Girard Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Girard Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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	20% 12% 4 8% 6% 3 5% 4 5% 6% 28%	74887
	Night	19% 13% 5% 8% 6% 3 4 4 4 5% 27%	75156
Transparency	Day	57% 6% 11% 17% 8% 1	74592
	Night	59% 6% 10% 16% 8% 1	74985
Seeing	Day	27% 17% 29% 18% 7% 2	13588
	Night	26% 16% 27% 23% 8% 1	22648

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	18% 22% 9% 14% 7% 3 4 3 3 4 12%	1517
	11th to 20th	Day	17% 19% 8% 15% 7% 4 4 4 4 4 14%	1520
	21st to end	Day	11% 16% 7% 13% 9% 5% 8% 5% 5% 5% 18%	1632
February	1st to 10th	Day	15% 18% 7% 13% 9% 5% 6% 4 3 4 16%	1534
	11th to 20th	Day	14% 19% 8% 13% 8% 3 6% 4 3 4 18%	1726
	21st to end	Day	15% 16% 8% 12% 8% 5% 5% 4 4 4 20%	1480
March	1st to 10th	Day	16% 15% 7% 11% 7% 4 4 4 4 4 23%	1797
	11th to 20th	Day	17% 16% 6% 8% 7% 3 4 3 3 5% 28%	1840
	21st to end	Day	20% 14% 6% 9% 6% 3 3 3 3 4 29%	2376
April	1st to 10th	Day	23% 15% 5% 9% 5% 3 4 3 4 5% 25%	2160
	11th to 20th	Day	21% 13% 4 6% 5% 3 4 4 4 4 33%	2141
	21st to end	Day	26% 11% 4 6% 6% 3 4 4 4 6% 26%	2340
May	1st to 10th	Day	22% 10% 3 7% 5% 3 4 4 4 7% 31%	2402
	11th to 20th	Day	21% 7% 3 7% 5% 3 5% 4 5% 6% 34%	2478
	21st to end	Day	19% 7% 3 6% 6% 3 5% 6% 6% 7% 31%	2725
June	1st to 10th	Day	28% 8% 2 5% 4 3 5% 6% 5% 7% 28%	2520
	11th to 20th	Day	20% 6% 2 6% 5% 3 4 4 7% 8% 35%	2520
	21st to end	Day	25% 7% 3 6% 5% 3 6% 5% 5% 7% 28%	2520
July	1st to 10th	Day	17% 6% 3 5% 5% 3 5% 5% 6% 8% 36%	2520
	11th to 20th	Day	17% 6% 3 6% 5% 3 5% 6% 6% 9% 33%	2520
	21st to end	Day	17% 7% 3 5% 6% 4 6% 6% 7% 8% 32%	2744
August	1st to 10th	Day	18% 6% 2 6% 6% 4 5% 5% 7% 9% 31%	2506
	11th to 20th	Day	17% 6% 3 5% 6% 4 4 5% 7% 8% 34%	2283
	21st to end	Day	17% 7% 3 5% 5% 3 4 6% 6% 8% 36%	2376
September	1st to 10th	Day	18% 6% 3 5% 4 3 5% 5% 5% 8% 39%	2160
	11th to 20th	Day	20% 6% 2 4 5% 2 4 4 4 7% 43%	2040
	21st to end	Day	20% 6% 3 4 5% 3 4 6% 6% 8% 35%	1980
October	1st to 10th	Day	25% 9% 3 6% 6% 3 5% 5% 5% 6% 27%	1926
	11th to 20th	Day	23% 11% 4 6% 5% 3 5% 4 5% 6% 27%	1720
	21st to end	Day	28% 17% 4 6% 5% 3 4 3 3 3 23%	1782
November	1st to 10th	Day	20% 15% 4 6% 5% 3 4 5% 5% 6% 27%	1671
	11th to 20th	Day	18% 18% 7% 10% 6% 3 4 4 4 4 23%	1710
	21st to end	Day	19% 20% 6% 11% 7% 3 4 4 5% 5% 18%	1523
December	1st to 10th	Day	22% 21% 8% 11% 7% 3 3 3 4 3 14%	1520
	11th to 20th	Day	20% 19% 8% 11% 5% 3 5% 3 3 4 19%	1906
	21st to end	Day	23% 18% 7% 14% 6% 3 5% 4 3 3 15%	2772
January	1st to 10th	Night	18% 21% 8% 12% 8% 5% 5% 5% 4 3 12%	2649
	11th to 20th	Night	15% 19% 8% 14% 8% 4 5% 4 4 3 17%	2660
	21st to end	Night	13% 15% 7% 12% 10% 5% 6% 4 4 4 20%	2868
February	1st to 10th	Night	16% 17% 7% 15% 8% 4 5% 3 4 4 16%	2423
	11th to 20th	Night	15% 15% 8% 12% 8% 5% 5% 4 4 4 20%	2120
	21st to end	Night	14% 17% 7% 11% 8% 4 4 4 3 4 24%	1776
March	1st to 10th	Night	16% 17% 7% 9% 7% 3 4 3 4 4 25%	2150
	11th to 20th	Night	19% 15% 6% 8% 7% 3 4 4 4 4 27%	2115
	21st to end	Night	18% 17% 5% 7% 4 2 4 3 3 4 32%	1980
April	1st to 10th	Night	23% 13% 6% 8% 6% 3 3 4 5% 4 26%	1800
	11th to 20th	Night	23% 11% 4 6% 5% 3 4 3 3 5% 33%	1767
	21st to end	Night	22% 11% 3 7% 6% 2 3 4 4 6% 31%	1620
May	1st to 10th	Night	23% 9% 3 7% 6% 3 5% 4 4 6% 30%	1571
	11th to 20th	Night	21% 8% 3 6% 5% 3 4 4 4 6% 36%	1415
	21st to end	Night	19% 7% 3 4 5% 4 5% 4 5% 7% 36%	1550
June	1st to 10th	Night	28% 7% 3 5% 4 3 3 5% 5% 8% 29%	1440
	11th to 20th	Night	21% 6% 3 5% 3 3 5% 5% 6% 8% 36%	1440
	21st to end	Night	19% 7% 3 5% 6% 3 5% 6% 7% 7% 31%	1440
July	1st to 10th	Night	15% 6% 3 6% 5% 3 6% 5% 7% 8% 36%	1440
	11th to 20th	Night	12% 6% 2 6% 6% 3 5% 6% 8% 9% 37%	1440
	21st to end	Night	16% 5% 2 6% 6% 4 5% 5% 6% 6% 38%	1574
August	1st to 10th	Night	16% 6% 3 4 5% 3 4 5% 7% 9% 38%	1432
	11th to 20th	Night	16% 7% 3 5% 5% 4 5% 5% 5% 7% 38%	1598
	21st to end	Night	15% 7% 3 5% 5% 3 5% 4 5% 8% 40%	1980
September	1st to 10th	Night	17% 7% 2 6% 5% 3 4 4 4 6% 41%	1800
	11th to 20th	Night	17% 5% 2 5% 4 3 4 4 5% 6% 45%	1919
	21st to end	Night	21% 6% 2 4 4 2 4 4 5% 7% 41%	1980
October	1st to 10th	Night	21% 8% 3 6% 6% 3 4 5% 5% 5% 34%	2031
	11th to 20th	Night	23% 11% 4 7% 6% 4 4 4 4 5% 28%	2240
	21st to end	Night	27% 14% 3 6% 6% 3 4 5% 4 5% 24%	2574
November	1st to 10th	Night	22% 14% 4 6% 5% 3 5% 4 5% 6% 27%	2407
	11th to 20th	Night	20% 15% 6% 8% 6% 3 4 4 3 5% 26%	2470
	21st to end	Night	20% 18% 7% 10% 6% 3 3 3 4 4 22%	2655
December	1st to 10th	Night	22% 23% 8% 10% 7% 3 4 3 4 3 13%	2660
	11th to 20th	Night	20% 20% 8% 13% 7% 3 4 3 3 3 15%	3288
	21st to end	Night	24% 19% 6% 12% 7% 3 4 3 3 3 16%	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	78% 4 8% 9% 2	1518
	11th to 20th	Day	73% 5% 8% 9% 5%	1520
	21st to end	Day	67% 6% 9% 11% 5% 1	1632
February	1st to 10th	Day	74% 5% 6% 10% 4 1	1534
	11th to 20th	Day	72% 4 9% 10% 4 1	1735
	21st to end	Day	67% 6% 8% 11% 6% 1	1480
March	1st to 10th	Day	65% 5% 9% 14% 5% 2	1800
	11th to 20th	Day	60% 6% 11% 13% 10% 1	1840
	21st to end	Day	61% 4 7% 18% 9% 2	2376
April	1st to 10th	Day	63% 5% 8% 16% 8% 1	2160
	11th to 20th	Day	55% 4 8% 19% 12% 1	2141
	21st to end	Day	60% 5% 9% 15% 8% 2	2340
May	1st to 10th	Day	54% 4 9% 19% 12% 2	2402
	11th to 20th	Day	51% 6% 10% 19% 12% 1	2482
	21st to end	Day	49% 9% 13% 17% 11% 1	2725
June	1st to 10th	Day	54% 7% 12% 17% 8% 1	2520
	11th to 20th	Day	46% 7% 11% 23% 11% 2	2520
	21st to end	Day	55% 8% 11% 18% 8% 1	2520
July	1st to 10th	Day	44% 8% 14% 24% 10%	2450
	11th to 20th	Day	46% 8% 14% 26% 6%	2445
	21st to end	Day	48% 9% 15% 21% 6%	2591
August	1st to 10th	Day	48% 9% 17% 21% 5%	2492
	11th to 20th	Day	46% 8% 17% 20% 9%	2283
	21st to end	Day	45% 8% 14% 23% 11%	2376
September	1st to 10th	Day	43% 8% 14% 25% 10% 1	2160
	11th to 20th	Day	41% 5% 11% 30% 12%	2040
	21st to end	Day	45% 8% 15% 23% 9% 1	1980
October	1st to 10th	Day	57% 7% 12% 16% 8%	1926
	11th to 20th	Day	56% 7% 15% 15% 6%	1720
	21st to end	Day	67% 4 9% 13% 5% 1	1782
November	1st to 10th	Day	58% 7% 11% 17% 7%	1671
	11th to 20th	Day	65% 7% 8% 12% 6% 1	1710
	21st to end	Day	68% 7% 12% 10% 3	1523
December	1st to 10th	Day	76% 5% 9% 8% 2	1520
	11th to 20th	Day	69% 5% 8% 13% 5%	1906
	21st to end	Day	74% 6% 7% 9% 3	2772
January	1st to 10th	Night	75% 7% 8% 7% 3	2649
	11th to 20th	Night	73% 5% 8% 10% 4	2660
	21st to end	Night	67% 6% 8% 11% 6% 1	2868
February	1st to 10th	Night	73% 5% 7% 11% 5%	2423
	11th to 20th	Night	68% 5% 8% 13% 5%	2122
	21st to end	Night	64% 4 8% 14% 9% 1	1776
March	1st to 10th	Night	64% 5% 8% 15% 7% 1	2160
	11th to 20th	Night	61% 5% 8% 16% 9% 2	2116
	21st to end	Night	57% 4 7% 18% 10% 3	1980
April	1st to 10th	Night	61% 4 9% 15% 10% 1	1800
	11th to 20th	Night	54% 5% 9% 19% 13% 1	1767
	21st to end	Night	55% 5% 9% 17% 11% 2	1620
May	1st to 10th	Night	55% 5% 7% 16% 14% 3	1571
	11th to 20th	Night	50% 6% 10% 17% 15% 2	1416
	21st to end	Night	47% 8% 12% 20% 12% 2	1551
June	1st to 10th	Night	53% 6% 10% 22% 8% 1	1440
	11th to 20th	Night	45% 8% 13% 21% 11% 3	1440
	21st to end	Night	49% 10% 14% 19% 8%	1440
July	1st to 10th	Night	43% 9% 14% 21% 13% 1	1400
	11th to 20th	Night	39% 10% 15% 28% 8%	1392
	21st to end	Night	42% 9% 16% 25% 8%	1486
August	1st to 10th	Night	39% 9% 20% 24% 8%	1422
	11th to 20th	Night	42% 9% 17% 22% 10%	1598
	21st to end	Night	42% 8% 15% 24% 10%	1980
September	1st to 10th	Night	43% 7% 13% 25% 11% 1	1800
	11th to 20th	Night	39% 6% 16% 28% 11% 1	1919
	21st to end	Night	43% 7% 12% 25% 12% 1	1980
October	1st to 10th	Night	51% 6% 10% 21% 11%	2031
	11th to 20th	Night	57% 7% 10% 19% 6%	2240
	21st to end	Night	61% 7% 10% 14% 7% 1	2574
November	1st to 10th	Night	58% 6% 11% 17% 7% 1	2407
	11th to 20th	Night	61% 6% 10% 14% 8% 2	2470
	21st to end	Night	67% 6% 11% 13% 4	2655
December	1st to 10th	Night	76% 6% 7% 8% 2	2660
	11th to 20th	Night	75% 5% 6% 10% 4	3288
	21st to end	Night	73% 6% 6% 11% 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	30% 18% 34% 13% 5%	180
	11th to 20th	Day	31% 26% 28% 13% 2 1	180
	21st to end	Day	27% 21% 34% 13% 5% 1	188
February	1st to 10th	Day	39% 32% 22% 6% 1	202
	11th to 20th	Day	33% 26% 32% 8% 2	394
	21st to end	Day	32% 20% 31% 16% 1	346
March	1st to 10th	Day	32% 19% 32% 15% 3	420
	11th to 20th	Day	31% 21% 31% 12% 4 1	420
	21st to end	Day	34% 20% 23% 15% 7% 1	462
April	1st to 10th	Day	35% 17% 27% 15% 5%	420
	11th to 20th	Day	35% 19% 24% 15% 6% 2	412
	21st to end	Day	33% 19% 29% 12% 5% 2	420
May	1st to 10th	Day	34% 18% 26% 15% 5% 2	423
	11th to 20th	Day	23% 19% 31% 18% 6% 3	447
	21st to end	Day	21% 19% 31% 22% 6% 1	485
June	1st to 10th	Day	32% 14% 31% 16% 6% 1	450
	11th to 20th	Day	20% 16% 30% 23% 10% 1	450
	21st to end	Day	26% 12% 35% 17% 8% 2	450
July	1st to 10th	Day	19% 10% 32% 27% 11% 2	445
	11th to 20th	Day	17% 10% 36% 26% 9% 2	444
	21st to end	Day	19% 10% 33% 27% 8% 1	488
August	1st to 10th	Day	19% 11% 28% 27% 14% 1	446
	11th to 20th	Day	20% 9% 28% 25% 15% 3	439
	21st to end	Day	21% 13% 23% 24% 12% 6%	495
September	1st to 10th	Day	19% 12% 29% 21% 14% 5%	450
	11th to 20th	Day	20% 12% 24% 21% 17% 7%	450
	21st to end	Day	18% 14% 26% 25% 12% 4	450
October	1st to 10th	Day	28% 17% 23% 18% 12% 2	450
	11th to 20th	Day	30% 20% 23% 19% 9% 1	410
	21st to end	Day	40% 16% 24% 15% 4	396
November	1st to 10th	Day	35% 16% 29% 15% 4 1	365
	11th to 20th	Day	31% 24% 25% 14% 5% 2	370
	21st to end	Day	33% 20% 33% 13% 1	183
December	1st to 10th	Day	34% 21% 28% 13% 4 1	180
	11th to 20th	Day	31% 29% 28% 10% 1 1	180
	21st to end	Day	38% 22% 29% 9% 3	198
January	1st to 10th	Night	29% 21% 31% 16% 3	760
	11th to 20th	Night	28% 24% 28% 16% 4	760
	21st to end	Night	21% 20% 34% 20% 5% 1	820
February	1st to 10th	Night	26% 22% 31% 18% 3	720
	11th to 20th	Night	22% 23% 33% 17% 3	703
	21st to end	Night	27% 19% 31% 21% 2	592
March	1st to 10th	Night	29% 17% 30% 21% 3 1	720
	11th to 20th	Night	29% 19% 26% 20% 6%	720
	21st to end	Night	28% 18% 26% 19% 8% 1	792
April	1st to 10th	Night	30% 17% 27% 21% 4	720
	11th to 20th	Night	30% 16% 25% 20% 8%	703
	21st to end	Night	30% 15% 29% 19% 7%	540
May	1st to 10th	Night	27% 11% 30% 23% 8%	491
	11th to 20th	Night	24% 11% 30% 27% 7% 1	354
	21st to end	Night	21% 12% 28% 31% 8% 1	389
June	1st to 10th	Night	30% 8% 27% 26% 8% 1	360
	11th to 20th	Night	21% 7% 26% 32% 13%	360
	21st to end	Night	19% 9% 25% 36% 10% 1	360
July	1st to 10th	Night	15% 5% 31% 36% 12% 1	350
	11th to 20th	Night	12% 4 27% 38% 18% 1	348
	21st to end	Night	15% 6% 25% 40% 14%	394
August	1st to 10th	Night	17% 7% 18% 37% 21% 1	358
	11th to 20th	Night	16% 7% 21% 35% 19% 2	534
	21st to end	Night	16% 8% 25% 31% 19% 2	792
September	1st to 10th	Night	20% 7% 25% 33% 13% 2	720
	11th to 20th	Night	21% 8% 20% 30% 20% 2	720
	21st to end	Night	23% 8% 27% 24% 16% 1	720
October	1st to 10th	Night	27% 13% 24% 25% 11% 1	720
	11th to 20th	Night	29% 14% 25% 21% 10% 1	720
	21st to end	Night	34% 16% 26% 20% 4	792
November	1st to 10th	Night	30% 16% 29% 19% 5% 1	740
	11th to 20th	Night	26% 21% 27% 16% 8% 1	760
	21st to end	Night	31% 23% 23% 17% 5% 1	760
December	1st to 10th	Night	34% 22% 25% 15% 4	760
	11th to 20th	Night	30% 22% 27% 16% 4 1	760
	21st to end	Night	33% 22% 27% 16% 2	836

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