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 2024-09-14.

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	16% 11% 4 9% 5% 5% 5% 4 5% 6% 30%	100995
	Night	16% 12% 4 10% 6% 5% 4 3 5% 6% 29%	96927
Transparency	Day	55% 2 11% 22% 9% 1	100756
	Night	58% 2 11% 20% 9% 1	96786
Seeing	Day	27% 18% 30% 17% 6% 2	14422
	Night	25% 19% 30% 19% 6% 1	23377

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% 14% 7% 16% 8% 6% 6% 3 5% 5% 16%	2517
	11th to 20th	Day	10% 16% 8% 17% 8% 7% 6% 3 4 3 17%	2520
	21st to end	Day	12% 19% 9% 13% 7% 7% 5% 4 4 5% 15%	2772
February	1st to 10th	Day	13% 19% 5% 14% 8% 6% 5% 3 5% 4 18%	1695
	11th to 20th	Day	10% 15% 6% 14% 8% 6% 5% 3 5% 5% 22%	1764
	21st to end	Day	16% 14% 5% 12% 8% 5% 5% 3 5% 5% 22%	1570
March	1st to 10th	Day	21% 15% 4 10% 6% 4 4 3 4 5% 24%	3116
	11th to 20th	Day	11% 20% 6% 13% 7% 4 4 3 4 5% 23%	3306
	21st to end	Day	14% 11% 4 9% 5% 4 4 3 4 5% 36%	4156
April	1st to 10th	Day	20% 17% 4 7% 4 4 4 3 4 5% 29%	2280
	11th to 20th	Day	19% 12% 4 7% 4 4 4 3 4 5% 34%	2254
	21st to end	Day	22% 11% 3 8% 5% 4 3 3 5% 5% 31%	2413
May	1st to 10th	Day	21% 12% 3 9% 5% 5% 4 4 5% 5% 27%	4352
	11th to 20th	Day	18% 11% 3 8% 5% 5% 5% 4 6% 7% 29%	4398
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 5% 7% 8% 35%	4837
June	1st to 10th	Day	24% 8% 3 6% 5% 5% 4 5% 6% 7% 28%	2660
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 4 6% 9% 34%	2660
	21st to end	Day	21% 8% 3 7% 6% 5% 5% 4 6% 7% 30%	2660
July	1st to 10th	Day	12% 6% 3 7% 6% 5% 6% 5% 7% 9% 34%	4440
	11th to 20th	Day	12% 6% 2 7% 5% 6% 6% 5% 8% 9% 35%	4440
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 6% 8% 40%	4814
August	1st to 10th	Day	15% 9% 3 8% 5% 5% 6% 4 6% 8% 30%	4423
	11th to 20th	Day	15% 7% 2 8% 5% 5% 5% 4 5% 8% 36%	3935
	21st to end	Day	17% 6% 2 6% 4 4 4 4 7% 8% 38%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	2280
	11th to 20th	Day	16% 7% 2 5% 3 4 3 3 4 7% 46%	2127
	21st to end	Day	13% 7% 3 7% 4 5% 4 4 7% 7% 39%	1980
October	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 7% 30%	1852
	11th to 20th	Day	17% 12% 3 8% 6% 6% 5% 4 5% 6% 28%	1778
	21st to end	Day	26% 15% 4 8% 4 4 4 2 4 4 25%	1782
November	1st to 10th	Day	16% 12% 4 8% 6% 4 5% 3 5% 8% 29%	1671
	11th to 20th	Day	16% 15% 5% 10% 4 5% 5% 3 5% 6% 26%	1645
	21st to end	Day	17% 16% 6% 12% 6% 6% 5% 4 5% 4 20%	1520
December	1st to 10th	Day	18% 19% 7% 12% 8% 5% 3 4 4 5% 15%	1520
	11th to 20th	Day	18% 16% 7% 11% 5% 4 4 4 5% 5% 21%	1906
	21st to end	Day	21% 18% 6% 12% 6% 5% 4 3 4 3 18%	2772
January	1st to 10th	Night	13% 15% 7% 15% 8% 7% 5% 4 4 5% 17%	4429
	11th to 20th	Night	10% 14% 7% 17% 10% 7% 6% 4 5% 5% 16%	4440
	21st to end	Night	16% 18% 7% 13% 8% 6% 4 3 4 4 18%	4884
February	1st to 10th	Night	12% 15% 6% 14% 9% 6% 6% 3 5% 4 19%	2483
	11th to 20th	Night	11% 12% 6% 12% 8% 7% 6% 4 5% 5% 24%	2302
	21st to end	Night	12% 15% 5% 12% 7% 5% 4 4 4 4 27%	1884
March	1st to 10th	Night	19% 11% 4 7% 6% 5% 4 2 5% 5% 32%	3724
	11th to 20th	Night	14% 19% 5% 10% 6% 5% 4 3 4 5% 26%	3652
	21st to end	Night	12% 13% 6% 11% 6% 4 4 3 4 4 34%	3476
April	1st to 10th	Night	19% 15% 3 9% 5% 5% 4 3 4 5% 26%	1900
	11th to 20th	Night	22% 11% 3 8% 5% 4 5% 3 4 7% 30%	1875
	21st to end	Night	21% 11% 4 7% 4 4 3 3 4 6% 33%	1767
May	1st to 10th	Night	22% 15% 2 7% 5% 4 5% 4 5% 6% 25%	2615
	11th to 20th	Night	20% 10% 3 7% 4 3 4 3 6% 7% 34%	2495
	21st to end	Night	14% 7% 3 7% 5% 5% 4 4 6% 8% 38%	2738
June	1st to 10th	Night	21% 8% 2 6% 5% 5% 5% 4 6% 9% 29%	1520
	11th to 20th	Night	17% 7% 3 6% 5% 4 4 4 6% 7% 38%	1520
	21st to end	Night	17% 8% 2 7% 5% 4 5% 5% 4 7% 35%	1520
July	1st to 10th	Night	12% 8% 4 8% 5% 5% 4 5% 6% 7% 35%	2520
	11th to 20th	Night	13% 5% 2 6% 4 5% 4 4 6% 9% 42%	2520
	21st to end	Night	12% 7% 3 8% 5% 4 4 4 7% 6% 40%	2739
August	1st to 10th	Night	15% 9% 3 7% 4 4 4 4 5% 7% 38%	2506
	11th to 20th	Night	14% 9% 3 8% 4 5% 5% 3 5% 8% 38%	2860
	21st to end	Night	15% 6% 2 6% 3 4 4 4 6% 7% 43%	3476
September	1st to 10th	Night	12% 7% 3 5% 5% 4 5% 3 6% 6% 44%	1900
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 5% 7% 47%	1910
	21st to end	Night	18% 6% 2 6% 5% 4 4 3 5% 7% 41%	1980
October	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 7% 6% 35%	2106
	11th to 20th	Night	18% 11% 3 7% 5% 5% 5% 4 5% 7% 30%	2182
	21st to end	Night	24% 14% 4 8% 4 4 4 3 5% 5% 26%	2574
November	1st to 10th	Night	17% 13% 4 8% 5% 5% 5% 4 5% 5% 29%	2407
	11th to 20th	Night	17% 12% 4 10% 5% 5% 5% 4 4 5% 29%	2531
	21st to end	Night	17% 14% 7% 11% 5% 5% 4 4 4 6% 23%	2660
December	1st to 10th	Night	17% 20% 7% 12% 7% 6% 4 4 4 5% 15%	2660
	11th to 20th	Night	18% 18% 6% 13% 5% 4 5% 3 4 4 19%	3288
	21st to end	Night	23% 16% 6% 14% 5% 5% 4 3 4 4 17%	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	71% 2 12% 13% 3	2518
	11th to 20th	Day	74% 1 10% 9% 6%	2520
	21st to end	Day	73% 2 11% 9% 5% 1	2772
February	1st to 10th	Day	70% 2 6% 16% 4 1	1695
	11th to 20th	Day	65% 2 8% 15% 8% 1	1773
	21st to end	Day	65% 3 8% 13% 9% 2	1570
March	1st to 10th	Day	65% 1 7% 18% 7% 1	3119
	11th to 20th	Day	65% 1 9% 13% 10% 2	3306
	21st to end	Day	53% 1 6% 20% 18% 2	4156
April	1st to 10th	Day	59% 2 5% 20% 12% 2	2280
	11th to 20th	Day	55% 1 6% 21% 15% 2	2254
	21st to end	Day	57% 2 8% 18% 12% 3	2413
May	1st to 10th	Day	60% 1 7% 20% 10% 1	4366
	11th to 20th	Day	56% 2 12% 20% 9% 1	4402
	21st to end	Day	47% 2 11% 27% 12% 1	4837
June	1st to 10th	Day	56% 3 9% 20% 12% 1	2660
	11th to 20th	Day	47% 3 10% 25% 13% 2	2660
	21st to end	Day	54% 3 11% 22% 9% 1	2660
July	1st to 10th	Day	46% 2 12% 30% 9%	4370
	11th to 20th	Day	43% 2 15% 33% 7%	4365
	21st to end	Day	42% 3 12% 35% 8%	4661
August	1st to 10th	Day	52% 2 14% 23% 9%	4409
	11th to 20th	Day	47% 2 15% 29% 6%	3977
	21st to end	Day	44% 2 17% 24% 13%	4180
September	1st to 10th	Day	41% 4 11% 30% 14%	2280
	11th to 20th	Day	40% 2 9% 35% 14% 1	2127
	21st to end	Day	44% 3 14% 28% 10% 1	1980
October	1st to 10th	Day	53% 4 9% 22% 11%	1852
	11th to 20th	Day	57% 3 14% 18% 8% 1	1778
	21st to end	Day	65% 2 9% 16% 7% 1	1782
November	1st to 10th	Day	55% 2 12% 21% 8% 1	1671
	11th to 20th	Day	60% 3 10% 16% 9% 2	1645
	21st to end	Day	67% 3 11% 13% 5%	1520
December	1st to 10th	Day	72% 3 11% 12% 2	1520
	11th to 20th	Day	67% 3 10% 15% 5%	1906
	21st to end	Day	72% 2 7% 15% 4	2772
January	1st to 10th	Night	71% 1 13% 11% 4	4429
	11th to 20th	Night	72% 2 9% 12% 5% 1	4440
	21st to end	Night	71% 2 9% 11% 6% 1	4884
February	1st to 10th	Night	69% 3 8% 14% 6% 1	2483
	11th to 20th	Night	63% 2 10% 17% 9%	2304
	21st to end	Night	61% 2 7% 16% 12% 1	1884
March	1st to 10th	Night	56% 1 8% 22% 10% 2	3734
	11th to 20th	Night	64% 1 9% 16% 9% 2	3653
	21st to end	Night	56% 1 4 21% 15% 2	3476
April	1st to 10th	Night	61% 2 7% 18% 11% 2	1900
	11th to 20th	Night	57% 2 8% 21% 11% 1	1875
	21st to end	Night	54% 2 8% 18% 14% 3	1767
May	1st to 10th	Night	61% 1 8% 18% 9% 2	2621
	11th to 20th	Night	51% 1 11% 24% 10% 2	2496
	21st to end	Night	45% 2 12% 24% 16% 1	2739
June	1st to 10th	Night	53% 3 10% 21% 12% 1	1520
	11th to 20th	Night	46% 3 12% 23% 14% 2	1520
	21st to end	Night	49% 3 13% 24% 10% 1	1520
July	1st to 10th	Night	49% 2 13% 24% 12%	2480
	11th to 20th	Night	39% 2 17% 34% 8%	2472
	21st to end	Night	44% 4 14% 27% 11%	2651
August	1st to 10th	Night	47% 2 15% 27% 9%	2496
	11th to 20th	Night	47% 2 20% 23% 8%	2884
	21st to end	Night	41% 2 16% 26% 13% 1	3476
September	1st to 10th	Night	41% 3 12% 29% 13% 1	1900
	11th to 20th	Night	38% 2 14% 34% 12% 1	1910
	21st to end	Night	44% 3 13% 27% 13% 1	1980
October	1st to 10th	Night	48% 3 11% 26% 11% 1	2106
	11th to 20th	Night	54% 2 13% 21% 10%	2182
	21st to end	Night	61% 2 11% 16% 9% 1	2574
November	1st to 10th	Night	56% 2 11% 19% 10% 1	2407
	11th to 20th	Night	59% 3 10% 18% 8% 3	2531
	21st to end	Night	64% 3 13% 16% 5%	2660
December	1st to 10th	Night	73% 3 8% 13% 3	2660
	11th to 20th	Night	69% 3 7% 14% 6%	3288
	21st to end	Night	72% 2 8% 14% 3	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	29% 21% 31% 16% 2 1	180
	11th to 20th	Day	32% 27% 29% 8% 3	180
	21st to end	Day	25% 28% 31% 12% 4	198
February	1st to 10th	Day	34% 29% 30% 5% 2	263
	11th to 20th	Day	32% 27% 33% 8% 1	371
	21st to end	Day	34% 23% 31% 11% 1	382
March	1st to 10th	Day	33% 23% 28% 11% 4	460
	11th to 20th	Day	30% 27% 30% 9% 3 1	460
	21st to end	Day	34% 22% 25% 13% 6%	506
April	1st to 10th	Day	36% 18% 30% 12% 3	460
	11th to 20th	Day	36% 18% 26% 14% 4 1	452
	21st to end	Day	33% 18% 30% 13% 5% 1	460
May	1st to 10th	Day	30% 18% 31% 14% 5% 2	463
	11th to 20th	Day	23% 19% 32% 18% 6% 2	487
	21st to end	Day	22% 17% 32% 22% 5% 2	529
June	1st to 10th	Day	31% 14% 32% 16% 5% 2	490
	11th to 20th	Day	21% 15% 34% 21% 7% 2	490
	21st to end	Day	29% 15% 31% 17% 7% 2	490
July	1st to 10th	Day	18% 11% 33% 26% 10% 3	485
	11th to 20th	Day	18% 12% 37% 25% 5% 3	484
	21st to end	Day	16% 11% 42% 23% 7% 2	532
August	1st to 10th	Day	18% 12% 33% 24% 12% 1	486
	11th to 20th	Day	19% 6% 33% 28% 10% 4	479
	21st to end	Day	19% 14% 29% 23% 11% 4	539
September	1st to 10th	Day	18% 12% 33% 20% 13% 4	490
	11th to 20th	Day	20% 10% 26% 25% 14% 5%	463
	21st to end	Day	15% 17% 31% 25% 9% 3	450
October	1st to 10th	Day	26% 16% 29% 17% 11% 2	450
	11th to 20th	Day	28% 22% 26% 14% 8% 2	439
	21st to end	Day	38% 16% 27% 15% 4	396
November	1st to 10th	Day	34% 19% 26% 14% 6% 1	365
	11th to 20th	Day	34% 28% 21% 10% 6% 2	305
	21st to end	Day	33% 23% 27% 12% 4	180
December	1st to 10th	Day	36% 22% 26% 12% 4	180
	11th to 20th	Day	36% 29% 27% 6% 3	180
	21st to end	Day	32% 32% 21% 11% 4	198
January	1st to 10th	Night	27% 26% 35% 9% 2	760
	11th to 20th	Night	28% 25% 31% 12% 3	760
	21st to end	Night	20% 26% 34% 15% 5%	836
February	1st to 10th	Night	24% 27% 34% 13% 2	760
	11th to 20th	Night	24% 30% 32% 14% 1	743
	21st to end	Night	26% 26% 31% 15% 2	628
March	1st to 10th	Night	30% 23% 27% 16% 3	760
	11th to 20th	Night	28% 25% 29% 16% 2 1	760
	21st to end	Night	29% 19% 28% 19% 5%	836
April	1st to 10th	Night	32% 20% 30% 15% 3	760
	11th to 20th	Night	31% 21% 25% 18% 5%	751
	21st to end	Night	29% 17% 30% 18% 6%	627
May	1st to 10th	Night	31% 13% 29% 21% 6%	442
	11th to 20th	Night	24% 14% 31% 24% 7% 1	374
	21st to end	Night	18% 14% 37% 21% 9%	411
June	1st to 10th	Night	28% 11% 31% 23% 7% 1	380
	11th to 20th	Night	20% 10% 32% 28% 9% 1	380
	21st to end	Night	22% 11% 33% 24% 10% 1	380
July	1st to 10th	Night	13% 11% 37% 29% 10% 1	370
	11th to 20th	Night	10% 8% 37% 33% 11% 1	368
	21st to end	Night	16% 9% 35% 30% 10% 1	416
August	1st to 10th	Night	16% 7% 29% 34% 13% 1	378
	11th to 20th	Night	16% 5% 26% 35% 17% 2	597
	21st to end	Night	18% 9% 30% 29% 13% 1	836
September	1st to 10th	Night	17% 10% 34% 28% 10% 1	760
	11th to 20th	Night	18% 9% 26% 29% 14% 3	736
	21st to end	Night	22% 10% 29% 22% 15% 2	720
October	1st to 10th	Night	26% 15% 28% 18% 12% 1	720
	11th to 20th	Night	28% 18% 27% 21% 6% 1	720
	21st to end	Night	33% 19% 29% 16% 4	792
November	1st to 10th	Night	29% 21% 28% 17% 5% 1	740
	11th to 20th	Night	29% 22% 28% 11% 7% 1	760
	21st to end	Night	29% 25% 25% 16% 5% 1	760
December	1st to 10th	Night	32% 24% 27% 14% 3	760
	11th to 20th	Night	30% 27% 25% 14% 3 1	760
	21st to end	Night	33% 25% 26% 12% 3	836

Page generated at 2024-09-11 10:00:00. Version ac07

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