Joseph D. Tatem Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph D. Tatem Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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% 7% 2 6% 4 4 4 4 5% 7% 40%	101327
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	97783
Transparency	Day	42% 5% 11% 23% 16% 3	104886
	Night	41% 5% 11% 23% 17% 3	102313
Seeing	Day	21% 16% 29% 21% 10% 3	15064
	Night	19% 13% 31% 27% 9% 1	24295

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% 9% 2 5% 3 3 3 3 4 5% 46%	2517
	11th to 20th	Day	15% 8% 3 10% 4 5% 3 3 4 5% 39%	2554
	21st to end	Day	23% 10% 3 6% 4 4 3 3 4 5% 35%	3124
February	1st to 10th	Day	15% 8% 3 6% 4 4 5% 4 5% 6% 39%	1710
	11th to 20th	Day	17% 7% 2 6% 4 3 5% 4 5% 6% 42%	1838
	21st to end	Day	18% 10% 3 6% 4 4 3 3 4 5% 39%	1570
March	1st to 10th	Day	27% 8% 2 4 3 3 3 3 3 5% 39%	3116
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 3 6% 7% 49%	3434
	21st to end	Day	21% 7% 2 6% 3 3 3 3 4 5% 44%	4084
April	1st to 10th	Day	17% 8% 3 6% 3 4 4 3 5% 6% 41%	2280
	11th to 20th	Day	15% 7% 2 5% 4 4 4 4 6% 7% 43%	2254
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 7% 40%	2280
May	1st to 10th	Day	26% 9% 3 7% 4 4 3 4 5% 7% 28%	4028
	11th to 20th	Day	26% 7% 3 8% 5% 4 4 3 5% 6% 29%	4388
	21st to end	Day	12% 6% 2 5% 3 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 7% 9% 40%	2660
	11th to 20th	Day	13% 5% 2 6% 6% 5% 5% 4 7% 9% 39%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 7% 43%	2660
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 7% 5% 8% 10% 35%	4440
	11th to 20th	Day	12% 5% 2 6% 5% 5% 6% 5% 7% 10% 37%	4440
	21st to end	Day	13% 7% 2 9% 7% 6% 6% 5% 7% 8% 31%	4784
August	1st to 10th	Day	19% 6% 3 7% 5% 6% 5% 4 6% 8% 31%	4050
	11th to 20th	Day	14% 5% 2 6% 4 5% 5% 4 7% 9% 39%	3730
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 46%	4180
September	1st to 10th	Day	13% 5% 2 7% 5% 4 4 3 5% 7% 44%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 8% 49%	2228
	21st to end	Day	19% 7% 2 7% 4 4 4 3 4 5% 39%	2090
October	1st to 10th	Day	19% 6% 2 5% 4 3 3 3 4 6% 45%	1938
	11th to 20th	Day	16% 5% 2 4 3 3 3 2 4 6% 51%	1900
	21st to end	Day	17% 5% 2 6% 3 3 4 3 4 6% 47%	2001
November	1st to 10th	Day	13% 5% 2 5% 4 4 3 3 5% 6% 52%	1734
	11th to 20th	Day	15% 5% 2 6% 4 3 3 3 5% 6% 46%	1710
	21st to end	Day	15% 5% 2 5% 4 4 4 4 5% 6% 47%	1630
December	1st to 10th	Day	19% 8% 2 6% 4 4 3 3 5% 7% 40%	1520
	11th to 20th	Day	20% 8% 3 4 3 3 3 3 4 4 45%	1906
	21st to end	Day	17% 6% 3 6% 4 5% 5% 5% 6% 7% 37%	2772
January	1st to 10th	Night	18% 6% 2 5% 3 4 4 3 5% 6% 46%	4429
	11th to 20th	Night	18% 9% 2 6% 3 3 3 3 4 5% 44%	4404
	21st to end	Night	22% 9% 3 6% 4 4 4 3 4 5% 36%	4532
February	1st to 10th	Night	16% 9% 3 5% 5% 3 4 3 5% 6% 42%	2470
	11th to 20th	Night	18% 8% 2 6% 4 3 3 3 5% 6% 41%	2227
	21st to end	Night	19% 7% 2 6% 3 4 3 3 4 6% 43%	1884
March	1st to 10th	Night	23% 11% 2 5% 4 3 3 3 4 4 38%	3724
	11th to 20th	Night	12% 6% 2 6% 4 3 3 3 5% 6% 49%	3521
	21st to end	Night	18% 9% 2 6% 4 3 4 3 3 4 45%	3546
April	1st to 10th	Night	17% 7% 3 7% 5% 4 3 3 5% 6% 41%	1900
	11th to 20th	Night	14% 8% 3 6% 5% 4 3 3 5% 6% 43%	1875
	21st to end	Night	18% 6% 2 5% 4 3 5% 3 5% 6% 44%	1900
May	1st to 10th	Night	21% 9% 3 8% 5% 4 4 4 5% 8% 30%	2910
	11th to 20th	Night	26% 9% 3 6% 5% 4 3 3 5% 6% 31%	2532
	21st to end	Night	14% 6% 3 6% 4 3 4 3 5% 10% 42%	2738
June	1st to 10th	Night	14% 6% 2 5% 4 3 5% 4 4 9% 43%	1520
	11th to 20th	Night	10% 7% 2 6% 6% 5% 5% 4 6% 8% 42%	1520
	21st to end	Night	9% 5% 1 6% 5% 4 6% 4 8% 11% 41%	1520
July	1st to 10th	Night	8% 5% 3 7% 7% 6% 6% 6% 7% 9% 36%	2520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 4 7% 10% 39%	2520
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	2764
August	1st to 10th	Night	21% 6% 3 6% 4 5% 5% 4 6% 8% 33%	2847
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 4 5% 7% 44%	3097
	21st to end	Night	10% 5% 1 5% 4 3 5% 3 5% 8% 51%	3476
September	1st to 10th	Night	14% 5% 1 7% 5% 4 4 3 6% 7% 45%	1900
	11th to 20th	Night	11% 6% 1 5% 3 3 3 3 5% 7% 54%	1952
	21st to end	Night	20% 6% 3 5% 4 3 4 3 5% 7% 41%	2090
October	1st to 10th	Night	18% 7% 2 5% 3 4 3 3 5% 6% 44%	2237
	11th to 20th	Night	16% 4 1 4 3 3 3 3 5% 6% 51%	2280
	21st to end	Night	16% 6% 1 5% 4 3 3 2 5% 6% 48%	2595
November	1st to 10th	Night	11% 5% 2 6% 3 3 3 3 5% 6% 53%	2505
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 4 4 48%	2470
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 6% 6% 46%	2546
December	1st to 10th	Night	20% 7% 3 6% 3 3 3 3 5% 7% 40%	2660
	11th to 20th	Night	17% 6% 2 6% 4 3 4 3 6% 7% 41%	3288
	21st to end	Night	17% 8% 3 6% 4 4 4 4 6% 7% 38%	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	40% 4 10% 26% 15% 4	2518
	11th to 20th	Day	47% 5% 9% 22% 14% 3	2554
	21st to end	Day	53% 3 11% 22% 10% 2	3124
February	1st to 10th	Day	44% 5% 12% 25% 12% 2	1710
	11th to 20th	Day	43% 5% 9% 24% 15% 3	1847
	21st to end	Day	48% 4 9% 21% 14% 5%	1570
March	1st to 10th	Day	49% 4 7% 28% 10% 2	3119
	11th to 20th	Day	32% 5% 9% 32% 19% 3	3434
	21st to end	Day	45% 3 8% 19% 21% 4	4084
April	1st to 10th	Day	43% 5% 10% 21% 16% 5%	2280
	11th to 20th	Day	39% 5% 12% 23% 17% 4	2254
	21st to end	Day	42% 6% 9% 20% 19% 4	2280
May	1st to 10th	Day	56% 5% 10% 18% 9% 2	4042
	11th to 20th	Day	54% 5% 12% 19% 8% 1	4392
	21st to end	Day	36% 6% 12% 27% 17% 2	4837
June	1st to 10th	Day	40% 6% 13% 23% 15% 2	2660
	11th to 20th	Day	40% 8% 12% 26% 12% 3	2660
	21st to end	Day	38% 7% 13% 23% 17% 2	2660
July	1st to 10th	Day	41% 10% 16% 18% 12% 2	4370
	11th to 20th	Day	40% 8% 16% 22% 13% 1	4365
	21st to end	Day	48% 7% 12% 19% 14% 1	4631
August	1st to 10th	Day	49% 7% 14% 21% 9% 1	4037
	11th to 20th	Day	40% 6% 14% 25% 14% 1	3766
	21st to end	Day	33% 7% 12% 27% 19% 2	4180
September	1st to 10th	Day	40% 5% 11% 19% 21% 3	2280
	11th to 20th	Day	35% 4 10% 23% 25% 4	2228
	21st to end	Day	47% 4 8% 18% 20% 2	2090
October	1st to 10th	Day	37% 3 9% 18% 28% 5%	3224
	11th to 20th	Day	26% 2 9% 24% 31% 8%	3140
	21st to end	Day	26% 3 12% 26% 28% 5%	3278
November	1st to 10th	Day	34% 3 12% 27% 20% 3	1734
	11th to 20th	Day	38% 4 9% 23% 23% 3	1710
	21st to end	Day	38% 4 10% 31% 13% 3	1630
December	1st to 10th	Day	44% 4 14% 23% 14% 1	1520
	11th to 20th	Day	42% 5% 11% 28% 12% 3	1906
	21st to end	Day	44% 6% 13% 23% 14% 1	2772
January	1st to 10th	Night	40% 3 9% 30% 14% 4	4429
	11th to 20th	Night	44% 3 11% 24% 15% 3	4404
	21st to end	Night	50% 4 7% 25% 13% 1	4532
February	1st to 10th	Night	44% 3 14% 23% 14% 2	2470
	11th to 20th	Night	45% 3 8% 20% 19% 5%	2229
	21st to end	Night	43% 3 8% 23% 17% 5%	1884
March	1st to 10th	Night	50% 4 7% 23% 13% 2	3734
	11th to 20th	Night	36% 5% 12% 27% 17% 3	3522
	21st to end	Night	46% 3 7% 17% 23% 5%	3546
April	1st to 10th	Night	44% 4 9% 20% 19% 5%	1900
	11th to 20th	Night	43% 4 10% 21% 19% 4	1875
	21st to end	Night	41% 5% 9% 20% 20% 5%	1900
May	1st to 10th	Night	53% 5% 10% 20% 11% 1	2916
	11th to 20th	Night	54% 5% 13% 16% 10% 2	2533
	21st to end	Night	39% 6% 16% 20% 16% 3	2739
June	1st to 10th	Night	38% 7% 13% 21% 19% 2	1520
	11th to 20th	Night	40% 7% 12% 26% 13% 2	1520
	21st to end	Night	34% 8% 16% 24% 16% 2	1520
July	1st to 10th	Night	42% 9% 19% 17% 11% 2	2480
	11th to 20th	Night	38% 7% 19% 25% 10% 1	2472
	21st to end	Night	46% 7% 15% 19% 13% 1	2676
August	1st to 10th	Night	49% 7% 18% 16% 9% 1	2836
	11th to 20th	Night	39% 5% 12% 30% 13% 1	3127
	21st to end	Night	33% 5% 10% 29% 21% 2	3476
September	1st to 10th	Night	38% 5% 13% 19% 21% 3	1900
	11th to 20th	Night	30% 5% 11% 25% 25% 4	1952
	21st to end	Night	43% 5% 8% 22% 19% 2	2090
October	1st to 10th	Night	38% 4 10% 23% 22% 3	3731
	11th to 20th	Night	24% 3 10% 27% 30% 6%	3779
	21st to end	Night	26% 3 11% 31% 25% 4	4268
November	1st to 10th	Night	33% 5% 11% 25% 21% 5%	2505
	11th to 20th	Night	40% 4 9% 22% 20% 6%	2470
	21st to end	Night	37% 5% 11% 27% 18% 2	2546
December	1st to 10th	Night	45% 3 12% 23% 15% 2	2660
	11th to 20th	Night	42% 4 12% 20% 18% 4	3288
	21st to end	Night	45% 5% 13% 25% 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	25% 27% 31% 12% 4 1	180
	11th to 20th	Day	34% 29% 25% 12% 1	191
	21st to end	Day	28% 30% 26% 12% 4 1	297
February	1st to 10th	Day	29% 30% 28% 11% 2	270
	11th to 20th	Day	31% 26% 31% 10% 2	446
	21st to end	Day	31% 24% 31% 12% 2	382
March	1st to 10th	Day	27% 25% 29% 16% 3	460
	11th to 20th	Day	27% 26% 31% 11% 3 2	460
	21st to end	Day	29% 22% 30% 13% 4 1	506
April	1st to 10th	Day	27% 27% 29% 11% 5% 1	460
	11th to 20th	Day	22% 21% 34% 14% 6% 1	452
	21st to end	Day	28% 19% 33% 14% 5% 2	460
May	1st to 10th	Day	31% 21% 26% 15% 6% 2	463
	11th to 20th	Day	24% 15% 33% 21% 6% 2	487
	21st to end	Day	19% 11% 23% 30% 14% 3	529
June	1st to 10th	Day	15% 13% 30% 27% 12% 2	490
	11th to 20th	Day	17% 11% 32% 25% 11% 3	490
	21st to end	Day	12% 10% 33% 30% 11% 3	490
July	1st to 10th	Day	13% 6% 26% 29% 20% 6%	485
	11th to 20th	Day	10% 3 27% 35% 21% 4	484
	21st to end	Day	12% 8% 30% 30% 15% 5%	532
August	1st to 10th	Day	13% 4 29% 33% 15% 6%	486
	11th to 20th	Day	14% 5% 22% 35% 19% 5%	479
	21st to end	Day	13% 9% 26% 26% 19% 8%	539
September	1st to 10th	Day	17% 9% 27% 24% 16% 8%	490
	11th to 20th	Day	14% 8% 27% 29% 13% 9%	490
	21st to end	Day	27% 11% 27% 21% 10% 4	490
October	1st to 10th	Day	25% 10% 25% 22% 12% 6%	490
	11th to 20th	Day	20% 17% 26% 21% 12% 3	490
	21st to end	Day	18% 17% 34% 18% 11% 1	492
November	1st to 10th	Day	17% 15% 35% 20% 11% 2	386
	11th to 20th	Day	24% 21% 31% 17% 7%	370
	21st to end	Day	21% 19% 32% 20% 7% 1	290
December	1st to 10th	Day	28% 28% 30% 9% 5% 1	180
	11th to 20th	Day	34% 16% 35% 13% 2 1	180
	21st to end	Day	29% 17% 31% 16% 5% 1	198
January	1st to 10th	Night	21% 21% 36% 18% 3	760
	11th to 20th	Night	26% 19% 31% 21% 3	760
	21st to end	Night	19% 21% 37% 22% 2	836
February	1st to 10th	Night	21% 20% 36% 21% 2	760
	11th to 20th	Night	26% 26% 33% 15% 1	743
	21st to end	Night	26% 24% 30% 19% 2	628
March	1st to 10th	Night	23% 21% 33% 19% 4	760
	11th to 20th	Night	24% 22% 32% 16% 6%	760
	21st to end	Night	22% 16% 39% 17% 5%	836
April	1st to 10th	Night	22% 23% 34% 16% 5%	760
	11th to 20th	Night	20% 15% 38% 23% 4	751
	21st to end	Night	22% 14% 35% 24% 5%	760
May	1st to 10th	Night	22% 12% 36% 22% 6%	620
	11th to 20th	Night	21% 11% 38% 26% 4	398
	21st to end	Night	19% 4 25% 39% 10% 1	411
June	1st to 10th	Night	19% 5% 30% 36% 10% 1	380
	11th to 20th	Night	12% 5% 30% 39% 13%	380
	21st to end	Night	10% 3 32% 43% 11%	380
July	1st to 10th	Night	10% 2 29% 43% 15% 1	370
	11th to 20th	Night	9% 3 23% 46% 17% 2	368
	21st to end	Night	11% 4 26% 41% 17%	429
August	1st to 10th	Night	12% 3 18% 45% 21% 1	576
	11th to 20th	Night	14% 4 18% 43% 19% 2	750
	21st to end	Night	14% 3 20% 40% 21% 3	836
September	1st to 10th	Night	16% 5% 26% 34% 16% 3	760
	11th to 20th	Night	14% 4 21% 34% 23% 4	760
	21st to end	Night	24% 6% 23% 30% 15% 1	760
October	1st to 10th	Night	21% 8% 26% 27% 15% 3	760
	11th to 20th	Night	17% 12% 28% 29% 12% 1	760
	21st to end	Night	19% 12% 36% 24% 10%	836
November	1st to 10th	Night	14% 13% 36% 26% 11% 1	771
	11th to 20th	Night	21% 16% 29% 26% 8%	760
	21st to end	Night	17% 17% 37% 21% 8% 1	760
December	1st to 10th	Night	26% 17% 32% 18% 7% 1	760
	11th to 20th	Night	20% 19% 33% 22% 6%	760
	21st to end	Night	22% 16% 35% 25% 2	836

Page generated at 2024-11-05 08:15:48. Version ac07

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