% 11% 21% 19% 3	2400
	11th to 20th	Day	37% 5% 12% 28% 15% 2	2240
	21st to end	Day	51% 6% 10% 19% 13% 1	2176
October	1st to 10th	Day	35% 5% 9% 28% 20% 3	3260
	11th to 20th	Day	45% 5% 10% 22% 12% 6%	3240
	21st to end	Day	38% 4 8% 29% 20% 1	3538
November	1st to 10th	Day	44% 5% 9% 20% 16% 4	2025
	11th to 20th	Day	55% 5% 8% 17% 12% 3	1732
	21st to end	Day	62% 4 10% 14% 8% 1	1680
December	1st to 10th	Day	72% 4 11% 9% 3	2680
	11th to 20th	Day	66% 3 10% 16% 5%	3066
	21st to end	Day	70% 4 9% 13% 5%	4048
January	1st to 10th	Night	76% 4 7% 9% 4	7658
	11th to 20th	Night	73% 4 6% 10% 5% 1	7680
	21st to end	Night	69% 3 8% 14% 6% 1	8401
February	1st to 10th	Night	63% 4 8% 16% 8%	2730
	11th to 20th	Night	65% 5% 8% 15% 7%	2665
	21st to end	Night	62% 4 9% 16% 8% 1	2058
March	1st to 10th	Night	57% 4 7% 23% 8% 1	5028
	11th to 20th	Night	45% 7% 9% 28% 8% 3	5090
	21st to end	Night	51% 5% 9% 20% 13% 2	5588
April	1st to 10th	Night	55% 5% 9% 18% 12% 2	2179
	11th to 20th	Night	46% 7% 12% 18% 14% 3	2075
	21st to end	Night	47% 7% 10% 25% 9% 3	1924
May	1st to 10th	Night	58% 6% 11% 14% 9% 2	5264
	11th to 20th	Night	49% 8% 10% 24% 8% 1	5235
	21st to end	Night	53% 8% 9% 17% 10% 2	5233
June	1st to 10th	Night	40% 7% 10% 25% 16% 1	1680
	11th to 20th	Night	42% 8% 13% 24% 11% 1	1680
	21st to end	Night	39% 9% 13% 24% 15% 1	1680
July	1st to 10th	Night	45% 7% 10% 21% 15% 2	4834
	11th to 20th	Night	45% 7% 13% 23% 11%	5226
	21st to end	Night	44% 7% 10% 18% 20% 1	5606
August	1st to 10th	Night	46% 9% 15% 21% 8% 1	2994
	11th to 20th	Night	44% 6% 9% 23% 17% 1	2992
	21st to end	Night	34% 7% 12% 33% 12% 2	3559
September	1st to 10th	Night	37% 7% 9% 23% 19% 4	2000
	11th to 20th	Night	30% 7% 13% 28% 21% 1	2155
	21st to end	Night	45% 7% 9% 21% 17% 1	2222
October	1st to 10th	Night	37% 5% 8% 27% 21% 2	3920
	11th to 20th	Night	39% 5% 9% 24% 15% 8%	3899
	21st to end	Night	38% 3 7% 26% 23% 2	4252
November	1st to 10th	Night	42% 5% 10% 21% 18% 4	2491
	11th to 20th	Night	57% 5% 7% 18% 11% 2	2886
	21st to end	Night	60% 5% 9% 19% 7% 1	2940
December	1st to 10th	Night	70% 3 10% 14% 3	4720
	11th to 20th	Night	67% 3 12% 14% 4	5348
	21st to end	Night	69% 5% 9% 11% 5%	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	34% 33% 18% 10% 3	325
	11th to 20th	Day	45% 27% 22% 4 1	330
	21st to end	Day	37% 32% 24% 4 2	363
February	1st to 10th	Day	39% 28% 26% 6% 1	330
	11th to 20th	Day	43% 30% 24% 3 1	330
	21st to end	Day	38% 26% 30% 5% 1	340
March	1st to 10th	Day	32% 28% 25% 13% 2	540
	11th to 20th	Day	39% 22% 25% 10% 4 1	540
	21st to end	Day	33% 22% 29% 13% 3	594
April	1st to 10th	Day	33% 24% 24% 13% 6%	540
	11th to 20th	Day	30% 20% 28% 14% 4 4	532
	21st to end	Day	31% 19% 29% 15% 6% 1	540
May	1st to 10th	Day	30% 22% 26% 14% 6% 2	543
	11th to 20th	Day	27% 15% 31% 17% 8% 2	567
	21st to end	Day	22% 13% 25% 24% 11% 6%	617
June	1st to 10th	Day	17% 12% 30% 24% 14% 4	570
	11th to 20th	Day	20% 15% 25% 22% 15% 3	570
	21st to end	Day	16% 13% 28% 26% 15% 3	570
July	1st to 10th	Day	16% 7% 20% 28% 19% 10%	565
	11th to 20th	Day	14% 8% 26% 29% 16% 7%	564
	21st to end	Day	17% 9% 23% 28% 17% 5%	620
August	1st to 10th	Day	16% 7% 25% 29% 17% 6%	566
	11th to 20th	Day	17% 9% 21% 28% 18% 7%	559
	21st to end	Day	14% 6% 24% 29% 19% 8%	619
September	1st to 10th	Day	18% 10% 18% 26% 18% 9%	530
	11th to 20th	Day	14% 4 22% 26% 27% 8%	530
	21st to end	Day	26% 13% 25% 20% 12% 4	530
October	1st to 10th	Day	19% 10% 24% 27% 16% 4	530
	11th to 20th	Day	26% 15% 27% 22% 8% 2	530
	21st to end	Day	31% 19% 25% 17% 7%	583
November	1st to 10th	Day	22% 18% 28% 20% 9% 2	503
	11th to 20th	Day	38% 19% 25% 13% 4 1	247
	21st to end	Day	37% 22% 30% 10% 2	220
December	1st to 10th	Day	37% 33% 20% 7% 2 1	220
	11th to 20th	Day	37% 27% 26% 10%	220
	21st to end	Day	37% 29% 19% 12% 2 1	242
January	1st to 10th	Night	35% 24% 26% 12% 3	840
	11th to 20th	Night	37% 22% 26% 13% 1	840
	21st to end	Night	33% 26% 25% 15% 1	924
February	1st to 10th	Night	31% 21% 26% 19% 2	840
	11th to 20th	Night	35% 26% 26% 12% 1	823
	21st to end	Night	33% 22% 29% 14% 1	692
March	1st to 10th	Night	28% 20% 30% 19% 3	840
	11th to 20th	Night	32% 22% 22% 19% 5% 1	840
	21st to end	Night	28% 21% 28% 19% 3 1	924
April	1st to 10th	Night	28% 21% 28% 18% 4	840
	11th to 20th	Night	24% 15% 28% 23% 9%	831
	21st to end	Night	24% 13% 29% 27% 7%	664
May	1st to 10th	Night	26% 10% 25% 30% 8% 1	630
	11th to 20th	Night	25% 10% 24% 31% 9% 1	621
	21st to end	Night	15% 6% 26% 31% 20% 2	496
June	1st to 10th	Night	14% 6% 23% 39% 18%	420
	11th to 20th	Night	14% 6% 25% 37% 17% 1	420
	21st to end	Night	15% 5% 24% 36% 21%	420
July	1st to 10th	Night	13% 4 18% 37% 24% 3	474
	11th to 20th	Night	13% 5% 15% 35% 28% 3	612
	21st to end	Night	15% 5% 15% 39% 25% 1	690
August	1st to 10th	Night	11% 4 14% 44% 26% 1	627
	11th to 20th	Night	15% 4 14% 37% 27% 3	624
	21st to end	Night	15% 3 16% 37% 26% 3	838
September	1st to 10th	Night	15% 6% 18% 32% 25% 4	800
	11th to 20th	Night	13% 5% 17% 36% 26% 3	800
	21st to end	Night	21% 10% 22% 29% 18% 2	800
October	1st to 10th	Night	19% 9% 21% 30% 20% 2	800
	11th to 20th	Night	23% 11% 27% 28% 10% 1	800
	21st to end	Night	31% 11% 27% 24% 7%	880
November	1st to 10th	Night	22% 11% 25% 31% 9% 1	820
	11th to 20th	Night	31% 18% 22% 22% 6% 1	840
	21st to end	Night	32% 17% 27% 19% 4 1	840
December	1st to 10th	Night	34% 23% 24% 15% 4	840
	11th to 20th	Night	31% 24% 25% 17% 3	840
	21st to end	Night	33% 20% 27% 19% 1	924

Page generated at 2026-08-27 04:07:16. Version ac07

Copyright © 2026 Attilla Danko.