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 2024-11-25.

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	18% 8% 3 6% 5% 4 4 4 5% 7% 36%	101279
	Night	19% 9% 3 7% 4 4 4 4 5% 6% 36%	98194
Transparency	Day	47% 6% 12% 21% 13% 2	104804
	Night	47% 6% 12% 21% 12% 2	102759
Seeing	Day	24% 14% 29% 21% 9% 2	15499
	Night	22% 11% 31% 28% 8% 1	24029

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% 2 6% 6% 4 5% 4 5% 6% 38%	2517
	11th to 20th	Day	17% 8% 3 8% 5% 4 3 3 4 6% 38%	2520
	21st to end	Day	16% 11% 4 9% 5% 4 4 3 5% 5% 34%	2772
February	1st to 10th	Day	17% 11% 2 6% 4 5% 4 4 5% 5% 35%	1673
	11th to 20th	Day	13% 11% 3 9% 5% 5% 5% 3 5% 6% 34%	1810
	21st to end	Day	15% 10% 3 7% 6% 4 5% 4 5% 6% 34%	1570
March	1st to 10th	Day	25% 12% 3 7% 4 4 4 3 3 5% 29%	3116
	11th to 20th	Day	18% 10% 3 6% 6% 5% 5% 4 4 5% 34%	3438
	21st to end	Day	25% 10% 3 6% 4 3 3 3 6% 6% 32%	3876
April	1st to 10th	Day	22% 11% 3 7% 4 4 3 3 4 4 34%	2280
	11th to 20th	Day	19% 8% 2 5% 4 4 5% 3 5% 7% 38%	2380
	21st to end	Day	24% 9% 3 7% 4 4 3 3 6% 5% 33%	2470
May	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 4 6% 6% 30%	4106
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 7% 30%	4164
	21st to end	Day	16% 5% 2 6% 4 5% 5% 5% 7% 9% 36%	4837
June	1st to 10th	Day	24% 8% 3 5% 4 5% 4 4 6% 7% 30%	2826
	11th to 20th	Day	18% 7% 2 6% 4 4 5% 4 6% 7% 37%	2850
	21st to end	Day	25% 7% 2 5% 5% 4 5% 5% 5% 7% 30%	2818
July	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 5% 6% 8% 36%	4440
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 9% 37%	4440
	21st to end	Day	18% 7% 2 6% 5% 4 4 4 6% 8% 36%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 6% 9% 33%	4139
	11th to 20th	Day	16% 9% 3 7% 5% 4 5% 4 6% 7% 34%	4045
	21st to end	Day	12% 6% 2 6% 3 4 5% 4 7% 9% 43%	4414
September	1st to 10th	Day	11% 5% 3 5% 4 4 4 4 5% 7% 47%	2224
	11th to 20th	Day	13% 5% 1 5% 3 3 4 3 4 7% 52%	2090
	21st to end	Day	17% 5% 2 6% 4 4 4 3 6% 8% 43%	2090
October	1st to 10th	Day	18% 7% 3 6% 4 4 5% 3 5% 6% 38%	2076
	11th to 20th	Day	20% 6% 3 5% 4 4 4 2 5% 7% 40%	1830
	21st to end	Day	24% 8% 3 7% 4 4 4 3 4 5% 34%	1881
November	1st to 10th	Day	17% 6% 2 6% 3 4 5% 4 5% 7% 40%	1761
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 5% 7% 40%	1800
	21st to end	Day	23% 7% 2 5% 4 3 3 2 4 6% 40%	1604
December	1st to 10th	Day	22% 11% 3 7% 5% 5% 3 4 4 5% 32%	1345
	11th to 20th	Day	23% 10% 2 6% 3 3 3 3 4 5% 37%	1668
	21st to end	Day	26% 12% 2 5% 5% 4 4 3 4 4 29%	2595
January	1st to 10th	Night	15% 9% 3 8% 5% 4 4 3 5% 6% 38%	4429
	11th to 20th	Night	13% 9% 4 10% 6% 6% 5% 4 6% 7% 30%	4440
	21st to end	Night	15% 13% 4 9% 5% 5% 4 4 5% 6% 30%	4884
February	1st to 10th	Night	15% 10% 4 8% 6% 5% 5% 4 6% 7% 32%	2504
	11th to 20th	Night	13% 11% 4 7% 5% 5% 5% 4 4 6% 38%	2254
	21st to end	Night	15% 12% 3 6% 5% 5% 4 4 5% 5% 37%	1884
March	1st to 10th	Night	29% 12% 2 6% 4 4 3 3 4 4 31%	3724
	11th to 20th	Night	16% 11% 3 7% 5% 6% 6% 4 5% 6% 31%	3520
	21st to end	Night	26% 10% 3 6% 4 5% 4 3 5% 6% 28%	3740
April	1st to 10th	Night	22% 12% 3 6% 4 3 4 2 3 5% 35%	1900
	11th to 20th	Night	19% 7% 2 6% 5% 5% 4 4 5% 6% 36%	1749
	21st to end	Night	25% 9% 2 7% 4 3 3 3 5% 5% 34%	1710
May	1st to 10th	Night	23% 8% 3 6% 3 3 4 4 6% 8% 31%	2834
	11th to 20th	Night	18% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	2759
	21st to end	Night	16% 6% 2 7% 4 4 4 4 6% 7% 40%	2738
June	1st to 10th	Night	23% 9% 2 6% 4 4 5% 3 5% 7% 32%	1349
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 4 6% 8% 39%	1330
	21st to end	Night	25% 8% 2 6% 5% 3 3 4 6% 8% 30%	1357
July	1st to 10th	Night	12% 7% 2 6% 5% 5% 6% 5% 8% 9% 36%	2520
	11th to 20th	Night	16% 6% 3 7% 5% 4 4 3 6% 8% 38%	2520
	21st to end	Night	15% 5% 2 6% 4 4 5% 4 7% 8% 41%	2739
August	1st to 10th	Night	20% 8% 3 7% 5% 5% 5% 4 6% 6% 30%	2757
	11th to 20th	Night	16% 9% 3 8% 5% 4 5% 4 5% 8% 33%	2782
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 5% 7% 45%	3238
September	1st to 10th	Night	13% 5% 2 5% 3 4 4 4 5% 7% 49%	1955
	11th to 20th	Night	16% 6% 2 4 3 3 3 2 4 6% 52%	2090
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 6% 45%	2090
October	1st to 10th	Night	19% 6% 2 6% 4 5% 4 4 6% 5% 39%	2104
	11th to 20th	Night	18% 6% 2 6% 4 5% 4 3 5% 6% 41%	2346
	21st to end	Night	25% 8% 2 6% 4 4 3 3 4 6% 34%	2717
November	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 41%	2537
	11th to 20th	Night	17% 6% 2 6% 4 4 4 4 4 7% 41%	2600
	21st to end	Night	18% 8% 2 6% 4 4 4 4 5% 7% 37%	2668
December	1st to 10th	Night	22% 11% 3 7% 5% 4 4 3 5% 5% 32%	2833
	11th to 20th	Night	22% 8% 3 5% 4 4 4 3 4 5% 37%	3531
	21st to end	Night	24% 10% 3 5% 3 3 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	44% 7% 15% 23% 10% 1	2518
	11th to 20th	Day	45% 7% 12% 23% 11% 2	2520
	21st to end	Day	50% 6% 13% 17% 13% 1	2772
February	1st to 10th	Day	49% 6% 10% 19% 13% 3	1673
	11th to 20th	Day	50% 6% 11% 17% 13% 3	1819
	21st to end	Day	50% 5% 11% 18% 10% 6%	1570
March	1st to 10th	Day	59% 5% 8% 11% 13% 3	3119
	11th to 20th	Day	48% 9% 10% 19% 11% 3	3438
	21st to end	Day	51% 5% 6% 16% 18% 4	3876
April	1st to 10th	Day	53% 5% 9% 17% 13% 5%	2280
	11th to 20th	Day	45% 5% 9% 23% 15% 2	2380
	21st to end	Day	52% 5% 9% 18% 12% 4	2470
May	1st to 10th	Day	51% 10% 12% 17% 9% 2	4120
	11th to 20th	Day	50% 9% 15% 16% 9% 1	4168
	21st to end	Day	39% 9% 16% 20% 15% 1	4837
June	1st to 10th	Day	51% 7% 13% 20% 8% 1	2826
	11th to 20th	Day	44% 7% 11% 20% 15% 2	2850
	21st to end	Day	52% 7% 12% 17% 10% 1	2818
July	1st to 10th	Day	44% 5% 13% 25% 12%	4370
	11th to 20th	Day	42% 5% 15% 27% 10% 1	4365
	21st to end	Day	46% 6% 11% 23% 14%	4661
August	1st to 10th	Day	48% 6% 14% 25% 7%	4125
	11th to 20th	Day	49% 5% 16% 23% 7%	4084
	21st to end	Day	37% 5% 13% 27% 17% 2	4414
September	1st to 10th	Day	34% 7% 13% 27% 17% 1	2224
	11th to 20th	Day	33% 5% 10% 30% 20% 2	2090
	21st to end	Day	39% 5% 13% 27% 15% 1	2090
October	1st to 10th	Day	55% 3 9% 22% 11% 1	3453
	11th to 20th	Day	37% 4 12% 29% 18% 1	3020
	21st to end	Day	40% 3 14% 21% 19% 3	3081
November	1st to 10th	Day	43% 6% 13% 22% 14% 1	1761
	11th to 20th	Day	42% 6% 11% 21% 16% 4	1800
	21st to end	Day	46% 4 16% 23% 10% 1	1604
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	46% 5% 16% 22% 9% 1	4429
	11th to 20th	Night	49% 8% 15% 17% 9% 3	4440
	21st to end	Night	53% 7% 9% 18% 11% 1	4884
February	1st to 10th	Night	50% 6% 12% 19% 11% 3	2504
	11th to 20th	Night	47% 6% 10% 20% 15% 3	2256
	21st to end	Night	48% 6% 9% 19% 14% 4	1884
March	1st to 10th	Night	57% 5% 9% 16% 10% 4	3734
	11th to 20th	Night	51% 8% 13% 17% 9% 2	3521
	21st to end	Night	55% 5% 6% 15% 16% 2	3740
April	1st to 10th	Night	53% 4 7% 17% 15% 3	1900
	11th to 20th	Night	46% 6% 10% 20% 17% 1	1749
	21st to end	Night	51% 5% 11% 15% 14% 3	1710
May	1st to 10th	Night	47% 9% 17% 17% 8% 1	2840
	11th to 20th	Night	49% 10% 15% 17% 7% 1	2760
	21st to end	Night	40% 9% 17% 20% 13% 1	2739
June	1st to 10th	Night	51% 6% 13% 21% 8% 1	1349
	11th to 20th	Night	43% 6% 16% 21% 13% 2	1330
	21st to end	Night	50% 8% 13% 17% 10% 2	1357
July	1st to 10th	Night	43% 5% 17% 23% 12%	2480
	11th to 20th	Night	45% 4 13% 28% 9%	2472
	21st to end	Night	41% 6% 16% 25% 12% 1	2651
August	1st to 10th	Night	53% 7% 13% 21% 6%	2747
	11th to 20th	Night	48% 6% 17% 18% 11%	2809
	21st to end	Night	39% 4 14% 24% 17% 2	3238
September	1st to 10th	Night	35% 6% 14% 30% 15% 1	1955
	11th to 20th	Night	35% 5% 9% 31% 18% 1	2090
	21st to end	Night	39% 7% 11% 28% 14% 2	2090
October	1st to 10th	Night	48% 4 12% 23% 12% 1	3507
	11th to 20th	Night	38% 4 11% 29% 18% 1	3895
	21st to end	Night	41% 2 12% 26% 15% 2	4467
November	1st to 10th	Night	44% 6% 12% 24% 13% 1	2537
	11th to 20th	Night	42% 6% 11% 23% 16% 3	2600
	21st to end	Night	46% 6% 15% 20% 12% 1	2668
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	32% 18% 28% 17% 3 2	180
	21st to end	Day	22% 20% 31% 22% 5%	198
February	1st to 10th	Day	30% 15% 30% 20% 5%	333
	11th to 20th	Day	26% 20% 36% 17% 2	438
	21st to end	Day	25% 20% 31% 20% 5%	382
March	1st to 10th	Day	28% 18% 32% 17% 4	460
	11th to 20th	Day	25% 20% 33% 17% 4 1	460
	21st to end	Day	28% 21% 27% 15% 6% 2	506
April	1st to 10th	Day	35% 18% 26% 13% 6% 2	460
	11th to 20th	Day	31% 22% 27% 14% 5% 2	452
	21st to end	Day	30% 19% 30% 14% 5% 1	460
May	1st to 10th	Day	31% 17% 29% 18% 4 1	463
	11th to 20th	Day	23% 17% 34% 17% 7% 2	487
	21st to end	Day	21% 16% 36% 19% 8%	529
June	1st to 10th	Day	25% 15% 28% 21% 9% 2	656
	11th to 20th	Day	20% 14% 26% 24% 14% 2	680
	21st to end	Day	27% 11% 27% 24% 9% 2	648
July	1st to 10th	Day	18% 12% 32% 28% 9% 2	485
	11th to 20th	Day	14% 8% 36% 24% 13% 4	484
	21st to end	Day	16% 7% 36% 27% 10% 3	532
August	1st to 10th	Day	14% 7% 32% 30% 14% 2	486
	11th to 20th	Day	17% 9% 25% 29% 14% 5%	479
	21st to end	Day	17% 10% 29% 26% 14% 4	539
September	1st to 10th	Day	13% 12% 30% 26% 15% 4	490
	11th to 20th	Day	19% 9% 26% 22% 14% 10%	490
	21st to end	Day	18% 13% 27% 22% 14% 6%	490
October	1st to 10th	Day	22% 14% 23% 23% 15% 3	483
	11th to 20th	Day	26% 13% 28% 18% 12% 3	390
	21st to end	Day	33% 11% 27% 20% 8%	429
November	1st to 10th	Day	26% 11% 28% 23% 9% 2	395
	11th to 20th	Day	28% 14% 29% 22% 6% 1	400
	21st to end	Day	30% 15% 24% 23% 8% 1	382
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	760
	21st to end	Night	18% 15% 37% 26% 4	836
February	1st to 10th	Night	22% 15% 31% 28% 4	760
	11th to 20th	Night	20% 18% 38% 21% 3	743
	21st to end	Night	21% 14% 34% 28% 3	628
March	1st to 10th	Night	25% 14% 35% 24% 2	760
	11th to 20th	Night	23% 14% 33% 23% 6%	760
	21st to end	Night	24% 17% 30% 23% 6%	836
April	1st to 10th	Night	30% 14% 29% 23% 4	760
	11th to 20th	Night	23% 15% 35% 22% 5%	625
	21st to end	Night	30% 12% 32% 22% 4	570
May	1st to 10th	Night	26% 11% 28% 30% 4	570
	11th to 20th	Night	23% 10% 36% 24% 5% 1	534
	21st to end	Night	19% 8% 32% 36% 4	411
June	1st to 10th	Night	25% 7% 33% 29% 7%	380
	11th to 20th	Night	17% 7% 27% 38% 12%	380
	21st to end	Night	25% 4 26% 38% 6% 1	380
July	1st to 10th	Night	19% 2 29% 39% 10%	370
	11th to 20th	Night	17% 4 24% 39% 14% 1	368
	21st to end	Night	15% 3 24% 42% 15%	416
August	1st to 10th	Night	15% 4 26% 41% 14%	525
	11th to 20th	Night	16% 6% 22% 41% 14% 1	562
	21st to end	Night	15% 6% 26% 36% 15% 2	689
September	1st to 10th	Night	16% 4 30% 36% 14% 1	760
	11th to 20th	Night	18% 6% 24% 31% 18% 3	760
	21st to end	Night	21% 6% 28% 29% 14% 2	760
October	1st to 10th	Night	23% 7% 23% 33% 14% 1	760
	11th to 20th	Night	21% 9% 29% 29% 10% 2	760
	21st to end	Night	30% 10% 28% 25% 6%	836
November	1st to 10th	Night	23% 8% 32% 27% 9% 1	780
	11th to 20th	Night	22% 11% 32% 26% 8% 1	800
	21st to end	Night	24% 14% 28% 28% 5%	846
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

Page generated at 2024-11-22 08:02:24. Version ac07

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