COG Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the COG Observing Site clear sky chart from 2005-11-06 to 2024-01-14.

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	19% 7% 3 5% 5% 3 5% 5% 5% 7% 37%	74566
	Night	19% 8% 3 5% 5% 3 4 4 5% 6% 37%	75176
Transparency	Day	46% 6% 12% 21% 13% 2	74271
	Night	47% 6% 12% 21% 13% 2	75006
Seeing	Day	24% 14% 29% 21% 9% 2	14304
	Night	23% 11% 31% 28% 7% 1	22975

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	16% 8% 3 6% 7% 3 4 5% 5% 6% 35%	1517
	11th to 20th	Day	21% 10% 3 6% 5% 2 4 4 4 6% 35%	1464
	21st to end	Day	15% 9% 3 8% 6% 3 4 5% 4 5% 38%	1584
February	1st to 10th	Day	18% 11% 3 6% 5% 3 6% 5% 5% 6% 34%	1585
	11th to 20th	Day	14% 10% 4 7% 7% 3 6% 4 5% 5% 34%	1712
	21st to end	Day	15% 10% 3 7% 7% 3 6% 5% 5% 6% 33%	1480
March	1st to 10th	Day	16% 10% 4 7% 6% 3 4 5% 4 6% 35%	1797
	11th to 20th	Day	17% 7% 4 4 6% 3 4 5% 5% 6% 40%	1952
	21st to end	Day	18% 8% 3 6% 5% 3 4 4 5% 5% 40%	2212
April	1st to 10th	Day	22% 9% 3 6% 6% 3 4 4 4 5% 34%	2160
	11th to 20th	Day	19% 7% 3 5% 5% 3 5% 4 5% 7% 39%	2253
	21st to end	Day	25% 8% 3 6% 5% 3 3 3 5% 6% 32%	2340
May	1st to 10th	Day	24% 8% 3 5% 5% 3 4 4 5% 6% 34%	2340
	11th to 20th	Day	21% 7% 3 6% 6% 3 4 4 4 6% 35%	2342
	21st to end	Day	18% 5% 3 6% 4 3 5% 5% 6% 7% 38%	2725
June	1st to 10th	Day	25% 7% 3 5% 5% 4 5% 5% 6% 7% 30%	2677
	11th to 20th	Day	19% 5% 3 5% 4 3 5% 5% 5% 7% 38%	2700
	21st to end	Day	25% 7% 3 5% 5% 3 5% 6% 4 7% 31%	2670
July	1st to 10th	Day	15% 6% 2 5% 5% 3 5% 5% 5% 8% 40%	2520
	11th to 20th	Day	14% 5% 3 5% 6% 4 6% 6% 6% 8% 36%	2520
	21st to end	Day	15% 5% 2 6% 6% 3 5% 6% 7% 9% 35%	2744
August	1st to 10th	Day	14% 5% 2 5% 6% 4 5% 6% 7% 10% 38%	2347
	11th to 20th	Day	15% 5% 2 5% 5% 3 5% 5% 7% 9% 39%	2304
	21st to end	Day	14% 5% 2 5% 4 2 4 5% 6% 8% 45%	2512
September	1st to 10th	Day	12% 4 3 5% 5% 3 5% 5% 5% 8% 46%	2107
	11th to 20th	Day	13% 5% 2 4 3 2 4 4 4 6% 52%	1980
	21st to end	Day	17% 4 2 4 4 3 4 4 5% 8% 44%	1980
October	1st to 10th	Day	19% 7% 3 5% 5% 3 5% 4 5% 6% 39%	1967
	11th to 20th	Day	20% 6% 3 4 5% 3 4 4 5% 7% 39%	1734
	21st to end	Day	25% 8% 3 7% 4 3 4 4 4 5% 32%	1782
November	1st to 10th	Day	18% 6% 3 5% 4 3 5% 5% 4 7% 41%	1671
	11th to 20th	Day	20% 6% 2 4 5% 3 4 4 5% 8% 39%	1710
	21st to end	Day	22% 7% 3 5% 4 2 3 3 4 6% 41%	1570
December	1st to 10th	Day	22% 10% 3 7% 5% 3 3 4 4 5% 32%	1345
	11th to 20th	Day	23% 9% 3 6% 3 3 2 4 4 6% 37%	1668
	21st to end	Day	26% 12% 3 4 6% 3 5% 4 4 4 29%	2595
January	1st to 10th	Night	16% 10% 3 6% 5% 3 4 5% 5% 6% 37%	2649
	11th to 20th	Night	17% 12% 4 8% 6% 3 5% 5% 5% 5% 30%	2569
	21st to end	Night	12% 10% 4 6% 5% 3 4 5% 6% 8% 38%	2772
February	1st to 10th	Night	16% 10% 4 7% 7% 3 5% 6% 6% 7% 30%	2372
	11th to 20th	Night	14% 10% 4 7% 6% 4 4 5% 4 6% 37%	2132
	21st to end	Night	15% 10% 4 5% 6% 4 4 5% 4 6% 37%	1776
March	1st to 10th	Night	14% 13% 4 7% 6% 3 4 4 5% 5% 36%	2150
	11th to 20th	Night	17% 9% 4 5% 5% 4 5% 5% 4 6% 36%	2006
	21st to end	Night	18% 9% 3 6% 4 3 4 4 4 5% 39%	2142
April	1st to 10th	Night	23% 10% 4 6% 5% 3 4 3 3 5% 35%	1800
	11th to 20th	Night	19% 6% 3 5% 6% 4 4 6% 5% 6% 37%	1656
	21st to end	Night	27% 8% 2 6% 5% 2 3 4 5% 5% 32%	1620
May	1st to 10th	Night	23% 8% 4 6% 5% 2 5% 5% 5% 7% 30%	1620
	11th to 20th	Night	21% 7% 3 6% 6% 3 5% 5% 6% 6% 33%	1567
	21st to end	Night	19% 7% 3 6% 5% 2 5% 5% 5% 7% 36%	1550
June	1st to 10th	Night	23% 8% 3 6% 5% 3 5% 4 5% 7% 32%	1278
	11th to 20th	Night	16% 6% 3 6% 6% 2 4 4 6% 8% 39%	1260
	21st to end	Night	25% 6% 2 6% 5% 2 3 5% 6% 8% 32%	1285
July	1st to 10th	Night	15% 7% 3 5% 5% 3 5% 5% 6% 9% 37%	1440
	11th to 20th	Night	16% 5% 3 5% 5% 3 4 4 6% 9% 40%	1440
	21st to end	Night	15% 4 2 5% 5% 3 5% 5% 7% 8% 39%	1574
August	1st to 10th	Night	13% 6% 2 5% 5% 4 6% 6% 7% 7% 41%	1571
	11th to 20th	Night	14% 7% 3 6% 6% 3 5% 5% 5% 9% 37%	1596
	21st to end	Night	14% 5% 2 4 5% 2 3 4 5% 7% 49%	1840
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 7% 49%	1852
	11th to 20th	Night	16% 5% 2 3 4 2 3 3 4 6% 51%	1980
	21st to end	Night	17% 6% 2 4 4 3 3 4 5% 6% 46%	1980
October	1st to 10th	Night	20% 5% 2 5% 5% 3 5% 5% 6% 5% 40%	1993
	11th to 20th	Night	18% 6% 2 5% 5% 3 6% 4 5% 7% 39%	2222
	21st to end	Night	27% 8% 2 4 5% 3 3 3 4 6% 33%	2574
November	1st to 10th	Night	19% 6% 3 5% 5% 3 4 4 5% 5% 41%	2407
	11th to 20th	Night	18% 6% 2 5% 5% 3 4 5% 4 7% 40%	2470
	21st to end	Night	18% 7% 3 5% 5% 3 4 5% 5% 7% 38%	2607
December	1st to 10th	Night	22% 10% 3 6% 5% 3 4 4 5% 6% 32%	2833
	11th to 20th	Night	22% 8% 3 5% 4 3 4 4 4 5% 37%	3531
	21st to end	Night	24% 10% 3 5% 4 2 3 3 5% 6% 34%	5062

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	48% 6% 11% 19% 14% 1	1518
	11th to 20th	Day	51% 7% 10% 19% 11% 3	1464
	21st to end	Day	47% 5% 11% 19% 16% 2	1584
February	1st to 10th	Day	51% 6% 10% 19% 12% 2	1585
	11th to 20th	Day	51% 6% 11% 17% 12% 3	1721
	21st to end	Day	51% 5% 11% 17% 11% 5%	1480
March	1st to 10th	Day	50% 6% 11% 17% 13% 4	1800
	11th to 20th	Day	44% 6% 10% 20% 16% 5%	1952
	21st to end	Day	46% 6% 9% 17% 17% 5%	2212
April	1st to 10th	Day	53% 4 9% 16% 13% 5%	2160
	11th to 20th	Day	46% 5% 9% 23% 15% 2	2253
	21st to end	Day	53% 5% 9% 18% 11% 4	2340
May	1st to 10th	Day	51% 4 7% 21% 13% 3	2340
	11th to 20th	Day	49% 6% 9% 19% 15% 2	2346
	21st to end	Day	44% 7% 15% 20% 12% 2	2725
June	1st to 10th	Day	52% 7% 12% 20% 9% 1	2677
	11th to 20th	Day	45% 7% 11% 20% 16% 2	2700
	21st to end	Day	52% 7% 13% 17% 10% 1	2670
July	1st to 10th	Day	40% 9% 14% 23% 13% 1	2450
	11th to 20th	Day	43% 8% 14% 25% 10% 1	2445
	21st to end	Day	43% 9% 15% 23% 11% 1	2591
August	1st to 10th	Day	40% 9% 15% 26% 9%	2333
	11th to 20th	Day	40% 8% 16% 24% 11%	2304
	21st to end	Day	35% 8% 14% 24% 17% 1	2512
September	1st to 10th	Day	35% 7% 14% 27% 15% 1	2107
	11th to 20th	Day	33% 6% 9% 30% 20% 2	1980
	21st to end	Day	38% 5% 13% 27% 15% 1	1980
October	1st to 10th	Day	45% 5% 12% 23% 14% 1	1967
	11th to 20th	Day	44% 6% 16% 25% 9% 1	1734
	21st to end	Day	54% 5% 11% 17% 12% 1	1782
November	1st to 10th	Day	42% 7% 13% 23% 14% 1	1671
	11th to 20th	Day	43% 6% 12% 20% 16% 3	1710
	21st to end	Day	45% 4 16% 23% 10% 1	1570
December	1st to 10th	Day	54% 5% 11% 19% 10% 1	1345
	11th to 20th	Day	49% 5% 10% 19% 14% 3	1668
	21st to end	Day	58% 5% 9% 18% 7% 2	2595
January	1st to 10th	Night	48% 5% 13% 22% 12% 1	2649
	11th to 20th	Night	55% 7% 12% 16% 9% 2	2569
	21st to end	Night	44% 6% 11% 21% 16% 2	2772
February	1st to 10th	Night	51% 6% 11% 19% 11% 2	2372
	11th to 20th	Night	48% 5% 10% 20% 14% 2	2134
	21st to end	Night	48% 6% 9% 19% 14% 4	1776
March	1st to 10th	Night	50% 5% 8% 20% 14% 3	2160
	11th to 20th	Night	50% 6% 11% 16% 14% 4	2007
	21st to end	Night	47% 5% 9% 18% 18% 3	2142
April	1st to 10th	Night	54% 4 7% 17% 15% 4	1800
	11th to 20th	Night	46% 5% 10% 20% 17% 1	1656
	21st to end	Night	53% 5% 11% 15% 12% 3	1620
May	1st to 10th	Night	51% 7% 9% 19% 14% 2	1620
	11th to 20th	Night	50% 6% 11% 18% 12% 2	1568
	21st to end	Night	48% 6% 12% 21% 12% 1	1551
June	1st to 10th	Night	53% 5% 12% 20% 9% 1	1278
	11th to 20th	Night	43% 6% 14% 22% 13% 2	1260
	21st to end	Night	48% 9% 14% 18% 10% 1	1285
July	1st to 10th	Night	41% 10% 15% 22% 12% 1	1400
	11th to 20th	Night	40% 7% 16% 27% 11%	1392
	21st to end	Night	41% 9% 17% 21% 10% 1	1486
August	1st to 10th	Night	39% 10% 16% 27% 8%	1561
	11th to 20th	Night	42% 9% 16% 20% 12%	1596
	21st to end	Night	35% 6% 15% 27% 16% 1	1840
September	1st to 10th	Night	34% 7% 15% 30% 14% 1	1852
	11th to 20th	Night	35% 6% 9% 31% 18% 1	1980
	21st to end	Night	38% 7% 11% 29% 14% 2	1980
October	1st to 10th	Night	44% 7% 11% 23% 13% 1	1993
	11th to 20th	Night	44% 6% 15% 25% 10% 1	2222
	21st to end	Night	52% 4 13% 18% 12% 1	2574
November	1st to 10th	Night	44% 6% 12% 24% 13% 1	2407
	11th to 20th	Night	43% 6% 11% 22% 15% 2	2470
	21st to end	Night	45% 6% 15% 21% 13% 1	2607
December	1st to 10th	Night	54% 5% 11% 20% 10% 1	2833
	11th to 20th	Night	49% 6% 9% 18% 17% 1	3532
	21st to end	Night	51% 7% 13% 18% 11% 1	5062

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% 18% 29% 19% 7%	180
	11th to 20th	Day	33% 17% 27% 18% 4 2	166
	21st to end	Day	23% 19% 32% 20% 5%	176
February	1st to 10th	Day	31% 15% 30% 19% 4	305
	11th to 20th	Day	26% 19% 35% 18% 2	400
	21st to end	Day	23% 21% 31% 20% 4	346
March	1st to 10th	Day	27% 19% 33% 17% 4	420
	11th to 20th	Day	25% 20% 34% 17% 4 1	420
	21st to end	Day	26% 21% 27% 16% 7% 3	462
April	1st to 10th	Day	35% 18% 26% 13% 7% 2	420
	11th to 20th	Day	31% 20% 28% 14% 6% 2	412
	21st to end	Day	32% 17% 30% 14% 5% 1	420
May	1st to 10th	Day	31% 18% 28% 18% 4 1	423
	11th to 20th	Day	24% 17% 35% 15% 7% 2	447
	21st to end	Day	21% 16% 36% 19% 8%	485
June	1st to 10th	Day	25% 15% 29% 20% 9% 1	607
	11th to 20th	Day	21% 15% 26% 24% 13% 1	630
	21st to end	Day	26% 11% 27% 26% 9% 2	600
July	1st to 10th	Day	19% 11% 31% 28% 10% 2	445
	11th to 20th	Day	14% 8% 36% 24% 14% 5%	444
	21st to end	Day	15% 7% 36% 29% 10% 3	488
August	1st to 10th	Day	14% 7% 32% 29% 15% 2	446
	11th to 20th	Day	17% 9% 25% 29% 15% 5%	439
	21st to end	Day	17% 10% 29% 27% 14% 4	495
September	1st to 10th	Day	13% 11% 30% 27% 15% 4	450
	11th to 20th	Day	20% 10% 27% 20% 14% 9%	450
	21st to end	Day	17% 14% 28% 21% 14% 6%	450
October	1st to 10th	Day	22% 14% 23% 24% 14% 3	444
	11th to 20th	Day	26% 13% 28% 19% 12% 3	360
	21st to end	Day	34% 11% 27% 20% 8% 1	396
November	1st to 10th	Day	25% 12% 27% 25% 10% 2	365
	11th to 20th	Day	29% 13% 28% 23% 6% 1	370
	21st to end	Day	28% 15% 25% 23% 8% 1	370
December	1st to 10th	Day	29% 13% 34% 15% 8%	195
	11th to 20th	Day	33% 18% 33% 13% 3 1	180
	21st to end	Day	34% 17% 26% 17% 6% 1	198
January	1st to 10th	Night	21% 15% 34% 27% 3	760
	11th to 20th	Night	24% 14% 33% 24% 4	735
	21st to end	Night	18% 15% 36% 27% 4	792
February	1st to 10th	Night	23% 14% 31% 28% 3	720
	11th to 20th	Night	21% 17% 37% 21% 3	703
	21st to end	Night	21% 14% 34% 28% 3	592
March	1st to 10th	Night	23% 14% 37% 24% 2	720
	11th to 20th	Night	24% 13% 33% 23% 6%	720
	21st to end	Night	23% 17% 30% 23% 6%	792
April	1st to 10th	Night	30% 14% 29% 22% 4	720
	11th to 20th	Night	22% 15% 34% 23% 6%	592
	21st to end	Night	31% 12% 30% 22% 4	540
May	1st to 10th	Night	26% 11% 29% 29% 4	540
	11th to 20th	Night	24% 11% 37% 23% 5% 1	506
	21st to end	Night	20% 8% 32% 36% 4	389
June	1st to 10th	Night	25% 6% 33% 28% 7%	360
	11th to 20th	Night	18% 6% 26% 38% 12%	360
	21st to end	Night	23% 4 26% 41% 6% 1	360
July	1st to 10th	Night	19% 2 29% 39% 10%	350
	11th to 20th	Night	16% 4 25% 41% 13% 1	348
	21st to end	Night	15% 3 24% 42% 15% 1	394
August	1st to 10th	Night	14% 3 25% 42% 15%	497
	11th to 20th	Night	16% 5% 22% 41% 14% 1	532
	21st to end	Night	15% 5% 26% 37% 15% 2	652
September	1st to 10th	Night	16% 3 30% 36% 14% 1	720
	11th to 20th	Night	19% 6% 25% 31% 16% 2	720
	21st to end	Night	20% 7% 28% 29% 13% 2	720
October	1st to 10th	Night	22% 7% 23% 33% 14% 1	720
	11th to 20th	Night	21% 9% 29% 30% 9% 2	720
	21st to end	Night	31% 10% 28% 25% 6%	792
November	1st to 10th	Night	23% 8% 31% 28% 9% 1	740
	11th to 20th	Night	23% 10% 31% 26% 9% 1	760
	21st to end	Night	23% 14% 29% 29% 5%	825
December	1st to 10th	Night	29% 10% 34% 22% 5%	850
	11th to 20th	Night	24% 14% 33% 25% 4	850
	21st to end	Night	29% 12% 32% 23% 4	884

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