Cooke Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cooke Observatory 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	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	101120
	Night	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	98311
Transparency	Day	36% 11% 16% 18% 13% 5%	104924
	Night	36% 10% 16% 19% 14% 5%	102646
Seeing	Day	9% 7% 21% 27% 23% 14%	15485
	Night	10% 5% 20% 32% 25% 8%	25521

By Month

Percentage Cloud Cover SE (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	12% 6% 2 7% 5% 4 5% 4 5% 8% 41%	2836
	11th to 20th	Day	21% 9% 3 7% 4 4 4 3 5% 7% 34%	2840
	21st to end	Day	11% 5% 2 6% 4 5% 4 4 5% 7% 46%	3357
February	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 8% 44%	1900
	11th to 20th	Day	10% 4 2 4 4 4 4 3 5% 6% 54%	1848
	21st to end	Day	7% 6% 2 6% 5% 4 4 4 6% 7% 50%	1603
March	1st to 10th	Day	8% 6% 2 8% 4 5% 5% 4 7% 9% 40%	3435
	11th to 20th	Day	7% 4 2 5% 3 4 4 4 7% 10% 50%	3480
	21st to end	Day	11% 4 1 5% 4 3 5% 4 5% 7% 51%	3815
April	1st to 10th	Day	7% 3 2 4 4 4 5% 4 7% 9% 51%	2090
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 7% 45%	2066
	21st to end	Day	5% 2 2 5% 4 5% 5% 4 6% 9% 54%	2320
May	1st to 10th	Day	3 2 1 3 3 4 5% 4 7% 10% 60%	4082
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 7% 10% 40%	4081
	21st to end	Day	4 3 1 4 4 4 5% 5% 6% 9% 55%	4476
June	1st to 10th	Day	10% 7% 2 8% 6% 6% 6% 5% 7% 11% 31%	2470
	11th to 20th	Day	6% 5% 2 6% 6% 7% 8% 7% 11% 12% 31%	2470
	21st to end	Day	7% 6% 2 8% 6% 7% 7% 6% 10% 11% 30%	2470
July	1st to 10th	Day	5% 5% 2 8% 8% 8% 10% 8% 10% 14% 22%	4120
	11th to 20th	Day	4 5% 2 8% 7% 9% 10% 8% 10% 13% 24%	4120
	21st to end	Day	4 4 3 8% 7% 8% 10% 9% 11% 14% 23%	4466
August	1st to 10th	Day	8% 5% 2 8% 6% 7% 7% 6% 8% 12% 31%	4104
	11th to 20th	Day	4 4 2 6% 5% 6% 6% 6% 10% 14% 39%	3733
	21st to end	Day	7% 6% 3 8% 6% 7% 8% 6% 10% 13% 27%	3860
September	1st to 10th	Day	7% 6% 3 9% 8% 8% 7% 6% 10% 12% 23%	2090
	11th to 20th	Day	9% 7% 3 9% 7% 6% 7% 6% 8% 9% 28%	2090
	21st to end	Day	7% 6% 3 8% 6% 7% 7% 4 9% 9% 35%	2090
October	1st to 10th	Day	11% 6% 3 8% 5% 6% 6% 4 6% 8% 38%	2090
	11th to 20th	Day	6% 4 2 5% 3 4 4 3 6% 7% 54%	1943
	21st to end	Day	6% 3 2 6% 4 4 5% 4 5% 8% 52%	2090
November	1st to 10th	Day	7% 5% 2 6% 4 4 3 4 5% 7% 51%	1957
	11th to 20th	Day	7% 6% 2 6% 5% 4 4 3 5% 7% 49%	1995
	21st to end	Day	8% 6% 3 6% 4 5% 5% 4 7% 9% 44%	1746
December	1st to 10th	Day	8% 4 2 5% 5% 4 5% 5% 7% 8% 47%	1710
	11th to 20th	Day	12% 6% 2 6% 5% 3 4 4 5% 8% 46%	2153
	21st to end	Day	14% 7% 2 8% 5% 4 5% 4 6% 7% 36%	3124
January	1st to 10th	Night	13% 7% 3 7% 5% 4 4 4 5% 8% 40%	4109
	11th to 20th	Night	18% 9% 3 7% 4 4 4 4 6% 7% 33%	4120
	21st to end	Night	13% 7% 2 7% 4 4 5% 4 6% 9% 40%	4295
February	1st to 10th	Night	13% 8% 2 5% 4 4 4 4 6% 7% 45%	2280
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 6% 48%	2218
	21st to end	Night	8% 7% 2 7% 5% 4 3 4 5% 7% 48%	1850
March	1st to 10th	Night	15% 9% 3 8% 5% 5% 5% 4 6% 5% 37%	3418
	11th to 20th	Night	6% 6% 2 4 3 4 5% 4 8% 10% 48%	3480
	21st to end	Night	11% 5% 2 5% 3 4 4 3 6% 7% 51%	3817
April	1st to 10th	Night	7% 5% 2 5% 3 3 4 4 7% 10% 50%	2090
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 5% 9% 51%	2063
	21st to end	Night	7% 3 1 5% 3 4 3 3 5% 8% 57%	1873
May	1st to 10th	Night	4 3 1 3 2 3 4 4 5% 9% 61%	2813
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 6% 9% 47%	2812
	21st to end	Night	4 2 1 3 3 4 3 3 5% 8% 63%	3079
June	1st to 10th	Night	10% 7% 2 7% 4 5% 5% 5% 8% 9% 40%	1710
	11th to 20th	Night	5% 5% 1 6% 6% 5% 6% 6% 9% 13% 37%	1710
	21st to end	Night	6% 5% 3 7% 6% 6% 7% 6% 8% 12% 33%	1710
July	1st to 10th	Night	5% 6% 3 9% 7% 8% 8% 6% 9% 14% 24%	2840
	11th to 20th	Night	2 5% 2 7% 6% 7% 8% 8% 12% 15% 27%	2840
	21st to end	Night	4 7% 3 11% 9% 9% 9% 7% 10% 11% 20%	3087
August	1st to 10th	Night	7% 4 2 7% 6% 6% 6% 6% 10% 13% 34%	2825
	11th to 20th	Night	3 3 1 5% 4 5% 6% 5% 10% 17% 41%	3091
	21st to end	Night	7% 6% 3 9% 6% 6% 8% 6% 9% 12% 28%	3762
September	1st to 10th	Night	6% 6% 3 8% 7% 6% 8% 6% 10% 11% 29%	2090
	11th to 20th	Night	10% 6% 3 9% 6% 7% 6% 6% 8% 11% 29%	2090
	21st to end	Night	8% 6% 2 8% 6% 6% 6% 5% 7% 10% 37%	2090
October	1st to 10th	Night	10% 6% 2 8% 4 5% 6% 4 7% 9% 40%	2090
	11th to 20th	Night	5% 5% 1 4 5% 3 4 3 6% 9% 55%	2237
	21st to end	Night	8% 4 2 7% 4 4 3 3 6% 8% 51%	2508
November	1st to 10th	Night	8% 5% 2 5% 3 3 4 4 6% 8% 52%	2341
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 5% 7% 49%	2400
	21st to end	Night	7% 6% 3 7% 4 4 4 3 6% 9% 47%	2529
December	1st to 10th	Night	8% 5% 2 6% 5% 4 4 4 6% 8% 48%	2470
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 5% 7% 48%	3042
	21st to end	Night	18% 8% 3 7% 4 4 4 4 6% 7% 35%	4532

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% 7% 9% 18% 18% 9%	2837
	11th to 20th	Day	49% 5% 6% 16% 15% 10%	2840
	21st to end	Day	34% 7% 9% 20% 22% 7%	3357
February	1st to 10th	Day	38% 7% 9% 15% 19% 12%	1900
	11th to 20th	Day	30% 6% 10% 18% 24% 12%	1858
	21st to end	Day	32% 7% 11% 18% 21% 11%	1603
March	1st to 10th	Day	36% 11% 14% 18% 15% 6%	3435
	11th to 20th	Day	28% 7% 13% 25% 21% 6%	3480
	21st to end	Day	31% 7% 10% 28% 17% 7%	3815
April	1st to 10th	Day	28% 9% 15% 21% 19% 8%	2090
	11th to 20th	Day	36% 8% 11% 17% 22% 7%	2066
	21st to end	Day	24% 8% 13% 23% 24% 8%	2320
May	1st to 10th	Day	15% 10% 20% 31% 18% 5%	4120
	11th to 20th	Day	34% 14% 18% 21% 12% 2	4085
	21st to end	Day	21% 10% 16% 31% 20% 2	4489
June	1st to 10th	Day	45% 14% 18% 18% 6%	2470
	11th to 20th	Day	40% 17% 21% 19% 4	2470
	21st to end	Day	44% 16% 21% 14% 4	2470
July	1st to 10th	Day	46% 16% 31% 7% 1	4055
	11th to 20th	Day	45% 18% 31% 5% 1	4051
	21st to end	Day	44% 19% 29% 8% 1	4324
August	1st to 10th	Day	42% 14% 28% 13% 3	4091
	11th to 20th	Day	31% 15% 26% 24% 4	3769
	21st to end	Day	43% 16% 24% 16% 2	3860
September	1st to 10th	Day	49% 20% 17% 11% 4	2090
	11th to 20th	Day	49% 15% 19% 13% 4	2090
	21st to end	Day	42% 12% 16% 16% 10% 3	2090
October	1st to 10th	Day	56% 10% 12% 10% 7% 5%	3480
	11th to 20th	Day	23% 6% 7% 22% 33% 8%	3209
	21st to end	Day	27% 5% 8% 23% 30% 7%	3428
November	1st to 10th	Day	32% 7% 10% 22% 18% 11%	1957
	11th to 20th	Day	33% 7% 11% 18% 20% 11%	1995
	21st to end	Day	35% 6% 9% 19% 21% 9%	1746
December	1st to 10th	Day	31% 9% 14% 24% 17% 6%	1710
	11th to 20th	Day	37% 6% 10% 18% 16% 13%	2150
	21st to end	Day	44% 9% 9% 19% 14% 5%	3124
January	1st to 10th	Night	41% 7% 10% 14% 21% 7%	4109
	11th to 20th	Night	48% 6% 9% 17% 15% 5%	4120
	21st to end	Night	38% 9% 11% 15% 19% 8%	4295
February	1st to 10th	Night	37% 6% 11% 16% 19% 11%	2280
	11th to 20th	Night	33% 7% 11% 18% 22% 10%	2219
	21st to end	Night	34% 6% 13% 18% 22% 8%	1850
March	1st to 10th	Night	45% 10% 12% 15% 14% 5%	3418
	11th to 20th	Night	26% 10% 21% 24% 14% 6%	3480
	21st to end	Night	30% 8% 10% 26% 19% 6%	3817
April	1st to 10th	Night	27% 8% 17% 23% 18% 6%	2090
	11th to 20th	Night	29% 7% 14% 25% 20% 6%	2063
	21st to end	Night	25% 6% 14% 25% 23% 6%	1873
May	1st to 10th	Night	16% 12% 21% 31% 15% 5%	2840
	11th to 20th	Night	28% 14% 19% 25% 12% 1	2813
	21st to end	Night	17% 7% 15% 40% 18% 1	3087
June	1st to 10th	Night	36% 12% 23% 22% 6%	1710
	11th to 20th	Night	34% 15% 28% 20% 3	1710
	21st to end	Night	38% 17% 24% 16% 5%	1710
July	1st to 10th	Night	47% 17% 29% 6% 1	2795
	11th to 20th	Night	38% 19% 37% 5% 1	2786
	21st to end	Night	52% 18% 23% 7% 1	2988
August	1st to 10th	Night	37% 15% 33% 12% 2	2814
	11th to 20th	Night	26% 15% 33% 23% 2	3121
	21st to end	Night	44% 16% 24% 13% 3	3762
September	1st to 10th	Night	43% 19% 23% 11% 4	2090
	11th to 20th	Night	45% 14% 21% 16% 3 1	2090
	21st to end	Night	38% 13% 17% 19% 11% 2	2090
October	1st to 10th	Night	56% 10% 11% 11% 7% 4	3480
	11th to 20th	Night	22% 5% 8% 24% 35% 6%	3710
	21st to end	Night	30% 6% 7% 19% 31% 6%	4120
November	1st to 10th	Night	29% 7% 12% 21% 20% 11%	2341
	11th to 20th	Night	33% 6% 10% 20% 20% 10%	2400
	21st to end	Night	34% 6% 10% 23% 18% 9%	2529
December	1st to 10th	Night	33% 7% 11% 27% 16% 5%	2470
	11th to 20th	Night	35% 6% 10% 18% 19% 11%	3044
	21st to end	Night	44% 9% 11% 18% 14% 4	4532

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	11% 13% 49% 19% 6% 1	270
	11th to 20th	Day	13% 14% 37% 24% 10% 2	270
	21st to end	Day	15% 18% 46% 18% 3	297
February	1st to 10th	Day	14% 14% 38% 23% 10% 1	270
	11th to 20th	Day	13% 13% 38% 24% 10% 2	270
	21st to end	Day	7% 14% 39% 26% 13%	258
March	1st to 10th	Day	6% 15% 40% 29% 8% 1	460
	11th to 20th	Day	12% 14% 31% 27% 14% 2	460
	21st to end	Day	9% 12% 34% 28% 15% 2	506
April	1st to 10th	Day	10% 13% 34% 28% 13% 2	460
	11th to 20th	Day	11% 14% 27% 28% 16% 4	452
	21st to end	Day	7% 7% 27% 35% 20% 5%	460
May	1st to 10th	Day	4 7% 27% 34% 19% 9%	463
	11th to 20th	Day	7% 4 25% 30% 23% 11%	487
	21st to end	Day	6% 3 12% 30% 30% 18%	529
June	1st to 10th	Day	11% 4 12% 28% 29% 17%	490
	11th to 20th	Day	9% 3 10% 26% 28% 24%	490
	21st to end	Day	6% 1 6% 21% 33% 33%	490
July	1st to 10th	Day	6% 1 4 18% 32% 39%	485
	11th to 20th	Day	5% 1 4 14% 36% 40%	484
	21st to end	Day	8% 1 4 23% 29% 34%	532
August	1st to 10th	Day	9% 3 5% 18% 32% 33%	486
	11th to 20th	Day	8% 1 5% 15% 33% 39%	479
	21st to end	Day	9% 2 6% 20% 35% 28%	539
September	1st to 10th	Day	9% 4 6% 19% 34% 27%	490
	11th to 20th	Day	8% 3 6% 29% 36% 19%	490
	21st to end	Day	9% 3 9% 28% 33% 19%	490
October	1st to 10th	Day	12% 6% 22% 32% 20% 8%	490
	11th to 20th	Day	6% 6% 22% 32% 26% 8%	490
	21st to end	Day	7% 5% 22% 40% 22% 4	539
November	1st to 10th	Day	9% 5% 24% 39% 19% 5%	495
	11th to 20th	Day	10% 8% 26% 36% 18% 2	495
	21st to end	Day	8% 10% 37% 31% 12% 3	282
December	1st to 10th	Day	6% 12% 39% 27% 12% 4	270
	11th to 20th	Day	14% 9% 36% 28% 13% 1	270
	21st to end	Day	9% 11% 32% 35% 10% 1	297
January	1st to 10th	Night	13% 11% 32% 31% 12% 1	760
	11th to 20th	Night	13% 10% 24% 39% 14%	760
	21st to end	Night	16% 12% 34% 30% 8%	836
February	1st to 10th	Night	16% 10% 30% 30% 14% 1	760
	11th to 20th	Night	12% 10% 28% 36% 12% 1	743
	21st to end	Night	10% 7% 34% 39% 9%	628
March	1st to 10th	Night	9% 9% 37% 35% 10% 1	760
	11th to 20th	Night	8% 11% 27% 38% 15% 1	760
	21st to end	Night	9% 8% 31% 37% 14% 1	836
April	1st to 10th	Night	8% 9% 29% 38% 14% 1	760
	11th to 20th	Night	9% 8% 32% 37% 13%	751
	21st to end	Night	7% 4 26% 40% 22%	700
May	1st to 10th	Night	5% 4 15% 46% 27% 3	570
	11th to 20th	Night	7% 4 16% 40% 28% 4	561
	21st to end	Night	7% 1 11% 36% 36% 8%	617
June	1st to 10th	Night	11% 2 10% 29% 38% 9%	570
	11th to 20th	Night	7% 1 5% 23% 42% 21%	570
	21st to end	Night	9% 1 5% 16% 45% 24%	570
July	1st to 10th	Night	7% 1 3 14% 44% 30%	555
	11th to 20th	Night	5% 1 2 11% 44% 36%	552
	21st to end	Night	5% 1 5% 13% 47% 29%	624
August	1st to 10th	Night	8% 1 3 12% 48% 28%	567
	11th to 20th	Night	5% 1 2 12% 45% 34%	564
	21st to end	Night	12% 3 5% 22% 37% 21%	796
September	1st to 10th	Night	9% 1 6% 24% 42% 19%	760
	11th to 20th	Night	11% 3 8% 28% 39% 11%	760
	21st to end	Night	11% 3 10% 28% 41% 7%	760
October	1st to 10th	Night	13% 3 18% 38% 24% 4	760
	11th to 20th	Night	5% 5% 17% 39% 32% 2	760
	21st to end	Night	10% 3 23% 39% 22% 2	836
November	1st to 10th	Night	10% 3 28% 36% 20% 3	780
	11th to 20th	Night	9% 6% 25% 41% 18% 2	800
	21st to end	Night	8% 6% 28% 42% 15% 1	779
December	1st to 10th	Night	10% 6% 29% 33% 21% 1	760
	11th to 20th	Night	12% 7% 28% 35% 17% 1	760
	21st to end	Night	12% 8% 27% 35% 17% 1	836

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