Covey-Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Covey-Hill clear sky chart from 2005-11-06 to 2026-07-01.

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% 11% 4 9% 6% 5% 5% 4 5% 6% 29%	138033
	Night	15% 12% 5% 11% 6% 5% 5% 4 5% 6% 27%	134342
Transparency	Day	56% 2 10% 22% 10% 1	141480
	Night	59% 2 10% 19% 9% 1	138971
Seeing	Day	27% 18% 30% 17% 6% 2	16650
	Night	26% 19% 30% 19% 6% 1	25651

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	11% 13% 8% 19% 10% 8% 6% 3 5% 5% 13%	4661
	11th to 20th	Day	7% 16% 9% 19% 11% 7% 6% 3 4 4 16%	4650
	21st to end	Day	8% 15% 7% 13% 10% 10% 6% 4 5% 5% 17%	5124
February	1st to 10th	Day	12% 18% 5% 14% 8% 6% 5% 3 5% 4 18%	1874
	11th to 20th	Day	10% 14% 6% 15% 8% 6% 5% 3 5% 6% 22%	1956
	21st to end	Day	15% 15% 5% 13% 8% 5% 5% 3 5% 5% 21%	1730
March	1st to 10th	Day	17% 14% 6% 14% 6% 4 5% 3 4 5% 23%	4576
	11th to 20th	Day	11% 16% 5% 11% 6% 4 5% 4 5% 6% 27%	4826
	21st to end	Day	17% 12% 4 10% 6% 5% 4 3 4 4 32%	6091
April	1st to 10th	Day	19% 17% 4 8% 4 4 3 3 4 5% 28%	2520
	11th to 20th	Day	19% 12% 4 8% 5% 4 4 3 4 5% 33%	2494
	21st to end	Day	22% 12% 3 8% 5% 4 3 3 5% 5% 31%	2667
May	1st to 10th	Day	23% 12% 4 8% 5% 5% 5% 4 6% 6% 23%	8107
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 4 5% 7% 30%	8221
	21st to end	Day	22% 9% 3 7% 4 4 4 4 6% 7% 31%	9060
June	1st to 10th	Day	23% 8% 3 7% 4 5% 4 4 6% 7% 29%	2940
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 9% 34%	2940
	21st to end	Day	20% 8% 3 6% 5% 5% 4 4 6% 7% 30%	2940
July	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 5% 7% 8% 33%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 8% 39%	6360
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 10% 41%	6926
August	1st to 10th	Day	15% 9% 3 7% 5% 5% 6% 4 6% 8% 31%	4563
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 5% 8% 36%	4061
	21st to end	Day	17% 6% 2 6% 4 4 4 4 7% 8% 38%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 43%	2400
	11th to 20th	Day	15% 7% 2 5% 3 4 3 3 4 7% 47%	2321
	21st to end	Day	14% 7% 3 7% 4 5% 4 4 7% 7% 39%	2200
October	1st to 10th	Day	19% 8% 3 6% 5% 5% 5% 4 5% 7% 33%	2057
	11th to 20th	Day	16% 12% 3 8% 5% 6% 5% 4 5% 6% 30%	1974
	21st to end	Day	24% 14% 4 8% 4 4 4 2 4 5% 26%	1980
November	1st to 10th	Day	16% 14% 4 8% 6% 4 4 3 5% 7% 28%	1851
	11th to 20th	Day	15% 15% 6% 11% 4 5% 5% 3 5% 6% 26%	1817
	21st to end	Day	17% 16% 6% 12% 6% 6% 4 4 5% 4 19%	1680
December	1st to 10th	Day	13% 22% 8% 17% 9% 5% 4 3 4 4 11%	2680
	11th to 20th	Day	15% 14% 7% 14% 5% 5% 4 4 5% 5% 22%	3066
	21st to end	Day	16% 18% 6% 12% 6% 5% 4 3 4 4 22%	4048
January	1st to 10th	Night	9% 13% 7% 21% 12% 6% 4 3 4 5% 15%	8234
	11th to 20th	Night	9% 15% 7% 19% 11% 7% 6% 4 5% 5% 13%	8235
	21st to end	Night	10% 12% 5% 13% 10% 11% 6% 4 5% 5% 18%	9057
February	1st to 10th	Night	12% 15% 6% 15% 9% 7% 6% 3 5% 4 19%	2743
	11th to 20th	Night	11% 12% 6% 13% 8% 7% 6% 4 5% 5% 24%	2550
	21st to end	Night	11% 15% 5% 13% 7% 6% 4 4 4 5% 27%	2076
March	1st to 10th	Night	17% 12% 4 11% 7% 6% 4 2 4 4 29%	5484
	11th to 20th	Night	12% 15% 5% 9% 5% 5% 5% 4 5% 7% 28%	5338
	21st to end	Night	14% 12% 6% 13% 6% 5% 4 3 4 4 28%	5083
April	1st to 10th	Night	20% 15% 4 9% 5% 5% 4 3 4 5% 26%	2100
	11th to 20th	Night	22% 12% 3 9% 5% 4 4 3 4 6% 29%	2075
	21st to end	Night	20% 11% 4 7% 4 4 3 3 5% 6% 34%	1953
May	1st to 10th	Night	23% 14% 3 7% 5% 4 5% 4 6% 7% 23%	4845
	11th to 20th	Night	20% 10% 3 7% 4 4 4 3 6% 8% 30%	4639
	21st to end	Night	17% 10% 3 8% 4 4 4 4 6% 8% 31%	5114
June	1st to 10th	Night	21% 8% 2 7% 5% 5% 5% 4 6% 9% 30%	1680
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 4 5% 8% 37%	1680
	21st to end	Night	17% 8% 2 7% 5% 4 5% 5% 5% 7% 35%	1680
July	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 5% 7% 8% 36%	3600
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 8% 43%	3600
	21st to end	Night	11% 7% 3 7% 5% 5% 4 4 7% 7% 40%	3926
August	1st to 10th	Night	15% 8% 3 7% 4 4 4 4 5% 7% 39%	2586
	11th to 20th	Night	14% 9% 3 8% 4 5% 5% 3 5% 8% 38%	2952
	21st to end	Night	15% 6% 2 6% 3 4 4 4 6% 7% 43%	3586
September	1st to 10th	Night	12% 7% 3 5% 5% 4 5% 3 6% 6% 44%	2000
	11th to 20th	Night	13% 6% 2 6% 3 3 4 3 5% 7% 48%	2078
	21st to end	Night	18% 6% 2 6% 5% 4 4 3 5% 7% 42%	2200
October	1st to 10th	Night	19% 8% 2 6% 4 3 4 4 7% 6% 37%	2340
	11th to 20th	Night	18% 11% 3 7% 5% 4 5% 4 5% 7% 32%	2425
	21st to end	Night	23% 14% 4 7% 4 4 4 3 5% 5% 27%	2860
November	1st to 10th	Night	17% 14% 4 7% 5% 4 5% 4 5% 5% 29%	2667
	11th to 20th	Night	16% 13% 5% 10% 5% 5% 4 4 4 5% 29%	2798
	21st to end	Night	16% 15% 7% 11% 5% 5% 4 4 4 6% 22%	2940
December	1st to 10th	Night	13% 21% 8% 16% 7% 6% 5% 3 4 4 12%	4720
	11th to 20th	Night	17% 18% 7% 13% 5% 4 5% 3 4 4 20%	5348
	21st to end	Night	19% 16% 6% 13% 5% 5% 5% 3 4 4 22%	7150

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	76% 1 9% 12% 2	4662
	11th to 20th	Day	75% 1 6% 9% 9% 1	4680
	21st to end	Day	71% 1 8% 10% 9% 1	5124
February	1st to 10th	Day	70% 2 5% 17% 5% 1	1874
	11th to 20th	Day	65% 2 8% 16% 8% 1	1965
	21st to end	Day	67% 2 8% 13% 9% 2	1730
March	1st to 10th	Day	66% 1 7% 14% 9% 3	4579
	11th to 20th	Day	59% 1 13% 14% 10% 2	4826
	21st to end	Day	58% 1 5% 17% 18% 2	6091
April	1st to 10th	Day	59% 2 5% 19% 12% 2	2520
	11th to 20th	Day	56% 1 5% 21% 15% 2	2494
	21st to end	Day	56% 2 7% 19% 13% 3	2667
May	1st to 10th	Day	63% 1 7% 19% 10% 1	8119
	11th to 20th	Day	56% 1 11% 23% 8% 1	8225
	21st to end	Day	55% 1 9% 25% 10% 1	9061
June	1st to 10th	Day	55% 3 9% 21% 12% 1	2940
	11th to 20th	Day	47% 3 10% 26% 12% 2	2940
	21st to end	Day	53% 3 11% 22% 10% 1	2940
July	1st to 10th	Day	49% 2 12% 28% 9%	6290
	11th to 20th	Day	42% 2 13% 31% 12%	6285
	21st to end	Day	40% 2 11% 39% 7%	6728
August	1st to 10th	Day	51% 2 14% 24% 9%	4549
	11th to 20th	Day	47% 2 15% 29% 6%	4103
	21st to end	Day	44% 2 16% 25% 12%	4312
September	1st to 10th	Day	40% 3 11% 32% 13%	2400
	11th to 20th	Day	39% 2 9% 35% 15% 1	2321
	21st to end	Day	44% 2 14% 27% 12% 1	2200
October	1st to 10th	Day	48% 2 10% 29% 11%	3343
	11th to 20th	Day	50% 1 14% 24% 10% 1	3190
	21st to end	Day	52% 1 16% 24% 7% 1	3180
November	1st to 10th	Day	57% 2 11% 21% 8% 1	1851
	11th to 20th	Day	60% 3 10% 16% 10% 2	1817
	21st to end	Day	69% 3 11% 13% 5%	1680
December	1st to 10th	Day	78% 2 9% 10% 2	2680
	11th to 20th	Day	65% 2 8% 17% 7%	3066
	21st to end	Day	67% 2 8% 16% 8%	4048
January	1st to 10th	Night	74% 1 9% 11% 4	8234
	11th to 20th	Night	75% 1 6% 11% 7%	8280
	21st to end	Night	70% 1 6% 12% 10%	9057
February	1st to 10th	Night	69% 2 7% 15% 6% 1	2743
	11th to 20th	Night	63% 2 9% 18% 8%	2552
	21st to end	Night	62% 2 7% 17% 11% 1	2076
March	1st to 10th	Night	62% 1 6% 17% 12% 2	5494
	11th to 20th	Night	58% 1 14% 16% 10% 2	5339
	21st to end	Night	62% 1 3 20% 13% 1	5083
April	1st to 10th	Night	61% 2 6% 18% 11% 2	2100
	11th to 20th	Night	59% 2 8% 20% 11% 1	2075
	21st to end	Night	54% 2 8% 19% 15% 3	1953
May	1st to 10th	Night	61% 1 10% 19% 7% 1	4851
	11th to 20th	Night	55% 1 9% 25% 9% 1	4640
	21st to end	Night	54% 2 10% 21% 13% 1	5115
June	1st to 10th	Night	52% 3 10% 21% 13% 1	1680
	11th to 20th	Night	46% 3 11% 24% 14% 2	1680
	21st to end	Night	49% 3 12% 24% 10% 1	1680
July	1st to 10th	Night	46% 1 18% 25% 9%	3560
	11th to 20th	Night	41% 2 15% 31% 12%	3552
	21st to end	Night	44% 2 13% 31% 10%	3809
August	1st to 10th	Night	46% 2 15% 28% 9%	2576
	11th to 20th	Night	47% 2 19% 23% 9%	2976
	21st to end	Night	42% 2 16% 26% 13% 1	3586
September	1st to 10th	Night	41% 3 12% 31% 13% 1	2000
	11th to 20th	Night	38% 2 13% 34% 13% 1	2078
	21st to end	Night	44% 2 13% 26% 14% 1	2200
October	1st to 10th	Night	47% 2 11% 29% 11%	3821
	11th to 20th	Night	49% 2 12% 25% 12% 1	3948
	21st to end	Night	54% 1 14% 21% 10% 1	4610
November	1st to 10th	Night	57% 2 11% 19% 9% 1	2667
	11th to 20th	Night	59% 2 10% 18% 8% 3	2798
	21st to end	Night	65% 2 12% 16% 5%	2940
December	1st to 10th	Night	77% 2 8% 11% 2	4720
	11th to 20th	Night	69% 2 6% 13% 9%	5348
	21st to end	Night	68% 2 7% 13% 9% 1	7150

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	27% 23% 32% 15% 2	220
	11th to 20th	Day	33% 28% 29% 8% 3	220
	21st to end	Day	29% 26% 31% 11% 3	242
February	1st to 10th	Day	33% 28% 32% 6% 2	322
	11th to 20th	Day	33% 26% 33% 8%	443
	21st to end	Day	33% 23% 32% 10% 1	446
March	1st to 10th	Day	34% 25% 27% 10% 4	540
	11th to 20th	Day	32% 25% 29% 10% 3 1	540
	21st to end	Day	36% 24% 24% 12% 5%	594
April	1st to 10th	Day	38% 19% 29% 11% 3	540
	11th to 20th	Day	38% 18% 24% 14% 4 1	532
	21st to end	Day	32% 20% 29% 13% 5% 1	540
May	1st to 10th	Day	34% 18% 29% 13% 4 1	543
	11th to 20th	Day	25% 19% 30% 18% 6% 2	567
	21st to end	Day	25% 18% 31% 20% 5% 1	617
June	1st to 10th	Day	29% 14% 33% 17% 5% 1	570
	11th to 20th	Day	23% 16% 34% 19% 6% 1	570
	21st to end	Day	28% 15% 32% 17% 6% 2	570
July	1st to 10th	Day	19% 11% 33% 25% 9% 2	525
	11th to 20th	Day	17% 12% 38% 26% 5% 3	524
	21st to end	Day	16% 11% 41% 22% 7% 2	576
August	1st to 10th	Day	17% 11% 33% 25% 12% 2	526
	11th to 20th	Day	18% 6% 33% 27% 11% 4	519
	21st to end	Day	19% 14% 30% 22% 11% 3	583
September	1st to 10th	Day	18% 12% 32% 20% 13% 4	530
	11th to 20th	Day	18% 9% 25% 29% 15% 5%	530
	21st to end	Day	17% 16% 31% 24% 9% 3	530
October	1st to 10th	Day	24% 16% 30% 18% 10% 2	530
	11th to 20th	Day	27% 22% 27% 15% 7% 2	515
	21st to end	Day	36% 16% 28% 15% 5%	462
November	1st to 10th	Day	37% 20% 25% 13% 5% 1	425
	11th to 20th	Day	32% 26% 25% 10% 6% 2	357
	21st to end	Day	36% 24% 25% 11% 4	220
December	1st to 10th	Day	37% 20% 27% 11% 4	220
	11th to 20th	Day	36% 29% 26% 6% 3	220
	21st to end	Day	29% 31% 24% 12% 4	242
January	1st to 10th	Night	26% 27% 36% 10% 2	840
	11th to 20th	Night	29% 25% 31% 12% 3	840
	21st to end	Night	21% 27% 33% 15% 4	924
February	1st to 10th	Night	25% 26% 33% 14% 2	840
	11th to 20th	Night	25% 29% 31% 14% 1	823
	21st to end	Night	26% 25% 32% 15% 2	692
March	1st to 10th	Night	31% 24% 27% 15% 3	840
	11th to 20th	Night	28% 26% 28% 16% 2	840
	21st to end	Night	30% 19% 27% 18% 5%	924
April	1st to 10th	Night	33% 19% 29% 15% 3	840
	11th to 20th	Night	33% 20% 25% 18% 4	831
	21st to end	Night	30% 16% 30% 18% 5%	693
May	1st to 10th	Night	33% 14% 28% 19% 6%	488
	11th to 20th	Night	24% 15% 31% 23% 6% 1	414
	21st to end	Night	20% 15% 36% 20% 8%	455
June	1st to 10th	Night	27% 11% 31% 24% 6% 1	420
	11th to 20th	Night	23% 10% 31% 27% 8%	420
	21st to end	Night	22% 11% 33% 25% 9%	420
July	1st to 10th	Night	13% 11% 37% 29% 9% 1	390
	11th to 20th	Night	11% 8% 37% 32% 10% 1	388
	21st to end	Night	16% 9% 35% 30% 9% 1	438
August	1st to 10th	Night	15% 7% 29% 35% 13% 1	398
	11th to 20th	Night	16% 4 26% 36% 17% 2	629
	21st to end	Night	18% 9% 30% 29% 12% 1	880
September	1st to 10th	Night	17% 10% 33% 29% 11% 1	800
	11th to 20th	Night	17% 8% 25% 30% 16% 3	800
	21st to end	Night	22% 10% 29% 21% 16% 2	800
October	1st to 10th	Night	25% 14% 30% 19% 12% 1	800
	11th to 20th	Night	28% 17% 28% 21% 6% 1	800
	21st to end	Night	33% 18% 29% 17% 4	880
November	1st to 10th	Night	31% 20% 27% 16% 5% 1	820
	11th to 20th	Night	30% 22% 29% 11% 7% 1	840
	21st to end	Night	30% 25% 25% 15% 4	840
December	1st to 10th	Night	32% 24% 28% 13% 3	840
	11th to 20th	Night	31% 27% 24% 14% 3 1	840
	21st to end	Night	33% 24% 27% 12% 4	924

Page generated at 2026-06-28 02:10:45. Version ac07

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