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 2025-04-26.

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% 12% 4 9% 5% 4 4 3 5% 6% 30%	113676
	Night	17% 13% 5% 9% 5% 4 4 3 4 6% 30%	114944
Transparency	Day	52% 6% 10% 20% 10% 1	117324
	Night	54% 6% 9% 19% 10% 2	119435
Seeing	Day	26% 17% 25% 19% 10% 3	15313
	Night	25% 15% 24% 25% 11% 1	25128

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	15% 20% 8% 23% 5% 4 2 2 3 3 15%	4030
	11th to 20th	Day	12% 24% 10% 16% 11% 5% 2 1 2 3 13%	4060
	21st to end	Day	19% 19% 9% 9% 7% 4 4 2 4 4 18%	4466
February	1st to 10th	Day	19% 19% 6% 11% 6% 4 3 3 4 5% 20%	1800
	11th to 20th	Day	18% 17% 8% 12% 5% 4 3 3 5% 5% 21%	1753
	21st to end	Day	21% 17% 6% 10% 4 3 3 3 4 5% 24%	1664
March	1st to 10th	Day	15% 17% 6% 8% 4 3 4 3 4 5% 31%	4935
	11th to 20th	Day	14% 12% 4 8% 5% 4 5% 3 4 6% 35%	4980
	21st to end	Day	18% 13% 4 9% 5% 4 4 3 4 6% 29%	5465
April	1st to 10th	Day	18% 16% 4 6% 4 4 5% 4 5% 7% 27%	2325
	11th to 20th	Day	16% 14% 3 6% 5% 4 5% 4 5% 6% 33%	2374
	21st to end	Day	23% 10% 2 6% 4 4 4 3 5% 6% 31%	2520
May	1st to 10th	Day	25% 9% 3 8% 5% 4 4 3 5% 6% 27%	4082
	11th to 20th	Day	22% 8% 3 8% 5% 5% 5% 4 5% 7% 29%	4081
	21st to end	Day	15% 6% 2 6% 5% 5% 6% 4 6% 9% 35%	4756
June	1st to 10th	Day	17% 6% 2 7% 5% 5% 5% 4 7% 8% 33%	2660
	11th to 20th	Day	15% 7% 2 7% 6% 5% 6% 4 6% 9% 34%	2660
	21st to end	Day	17% 6% 2 6% 5% 4 5% 5% 6% 9% 34%	2660
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 6% 8% 39%	4322
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 7% 9% 38%	4120
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	4466
August	1st to 10th	Day	16% 7% 3 8% 5% 5% 6% 4 7% 9% 29%	4104
	11th to 20th	Day	15% 6% 3 7% 5% 5% 5% 4 7% 8% 35%	4044
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 43%	4257
September	1st to 10th	Day	17% 6% 2 5% 4 3 4 4 4 7% 44%	2280
	11th to 20th	Day	14% 6% 2 4 4 4 4 3 5% 7% 45%	2128
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 34%	2067
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 40%	1900
	11th to 20th	Day	21% 9% 3 7% 4 4 4 3 4 5% 36%	1900
	21st to end	Day	24% 12% 2 5% 3 3 3 2 3 5% 36%	2090
November	1st to 10th	Day	13% 13% 3 6% 4 4 4 3 3 6% 42%	1927
	11th to 20th	Day	16% 16% 4 8% 4 4 5% 3 5% 5% 31%	1650
	21st to end	Day	20% 20% 6% 8% 3 4 2 2 4 5% 26%	1600
December	1st to 10th	Day	17% 14% 9% 21% 5% 4 3 3 3 4 17%	2600
	11th to 20th	Day	22% 20% 7% 10% 3 2 2 2 4 5% 23%	2990
	21st to end	Day	18% 21% 6% 12% 5% 4 4 3 5% 6% 16%	3960
January	1st to 10th	Night	15% 20% 8% 23% 6% 3 3 2 3 3 14%	5911
	11th to 20th	Night	14% 20% 10% 16% 8% 5% 3 3 3 3 15%	5900
	21st to end	Night	20% 17% 8% 11% 7% 5% 4 2 3 3 21%	6490
February	1st to 10th	Night	20% 17% 5% 10% 5% 3 3 3 4 4 25%	2600
	11th to 20th	Night	19% 17% 6% 13% 4 4 4 3 4 5% 21%	2533
	21st to end	Night	19% 16% 6% 10% 5% 4 4 3 4 5% 24%	1963
March	1st to 10th	Night	20% 13% 6% 9% 6% 3 3 3 3 5% 30%	4918
	11th to 20th	Night	14% 14% 5% 6% 3 3 3 2 4 6% 39%	4980
	21st to end	Night	16% 14% 3 7% 5% 4 4 3 5% 5% 33%	5467
April	1st to 10th	Night	20% 16% 4 6% 4 4 4 3 4 5% 32%	2075
	11th to 20th	Night	17% 11% 2 7% 4 4 4 3 5% 7% 35%	1975
	21st to end	Night	17% 10% 3 7% 3 5% 4 4 5% 7% 36%	1793
May	1st to 10th	Night	28% 11% 3 8% 4 4 4 3 4 6% 25%	2813
	11th to 20th	Night	20% 10% 4 7% 4 4 4 3 5% 6% 33%	2812
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 6% 8% 36%	2794
June	1st to 10th	Night	13% 6% 2 7% 3 4 5% 4 5% 7% 43%	1520
	11th to 20th	Night	13% 6% 2 7% 5% 4 4 4 7% 10% 38%	1520
	21st to end	Night	13% 6% 3 5% 4 4 4 5% 6% 10% 40%	1520
July	1st to 10th	Night	13% 8% 3 7% 5% 4 4 4 6% 8% 38%	2633
	11th to 20th	Night	13% 7% 3 7% 5% 4 5% 4 6% 8% 39%	2840
	21st to end	Night	15% 8% 2 7% 5% 5% 5% 4 7% 6% 37%	3087
August	1st to 10th	Night	14% 9% 3 7% 5% 5% 6% 4 7% 8% 33%	2825
	11th to 20th	Night	18% 7% 3 7% 4 3 4 3 5% 7% 40%	2785
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 6% 9% 45%	3366
September	1st to 10th	Night	14% 5% 2 6% 3 5% 4 3 5% 6% 45%	1900
	11th to 20th	Night	13% 4 2 4 3 3 4 3 6% 7% 52%	2047
	21st to end	Night	18% 8% 3 6% 5% 4 4 3 5% 6% 40%	2111
October	1st to 10th	Night	17% 6% 2 6% 4 4 4 4 5% 6% 43%	2280
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 6% 6% 39%	2280
	21st to end	Night	26% 9% 2 5% 3 3 4 3 4 4 36%	2508
November	1st to 10th	Night	14% 10% 2 6% 5% 3 3 2 4 6% 45%	2369
	11th to 20th	Night	19% 16% 4 7% 5% 4 3 3 4 5% 30%	2748
	21st to end	Night	19% 16% 5% 8% 5% 4 3 3 4 5% 28%	2800
December	1st to 10th	Night	19% 16% 9% 18% 4 3 3 2 4 5% 19%	4580
	11th to 20th	Night	21% 17% 9% 11% 5% 3 3 2 3 5% 22%	5205
	21st to end	Night	19% 21% 7% 11% 5% 4 4 3 4 5% 17%	6996

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% 4 8% 9% 4 1	4031
	11th to 20th	Day	80% 3 5% 7% 4 1	4060
	21st to end	Day	70% 3 8% 12% 7%	4466
February	1st to 10th	Day	67% 4 9% 13% 7%	1800
	11th to 20th	Day	64% 6% 8% 14% 8%	1762
	21st to end	Day	62% 5% 10% 12% 8% 2	1664
March	1st to 10th	Day	56% 4 9% 22% 8% 1	4935
	11th to 20th	Day	48% 5% 10% 26% 9% 2	4980
	21st to end	Day	56% 4 8% 18% 13% 1	5465
April	1st to 10th	Day	56% 5% 9% 17% 11% 2	2325
	11th to 20th	Day	50% 7% 10% 15% 15% 2	2374
	21st to end	Day	52% 6% 9% 22% 9% 2	2520
May	1st to 10th	Day	57% 8% 8% 20% 7% 1	4120
	11th to 20th	Day	52% 9% 9% 22% 7% 1	4085
	21st to end	Day	42% 10% 13% 21% 11% 2	4770
June	1st to 10th	Day	46% 9% 12% 23% 10% 1	2660
	11th to 20th	Day	45% 9% 12% 23% 11% 1	2660
	21st to end	Day	44% 8% 11% 25% 11% 1	2660
July	1st to 10th	Day	41% 8% 12% 22% 14% 2	4255
	11th to 20th	Day	40% 7% 13% 26% 13%	4051
	21st to end	Day	48% 6% 9% 21% 15% 1	4324
August	1st to 10th	Day	48% 9% 13% 22% 8%	4091
	11th to 20th	Day	42% 7% 11% 28% 11%	4083
	21st to end	Day	35% 8% 14% 27% 14% 2	4257
September	1st to 10th	Day	39% 7% 11% 20% 19% 3	2280
	11th to 20th	Day	38% 5% 12% 28% 15% 2	2128
	21st to end	Day	50% 6% 10% 20% 13% 1	2067
October	1st to 10th	Day	35% 5% 9% 28% 19% 3	3160
	11th to 20th	Day	45% 5% 10% 22% 12% 6%	3140
	21st to end	Day	37% 4 9% 29% 20% 1	3428
November	1st to 10th	Day	45% 5% 9% 20% 16% 4	1927
	11th to 20th	Day	54% 5% 8% 17% 12% 3	1650
	21st to end	Day	62% 4 10% 14% 9% 1	1600
December	1st to 10th	Day	72% 5% 11% 10% 3	2600
	11th to 20th	Day	66% 3 10% 16% 4	2986
	21st to end	Day	70% 4 9% 13% 5%	3960
January	1st to 10th	Night	77% 4 5% 9% 5%	5911
	11th to 20th	Night	75% 4 6% 10% 3 1	5900
	21st to end	Night	69% 3 8% 13% 6% 1	6490
February	1st to 10th	Night	63% 5% 8% 17% 7%	2600
	11th to 20th	Night	65% 5% 8% 15% 7%	2535
	21st to end	Night	62% 4 8% 15% 8% 1	1963
March	1st to 10th	Night	57% 4 6% 23% 8% 1	4918
	11th to 20th	Night	45% 7% 9% 28% 8% 3	4980
	21st to end	Night	51% 5% 9% 20% 13% 2	5467
April	1st to 10th	Night	56% 5% 8% 17% 12% 2	2075
	11th to 20th	Night	47% 7% 11% 18% 14% 3	1975
	21st to end	Night	46% 7% 10% 25% 9% 3	1793
May	1st to 10th	Night	59% 6% 8% 16% 9% 2	2840
	11th to 20th	Night	50% 9% 9% 22% 8% 1	2813
	21st to end	Night	41% 10% 13% 22% 9% 4	2801
June	1st to 10th	Night	40% 8% 11% 26% 14% 1	1520
	11th to 20th	Night	41% 8% 13% 26% 11% 1	1520
	21st to end	Night	38% 9% 13% 24% 14% 2	1520
July	1st to 10th	Night	43% 6% 11% 23% 14% 4	2590
	11th to 20th	Night	41% 8% 13% 24% 14% 1	2786
	21st to end	Night	45% 8% 10% 18% 17% 2	2988
August	1st to 10th	Night	46% 10% 15% 21% 8%	2814
	11th to 20th	Night	44% 6% 9% 23% 17% 1	2812
	21st to end	Night	34% 7% 12% 33% 12% 2	3366
September	1st to 10th	Night	38% 7% 9% 23% 18% 4	1900
	11th to 20th	Night	30% 7% 13% 28% 22% 1	2047
	21st to end	Night	43% 8% 9% 22% 17% 1	2111
October	1st to 10th	Night	37% 5% 8% 28% 20% 2	3800
	11th to 20th	Night	39% 5% 9% 24% 15% 8%	3779
	21st to end	Night	38% 3 7% 26% 23% 2	4120
November	1st to 10th	Night	42% 5% 9% 21% 18% 5%	2369
	11th to 20th	Night	56% 5% 7% 18% 12% 3	2748
	21st to end	Night	59% 5% 8% 19% 8% 1	2800
December	1st to 10th	Night	70% 3 9% 14% 3	4580
	11th to 20th	Night	67% 3 12% 14% 3	5208
	21st to end	Night	69% 5% 9% 11% 5%	6996

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% 34% 18% 10% 4	295
	11th to 20th	Day	43% 27% 24% 5% 1	300
	21st to end	Day	35% 33% 24% 4 3	330
February	1st to 10th	Day	40% 28% 26% 5% 1	300
	11th to 20th	Day	42% 31% 23% 3 1	300
	21st to end	Day	39% 26% 28% 5% 1	313
March	1st to 10th	Day	31% 27% 26% 14% 2	500
	11th to 20th	Day	37% 23% 26% 11% 4 1	500
	21st to end	Day	34% 21% 28% 13% 4	550
April	1st to 10th	Day	33% 25% 23% 13% 6%	500
	11th to 20th	Day	30% 19% 27% 15% 4 4	492
	21st to end	Day	31% 19% 27% 15% 6% 1	480
May	1st to 10th	Day	30% 21% 25% 15% 7% 3	463
	11th to 20th	Day	26% 16% 29% 17% 9% 3	487
	21st to end	Day	18% 11% 26% 26% 13% 6%	529
June	1st to 10th	Day	18% 11% 29% 24% 13% 4	490
	11th to 20th	Day	18% 13% 24% 23% 17% 4	490
	21st to end	Day	17% 13% 29% 25% 13% 2	490
July	1st to 10th	Day	17% 8% 21% 26% 17% 11%	485
	11th to 20th	Day	13% 8% 26% 30% 16% 7%	484
	21st to end	Day	18% 8% 24% 28% 17% 5%	532
August	1st to 10th	Day	15% 6% 25% 29% 18% 7%	486
	11th to 20th	Day	16% 8% 22% 28% 18% 8%	479
	21st to end	Day	15% 6% 23% 28% 19% 9%	539
September	1st to 10th	Day	19% 10% 18% 27% 17% 9%	490
	11th to 20th	Day	15% 4 22% 26% 25% 8%	490
	21st to end	Day	26% 14% 24% 20% 12% 4	490
October	1st to 10th	Day	20% 10% 25% 26% 15% 4	490
	11th to 20th	Day	26% 15% 26% 22% 9% 2	490
	21st to end	Day	32% 18% 25% 17% 8%	539
November	1st to 10th	Day	21% 16% 28% 22% 10% 2	465
	11th to 20th	Day	36% 20% 26% 13% 4 1	225
	21st to end	Day	35% 23% 32% 9% 2	200
December	1st to 10th	Day	35% 33% 23% 7% 3 1	200
	11th to 20th	Day	38% 25% 27% 11% 1	200
	21st to end	Day	34% 31% 19% 13% 2 1	220
January	1st to 10th	Night	34% 25% 26% 12% 4	800
	11th to 20th	Night	37% 22% 27% 14% 1	800
	21st to end	Night	32% 26% 26% 15% 1	880
February	1st to 10th	Night	32% 21% 26% 19% 2	800
	11th to 20th	Night	36% 26% 26% 12% 1	783
	21st to end	Night	33% 22% 29% 14% 1	660
March	1st to 10th	Night	28% 20% 28% 20% 4	800
	11th to 20th	Night	31% 22% 22% 19% 6% 1	800
	21st to end	Night	28% 20% 28% 20% 3 1	880
April	1st to 10th	Night	29% 20% 28% 18% 5%	800
	11th to 20th	Night	24% 15% 28% 24% 9% 1	791
	21st to end	Night	24% 12% 31% 26% 7%	619
May	1st to 10th	Night	26% 9% 24% 31% 9% 1	570
	11th to 20th	Night	24% 10% 24% 31% 10% 1	561
	21st to end	Night	11% 5% 26% 33% 22% 2	448
June	1st to 10th	Night	14% 5% 22% 39% 19%	380
	11th to 20th	Night	14% 5% 24% 37% 18% 1	380
	21st to end	Night	14% 4 24% 37% 19%	380
July	1st to 10th	Night	13% 4 18% 37% 24% 4	428
	11th to 20th	Night	12% 4 16% 36% 28% 4	552
	21st to end	Night	15% 5% 15% 38% 26% 1	624
August	1st to 10th	Night	10% 4 13% 45% 27% 2	567
	11th to 20th	Night	14% 4 13% 37% 28% 3	564
	21st to end	Night	14% 3 16% 36% 27% 3	765
September	1st to 10th	Night	16% 6% 18% 31% 25% 4	760
	11th to 20th	Night	14% 5% 17% 35% 26% 3	760
	21st to end	Night	21% 9% 21% 29% 19% 2	760
October	1st to 10th	Night	19% 9% 21% 29% 19% 2	760
	11th to 20th	Night	23% 12% 26% 28% 10% 1	760
	21st to end	Night	31% 12% 26% 24% 7%	836
November	1st to 10th	Night	21% 12% 25% 32% 10% 1	780
	11th to 20th	Night	30% 18% 23% 23% 6% 1	800
	21st to end	Night	32% 17% 27% 20% 5% 1	800
December	1st to 10th	Night	34% 22% 24% 15% 5%	800
	11th to 20th	Night	31% 23% 25% 18% 3	800
	21st to end	Night	32% 20% 27% 20% 1	880

Page generated at 2025-04-23 09:25:04. Version ac07

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