Pilot's Cross Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pilot's Cross Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 9% 3 8% 5% 4 4 4 5% 7% 38%	120955
	Night	12% 9% 4 8% 5% 4 4 4 5% 6% 38%	118402
Transparency	Day	44% 6% 13% 25% 11% 1	124564
	Night	45% 5% 13% 24% 12% 2	122936
Seeing	Day	16% 9% 30% 27% 14% 4	17022
	Night	16% 8% 31% 33% 11% 1	26906

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	8% 14% 6% 17% 6% 5% 5% 3 4 6% 25%	3617
	11th to 20th	Day	6% 11% 6% 13% 7% 6% 9% 8% 8% 5% 22%	3638
	21st to end	Day	6% 16% 7% 13% 5% 4 4 3 5% 5% 32%	4488
February	1st to 10th	Day	8% 13% 5% 11% 7% 4 5% 4 5% 6% 32%	1800
	11th to 20th	Day	8% 11% 5% 10% 6% 5% 4 4 5% 6% 36%	1754
	21st to end	Day	9% 12% 5% 10% 6% 5% 4 3 6% 8% 33%	1536
March	1st to 10th	Day	12% 9% 3 6% 3 4 4 3 5% 8% 44%	4837
	11th to 20th	Day	10% 10% 3 7% 4 5% 4 3 6% 7% 41%	4980
	21st to end	Day	21% 11% 4 10% 5% 4 4 4 4 5% 27%	5522
April	1st to 10th	Day	17% 10% 3 7% 5% 4 4 3 5% 7% 35%	2400
	11th to 20th	Day	19% 13% 3 7% 5% 4 4 4 3 5% 33%	2398
	21st to end	Day	21% 10% 3 7% 5% 5% 4 3 4 6% 31%	2600
May	1st to 10th	Day	14% 5% 2 5% 4 4 3 3 4 6% 50%	6131
	11th to 20th	Day	18% 7% 2 6% 4 3 3 3 5% 6% 43%	6319
	21st to end	Day	17% 7% 3 7% 4 4 4 3 5% 8% 38%	6935
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 9% 45%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 36%	2828
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 5% 8% 9% 41%	3000
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 39%	4713
	11th to 20th	Day	6% 3 2 4 3 3 3 3 6% 8% 60%	4580
	21st to end	Day	7% 4 2 5% 4 4 4 4 7% 9% 50%	4768
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 9% 44%	4104
	11th to 20th	Day	13% 5% 2 6% 4 5% 4 4 7% 8% 44%	4041
	21st to end	Day	10% 6% 2 7% 4 4 4 4 5% 7% 47%	4347
September	1st to 10th	Day	11% 4 2 6% 4 4 4 4 6% 8% 48%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 8% 46%	2247
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 45%	2001
October	1st to 10th	Day	17% 7% 3 5% 4 4 4 3 5% 6% 42%	1900
	11th to 20th	Day	15% 11% 3 7% 4 4 3 3 5% 6% 39%	1900
	21st to end	Day	17% 11% 3 8% 3 3 4 3 4 5% 38%	2062
November	1st to 10th	Day	12% 13% 3 8% 4 4 5% 3 5% 6% 37%	1629
	11th to 20th	Day	12% 14% 6% 11% 6% 4 4 3 4 6% 29%	1600
	21st to end	Day	13% 12% 4 8% 4 4 5% 4 5% 7% 33%	1600
December	1st to 10th	Day	8% 10% 5% 15% 10% 6% 6% 4 6% 6% 23%	2610
	11th to 20th	Day	10% 17% 6% 11% 5% 4 4 3 5% 5% 29%	3008
	21st to end	Day	20% 20% 8% 19% 5% 4 3 2 2 4 14%	3982
January	1st to 10th	Night	8% 11% 5% 15% 8% 5% 4 3 4 5% 32%	6348
	11th to 20th	Night	7% 11% 5% 11% 7% 7% 10% 8% 7% 5% 21%	6337
	21st to end	Night	8% 19% 6% 9% 5% 4 4 3 5% 5% 33%	6490
February	1st to 10th	Night	8% 12% 4 10% 7% 5% 4 3 5% 6% 36%	2600
	11th to 20th	Night	6% 10% 4 8% 6% 5% 6% 4 5% 6% 40%	2533
	21st to end	Night	12% 10% 3 9% 6% 5% 5% 2 5% 7% 37%	2093
March	1st to 10th	Night	10% 7% 3 6% 4 4 4 3 5% 7% 48%	5057
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 8% 9% 39%	4980
	21st to end	Night	25% 11% 3 9% 4 4 3 3 4 5% 28%	5406
April	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 5% 6% 38%	2000
	11th to 20th	Night	20% 10% 4 7% 4 4 3 3 5% 6% 36%	1947
	21st to end	Night	20% 10% 3 6% 4 4 4 4 4 6% 35%	1800
May	1st to 10th	Night	11% 7% 3 5% 4 3 4 3 4 5% 52%	3782
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 5% 7% 49%	3596
	21st to end	Night	16% 8% 3 6% 4 4 4 4 4 7% 40%	3942
June	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 48%	1600
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 5% 6% 8% 46%	1563
	21st to end	Night	8% 4 1 5% 3 4 4 4 8% 8% 51%	1400
July	1st to 10th	Night	11% 4 2 4 3 4 5% 4 7% 9% 47%	2467
	11th to 20th	Night	4 3 1 4 4 3 3 3 5% 9% 60%	2610
	21st to end	Night	5% 4 1 4 3 3 4 3 5% 8% 59%	2781
August	1st to 10th	Night	12% 5% 2 6% 4 4 3 3 4 7% 50%	2836
	11th to 20th	Night	13% 4 1 5% 3 3 4 4 5% 8% 50%	2798
	21st to end	Night	9% 6% 2 7% 4 4 5% 3 5% 7% 47%	3313
September	1st to 10th	Night	9% 4 1 5% 4 4 3 3 5% 8% 54%	1900
	11th to 20th	Night	9% 5% 1 4 3 4 4 3 6% 6% 55%	1928
	21st to end	Night	14% 4 1 4 4 4 4 3 5% 6% 52%	2159
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 6% 50%	2280
	11th to 20th	Night	15% 10% 3 6% 4 4 4 3 6% 6% 40%	2280
	21st to end	Night	18% 11% 3 7% 4 3 4 3 4 6% 38%	2534
November	1st to 10th	Night	13% 10% 2 7% 4 4 4 4 5% 6% 39%	2667
	11th to 20th	Night	13% 12% 5% 10% 5% 4 4 3 4 6% 34%	2800
	21st to end	Night	13% 10% 4 9% 6% 5% 4 3 5% 7% 35%	2800
December	1st to 10th	Night	7% 9% 4 14% 7% 7% 6% 4 6% 7% 29%	4580
	11th to 20th	Night	10% 17% 6% 13% 6% 5% 4 4 5% 4 25%	5199
	21st to end	Night	20% 17% 8% 17% 5% 4 3 2 3 4 18%	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	61% 4 10% 18% 7%	3618
	11th to 20th	Day	59% 6% 10% 17% 9%	3638
	21st to end	Day	55% 4 16% 17% 8%	4488
February	1st to 10th	Day	53% 5% 14% 19% 8% 2	1800
	11th to 20th	Day	48% 6% 12% 25% 8% 1	1763
	21st to end	Day	51% 5% 13% 21% 10% 1	1536
March	1st to 10th	Day	39% 5% 19% 24% 11% 2	4837
	11th to 20th	Day	42% 5% 14% 29% 9% 1	4980
	21st to end	Day	59% 4 10% 19% 7% 1	5524
April	1st to 10th	Day	49% 5% 12% 21% 13% 1	2400
	11th to 20th	Day	54% 5% 9% 18% 12% 1	2398
	21st to end	Day	54% 6% 12% 19% 9%	2600
May	1st to 10th	Day	35% 5% 13% 28% 17% 2	6169
	11th to 20th	Day	41% 5% 10% 22% 15% 5%	6323
	21st to end	Day	44% 6% 10% 29% 11% 1	6949
June	1st to 10th	Day	34% 8% 14% 31% 12% 2	2800
	11th to 20th	Day	44% 7% 14% 21% 13% 1	2828
	21st to end	Day	36% 8% 16% 28% 10% 2	3000
July	1st to 10th	Day	38% 8% 18% 24% 11% 1	4644
	11th to 20th	Day	22% 6% 16% 37% 18% 1	4505
	21st to end	Day	30% 8% 16% 28% 17% 1	4618
August	1st to 10th	Day	37% 5% 14% 30% 12% 2	4090
	11th to 20th	Day	36% 7% 14% 34% 10%	4080
	21st to end	Day	36% 6% 17% 28% 13% 1	4346
September	1st to 10th	Day	34% 7% 14% 34% 11%	2280
	11th to 20th	Day	35% 7% 17% 32% 8%	2247
	21st to end	Day	38% 6% 12% 32% 11%	2001
October	1st to 10th	Day	32% 3 9% 35% 21%	3160
	11th to 20th	Day	39% 4 12% 29% 15% 1	3140
	21st to end	Day	44% 4 14% 28% 9%	3376
November	1st to 10th	Day	49% 6% 12% 19% 12% 2	1629
	11th to 20th	Day	58% 5% 13% 16% 9% 1	1600
	21st to end	Day	50% 7% 15% 20% 6% 2	1600
December	1st to 10th	Day	62% 6% 15% 13% 4 1	2610
	11th to 20th	Day	58% 4 14% 19% 6%	3005
	21st to end	Day	77% 3 8% 8% 4	3982
January	1st to 10th	Night	56% 4 12% 20% 8%	6348
	11th to 20th	Night	60% 5% 12% 13% 10%	6337
	21st to end	Night	54% 5% 11% 23% 6%	6490
February	1st to 10th	Night	49% 5% 13% 22% 10% 1	2600
	11th to 20th	Night	44% 6% 14% 25% 10% 1	2534
	21st to end	Night	48% 5% 13% 21% 12% 1	2093
March	1st to 10th	Night	36% 4 14% 26% 17% 3	5057
	11th to 20th	Night	37% 6% 17% 26% 13% 1	4980
	21st to end	Night	60% 3 10% 17% 9% 1	5406
April	1st to 10th	Night	48% 4 9% 23% 15% 1	2000
	11th to 20th	Night	49% 5% 10% 20% 14% 2	1947
	21st to end	Night	49% 6% 9% 20% 14% 2	1800
May	1st to 10th	Night	35% 5% 11% 27% 21% 2	3809
	11th to 20th	Night	34% 5% 11% 23% 18% 8%	3597
	21st to end	Night	44% 6% 10% 26% 13% 1	3950
June	1st to 10th	Night	32% 7% 14% 28% 17% 2	1600
	11th to 20th	Night	34% 7% 16% 24% 17% 2	1563
	21st to end	Night	28% 5% 16% 31% 16% 3	1400
July	1st to 10th	Night	31% 8% 20% 25% 14% 1	2427
	11th to 20th	Night	22% 5% 15% 38% 19% 2	2562
	21st to end	Night	24% 6% 16% 33% 19% 3	2692
August	1st to 10th	Night	36% 5% 12% 29% 14% 5%	2826
	11th to 20th	Night	33% 6% 14% 32% 15% 1	2825
	21st to end	Night	35% 7% 16% 29% 11% 2	3313
September	1st to 10th	Night	29% 6% 14% 33% 17% 1	1900
	11th to 20th	Night	28% 6% 17% 31% 16%	1928
	21st to end	Night	34% 5% 13% 32% 15% 1	2159
October	1st to 10th	Night	24% 3 7% 35% 27% 4	3800
	11th to 20th	Night	35% 5% 12% 25% 21% 3	3779
	21st to end	Night	44% 4 12% 27% 12%	4170
November	1st to 10th	Night	44% 6% 12% 23% 13% 1	2667
	11th to 20th	Night	53% 5% 12% 19% 11% 1	2800
	21st to end	Night	50% 5% 16% 22% 6% 1	2800
December	1st to 10th	Night	54% 6% 17% 18% 4	4580
	11th to 20th	Night	62% 4 10% 17% 5% 1	5201
	21st to end	Night	71% 3 9% 12% 4	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	12% 11% 33% 28% 11% 4	300
	11th to 20th	Day	17% 8% 39% 30% 5% 1	300
	21st to end	Day	16% 9% 40% 22% 10% 2	330
February	1st to 10th	Day	17% 15% 39% 23% 6%	300
	11th to 20th	Day	16% 14% 39% 25% 5% 1	300
	21st to end	Day	14% 15% 42% 25% 3	249
March	1st to 10th	Day	18% 12% 42% 21% 7%	300
	11th to 20th	Day	18% 10% 40% 20% 10% 1	300
	21st to end	Day	26% 13% 30% 21% 8% 3	352
April	1st to 10th	Day	23% 15% 33% 23% 6%	500
	11th to 20th	Day	29% 12% 27% 21% 9% 1	516
	21st to end	Day	28% 11% 30% 21% 9% 2	700
May	1st to 10th	Day	18% 14% 29% 26% 11% 2	703
	11th to 20th	Day	19% 12% 28% 24% 13% 3	725
	21st to end	Day	17% 8% 27% 27% 16% 4	789
June	1st to 10th	Day	11% 7% 28% 30% 19% 5%	730
	11th to 20th	Day	15% 7% 34% 32% 10% 3	730
	21st to end	Day	15% 6% 29% 29% 16% 5%	730
July	1st to 10th	Day	10% 3 19% 33% 23% 11%	721
	11th to 20th	Day	9% 4 27% 30% 21% 8%	718
	21st to end	Day	6% 4 24% 34% 24% 8%	740
August	1st to 10th	Day	10% 4 23% 34% 24% 6%	676
	11th to 20th	Day	13% 4 20% 32% 23% 7%	665
	21st to end	Day	11% 6% 24% 31% 22% 6%	641
September	1st to 10th	Day	14% 6% 26% 30% 21% 3	490
	11th to 20th	Day	12% 5% 25% 32% 20% 5%	457
	21st to end	Day	16% 11% 30% 23% 12% 8%	300
October	1st to 10th	Day	19% 9% 27% 22% 17% 6%	300
	11th to 20th	Day	17% 14% 37% 23% 8% 1	300
	21st to end	Day	22% 14% 39% 18% 6% 1	330
November	1st to 10th	Day	21% 12% 33% 24% 9% 2	300
	11th to 20th	Day	18% 17% 40% 20% 5%	300
	21st to end	Day	20% 13% 42% 21% 4	300
December	1st to 10th	Day	12% 12% 42% 23% 11%	300
	11th to 20th	Day	20% 16% 34% 27% 3	300
	21st to end	Day	28% 15% 32% 22% 3 1	330
January	1st to 10th	Night	13% 12% 33% 33% 9% 1	1000
	11th to 20th	Night	16% 11% 36% 30% 7% 1	1000
	21st to end	Night	18% 10% 36% 29% 6%	1100
February	1st to 10th	Night	13% 12% 38% 29% 8%	1000
	11th to 20th	Night	11% 11% 42% 28% 7% 1	979
	21st to end	Night	17% 10% 35% 29% 9%	773
March	1st to 10th	Night	16% 10% 35% 31% 8%	665
	11th to 20th	Night	17% 9% 40% 27% 7%	600
	21st to end	Night	23% 10% 30% 25% 10% 2	660
April	1st to 10th	Night	21% 11% 32% 29% 7%	600
	11th to 20th	Night	24% 8% 32% 28% 7%	593
	21st to end	Night	25% 8% 30% 27% 9% 1	600
May	1st to 10th	Night	16% 11% 34% 31% 7% 1	600
	11th to 20th	Night	16% 8% 32% 29% 13% 1	591
	21st to end	Night	14% 7% 25% 38% 16%	650
June	1st to 10th	Night	11% 5% 27% 41% 15% 1	600
	11th to 20th	Night	11% 4 30% 45% 10%	600
	21st to end	Night	10% 4 24% 43% 17% 2	600
July	1st to 10th	Night	7% 1 19% 44% 28% 2	585
	11th to 20th	Night	8% 3 21% 46% 21% 1	582
	21st to end	Night	5% 2 22% 49% 21% 2	624
August	1st to 10th	Night	8% 4 19% 50% 19% 1	567
	11th to 20th	Night	13% 2 18% 35% 28% 3	564
	21st to end	Night	8% 4 24% 37% 24% 2	627
September	1st to 10th	Night	11% 3 23% 41% 19% 3	570
	11th to 20th	Night	11% 5% 27% 37% 18% 3	570
	21st to end	Night	15% 7% 29% 32% 15% 1	639
October	1st to 10th	Night	18% 8% 28% 33% 12% 1	760
	11th to 20th	Night	20% 9% 33% 30% 8% 1	760
	21st to end	Night	25% 9% 26% 33% 7%	836
November	1st to 10th	Night	19% 10% 30% 32% 8% 1	911
	11th to 20th	Night	19% 12% 34% 27% 8%	1000
	21st to end	Night	18% 12% 37% 26% 7%	1000
December	1st to 10th	Night	15% 10% 37% 30% 7% 1	1000
	11th to 20th	Night	19% 10% 38% 27% 7%	1000
	21st to end	Night	25% 10% 33% 26% 6% 1	1100

Page generated at 2025-07-18 00:09:57. Version ac07

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