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 2024-02-25.

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	18% 11% 5% 7% 6% 3 4 4 5% 7% 30%	78487
	Night	18% 12% 5% 7% 5% 3 4 4 4 6% 31%	80811
Transparency	Day	54% 6% 10% 18% 10% 1	78206
	Night	54% 6% 9% 19% 10% 1	80611
Seeing	Day	25% 17% 25% 19% 10% 4	13768
	Night	25% 14% 23% 25% 11% 1	23598

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% 20% 9% 13% 6% 3 3 3 3 4 19%	2810
	11th to 20th	Day	12% 23% 11% 15% 11% 3 3 2 2 3 15%	2840
	21st to end	Day	24% 19% 10% 9% 8% 3 4 3 3 4 13%	3124
February	1st to 10th	Day	20% 18% 6% 10% 7% 3 4 3 4 6% 20%	1710
	11th to 20th	Day	17% 15% 9% 11% 5% 3 4 4 5% 5% 22%	1663
	21st to end	Day	21% 16% 6% 10% 4 3 3 4 4 5% 24%	1529
March	1st to 10th	Day	17% 13% 6% 8% 5% 3 4 4 4 6% 31%	1980
	11th to 20th	Day	20% 14% 6% 6% 5% 3 5% 4 4 5% 27%	1980
	21st to end	Day	21% 14% 5% 8% 5% 2 4 3 5% 6% 27%	2178
April	1st to 10th	Day	16% 14% 5% 6% 4 3 4 5% 5% 8% 29%	2092
	11th to 20th	Day	18% 12% 3 6% 5% 3 4 5% 5% 6% 33%	2134
	21st to end	Day	24% 9% 3 5% 5% 3 4 4 4 6% 31%	2326
May	1st to 10th	Day	22% 9% 3 7% 6% 4 5% 4 5% 7% 29%	2340
	11th to 20th	Day	24% 8% 3 7% 5% 3 6% 4 4 7% 29%	2301
	21st to end	Day	14% 6% 2 6% 6% 4 6% 5% 7% 9% 34%	2686
June	1st to 10th	Day	17% 6% 3 5% 6% 4 6% 5% 7% 8% 33%	2520
	11th to 20th	Day	16% 7% 2 7% 6% 3 5% 6% 6% 9% 33%	2520
	21st to end	Day	17% 5% 2 5% 6% 3 6% 6% 6% 9% 35%	2520
July	1st to 10th	Day	15% 6% 2 6% 6% 4 6% 5% 6% 8% 36%	2458
	11th to 20th	Day	14% 6% 2 6% 6% 4 6% 5% 6% 9% 36%	2340
	21st to end	Day	15% 7% 3 7% 6% 4 6% 6% 6% 9% 32%	2547
August	1st to 10th	Day	14% 5% 3 6% 7% 4 7% 6% 8% 9% 31%	2327
	11th to 20th	Day	14% 6% 3 5% 7% 3 5% 5% 7% 10% 33%	2303
	21st to end	Day	15% 6% 2 6% 5% 3 5% 6% 6% 9% 37%	2425
September	1st to 10th	Day	17% 6% 3 4 5% 3 3 5% 5% 7% 42%	2160
	11th to 20th	Day	15% 6% 3 4 4 3 5% 4 5% 7% 44%	2016
	21st to end	Day	19% 8% 4 5% 5% 4 3 4 5% 6% 35%	1959
October	1st to 10th	Day	18% 7% 3 5% 5% 3 4 5% 5% 7% 39%	1800
	11th to 20th	Day	22% 9% 3 6% 4 3 4 4 4 5% 35%	1800
	21st to end	Day	26% 12% 2 5% 4 2 3 3 3 5% 34%	1980
November	1st to 10th	Day	14% 12% 4 6% 4 2 5% 3 3 5% 41%	1829
	11th to 20th	Day	17% 14% 5% 7% 5% 3 5% 4 4 6% 31%	1568
	21st to end	Day	20% 18% 6% 7% 4 3 3 3 4 5% 27%	1520
December	1st to 10th	Day	21% 18% 9% 11% 5% 3 4 4 4 4 17%	1520
	11th to 20th	Day	19% 19% 8% 10% 5% 2 3 2 4 5% 21%	1910
	21st to end	Day	22% 21% 8% 9% 5% 3 3 3 4 5% 16%	2772
January	1st to 10th	Night	17% 23% 8% 11% 5% 3 4 4 3 4 17%	4131
	11th to 20th	Night	17% 19% 10% 13% 6% 5% 5% 4 4 3 15%	4120
	21st to end	Night	26% 18% 9% 9% 9% 3 3 2 2 3 17%	4532
February	1st to 10th	Night	20% 15% 6% 10% 6% 3 3 4 4 4 26%	2470
	11th to 20th	Night	20% 15% 6% 12% 5% 4 4 3 4 5% 22%	2403
	21st to end	Night	21% 15% 6% 9% 6% 3 4 4 4 5% 23%	1819
March	1st to 10th	Night	17% 13% 6% 7% 7% 2 5% 4 4 6% 30%	1980
	11th to 20th	Night	20% 11% 5% 7% 5% 3 4 3 4 5% 32%	1980
	21st to end	Night	20% 13% 4 6% 6% 3 4 3 4 5% 32%	2178
April	1st to 10th	Night	19% 13% 5% 5% 5% 3 4 3 4 5% 33%	1868
	11th to 20th	Night	17% 9% 2 6% 5% 4 4 4 5% 8% 36%	1775
	21st to end	Night	17% 10% 4 6% 4 3 4 5% 4 7% 36%	1650
May	1st to 10th	Night	21% 9% 3 5% 4 3 5% 4 6% 6% 32%	1620
	11th to 20th	Night	20% 9% 3 6% 4 3 5% 4 6% 7% 32%	1592
	21st to end	Night	10% 6% 3 5% 5% 3 4 5% 6% 10% 43%	1584
June	1st to 10th	Night	14% 5% 3 7% 4 3 5% 5% 4 7% 44%	1440
	11th to 20th	Night	14% 6% 2 6% 6% 4 4 5% 7% 9% 37%	1440
	21st to end	Night	14% 6% 2 5% 4 3 4 5% 6% 10% 41%	1440
July	1st to 10th	Night	14% 6% 3 5% 5% 2 4 5% 6% 8% 43%	1497
	11th to 20th	Night	12% 5% 3 5% 5% 3 4 5% 6% 9% 45%	1620
	21st to end	Night	14% 5% 2 5% 5% 4 6% 6% 8% 9% 37%	1771
August	1st to 10th	Night	10% 6% 2 4 6% 4 5% 6% 7% 9% 40%	1611
	11th to 20th	Night	12% 6% 3 6% 6% 3 4 5% 5% 7% 44%	1598
	21st to end	Night	13% 5% 2 5% 5% 3 5% 4 5% 7% 46%	1912
September	1st to 10th	Night	15% 5% 2 5% 4 4 5% 4 5% 6% 44%	1800
	11th to 20th	Night	14% 3 2 3 3 2 4 4 5% 8% 51%	1939
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 41%	1999
October	1st to 10th	Night	18% 6% 2 5% 4 3 4 5% 5% 6% 42%	2160
	11th to 20th	Night	19% 8% 2 5% 6% 3 4 4 6% 7% 37%	2160
	21st to end	Night	27% 9% 2 5% 4 2 4 4 4 4 35%	2376
November	1st to 10th	Night	14% 10% 2 5% 5% 3 4 3 4 6% 44%	2247
	11th to 20th	Night	19% 15% 5% 6% 5% 3 4 3 4 5% 31%	2610
	21st to end	Night	19% 14% 6% 7% 5% 3 4 4 3 5% 29%	2660
December	1st to 10th	Night	23% 18% 9% 10% 5% 3 3 3 3 5% 19%	2660
	11th to 20th	Night	20% 18% 7% 10% 6% 3 3 3 3 5% 22%	3285
	21st to end	Night	22% 21% 8% 8% 7% 3 4 4 4 5% 16%	4884

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	71% 5% 8% 12% 4	2811
	11th to 20th	Day	75% 4 6% 9% 5% 1	2840
	21st to end	Day	76% 4 7% 11% 3	3124
February	1st to 10th	Day	67% 4 9% 13% 7%	1710
	11th to 20th	Day	64% 6% 8% 14% 8%	1672
	21st to end	Day	63% 5% 9% 12% 9% 2	1529
March	1st to 10th	Day	55% 5% 11% 20% 9% 1	1980
	11th to 20th	Day	59% 5% 9% 16% 9% 1	1980
	21st to end	Day	59% 4 7% 17% 11% 2	2178
April	1st to 10th	Day	53% 6% 10% 18% 11% 2	2092
	11th to 20th	Day	51% 6% 10% 14% 16% 2	2134
	21st to end	Day	53% 6% 9% 21% 9% 2	2326
May	1st to 10th	Day	54% 7% 10% 17% 10% 1	2340
	11th to 20th	Day	55% 6% 11% 16% 10% 1	2305
	21st to end	Day	43% 9% 16% 21% 9% 2	2686
June	1st to 10th	Day	46% 9% 12% 22% 10% 1	2520
	11th to 20th	Day	46% 9% 12% 22% 11% 1	2520
	21st to end	Day	45% 8% 12% 25% 11% 1	2520
July	1st to 10th	Day	45% 10% 10% 20% 13% 3	2391
	11th to 20th	Day	42% 10% 12% 22% 13% 1	2271
	21st to end	Day	46% 8% 11% 21% 12% 1	2405
August	1st to 10th	Day	45% 10% 14% 22% 9%	2314
	11th to 20th	Day	43% 9% 13% 23% 12%	2303
	21st to end	Day	42% 9% 13% 22% 12% 3	2425
September	1st to 10th	Day	41% 7% 12% 20% 17% 3	2160
	11th to 20th	Day	39% 6% 12% 28% 14% 2	2016
	21st to end	Day	48% 6% 10% 21% 14% 1	1959
October	1st to 10th	Day	45% 7% 13% 20% 14% 2	1800
	11th to 20th	Day	50% 6% 10% 24% 10%	1800
	21st to end	Day	54% 4 8% 19% 12% 2	1980
November	1st to 10th	Day	45% 6% 9% 20% 15% 4	1829
	11th to 20th	Day	54% 5% 9% 17% 12% 3	1568
	21st to end	Day	60% 5% 11% 15% 9% 1	1520
December	1st to 10th	Day	71% 6% 7% 11% 4	1520
	11th to 20th	Day	66% 3 7% 18% 4 1	1906
	21st to end	Day	71% 4 9% 12% 4	2772
January	1st to 10th	Night	70% 5% 7% 12% 5% 1	4131
	11th to 20th	Night	73% 6% 8% 11% 2	4120
	21st to end	Night	75% 3 5% 13% 4 1	4532
February	1st to 10th	Night	63% 5% 8% 16% 7%	2470
	11th to 20th	Night	65% 5% 8% 15% 8%	2405
	21st to end	Night	63% 4 8% 15% 8% 2	1819
March	1st to 10th	Night	55% 6% 8% 20% 10% 1	1980
	11th to 20th	Night	54% 5% 9% 20% 9% 1	1980
	21st to end	Night	55% 4 7% 19% 12% 3	2178
April	1st to 10th	Night	54% 5% 9% 18% 13% 2	1868
	11th to 20th	Night	46% 8% 11% 17% 15% 3	1775
	21st to end	Night	47% 7% 9% 24% 9% 3	1650
May	1st to 10th	Night	51% 5% 11% 19% 11% 3	1620
	11th to 20th	Night	51% 8% 11% 15% 13% 2	1593
	21st to end	Night	36% 9% 18% 26% 11% 1	1585
June	1st to 10th	Night	40% 8% 11% 26% 14% 1	1440
	11th to 20th	Night	42% 8% 13% 25% 11% 1	1440
	21st to end	Night	38% 9% 13% 24% 14% 2	1440
July	1st to 10th	Night	39% 8% 14% 22% 13% 4	1454
	11th to 20th	Night	33% 9% 14% 24% 18% 1	1566
	21st to end	Night	39% 10% 14% 22% 13% 2	1672
August	1st to 10th	Night	37% 11% 14% 24% 14%	1600
	11th to 20th	Night	39% 9% 13% 23% 15% 1	1598
	21st to end	Night	37% 7% 12% 28% 13% 4	1912
September	1st to 10th	Night	38% 8% 10% 23% 17% 4	1800
	11th to 20th	Night	30% 7% 14% 27% 21% 1	1939
	21st to end	Night	42% 8% 9% 23% 18% 1	1999
October	1st to 10th	Night	42% 7% 11% 24% 14% 2	2160
	11th to 20th	Night	46% 6% 9% 25% 13% 1	2160
	21st to end	Night	53% 5% 8% 20% 12% 2	2376
November	1st to 10th	Night	43% 5% 10% 21% 17% 5%	2247
	11th to 20th	Night	56% 5% 7% 18% 12% 2	2610
	21st to end	Night	58% 5% 8% 19% 8% 1	2660
December	1st to 10th	Night	70% 5% 8% 13% 4 1	2660
	11th to 20th	Night	66% 5% 8% 16% 4 1	3288
	21st to end	Night	71% 6% 8% 11% 4	4884

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	33% 33% 19% 11% 4	265
	11th to 20th	Day	41% 28% 25% 5% 1	270
	21st to end	Day	34% 33% 26% 4 3	297
February	1st to 10th	Day	39% 27% 27% 6% 1	270
	11th to 20th	Day	39% 33% 24% 3 1	270
	21st to end	Day	39% 25% 29% 6% 1	267
March	1st to 10th	Day	30% 27% 27% 14% 2	420
	11th to 20th	Day	35% 23% 26% 11% 4 1	420
	21st to end	Day	34% 21% 26% 14% 4	462
April	1st to 10th	Day	29% 25% 24% 15% 7%	420
	11th to 20th	Day	27% 19% 29% 16% 5% 5%	412
	21st to end	Day	33% 19% 26% 14% 6% 1	420
May	1st to 10th	Day	31% 21% 25% 14% 7% 3	423
	11th to 20th	Day	28% 15% 30% 16% 8% 3	447
	21st to end	Day	18% 11% 25% 26% 13% 7%	485
June	1st to 10th	Day	18% 10% 30% 25% 14% 4	450
	11th to 20th	Day	19% 14% 24% 23% 16% 4	450
	21st to end	Day	18% 12% 27% 27% 13% 2	450
July	1st to 10th	Day	18% 7% 19% 26% 18% 12%	445
	11th to 20th	Day	13% 7% 26% 30% 17% 7%	444
	21st to end	Day	17% 8% 24% 27% 17% 6%	488
August	1st to 10th	Day	15% 6% 23% 29% 20% 7%	446
	11th to 20th	Day	17% 8% 20% 28% 18% 9%	439
	21st to end	Day	16% 6% 24% 27% 17% 9%	495
September	1st to 10th	Day	20% 10% 17% 26% 17% 10%	450
	11th to 20th	Day	16% 4 22% 26% 25% 8%	450
	21st to end	Day	24% 13% 24% 21% 13% 5%	450
October	1st to 10th	Day	20% 11% 24% 26% 16% 4	450
	11th to 20th	Day	27% 15% 27% 22% 8% 2	450
	21st to end	Day	34% 18% 24% 16% 7%	495
November	1st to 10th	Day	21% 16% 27% 23% 11% 2	427
	11th to 20th	Day	36% 20% 24% 14% 5% 1	203
	21st to end	Day	32% 23% 34% 9% 2	180
December	1st to 10th	Day	34% 32% 22% 8% 3 1	180
	11th to 20th	Day	36% 26% 27% 12% 1	180
	21st to end	Day	34% 32% 17% 14% 2 1	198
January	1st to 10th	Night	34% 25% 26% 12% 4	760
	11th to 20th	Night	36% 21% 27% 14% 1	760
	21st to end	Night	32% 26% 26% 15% 1	836
February	1st to 10th	Night	31% 21% 26% 20% 2	760
	11th to 20th	Night	34% 27% 26% 12% 1	743
	21st to end	Night	33% 22% 28% 14% 1	611
March	1st to 10th	Night	28% 20% 28% 20% 4	720
	11th to 20th	Night	30% 22% 21% 19% 6% 1	720
	21st to end	Night	28% 18% 28% 21% 4 1	792
April	1st to 10th	Night	28% 19% 28% 19% 5%	720
	11th to 20th	Night	23% 15% 28% 24% 10% 1	711
	21st to end	Night	24% 12% 31% 24% 8%	570
May	1st to 10th	Night	26% 10% 24% 31% 9% 1	540
	11th to 20th	Night	25% 10% 24% 31% 10% 1	531
	21st to end	Night	12% 4 26% 32% 23% 2	424
June	1st to 10th	Night	14% 4 22% 39% 20%	360
	11th to 20th	Night	14% 5% 24% 36% 19% 1	360
	21st to end	Night	14% 4 24% 38% 20%	360
July	1st to 10th	Night	13% 4 18% 37% 25% 4	404
	11th to 20th	Night	11% 4 15% 35% 30% 4	522
	21st to end	Night	15% 5% 15% 38% 27% 1	591
August	1st to 10th	Night	10% 4 13% 44% 28% 2	537
	11th to 20th	Night	13% 5% 13% 36% 30% 4	534
	21st to end	Night	15% 3 16% 35% 28% 3	724
September	1st to 10th	Night	16% 5% 18% 30% 27% 5%	720
	11th to 20th	Night	14% 5% 17% 35% 26% 4	720
	21st to end	Night	19% 9% 20% 29% 20% 2	720
October	1st to 10th	Night	20% 10% 21% 28% 19% 3	720
	11th to 20th	Night	24% 12% 26% 28% 9% 1	720
	21st to end	Night	32% 12% 26% 24% 6% 1	792
November	1st to 10th	Night	21% 11% 24% 32% 10% 1	740
	11th to 20th	Night	29% 18% 23% 22% 7% 1	760
	21st to end	Night	30% 17% 27% 20% 5% 1	760
December	1st to 10th	Night	35% 21% 24% 15% 5%	760
	11th to 20th	Night	30% 24% 25% 18% 3	760
	21st to end	Night	32% 21% 26% 20% 1	836

Page generated at 2024-02-22 06:05:42. Version ac07

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