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 2024-01-12.

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	15% 6% 2 5% 5% 3 4 5% 5% 7% 42%	74737
	Night	15% 6% 2 5% 5% 3 4 4 5% 7% 43%	74969
Transparency	Day	41% 5% 11% 23% 16% 3	74443
	Night	41% 5% 11% 23% 17% 3	74797
Seeing	Day	21% 16% 29% 21% 10% 3	13913
	Night	19% 13% 31% 27% 9% 1	23271

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	14% 7% 3 5% 5% 3 5% 5% 5% 6% 44%	1517
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 4 7% 42%	1474
	21st to end	Day	16% 8% 3 4 5% 3 4 4 5% 6% 42%	1782
February	1st to 10th	Day	16% 8% 3 5% 6% 3 5% 5% 5% 7% 38%	1620
	11th to 20th	Day	18% 7% 2 5% 5% 2 4 5% 5% 6% 41%	1739
	21st to end	Day	18% 10% 4 5% 4 3 4 4 4 6% 39%	1480
March	1st to 10th	Day	14% 7% 2 5% 4 3 4 4 3 6% 48%	1797
	11th to 20th	Day	17% 6% 2 6% 5% 3 3 5% 6% 6% 41%	1948
	21st to end	Day	20% 7% 3 5% 4 3 4 4 4 6% 40%	2322
April	1st to 10th	Day	16% 7% 3 5% 4 3 4 4 5% 6% 42%	2160
	11th to 20th	Day	15% 6% 2 4 4 3 4 5% 6% 8% 43%	2134
	21st to end	Day	17% 6% 3 5% 5% 3 4 4 6% 8% 39%	2160
May	1st to 10th	Day	20% 6% 2 5% 5% 3 5% 5% 5% 7% 37%	2290
	11th to 20th	Day	18% 5% 3 6% 6% 3 4 4 6% 8% 37%	2468
	21st to end	Day	14% 6% 2 5% 4 3 4 4 6% 9% 42%	2725
June	1st to 10th	Day	14% 6% 2 5% 5% 3 5% 5% 7% 9% 39%	2520
	11th to 20th	Day	14% 5% 3 5% 6% 4 5% 5% 6% 9% 38%	2520
	21st to end	Day	11% 5% 2 5% 6% 4 5% 6% 6% 8% 42%	2520
July	1st to 10th	Day	11% 5% 3 5% 6% 4 6% 6% 6% 8% 39%	2520
	11th to 20th	Day	8% 3 3 4 6% 4 6% 6% 7% 12% 40%	2520
	21st to end	Day	11% 5% 2 6% 7% 4 6% 6% 7% 9% 37%	2727
August	1st to 10th	Day	10% 6% 2 7% 7% 5% 6% 5% 6% 8% 39%	2300
	11th to 20th	Day	13% 5% 2 6% 6% 4 5% 5% 6% 9% 39%	2126
	21st to end	Day	13% 6% 2 5% 5% 3 5% 6% 7% 8% 40%	2376
September	1st to 10th	Day	14% 5% 2 6% 6% 3 4 4 6% 7% 43%	2160
	11th to 20th	Day	14% 5% 2 4 4 2 3 4 5% 8% 49%	2111
	21st to end	Day	18% 7% 2 6% 5% 3 4 4 4 6% 41%	1980
October	1st to 10th	Day	20% 5% 2 4 5% 2 3 4 4 6% 44%	1836
	11th to 20th	Day	17% 5% 2 4 4 2 3 3 4 6% 49%	1800
	21st to end	Day	18% 5% 3 5% 4 2 4 3 4 6% 45%	1896
November	1st to 10th	Day	13% 4 2 4 5% 3 3 3 5% 6% 52%	1671
	11th to 20th	Day	16% 5% 2 5% 5% 2 3 4 5% 6% 47%	1710
	21st to end	Day	15% 4 2 5% 4 3 4 4 5% 6% 47%	1630
December	1st to 10th	Day	19% 8% 2 4 5% 3 3 4 5% 8% 40%	1520
	11th to 20th	Day	20% 8% 3 4 3 2 4 3 4 4 45%	1906
	21st to end	Day	17% 6% 3 5% 4 4 6% 6% 6% 7% 37%	2772
January	1st to 10th	Night	17% 6% 2 5% 3 3 4 4 5% 6% 46%	2649
	11th to 20th	Night	18% 7% 2 4 5% 3 4 4 5% 7% 40%	2524
	21st to end	Night	14% 7% 3 4 5% 3 4 4 4 7% 44%	2574
February	1st to 10th	Night	17% 8% 3 5% 5% 3 4 4 5% 6% 40%	2340
	11th to 20th	Night	18% 8% 2 5% 4 3 4 4 5% 6% 41%	2106
	21st to end	Night	19% 7% 2 5% 4 2 4 3 4 5% 43%	1776
March	1st to 10th	Night	15% 5% 3 4 5% 2 3 4 5% 6% 47%	2150
	11th to 20th	Night	17% 7% 3 6% 4 3 3 3 5% 6% 44%	2007
	21st to end	Night	17% 8% 2 5% 5% 2 4 5% 4 5% 43%	2032
April	1st to 10th	Night	16% 7% 3 5% 6% 2 4 4 5% 6% 41%	1800
	11th to 20th	Night	14% 7% 3 5% 6% 3 4 3 5% 7% 44%	1775
	21st to end	Night	19% 6% 2 4 4 3 4 5% 4 6% 43%	1800
May	1st to 10th	Night	19% 6% 3 6% 6% 3 4 5% 4 7% 36%	1668
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 5% 7% 38%	1438
	21st to end	Night	16% 6% 3 6% 5% 3 5% 4 4 8% 39%	1550
June	1st to 10th	Night	15% 6% 2 5% 5% 3 5% 5% 4 9% 43%	1440
	11th to 20th	Night	11% 6% 3 5% 6% 3 6% 5% 6% 9% 40%	1440
	21st to end	Night	10% 4 2 5% 5% 3 5% 6% 7% 11% 42%	1440
July	1st to 10th	Night	10% 5% 3 6% 7% 4 5% 6% 6% 9% 39%	1440
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 6% 8% 9% 43%	1440
	21st to end	Night	9% 4 2 5% 5% 3 6% 5% 6% 10% 43%	1586
August	1st to 10th	Night	8% 6% 2 5% 5% 4 5% 6% 6% 9% 42%	1619
	11th to 20th	Night	10% 7% 2 6% 7% 4 4 5% 6% 8% 41%	1774
	21st to end	Night	13% 6% 2 5% 5% 3 5% 5% 5% 8% 42%	1980
September	1st to 10th	Night	15% 5% 2 5% 6% 3 4 4 6% 7% 44%	1800
	11th to 20th	Night	11% 5% 2 4 3 3 3 4 5% 7% 54%	1849
	21st to end	Night	18% 6% 3 4 4 2 4 4 6% 7% 42%	1980
October	1st to 10th	Night	19% 7% 2 4 3 2 3 4 5% 6% 44%	2119
	11th to 20th	Night	17% 5% 2 4 4 2 3 4 5% 6% 50%	2160
	21st to end	Night	17% 7% 2 5% 4 2 4 3 4 6% 47%	2458
November	1st to 10th	Night	12% 5% 2 5% 4 2 4 3 4 6% 53%	2407
	11th to 20th	Night	17% 6% 3 5% 4 2 4 4 4 5% 48%	2470
	21st to end	Night	13% 5% 2 4 5% 3 4 5% 6% 6% 46%	2546
December	1st to 10th	Night	20% 7% 3 5% 4 2 4 4 5% 7% 40%	2660
	11th to 20th	Night	17% 6% 3 5% 5% 3 4 4 5% 7% 42%	3288
	21st to end	Night	17% 7% 3 5% 5% 3 5% 4 5% 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	42% 4 11% 23% 15% 5%	1518
	11th to 20th	Day	42% 5% 14% 23% 13% 3	1474
	21st to end	Day	43% 3 10% 25% 16% 3	1782
February	1st to 10th	Day	45% 5% 11% 25% 11% 2	1620
	11th to 20th	Day	44% 5% 10% 24% 14% 3	1748
	21st to end	Day	48% 4 8% 21% 14% 5%	1480
March	1st to 10th	Day	40% 4 12% 28% 15% 3	1800
	11th to 20th	Day	42% 4 10% 23% 17% 4	1948
	21st to end	Day	46% 5% 7% 22% 17% 4	2322
April	1st to 10th	Day	43% 5% 10% 21% 17% 5%	2160
	11th to 20th	Day	38% 5% 12% 24% 17% 4	2134
	21st to end	Day	43% 6% 9% 20% 18% 4	2160
May	1st to 10th	Day	47% 5% 9% 21% 15% 3	2290
	11th to 20th	Day	45% 5% 12% 22% 14% 2	2472
	21st to end	Day	40% 6% 15% 24% 13% 2	2725
June	1st to 10th	Day	41% 6% 13% 23% 14% 2	2520
	11th to 20th	Day	41% 8% 12% 25% 13% 2	2520
	21st to end	Day	39% 7% 13% 23% 17% 1	2520
July	1st to 10th	Day	42% 8% 13% 21% 15% 2	2450
	11th to 20th	Day	35% 8% 14% 26% 15% 2	2445
	21st to end	Day	42% 7% 12% 22% 15% 2	2574
August	1st to 10th	Day	41% 6% 14% 24% 13% 1	2287
	11th to 20th	Day	41% 6% 16% 19% 16% 2	2126
	21st to end	Day	39% 7% 13% 21% 17% 3	2376
September	1st to 10th	Day	40% 6% 12% 19% 20% 3	2160
	11th to 20th	Day	35% 4 10% 23% 25% 4	2111
	21st to end	Day	45% 4 8% 19% 22% 2	1980
October	1st to 10th	Day	43% 4 10% 23% 18% 2	1836
	11th to 20th	Day	37% 3 9% 25% 23% 2	1800
	21st to end	Day	41% 4 9% 23% 20% 2	1896
November	1st to 10th	Day	34% 3 12% 27% 20% 3	1671
	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% 4 10% 24% 18% 4	2649
	11th to 20th	Night	44% 4 14% 23% 13% 2	2524
	21st to end	Night	41% 3 9% 27% 18% 3	2574
February	1st to 10th	Night	45% 3 14% 23% 13% 2	2340
	11th to 20th	Night	45% 4 8% 20% 17% 5%	2108
	21st to end	Night	44% 3 7% 23% 18% 5%	1776
March	1st to 10th	Night	38% 4 11% 25% 19% 4	2160
	11th to 20th	Night	43% 5% 9% 21% 18% 4	2008
	21st to end	Night	45% 3 8% 19% 19% 6%	2032
April	1st to 10th	Night	44% 4 10% 19% 19% 5%	1800
	11th to 20th	Night	42% 3 10% 21% 20% 4	1775
	21st to end	Night	42% 5% 10% 19% 20% 5%	1800
May	1st to 10th	Night	48% 6% 9% 19% 17% 2	1668
	11th to 20th	Night	46% 6% 12% 16% 17% 3	1439
	21st to end	Night	45% 7% 15% 19% 13% 2	1551
June	1st to 10th	Night	39% 7% 13% 21% 18% 2	1440
	11th to 20th	Night	42% 7% 13% 24% 13% 2	1440
	21st to end	Night	34% 8% 16% 24% 16% 2	1440
July	1st to 10th	Night	41% 9% 17% 19% 13% 2	1400
	11th to 20th	Night	34% 7% 16% 26% 15% 2	1392
	21st to end	Night	35% 7% 15% 27% 15% 2	1498
August	1st to 10th	Night	37% 8% 17% 22% 16% 1	1608
	11th to 20th	Night	40% 8% 14% 20% 16% 1	1774
	21st to end	Night	40% 7% 13% 19% 18% 3	1980
September	1st to 10th	Night	39% 6% 14% 20% 19% 3	1800
	11th to 20th	Night	30% 5% 11% 25% 25% 4	1849
	21st to end	Night	42% 5% 7% 24% 20% 2	1980
October	1st to 10th	Night	42% 5% 11% 23% 17% 3	2119
	11th to 20th	Night	36% 4 11% 29% 18% 3	2160
	21st to end	Night	40% 4 9% 25% 20% 2	2458
November	1st to 10th	Night	33% 5% 11% 25% 20% 5%	2407
	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	30% 30% 26% 13% 1	174
	21st to end	Day	27% 33% 26% 10% 3 1	264
February	1st to 10th	Day	31% 32% 26% 10% 2	240
	11th to 20th	Day	30% 27% 31% 10% 1	407
	21st to end	Day	31% 25% 31% 11% 1	346
March	1st to 10th	Day	25% 26% 29% 17% 3	420
	11th to 20th	Day	26% 26% 32% 11% 3 2	420
	21st to end	Day	30% 22% 30% 13% 5% 1	462
April	1st to 10th	Day	27% 26% 30% 11% 5% 1	420
	11th to 20th	Day	21% 20% 35% 15% 7% 1	412
	21st to end	Day	29% 20% 33% 12% 4 1	420
May	1st to 10th	Day	31% 21% 26% 15% 6% 1	423
	11th to 20th	Day	23% 15% 34% 20% 6% 1	447
	21st to end	Day	20% 11% 21% 31% 14% 3	485
June	1st to 10th	Day	16% 13% 30% 26% 13% 2	450
	11th to 20th	Day	18% 12% 32% 25% 10% 3	450
	21st to end	Day	12% 10% 31% 32% 12% 4	450
July	1st to 10th	Day	13% 6% 25% 29% 20% 6%	445
	11th to 20th	Day	9% 3 27% 34% 22% 5%	444
	21st to end	Day	11% 8% 30% 31% 15% 5%	488
August	1st to 10th	Day	12% 4 28% 34% 15% 7%	446
	11th to 20th	Day	15% 4 20% 36% 20% 5%	439
	21st to end	Day	13% 8% 26% 26% 18% 8%	495
September	1st to 10th	Day	18% 8% 26% 23% 16% 8%	450
	11th to 20th	Day	15% 8% 27% 28% 14% 9%	450
	21st to end	Day	25% 11% 28% 22% 11% 5%	450
October	1st to 10th	Day	26% 9% 25% 22% 13% 5%	450
	11th to 20th	Day	21% 17% 25% 22% 12% 2	450
	21st to end	Day	20% 18% 35% 17% 9% 1	453
November	1st to 10th	Day	17% 15% 34% 20% 12% 2	365
	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	25% 18% 32% 22% 4	728
	21st to end	Night	19% 21% 37% 22% 2	792
February	1st to 10th	Night	22% 21% 36% 20% 2	720
	11th to 20th	Night	26% 25% 33% 14% 1	703
	21st to end	Night	25% 24% 30% 18% 2	592
March	1st to 10th	Night	21% 21% 34% 20% 4	720
	11th to 20th	Night	24% 21% 32% 16% 6%	720
	21st to end	Night	22% 15% 40% 18% 5%	792
April	1st to 10th	Night	22% 23% 34% 16% 5%	720
	11th to 20th	Night	19% 14% 38% 24% 5%	711
	21st to end	Night	23% 13% 35% 24% 5%	720
May	1st to 10th	Night	22% 12% 36% 22% 7%	588
	11th to 20th	Night	20% 10% 39% 26% 5%	377
	21st to end	Night	20% 4 23% 40% 11% 2	389
June	1st to 10th	Night	19% 5% 29% 36% 11% 1	360
	11th to 20th	Night	13% 5% 31% 38% 13%	360
	21st to end	Night	11% 2 31% 45% 11%	360
July	1st to 10th	Night	11% 2 29% 42% 15% 1	350
	11th to 20th	Night	9% 3 22% 46% 18% 2	348
	21st to end	Night	10% 4 26% 41% 17%	406
August	1st to 10th	Night	10% 3 18% 46% 22% 1	545
	11th to 20th	Night	14% 4 17% 43% 20% 3	710
	21st to end	Night	14% 3 20% 39% 21% 3	792
September	1st to 10th	Night	17% 4 25% 34% 16% 3	720
	11th to 20th	Night	14% 5% 22% 34% 22% 4	720
	21st to end	Night	23% 6% 24% 31% 16% 1	720
October	1st to 10th	Night	22% 8% 25% 27% 15% 3	720
	11th to 20th	Night	18% 12% 28% 30% 12% 1	720
	21st to end	Night	21% 12% 36% 22% 9%	792
November	1st to 10th	Night	15% 13% 35% 26% 11% 1	740
	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 2024-01-09 16:24:28. Version ac07

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