JAT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JAT Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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	16% 6% 3 5% 5% 3 5% 5% 5% 7% 41%	83274
	Night	16% 6% 2 5% 5% 3 4 4 5% 6% 43%	85828
Transparency	Day	42% 4 11% 24% 16% 3	82980
	Night	43% 3 11% 24% 17% 3	85656
Seeing	Day	22% 13% 29% 22% 11% 3	14249
	Night	20% 11% 29% 29% 10% 1	23641

By Month

Percentage Cloud Cover NE (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% 7% 2 4 4 2 4 4 5% 5% 47%	2517
	11th to 20th	Day	14% 9% 3 8% 6% 3 4 4 5% 6% 37%	2531
	21st to end	Day	23% 7% 3 5% 5% 3 4 4 4 6% 36%	3124
February	1st to 10th	Day	16% 6% 3 5% 6% 3 5% 4 5% 7% 39%	1710
	11th to 20th	Day	17% 7% 3 5% 5% 3 4 5% 5% 6% 42%	1825
	21st to end	Day	19% 8% 3 5% 5% 3 4 4 5% 6% 37%	1570
March	1st to 10th	Day	26% 6% 3 5% 5% 3 4 4 3 5% 37%	3116
	11th to 20th	Day	10% 4 2 4 6% 4 5% 6% 6% 8% 46%	3438
	21st to end	Day	22% 5% 2 4 4 2 4 4 4 5% 43%	4070
April	1st to 10th	Day	17% 6% 3 5% 5% 2 4 4 5% 8% 39%	2280
	11th to 20th	Day	15% 6% 2 4 4 3 5% 4 6% 8% 42%	2254
	21st to end	Day	16% 5% 2 6% 5% 3 4 5% 6% 8% 40%	2210
May	1st to 10th	Day	20% 7% 3 5% 6% 3 5% 4 6% 7% 35%	2305
	11th to 20th	Day	19% 6% 3 5% 5% 3 5% 5% 6% 8% 36%	2478
	21st to end	Day	15% 5% 3 5% 5% 3 5% 5% 5% 8% 41%	2725
June	1st to 10th	Day	13% 5% 3 6% 5% 3 5% 5% 8% 10% 38%	2520
	11th to 20th	Day	13% 6% 3 5% 6% 4 5% 6% 6% 9% 38%	2520
	21st to end	Day	12% 5% 2 5% 5% 3 5% 5% 7% 10% 41%	2520
July	1st to 10th	Day	11% 5% 2 5% 7% 3 6% 6% 7% 9% 38%	2520
	11th to 20th	Day	9% 4 2 6% 5% 3 6% 6% 7% 11% 42%	2520
	21st to end	Day	11% 5% 3 6% 6% 3 6% 5% 6% 9% 39%	2738
August	1st to 10th	Day	9% 6% 2 6% 6% 4 6% 5% 7% 8% 40%	2312
	11th to 20th	Day	12% 5% 3 5% 5% 4 5% 6% 7% 9% 39%	2126
	21st to end	Day	14% 6% 2 5% 5% 3 4 5% 5% 9% 41%	2376
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 4 6% 7% 43%	2160
	11th to 20th	Day	13% 5% 2 3 4 3 3 4 6% 7% 51%	2116
	21st to end	Day	17% 7% 3 6% 5% 4 4 4 5% 8% 39%	1980
October	1st to 10th	Day	18% 6% 3 4 4 3 4 4 5% 6% 44%	1831
	11th to 20th	Day	16% 5% 2 4 4 2 4 5% 4 7% 47%	1800
	21st to end	Day	19% 6% 2 5% 4 2 3 4 5% 5% 44%	1892
November	1st to 10th	Day	13% 6% 2 3 4 2 3 3 4 6% 52%	1671
	11th to 20th	Day	17% 6% 2 3 4 2 4 4 5% 5% 48%	1710
	21st to end	Day	15% 4 2 5% 4 2 3 5% 4 8% 47%	1611
December	1st to 10th	Day	19% 7% 3 5% 4 2 3 4 5% 7% 39%	1520
	11th to 20th	Day	20% 6% 3 4 4 2 4 4 4 6% 43%	1906
	21st to end	Day	17% 6% 3 5% 5% 4 6% 6% 6% 7% 36%	2772
January	1st to 10th	Night	16% 6% 2 5% 4 2 5% 4 4 6% 46%	4429
	11th to 20th	Night	18% 8% 3 6% 5% 3 3 3 4 5% 44%	4426
	21st to end	Night	24% 8% 3 5% 4 2 3 4 4 6% 36%	4532
February	1st to 10th	Night	16% 8% 3 5% 4 3 4 4 5% 6% 41%	2470
	11th to 20th	Night	18% 8% 2 5% 5% 3 4 4 4 5% 42%	2239
	21st to end	Night	19% 7% 3 5% 5% 2 3 4 4 5% 43%	1884
March	1st to 10th	Night	24% 9% 2 5% 4 2 3 3 4 6% 37%	3724
	11th to 20th	Night	11% 6% 2 6% 5% 3 4 4 4 6% 50%	3519
	21st to end	Night	18% 8% 3 5% 4 2 3 3 3 5% 45%	3562
April	1st to 10th	Night	18% 5% 2 5% 5% 3 4 5% 5% 7% 41%	1900
	11th to 20th	Night	16% 6% 2 6% 5% 3 5% 4 4 6% 42%	1875
	21st to end	Night	19% 6% 2 5% 4 3 4 4 5% 7% 44%	1850
May	1st to 10th	Night	19% 7% 3 6% 5% 3 5% 5% 5% 7% 35%	1661
	11th to 20th	Night	18% 6% 3 6% 5% 3 4 6% 5% 6% 39%	1420
	21st to end	Night	15% 6% 3 5% 4 3 5% 4 6% 9% 40%	1550
June	1st to 10th	Night	16% 5% 2 5% 5% 3 4 4 6% 7% 43%	1440
	11th to 20th	Night	13% 5% 3 6% 5% 4 5% 6% 5% 8% 41%	1440
	21st to end	Night	10% 4 2 5% 5% 3 6% 6% 7% 10% 43%	1440
July	1st to 10th	Night	10% 6% 3 6% 4 4 7% 5% 8% 10% 38%	1440
	11th to 20th	Night	8% 4 3 5% 5% 3 6% 6% 7% 9% 43%	1440
	21st to end	Night	9% 5% 2 5% 5% 3 6% 7% 7% 9% 43%	1577
August	1st to 10th	Night	9% 6% 2 6% 5% 3 6% 6% 7% 9% 42%	1611
	11th to 20th	Night	11% 6% 2 6% 7% 4 6% 7% 5% 8% 38%	1770
	21st to end	Night	14% 6% 2 4 6% 3 5% 4 6% 9% 41%	1980
September	1st to 10th	Night	14% 7% 3 4 6% 2 4 5% 5% 6% 45%	1800
	11th to 20th	Night	11% 6% 2 4 3 2 4 4 5% 6% 53%	1844
	21st to end	Night	18% 6% 2 4 5% 3 3 4 5% 7% 42%	1980
October	1st to 10th	Night	19% 7% 2 3 4 3 3 4 5% 5% 44%	2127
	11th to 20th	Night	15% 4 2 4 4 2 4 3 4 7% 50%	2160
	21st to end	Night	19% 7% 2 4 4 2 3 3 4 5% 46%	2462
November	1st to 10th	Night	11% 5% 2 5% 5% 3 3 3 5% 6% 53%	2407
	11th to 20th	Night	18% 5% 2 5% 4 2 3 4 4 5% 48%	2470
	21st to end	Night	14% 5% 2 5% 5% 3 4 4 5% 7% 46%	2567
December	1st to 10th	Night	20% 6% 3 5% 4 3 3 4 5% 7% 40%	2660
	11th to 20th	Night	18% 6% 2 5% 4 3 4 5% 6% 7% 41%	3288
	21st to end	Night	18% 7% 3 6% 5% 3 4 4 5% 6% 39%	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	39% 3 11% 28% 14% 4	2518
	11th to 20th	Day	48% 5% 9% 22% 13% 3	2531
	21st to end	Day	52% 2 11% 22% 11% 2	3124
February	1st to 10th	Day	45% 4 13% 25% 11% 2	1710
	11th to 20th	Day	44% 4 8% 24% 16% 3	1834
	21st to end	Day	48% 4 9% 22% 13% 4	1570
March	1st to 10th	Day	51% 3 7% 27% 9% 2	3119
	11th to 20th	Day	36% 4 9% 31% 17% 3	3438
	21st to end	Day	45% 2 8% 18% 22% 4	4070
April	1st to 10th	Day	44% 4 9% 22% 16% 5%	2280
	11th to 20th	Day	41% 4 11% 23% 18% 4	2254
	21st to end	Day	43% 3 9% 21% 19% 4	2210
May	1st to 10th	Day	49% 4 9% 19% 17% 3	2305
	11th to 20th	Day	46% 3 11% 22% 15% 2	2482
	21st to end	Day	43% 4 14% 23% 14% 2	2725
June	1st to 10th	Day	41% 6% 13% 25% 15% 1	2520
	11th to 20th	Day	42% 6% 13% 24% 12% 3	2520
	21st to end	Day	38% 6% 14% 25% 17% 1	2520
July	1st to 10th	Day	41% 4 13% 24% 15% 3	2450
	11th to 20th	Day	35% 4 14% 28% 17% 2	2445
	21st to end	Day	41% 6% 10% 22% 18% 2	2585
August	1st to 10th	Day	40% 5% 13% 26% 16% 1	2299
	11th to 20th	Day	40% 5% 14% 23% 15% 2	2126
	21st to end	Day	40% 5% 11% 22% 19% 3	2376
September	1st to 10th	Day	41% 4 11% 21% 21% 3	2160
	11th to 20th	Day	33% 3 9% 27% 24% 4	2116
	21st to end	Day	46% 3 8% 19% 22% 2	1980
October	1st to 10th	Day	42% 3 11% 25% 17% 2	1831
	11th to 20th	Day	38% 3 10% 25% 22% 2	1800
	21st to end	Day	42% 4 9% 24% 19% 2	1892
November	1st to 10th	Day	35% 2 12% 26% 22% 3	1671
	11th to 20th	Day	39% 3 9% 25% 22% 3	1710
	21st to end	Day	37% 3 13% 31% 13% 3	1611
December	1st to 10th	Day	45% 2 15% 23% 14% 1	1520
	11th to 20th	Day	43% 4 12% 27% 12% 2	1906
	21st to end	Day	47% 4 11% 25% 12% 1	2772
January	1st to 10th	Night	41% 2 9% 31% 13% 4	4429
	11th to 20th	Night	45% 2 11% 25% 14% 3	4426
	21st to end	Night	51% 2 8% 25% 13% 2	4532
February	1st to 10th	Night	44% 3 13% 26% 12% 2	2470
	11th to 20th	Night	45% 3 7% 21% 19% 5%	2241
	21st to end	Night	44% 2 7% 24% 18% 5%	1884
March	1st to 10th	Night	51% 2 7% 25% 12% 3	3734
	11th to 20th	Night	37% 3 12% 27% 18% 3	3520
	21st to end	Night	45% 2 7% 17% 23% 6%	3562
April	1st to 10th	Night	44% 3 10% 21% 17% 5%	1900
	11th to 20th	Night	44% 3 8% 21% 21% 3	1875
	21st to end	Night	41% 4 9% 22% 20% 5%	1850
May	1st to 10th	Night	49% 4 9% 18% 18% 2	1661
	11th to 20th	Night	46% 5% 12% 17% 18% 3	1421
	21st to end	Night	42% 6% 16% 20% 14% 2	1551
June	1st to 10th	Night	41% 5% 11% 22% 19% 2	1440
	11th to 20th	Night	42% 4 14% 26% 13% 1	1440
	21st to end	Night	35% 6% 14% 28% 15% 2	1440
July	1st to 10th	Night	42% 7% 15% 21% 14% 2	1400
	11th to 20th	Night	35% 6% 15% 27% 15% 2	1392
	21st to end	Night	34% 8% 15% 25% 15% 3	1489
August	1st to 10th	Night	37% 6% 17% 23% 15% 2	1600
	11th to 20th	Night	44% 5% 13% 22% 15% 2	1770
	21st to end	Night	40% 4 14% 19% 20% 3	1980
September	1st to 10th	Night	41% 5% 10% 22% 20% 3	1800
	11th to 20th	Night	34% 3 9% 23% 26% 4	1844
	21st to end	Night	43% 3 7% 23% 23% 2	1980
October	1st to 10th	Night	44% 3 10% 25% 16% 2	2127
	11th to 20th	Night	36% 3 12% 29% 18% 3	2160
	21st to end	Night	42% 2 8% 27% 19% 2	2462
November	1st to 10th	Night	34% 2 12% 25% 22% 5%	2407
	11th to 20th	Night	40% 2 8% 24% 20% 5%	2470
	21st to end	Night	38% 3 12% 28% 16% 2	2567
December	1st to 10th	Night	46% 3 11% 24% 14% 2	2660
	11th to 20th	Night	42% 4 12% 20% 17% 4	3288
	21st to end	Night	47% 2 11% 27% 12% 1	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	28% 21% 33% 13% 4 1	180
	11th to 20th	Day	37% 23% 29% 10% 2	186
	21st to end	Day	27% 22% 31% 15% 4 1	297
February	1st to 10th	Day	31% 22% 33% 11% 3	270
	11th to 20th	Day	30% 20% 34% 13% 2	433
	21st to end	Day	30% 20% 31% 15% 3	382
March	1st to 10th	Day	28% 20% 30% 17% 4	460
	11th to 20th	Day	27% 22% 32% 14% 4 2	460
	21st to end	Day	29% 22% 28% 15% 5% 1	506
April	1st to 10th	Day	29% 20% 29% 16% 5% 1	460
	11th to 20th	Day	25% 20% 30% 15% 6% 2	452
	21st to end	Day	28% 17% 34% 15% 4 2	437
May	1st to 10th	Day	31% 14% 29% 16% 7% 2	423
	11th to 20th	Day	23% 12% 33% 21% 9% 2	447
	21st to end	Day	21% 6% 25% 29% 16% 4	485
June	1st to 10th	Day	16% 10% 30% 25% 15% 3	450
	11th to 20th	Day	18% 12% 27% 29% 11% 3	450
	21st to end	Day	17% 8% 29% 31% 12% 4	450
July	1st to 10th	Day	13% 4 22% 32% 20% 8%	445
	11th to 20th	Day	9% 3 23% 35% 24% 5%	444
	21st to end	Day	13% 5% 29% 32% 15% 6%	488
August	1st to 10th	Day	11% 4 24% 35% 19% 7%	446
	11th to 20th	Day	13% 4 18% 36% 23% 6%	439
	21st to end	Day	16% 4 25% 29% 17% 9%	495
September	1st to 10th	Day	18% 8% 23% 25% 19% 8%	450
	11th to 20th	Day	16% 5% 26% 28% 16% 10%	450
	21st to end	Day	24% 10% 23% 24% 13% 6%	450
October	1st to 10th	Day	26% 5% 26% 22% 13% 7%	450
	11th to 20th	Day	18% 13% 31% 24% 12% 2	450
	21st to end	Day	23% 13% 32% 20% 10% 2	450
November	1st to 10th	Day	21% 11% 31% 22% 12% 3	365
	11th to 20th	Day	25% 16% 32% 21% 6% 1	370
	21st to end	Day	21% 16% 38% 17% 7%	271
December	1st to 10th	Day	26% 24% 31% 12% 7%	180
	11th to 20th	Day	35% 19% 29% 12% 4	180
	21st to end	Day	31% 15% 32% 16% 6% 1	198
January	1st to 10th	Night	20% 19% 34% 22% 4	760
	11th to 20th	Night	24% 15% 33% 23% 4	760
	21st to end	Night	19% 17% 36% 25% 2	836
February	1st to 10th	Night	21% 15% 34% 28% 2	760
	11th to 20th	Night	25% 21% 34% 18% 1	743
	21st to end	Night	25% 17% 35% 20% 3	628
March	1st to 10th	Night	24% 17% 34% 21% 4	760
	11th to 20th	Night	24% 18% 34% 19% 5% 1	760
	21st to end	Night	24% 12% 37% 21% 6%	836
April	1st to 10th	Night	22% 18% 37% 18% 5%	760
	11th to 20th	Night	20% 13% 37% 24% 6%	751
	21st to end	Night	22% 12% 33% 26% 6%	740
May	1st to 10th	Night	23% 12% 31% 26% 7%	581
	11th to 20th	Night	21% 10% 34% 27% 7% 1	359
	21st to end	Night	17% 6% 21% 40% 15% 1	389
June	1st to 10th	Night	18% 5% 28% 35% 14%	360
	11th to 20th	Night	14% 4 26% 41% 14%	360
	21st to end	Night	10% 3 25% 49% 13% 1	360
July	1st to 10th	Night	11% 3 22% 42% 21% 1	350
	11th to 20th	Night	9% 2 22% 45% 22% 1	348
	21st to end	Night	10% 2 25% 44% 19% 1	397
August	1st to 10th	Night	13% 1 14% 45% 25% 2	537
	11th to 20th	Night	15% 2 15% 38% 27% 3	706
	21st to end	Night	17% 2 15% 38% 25% 3	792
September	1st to 10th	Night	17% 3 23% 34% 20% 3	720
	11th to 20th	Night	16% 4 20% 35% 20% 5%	720
	21st to end	Night	22% 4 21% 32% 20% 1	720
October	1st to 10th	Night	22% 7% 23% 26% 18% 4	720
	11th to 20th	Night	18% 8% 26% 34% 13%	720
	21st to end	Night	23% 10% 32% 26% 9% 1	792
November	1st to 10th	Night	15% 9% 34% 30% 11% 1	740
	11th to 20th	Night	21% 16% 27% 27% 8% 1	760
	21st to end	Night	17% 15% 32% 28% 8% 1	760
December	1st to 10th	Night	26% 14% 33% 20% 7% 1	760
	11th to 20th	Night	21% 14% 34% 25% 6%	760
	21st to end	Night	23% 14% 31% 29% 3	836

Page generated at 2024-04-22 09:25:41. Version ac07

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