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 2024-03-28.

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% 4 7% 6% 3 5% 5% 5% 7% 37%	78989
	Night	12% 9% 4 7% 6% 3 5% 4 5% 6% 38%	81039
Transparency	Day	46% 6% 14% 23% 10% 1	78691
	Night	46% 6% 13% 23% 11% 1	80857
Seeing	Day	16% 9% 30% 27% 14% 4	15075
	Night	16% 8% 31% 33% 11% 1	25194

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% 16% 8% 14% 7% 3 5% 4 4 6% 22%	2527
	11th to 20th	Day	6% 12% 5% 12% 10% 5% 9% 8% 5% 5% 22%	2534
	21st to end	Day	8% 19% 8% 12% 6% 4 5% 4 6% 6% 24%	3135
February	1st to 10th	Day	8% 12% 6% 9% 7% 3 5% 6% 5% 6% 33%	1710
	11th to 20th	Day	8% 11% 5% 9% 7% 3 6% 4 5% 6% 36%	1664
	21st to end	Day	10% 12% 6% 9% 6% 4 4 4 5% 8% 32%	1461
March	1st to 10th	Day	13% 12% 5% 7% 6% 3 5% 4 4 7% 33%	2042
	11th to 20th	Day	14% 9% 3 6% 5% 4 4 4 5% 7% 38%	2090
	21st to end	Day	21% 10% 5% 7% 6% 3 5% 4 4 6% 30%	2274
April	1st to 10th	Day	17% 9% 4 7% 6% 3 4 4 5% 7% 35%	2160
	11th to 20th	Day	20% 12% 4 6% 6% 3 5% 4 3 5% 33%	2155
	21st to end	Day	22% 9% 4 6% 6% 3 4 4 4 6% 31%	2340
May	1st to 10th	Day	18% 7% 3 6% 7% 4 5% 5% 5% 6% 35%	2441
	11th to 20th	Day	18% 7% 2 6% 5% 3 4 5% 5% 7% 38%	2479
	21st to end	Day	14% 6% 4 6% 6% 3 5% 5% 5% 8% 38%	2725
June	1st to 10th	Day	13% 4 2 5% 4 3 4 5% 6% 9% 45%	2520
	11th to 20th	Day	12% 6% 3 6% 6% 3 5% 5% 6% 9% 39%	2544
	21st to end	Day	10% 4 2 5% 5% 4 5% 6% 8% 9% 42%	2700
July	1st to 10th	Day	8% 3 1 4 4 3 5% 5% 7% 10% 50%	2597
	11th to 20th	Day	9% 3 2 4 5% 2 4 4 6% 9% 51%	2520
	21st to end	Day	7% 4 2 4 4 3 5% 5% 7% 9% 49%	2723
August	1st to 10th	Day	9% 5% 3 5% 6% 4 5% 5% 8% 9% 42%	2328
	11th to 20th	Day	13% 5% 2 4 5% 3 4 5% 6% 7% 45%	2301
	21st to end	Day	10% 5% 2 4 4 3 4 5% 5% 7% 52%	2473
September	1st to 10th	Day	11% 4 2 5% 5% 3 4 5% 6% 8% 46%	2160
	11th to 20th	Day	13% 4 2 4 5% 3 4 5% 6% 8% 46%	2129
	21st to end	Day	16% 6% 2 5% 4 2 4 5% 5% 7% 43%	1896
October	1st to 10th	Day	18% 7% 3 5% 5% 4 5% 4 5% 6% 40%	1800
	11th to 20th	Day	16% 11% 3 6% 5% 3 4 4 5% 6% 38%	1800
	21st to end	Day	17% 10% 4 7% 5% 2 4 3 4 5% 38%	1954
November	1st to 10th	Day	12% 12% 4 7% 4 3 5% 4 5% 5% 37%	1546
	11th to 20th	Day	12% 13% 7% 10% 8% 3 5% 4 4 6% 29%	1520
	21st to end	Day	14% 11% 4 7% 5% 3 5% 5% 5% 8% 33%	1520
December	1st to 10th	Day	12% 12% 7% 9% 8% 4 5% 6% 5% 7% 26%	1520
	11th to 20th	Day	11% 14% 6% 10% 7% 4 6% 4 4 6% 28%	1918
	21st to end	Day	21% 16% 7% 14% 6% 3 4 3 2 5% 19%	2783
January	1st to 10th	Night	12% 13% 7% 12% 7% 3 4 4 4 5% 29%	4428
	11th to 20th	Night	8% 11% 5% 11% 10% 5% 9% 8% 6% 6% 22%	4431
	21st to end	Night	10% 21% 8% 10% 6% 3 4 4 4 4 27%	4532
February	1st to 10th	Night	7% 11% 5% 9% 7% 4 5% 4 5% 6% 37%	2470
	11th to 20th	Night	6% 9% 4 7% 6% 4 5% 5% 5% 7% 40%	2403
	21st to end	Night	12% 9% 4 8% 7% 3 5% 4 4 7% 37%	1992
March	1st to 10th	Night	12% 10% 4 6% 6% 3 5% 5% 5% 6% 38%	2152
	11th to 20th	Night	13% 9% 4 7% 5% 3 4 4 5% 6% 40%	2090
	21st to end	Night	20% 9% 4 6% 5% 3 4 4 4 6% 36%	2235
April	1st to 10th	Night	16% 9% 3 6% 4 3 4 4 5% 7% 37%	1800
	11th to 20th	Night	21% 9% 5% 6% 5% 3 4 4 4 6% 35%	1751
	21st to end	Night	20% 10% 4 5% 4 3 4 5% 4 6% 35%	1620
May	1st to 10th	Night	17% 8% 3 6% 5% 2 4 4 5% 6% 40%	1517
	11th to 20th	Night	15% 5% 4 5% 5% 3 5% 4 4 7% 43%	1416
	21st to end	Night	12% 6% 2 5% 5% 3 5% 5% 5% 8% 44%	1551
June	1st to 10th	Night	11% 4 2 4 5% 2 5% 4 6% 8% 49%	1440
	11th to 20th	Night	8% 5% 2 6% 5% 3 5% 5% 6% 8% 47%	1407
	21st to end	Night	7% 4 1 4 4 3 4 4 7% 9% 52%	1260
July	1st to 10th	Night	5% 2 1 3 3 3 4 5% 6% 9% 58%	1354
	11th to 20th	Night	6% 3 1 4 3 3 4 4 5% 10% 57%	1440
	21st to end	Night	4 3 1 3 4 2 4 4 6% 8% 60%	1593
August	1st to 10th	Night	7% 4 2 5% 4 3 3 5% 5% 8% 53%	1611
	11th to 20th	Night	12% 4 1 4 4 3 4 4 5% 7% 53%	1600
	21st to end	Night	8% 4 1 5% 4 2 4 4 5% 6% 56%	1882
September	1st to 10th	Night	10% 4 2 4 5% 3 3 4 5% 8% 53%	1800
	11th to 20th	Night	10% 4 2 3 4 3 4 3 6% 7% 54%	1826
	21st to end	Night	15% 4 2 4 4 3 4 4 5% 6% 50%	2045
October	1st to 10th	Night	15% 6% 2 4 5% 3 3 4 4 6% 48%	2160
	11th to 20th	Night	16% 10% 3 5% 5% 3 4 4 6% 6% 39%	2160
	21st to end	Night	18% 10% 3 6% 5% 2 3 4 4 6% 38%	2400
November	1st to 10th	Night	13% 9% 3 6% 5% 3 5% 5% 5% 6% 40%	2530
	11th to 20th	Night	14% 11% 5% 10% 6% 3 4 4 4 5% 33%	2660
	21st to end	Night	14% 9% 4 7% 7% 3 4 4 5% 7% 37%	2660
December	1st to 10th	Night	11% 12% 5% 11% 8% 3 5% 4 5% 6% 31%	2660
	11th to 20th	Night	11% 14% 6% 9% 8% 4 5% 5% 5% 6% 28%	3279
	21st to end	Night	21% 15% 5% 11% 6% 3 4 3 3 5% 23%	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	64% 6% 11% 16% 4	2528
	11th to 20th	Day	61% 7% 12% 15% 5%	2534
	21st to end	Day	61% 5% 14% 16% 4	3135
February	1st to 10th	Day	52% 5% 15% 20% 8% 1	1710
	11th to 20th	Day	49% 6% 12% 24% 8% 1	1673
	21st to end	Day	52% 5% 12% 20% 10% 1	1461
March	1st to 10th	Day	51% 6% 15% 20% 8% 1	2042
	11th to 20th	Day	46% 6% 15% 24% 8% 1	2090
	21st to end	Day	56% 5% 10% 21% 6% 1	2274
April	1st to 10th	Day	49% 5% 12% 21% 12% 1	2160
	11th to 20th	Day	54% 5% 9% 19% 11% 2	2155
	21st to end	Day	54% 6% 12% 19% 9%	2340
May	1st to 10th	Day	49% 6% 12% 21% 10% 2	2441
	11th to 20th	Day	45% 6% 14% 23% 11% 1	2483
	21st to end	Day	44% 8% 11% 23% 13% 1	2725
June	1st to 10th	Day	34% 8% 15% 31% 11% 1	2520
	11th to 20th	Day	42% 7% 14% 23% 13% 1	2544
	21st to end	Day	35% 9% 17% 29% 9% 2	2700
July	1st to 10th	Day	27% 8% 17% 32% 14% 1	2528
	11th to 20th	Day	29% 8% 16% 27% 19% 3	2445
	21st to end	Day	29% 8% 18% 27% 16% 2	2572
August	1st to 10th	Day	37% 7% 16% 28% 9% 3	2314
	11th to 20th	Day	36% 8% 16% 30% 11%	2301
	21st to end	Day	30% 7% 14% 32% 16% 1	2473
September	1st to 10th	Day	36% 7% 14% 32% 11%	2160
	11th to 20th	Day	35% 7% 17% 32% 8%	2129
	21st to end	Day	40% 6% 12% 32% 9%	1896
October	1st to 10th	Day	45% 5% 13% 26% 11%	1800
	11th to 20th	Day	46% 6% 12% 24% 11% 1	1800
	21st to end	Day	50% 5% 12% 23% 9%	1954
November	1st to 10th	Day	49% 6% 12% 19% 12% 2	1546
	11th to 20th	Day	58% 5% 13% 15% 9%	1520
	21st to end	Day	49% 7% 16% 21% 6% 2	1520
December	1st to 10th	Day	58% 7% 16% 14% 5%	1520
	11th to 20th	Day	58% 5% 18% 16% 4	1915
	21st to end	Day	71% 4 9% 10% 5%	2783
January	1st to 10th	Night	58% 5% 13% 18% 5% 1	4428
	11th to 20th	Night	60% 6% 14% 14% 6%	4431
	21st to end	Night	62% 5% 9% 19% 5%	4532
February	1st to 10th	Night	49% 5% 13% 22% 10% 1	2470
	11th to 20th	Night	44% 6% 15% 25% 9% 2	2404
	21st to end	Night	48% 5% 12% 21% 12% 1	1992
March	1st to 10th	Night	46% 7% 15% 22% 9% 1	2152
	11th to 20th	Night	45% 5% 13% 22% 14% 1	2090
	21st to end	Night	50% 5% 11% 23% 9% 1	2235
April	1st to 10th	Night	47% 4 10% 23% 15% 1	1800
	11th to 20th	Night	51% 5% 9% 20% 14% 2	1751
	21st to end	Night	49% 6% 8% 20% 14% 2	1620
May	1st to 10th	Night	45% 6% 12% 21% 15% 1	1517
	11th to 20th	Night	40% 7% 14% 23% 15% 2	1417
	21st to end	Night	39% 7% 13% 22% 17% 2	1552
June	1st to 10th	Night	31% 8% 15% 28% 17% 2	1440
	11th to 20th	Night	33% 8% 15% 25% 18% 2	1407
	21st to end	Night	27% 5% 17% 32% 15% 4	1260
July	1st to 10th	Night	21% 7% 16% 34% 20% 2	1314
	11th to 20th	Night	24% 6% 17% 32% 17% 4	1392
	21st to end	Night	21% 6% 17% 33% 18% 5%	1504
August	1st to 10th	Night	30% 6% 15% 30% 15% 5%	1601
	11th to 20th	Night	30% 6% 16% 30% 16% 2	1600
	21st to end	Night	28% 6% 13% 31% 18% 4	1882
September	1st to 10th	Night	30% 6% 13% 33% 17% 1	1800
	11th to 20th	Night	29% 6% 17% 31% 17%	1826
	21st to end	Night	35% 5% 13% 32% 14% 1	2045
October	1st to 10th	Night	38% 5% 11% 30% 15% 2	2160
	11th to 20th	Night	44% 6% 13% 23% 13% 1	2160
	21st to end	Night	49% 5% 12% 22% 11% 1	2400
November	1st to 10th	Night	43% 6% 12% 23% 14% 2	2530
	11th to 20th	Night	54% 5% 12% 18% 10% 1	2660
	21st to end	Night	48% 5% 16% 23% 6% 1	2660
December	1st to 10th	Night	54% 5% 14% 20% 6%	2660
	11th to 20th	Night	57% 6% 13% 17% 5% 2	3281
	21st to end	Night	65% 4 12% 15% 4	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	13% 10% 34% 28% 11% 4	270
	11th to 20th	Day	16% 9% 40% 29% 6% 1	270
	21st to end	Day	17% 8% 40% 23% 10% 2	297
February	1st to 10th	Day	16% 15% 40% 23% 6%	270
	11th to 20th	Day	17% 16% 39% 23% 4 1	270
	21st to end	Day	15% 14% 43% 24% 3	225
March	1st to 10th	Day	18% 13% 39% 22% 7%	270
	11th to 20th	Day	19% 9% 40% 21% 10% 1	270
	21st to end	Day	27% 13% 27% 21% 8% 3	304
April	1st to 10th	Day	23% 15% 33% 23% 6%	420
	11th to 20th	Day	29% 11% 26% 22% 11% 1	433
	21st to end	Day	28% 11% 29% 20% 10% 2	600
May	1st to 10th	Day	19% 15% 29% 26% 10% 1	603
	11th to 20th	Day	20% 11% 29% 24% 13% 3	625
	21st to end	Day	16% 9% 27% 27% 17% 4	679
June	1st to 10th	Day	11% 6% 26% 31% 21% 5%	630
	11th to 20th	Day	13% 6% 34% 33% 10% 3	630
	21st to end	Day	13% 5% 27% 31% 17% 6%	630
July	1st to 10th	Day	9% 3 20% 33% 23% 11%	621
	11th to 20th	Day	9% 5% 28% 30% 21% 7%	618
	21st to end	Day	6% 4 25% 35% 23% 7%	684
August	1st to 10th	Day	9% 3 22% 35% 25% 6%	626
	11th to 20th	Day	13% 5% 21% 32% 23% 8%	615
	21st to end	Day	11% 6% 24% 30% 23% 6%	592
September	1st to 10th	Day	16% 6% 26% 29% 20% 3	450
	11th to 20th	Day	13% 5% 26% 32% 21% 4	419
	21st to end	Day	18% 12% 30% 24% 10% 6%	270
October	1st to 10th	Day	20% 9% 27% 21% 17% 7%	270
	11th to 20th	Day	16% 15% 36% 23% 9% 1	270
	21st to end	Day	22% 14% 40% 16% 6% 1	297
November	1st to 10th	Day	20% 13% 32% 24% 9% 2	270
	11th to 20th	Day	17% 17% 40% 21% 5%	270
	21st to end	Day	21% 13% 41% 21% 5%	270
December	1st to 10th	Day	11% 13% 43% 22% 11%	270
	11th to 20th	Day	20% 16% 34% 28% 3	270
	21st to end	Day	27% 16% 32% 22% 2 1	297
January	1st to 10th	Night	13% 12% 33% 33% 9% 1	950
	11th to 20th	Night	17% 11% 36% 30% 7% 1	950
	21st to end	Night	18% 9% 35% 30% 7%	1045
February	1st to 10th	Night	12% 11% 38% 30% 8%	950
	11th to 20th	Night	12% 11% 41% 28% 7% 1	929
	21st to end	Night	17% 10% 35% 29% 9%	736
March	1st to 10th	Night	16% 10% 35% 30% 8%	632
	11th to 20th	Night	17% 8% 39% 28% 8%	570
	21st to end	Night	23% 10% 29% 25% 11% 2	615
April	1st to 10th	Night	21% 10% 34% 28% 7%	540
	11th to 20th	Night	25% 7% 32% 29% 8%	533
	21st to end	Night	25% 8% 30% 27% 10% 1	540
May	1st to 10th	Night	17% 11% 34% 31% 6% 1	540
	11th to 20th	Night	16% 9% 32% 30% 12% 1	531
	21st to end	Night	13% 7% 25% 38% 17% 1	584
June	1st to 10th	Night	11% 5% 26% 42% 16% 1	540
	11th to 20th	Night	10% 4 30% 46% 11%	540
	21st to end	Night	9% 4 24% 44% 18% 2	540
July	1st to 10th	Night	6% 1 19% 44% 28% 2	525
	11th to 20th	Night	7% 3 22% 45% 20% 2	522
	21st to end	Night	5% 2 23% 49% 19% 2	591
August	1st to 10th	Night	7% 3 18% 51% 19% 1	537
	11th to 20th	Night	13% 3 19% 34% 28% 3	534
	21st to end	Night	8% 4 24% 37% 24% 3	594
September	1st to 10th	Night	11% 3 24% 40% 19% 3	540
	11th to 20th	Night	11% 5% 27% 36% 18% 3	540
	21st to end	Night	16% 7% 30% 32% 13% 1	605
October	1st to 10th	Night	19% 7% 28% 32% 13% 1	720
	11th to 20th	Night	21% 9% 32% 29% 8% 1	720
	21st to end	Night	25% 9% 26% 33% 7%	792
November	1st to 10th	Night	18% 10% 30% 32% 8% 1	864
	11th to 20th	Night	19% 12% 34% 27% 8%	950
	21st to end	Night	18% 12% 37% 26% 7%	950
December	1st to 10th	Night	15% 10% 37% 29% 8% 1	950
	11th to 20th	Night	19% 9% 38% 27% 7%	950
	21st to end	Night	25% 10% 33% 26% 6%	1045

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