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 2025-02-21.

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	15% 11% 4 9% 6% 5% 5% 4 5% 6% 29%	108590
	Night	15% 12% 5% 11% 6% 5% 5% 3 5% 6% 28%	110001
Transparency	Day	56% 2 10% 22% 9% 1	112083
	Night	59% 2 10% 19% 9% 1	114659
Seeing	Day	27% 18% 30% 17% 6% 2	14866
	Night	26% 19% 30% 19% 6% 1	24014

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	12% 13% 8% 19% 10% 8% 5% 3 4 5% 13%	3597
	11th to 20th	Day	8% 16% 8% 19% 10% 6% 6% 3 4 4 16%	3570
	21st to end	Day	10% 17% 8% 14% 7% 7% 6% 4 5% 5% 18%	3960
February	1st to 10th	Day	12% 19% 5% 15% 8% 6% 5% 3 5% 4 18%	1785
	11th to 20th	Day	10% 14% 6% 15% 8% 6% 6% 3 5% 5% 21%	1860
	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	15% 7% 2 5% 3 4 3 3 4 7% 46%	2205
	21st to end	Day	14% 7% 3 7% 4 5% 4 4 7% 7% 40%	2090
October	1st to 10th	Day	20% 9% 3 6% 5% 5% 5% 4 6% 7% 31%	1954
	11th to 20th	Day	16% 13% 3 8% 6% 6% 5% 4 5% 6% 30%	1876
	21st to end	Day	24% 15% 4 8% 4 4 4 2 4 5% 26%	1881
November	1st to 10th	Day	16% 12% 4 8% 6% 4 4 3 5% 8% 29%	1761
	11th to 20th	Day	16% 14% 5% 10% 5% 5% 5% 3 5% 6% 27%	1731
	21st to end	Day	17% 16% 6% 12% 6% 6% 4 4 5% 4 20%	1600
December	1st to 10th	Day	13% 22% 8% 17% 9% 5% 4 3 4 4 11%	2600
	11th to 20th	Day	16% 14% 7% 14% 5% 5% 4 4 5% 5% 22%	2986
	21st to end	Day	16% 18% 6% 12% 6% 5% 4 3 4 4 22%	3960
January	1st to 10th	Night	10% 14% 7% 20% 13% 7% 4 3 3 4 14%	6349
	11th to 20th	Night	8% 15% 8% 19% 10% 7% 6% 4 5% 5% 13%	6315
	21st to end	Night	11% 14% 6% 14% 10% 8% 5% 4 5% 5% 18%	6996
February	1st to 10th	Night	12% 15% 6% 15% 9% 7% 6% 3 5% 4 19%	2613
	11th to 20th	Night	11% 12% 5% 13% 8% 8% 6% 4 5% 5% 23%	2426
	21st to end	Night	12% 15% 5% 12% 7% 5% 4 4 4 4 27%	1885
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 4 3 5% 7% 47%	1974
	21st to end	Night	17% 6% 2 6% 5% 4 4 3 5% 7% 42%	2090
October	1st to 10th	Night	20% 8% 2 6% 4 3 4 4 7% 6% 35%	2223
	11th to 20th	Night	18% 11% 3 7% 5% 5% 5% 4 5% 7% 31%	2304
	21st to end	Night	23% 14% 4 7% 4 4 4 3 5% 5% 27%	2717
November	1st to 10th	Night	17% 13% 4 8% 5% 5% 5% 4 5% 5% 29%	2537
	11th to 20th	Night	17% 12% 4 10% 5% 5% 5% 4 4 5% 29%	2665
	21st to end	Night	17% 15% 7% 11% 5% 5% 4 4 4 6% 22%	2800
December	1st to 10th	Night	14% 21% 8% 16% 7% 6% 5% 3 4 4 11%	4580
	11th to 20th	Night	17% 18% 7% 13% 5% 4 4 3 4 4 20%	5208
	21st to end	Night	19% 16% 6% 13% 5% 5% 5% 3 4 4 22%	6996

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	3598
	11th to 20th	Day	75% 1 7% 9% 7% 1	3600
	21st to end	Day	70% 1 9% 10% 8% 1	3960
February	1st to 10th	Day	70% 2 5% 16% 5% 1	1785
	11th to 20th	Day	66% 2 8% 15% 8% 1	1869
	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	39% 2 10% 34% 14% 1	2205
	21st to end	Day	43% 3 14% 28% 11% 1	2090
October	1st to 10th	Day	49% 2 10% 29% 10%	3240
	11th to 20th	Day	50% 1 14% 24% 10%	3092
	21st to end	Day	51% 1 16% 24% 6% 1	3081
November	1st to 10th	Day	55% 2 12% 22% 8% 1	1761
	11th to 20th	Day	59% 3 10% 16% 10% 2	1731
	21st to end	Day	68% 3 11% 13% 5%	1600
December	1st to 10th	Day	78% 2 9% 10% 1	2600
	11th to 20th	Day	65% 2 8% 17% 7%	2986
	21st to end	Day	67% 2 8% 16% 8%	3960
January	1st to 10th	Night	76% 1 10% 10% 3	6349
	11th to 20th	Night	74% 1 7% 11% 5%	6360
	21st to end	Night	70% 1 7% 11% 11% 1	6996
February	1st to 10th	Night	69% 3 7% 14% 6% 1	2613
	11th to 20th	Night	63% 2 9% 16% 8%	2428
	21st to end	Night	61% 2 7% 16% 12% 1	1885
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% 33% 13% 1	1974
	21st to end	Night	43% 3 13% 27% 14% 1	2090
October	1st to 10th	Night	48% 2 11% 29% 10%	3704
	11th to 20th	Night	50% 2 12% 25% 12%	3827
	21st to end	Night	54% 1 14% 20% 10% 1	4467
November	1st to 10th	Night	56% 2 11% 19% 10% 1	2537
	11th to 20th	Night	58% 3 9% 18% 9% 3	2665
	21st to end	Night	64% 2 12% 16% 5%	2800
December	1st to 10th	Night	78% 2 9% 10% 2	4580
	11th to 20th	Night	69% 2 6% 13% 9%	5208
	21st to end	Night	68% 2 7% 13% 9% 1	6996

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% 23% 33% 15% 2 1	200
	11th to 20th	Day	31% 28% 31% 8% 3	200
	21st to end	Day	28% 28% 30% 11% 3	220
February	1st to 10th	Day	35% 29% 29% 5% 1	293
	11th to 20th	Day	33% 28% 32% 7%	407
	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	19% 9% 24% 27% 15% 5%	490
	21st to end	Day	16% 16% 31% 25% 9% 3	490
October	1st to 10th	Day	26% 16% 30% 17% 10% 2	490
	11th to 20th	Day	28% 22% 26% 15% 8% 2	477
	21st to end	Day	37% 17% 26% 15% 4	429
November	1st to 10th	Day	35% 19% 26% 14% 5% 1	395
	11th to 20th	Day	33% 27% 23% 10% 6% 2	331
	21st to end	Day	35% 25% 26% 11% 4	200
December	1st to 10th	Day	38% 22% 26% 12% 4	200
	11th to 20th	Day	37% 28% 26% 7% 3	200
	21st to end	Day	29% 30% 23% 13% 5%	220
January	1st to 10th	Night	26% 27% 36% 9% 2	800
	11th to 20th	Night	28% 25% 32% 12% 3	800
	21st to end	Night	20% 27% 34% 15% 5%	880
February	1st to 10th	Night	25% 27% 33% 13% 2	800
	11th to 20th	Night	25% 29% 31% 13% 1	783
	21st to end	Night	26% 26% 31% 15% 2	629
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% 16% 4	760
	21st to end	Night	22% 10% 29% 22% 16% 1	760
October	1st to 10th	Night	26% 14% 29% 18% 12% 1	760
	11th to 20th	Night	28% 18% 27% 20% 6% 1	760
	21st to end	Night	33% 19% 29% 16% 4	836
November	1st to 10th	Night	30% 21% 28% 16% 5% 1	780
	11th to 20th	Night	29% 22% 28% 12% 7% 1	800
	21st to end	Night	30% 25% 25% 15% 5% 1	800
December	1st to 10th	Night	32% 24% 28% 14% 3	800
	11th to 20th	Night	31% 27% 24% 15% 3 1	800
	21st to end	Night	32% 24% 27% 13% 4	880

Page generated at 2025-02-18 03:26:26. Version ac07

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