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 2024-11-05.

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% 2 6% 4 4 4 4 6% 7% 40%	101307
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	97751
Transparency	Day	24% 46% 6% 13% 9% 1	104853
	Night	24% 46% 6% 14% 9% 2	102294
Seeing	Day	22% 12% 29% 23% 11% 3	15016
	Night	20% 11% 29% 29% 10% 1	24236

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 5% 4 3 3 3 5% 5% 47%	2517
	11th to 20th	Day	14% 10% 3 9% 5% 4 4 3 5% 6% 37%	2531
	21st to end	Day	23% 8% 3 6% 5% 3 4 3 4 6% 36%	3124
February	1st to 10th	Day	16% 7% 3 6% 5% 4 4 3 5% 7% 39%	1710
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 5% 6% 41%	1825
	21st to end	Day	19% 8% 3 6% 4 4 4 4 5% 6% 37%	1570
March	1st to 10th	Day	26% 6% 2 6% 4 3 4 3 3 5% 37%	3116
	11th to 20th	Day	10% 4 2 5% 5% 5% 5% 5% 6% 7% 46%	3438
	21st to end	Day	22% 5% 2 5% 4 4 4 3 4 5% 43%	4070
April	1st to 10th	Day	16% 7% 3 6% 5% 4 4 3 6% 8% 39%	2280
	11th to 20th	Day	15% 7% 2 5% 4 4 5% 4 6% 8% 41%	2254
	21st to end	Day	16% 5% 2 7% 4 4 4 4 6% 8% 40%	2280
May	1st to 10th	Day	28% 8% 3 6% 4 4 4 3 5% 7% 28%	4057
	11th to 20th	Day	24% 8% 3 7% 5% 5% 5% 4 5% 6% 29%	4398
	21st to end	Day	13% 5% 2 6% 4 4 4 4 6% 9% 43%	4837
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 4 8% 9% 38%	2660
	11th to 20th	Day	12% 6% 2 5% 5% 5% 5% 4 7% 9% 39%	2660
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 10% 42%	2660
July	1st to 10th	Day	8% 5% 2 7% 7% 6% 7% 5% 8% 11% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	12% 8% 3 9% 6% 5% 5% 4 7% 8% 33%	4795
August	1st to 10th	Day	19% 6% 2 7% 5% 5% 5% 4 6% 8% 33%	4076
	11th to 20th	Day	13% 5% 2 6% 3 4 5% 4 7% 9% 41%	3730
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 8% 47%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	13% 5% 1 4 4 4 3 3 6% 7% 51%	2233
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 7% 38%	2090
October	1st to 10th	Day	17% 6% 2 5% 3 4 4 3 5% 6% 44%	1932
	11th to 20th	Day	15% 5% 2 4 3 3 4 3 4 7% 49%	1900
	21st to end	Day	18% 6% 2 5% 3 2 3 3 5% 5% 46%	1997
November	1st to 10th	Day	13% 6% 2 4 3 3 3 2 5% 6% 52%	1708
	11th to 20th	Day	16% 6% 2 4 4 3 3 3 5% 5% 48%	1710
	21st to end	Day	15% 5% 2 6% 4 2 3 4 5% 7% 47%	1611
December	1st to 10th	Day	19% 7% 3 6% 4 3 4 4 6% 7% 39%	1520
	11th to 20th	Day	20% 6% 3 5% 3 3 4 3 4 6% 43%	1906
	21st to end	Day	17% 7% 3 6% 5% 5% 6% 5% 6% 7% 36%	2772
January	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 5% 6% 46%	4429
	11th to 20th	Night	18% 8% 3 7% 3 3 3 3 4 5% 44%	4426
	21st to end	Night	24% 9% 2 6% 4 3 3 3 4 5% 36%	4532
February	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 6% 6% 41%	2470
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 5% 5% 42%	2239
	21st to end	Night	19% 7% 2 5% 4 3 3 3 5% 5% 43%	1884
March	1st to 10th	Night	24% 9% 2 6% 3 3 3 3 4 6% 37%	3724
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 4 5% 50%	3519
	21st to end	Night	18% 9% 3 6% 3 3 3 3 3 4 44%	3562
April	1st to 10th	Night	18% 5% 2 6% 5% 4 4 4 5% 7% 41%	1900
	11th to 20th	Night	16% 7% 2 7% 4 4 5% 4 4 6% 42%	1875
	21st to end	Night	18% 6% 2 5% 3 4 4 3 5% 6% 44%	1900
May	1st to 10th	Night	23% 9% 3 7% 4 4 4 4 5% 7% 29%	2903
	11th to 20th	Night	24% 8% 3 8% 5% 3 4 4 5% 6% 31%	2500
	21st to end	Night	14% 6% 2 5% 3 4 4 3 6% 11% 41%	2738
June	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 6% 7% 43%	1520
	11th to 20th	Night	12% 5% 3 6% 5% 5% 4 5% 5% 8% 42%	1520
	21st to end	Night	10% 4 2 6% 5% 4 6% 5% 7% 11% 42%	1520
July	1st to 10th	Night	8% 6% 2 7% 5% 6% 6% 4 9% 10% 36%	2520
	11th to 20th	Night	12% 6% 2 6% 5% 4 6% 4 8% 10% 37%	2520
	21st to end	Night	15% 7% 2 8% 5% 5% 5% 4 7% 7% 36%	2755
August	1st to 10th	Night	22% 6% 2 6% 4 4 5% 4 6% 8% 32%	2825
	11th to 20th	Night	13% 7% 2 6% 5% 4 5% 4 6% 7% 41%	3093
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 8% 50%	3476
September	1st to 10th	Night	14% 7% 2 5% 5% 4 4 4 5% 6% 46%	1900
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 6% 52%	1947
	21st to end	Night	19% 6% 2 6% 4 3 3 3 6% 7% 41%	2090
October	1st to 10th	Night	19% 7% 2 5% 4 4 4 3 5% 5% 44%	2246
	11th to 20th	Night	14% 4 2 5% 3 3 4 3 4 7% 52%	2280
	21st to end	Night	18% 7% 2 5% 3 3 3 3 5% 6% 46%	2599
November	1st to 10th	Night	11% 5% 2 5% 4 3 2 3 6% 5% 53%	2470
	11th to 20th	Night	18% 5% 2 5% 4 3 3 3 4 5% 48%	2470
	21st to end	Night	14% 6% 2 5% 5% 5% 3 4 5% 7% 46%	2567
December	1st to 10th	Night	20% 7% 3 6% 3 3 3 3 5% 7% 40%	2660
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 6% 7% 41%	3288
	21st to end	Night	18% 7% 3 6% 4 4 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	26% 37% 6% 18% 9% 4	2518
	11th to 20th	Day	30% 39% 5% 15% 9% 2	2531
	21st to end	Day	36% 38% 8% 12% 5% 1	3124
February	1st to 10th	Day	17% 66% 6% 7% 3 1	1710
	11th to 20th	Day	16% 63% 2 9% 8% 2	1834
	21st to end	Day	17% 65% 4 8% 4 1	1570
March	1st to 10th	Day	36% 41% 4 15% 4 2	3119
	11th to 20th	Day	19% 41% 5% 23% 11% 1	3438
	21st to end	Day	27% 39% 5% 9% 16% 3	4070
April	1st to 10th	Day	18% 65% 3 7% 6% 1	2280
	11th to 20th	Day	14% 66% 5% 8% 7%	2254
	21st to end	Day	16% 65% 4 8% 6% 1	2280
May	1st to 10th	Day	38% 39% 8% 10% 4	4071
	11th to 20th	Day	39% 39% 8% 12% 2	4402
	21st to end	Day	23% 40% 6% 19% 11% 1	4837
June	1st to 10th	Day	15% 65% 4 9% 6% 1	2660
	11th to 20th	Day	14% 66% 4 9% 7%	2660
	21st to end	Day	14% 65% 4 9% 8%	2660
July	1st to 10th	Day	28% 39% 11% 12% 9% 2	4370
	11th to 20th	Day	29% 37% 11% 14% 9%	4365
	21st to end	Day	33% 36% 6% 13% 11% 1	4642
August	1st to 10th	Day	35% 38% 8% 14% 5%	4063
	11th to 20th	Day	24% 37% 7% 22% 9%	3766
	21st to end	Day	19% 36% 6% 22% 16% 1	4180
September	1st to 10th	Day	16% 60% 6% 10% 7% 1	2280
	11th to 20th	Day	16% 60% 4 10% 9% 2	2233
	21st to end	Day	23% 60% 3 8% 7%	2090
October	1st to 10th	Day	24% 37% 6% 8% 21% 5%	3205
	11th to 20th	Day	11% 37% 6% 16% 25% 6%	3140
	21st to end	Day	11% 36% 9% 18% 23% 4	3274
November	1st to 10th	Day	16% 58% 6% 10% 9% 2	1708
	11th to 20th	Day	14% 59% 4 10% 13% 1	1710
	21st to end	Day	14% 59% 6% 14% 5% 2	1611
December	1st to 10th	Day	21% 59% 6% 7% 6%	1520
	11th to 20th	Day	22% 50% 5% 15% 7% 2	1906
	21st to end	Day	32% 38% 5% 18% 8%	2772
January	1st to 10th	Night	27% 36% 6% 21% 7% 3	4429
	11th to 20th	Night	28% 37% 6% 17% 10% 2	4426
	21st to end	Night	36% 39% 4 14% 7% 1	4532
February	1st to 10th	Night	16% 65% 6% 10% 3 1	2470
	11th to 20th	Night	16% 64% 2 7% 9% 2	2241
	21st to end	Night	16% 65% 4 7% 6% 2	1884
March	1st to 10th	Night	36% 40% 3 14% 6% 1	3734
	11th to 20th	Night	18% 40% 9% 19% 12% 1	3520
	21st to end	Night	28% 39% 3 10% 15% 4	3562
April	1st to 10th	Night	17% 65% 2 8% 7% 1	1900
	11th to 20th	Night	14% 65% 4 8% 7% 1	1875
	21st to end	Night	16% 65% 3 8% 8% 1	1900
May	1st to 10th	Night	36% 40% 7% 13% 3	2909
	11th to 20th	Night	38% 40% 10% 9% 3	2501
	21st to end	Night	24% 41% 10% 15% 9% 2	2739
June	1st to 10th	Night	14% 65% 3 9% 8%	1520
	11th to 20th	Night	14% 66% 3 10% 6%	1520
	21st to end	Night	14% 64% 5% 11% 6% 1	1520
July	1st to 10th	Night	27% 38% 14% 11% 8% 1	2480
	11th to 20th	Night	29% 36% 12% 16% 7%	2472
	21st to end	Night	35% 35% 8% 11% 10% 1	2667
August	1st to 10th	Night	37% 37% 13% 9% 4	2814
	11th to 20th	Night	26% 37% 8% 21% 8%	3123
	21st to end	Night	18% 36% 5% 24% 16%	3476
September	1st to 10th	Night	16% 61% 6% 10% 7% 1	1900
	11th to 20th	Night	14% 60% 4 9% 11% 2	1947
	21st to end	Night	22% 60% 3 8% 7%	2090
October	1st to 10th	Night	26% 37% 6% 14% 16% 2	3753
	11th to 20th	Night	11% 36% 5% 21% 23% 4	3779
	21st to end	Night	14% 36% 7% 23% 18% 3	4272
November	1st to 10th	Night	16% 57% 6% 11% 7% 2	2470
	11th to 20th	Night	15% 59% 3 9% 10% 3	2470
	21st to end	Night	13% 60% 6% 13% 6% 1	2567
December	1st to 10th	Night	21% 60% 4 10% 5%	2660
	11th to 20th	Night	21% 50% 6% 9% 11% 3	3288
	21st to end	Night	32% 37% 7% 17% 7%	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	27% 15% 34% 17% 4 2	460
May	1st to 10th	Day	32% 14% 29% 17% 7% 2	463
	11th to 20th	Day	24% 11% 32% 22% 9% 2	487
	21st to end	Day	19% 6% 27% 29% 15% 4	529
June	1st to 10th	Day	17% 10% 29% 26% 15% 3	490
	11th to 20th	Day	17% 11% 27% 30% 12% 3	490
	21st to end	Day	17% 8% 31% 30% 11% 3	490
July	1st to 10th	Day	12% 5% 22% 33% 20% 8%	485
	11th to 20th	Day	9% 4 23% 35% 24% 5%	484
	21st to end	Day	13% 5% 30% 32% 15% 5%	532
August	1st to 10th	Day	13% 4 24% 34% 18% 7%	486
	11th to 20th	Day	13% 5% 18% 36% 22% 6%	479
	21st to end	Day	16% 5% 24% 29% 17% 9%	539
September	1st to 10th	Day	17% 9% 23% 26% 18% 7%	490
	11th to 20th	Day	16% 4 25% 29% 17% 9%	490
	21st to end	Day	27% 9% 23% 24% 12% 6%	490
October	1st to 10th	Day	25% 5% 26% 23% 13% 8%	490
	11th to 20th	Day	17% 13% 31% 24% 11% 3	490
	21st to end	Day	21% 13% 31% 21% 12% 2	489
November	1st to 10th	Day	21% 10% 31% 22% 12% 3	378
	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	21% 12% 33% 27% 6%	760
May	1st to 10th	Night	24% 12% 31% 25% 7%	613
	11th to 20th	Night	22% 9% 33% 27% 7% 1	379
	21st to end	Night	16% 5% 21% 41% 15% 1	411
June	1st to 10th	Night	18% 5% 28% 35% 13%	380
	11th to 20th	Night	14% 4 27% 42% 14%	380
	21st to end	Night	10% 3 25% 48% 13% 1	380
July	1st to 10th	Night	12% 3 22% 42% 20% 1	370
	11th to 20th	Night	10% 2 21% 44% 21% 1	368
	21st to end	Night	11% 2 25% 44% 18% 1	420
August	1st to 10th	Night	14% 1 14% 44% 25% 2	567
	11th to 20th	Night	15% 3 16% 38% 26% 2	746
	21st to end	Night	16% 2 15% 38% 25% 3	836
September	1st to 10th	Night	17% 3 24% 34% 19% 3	760
	11th to 20th	Night	16% 3 20% 36% 21% 5%	760
	21st to end	Night	23% 4 20% 31% 19% 1	760
October	1st to 10th	Night	22% 7% 23% 26% 18% 4	760
	11th to 20th	Night	17% 8% 27% 34% 14% 1	760
	21st to end	Night	22% 10% 31% 27% 10% 1	836
November	1st to 10th	Night	15% 9% 34% 29% 11% 1	760
	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 2024-11-03 02:11:13. Version ac07

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