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 2025-02-06.

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	18% 13% 4 10% 5% 5% 4 3 5% 6% 27%	108425
	Night	18% 15% 5% 11% 6% 4 4 3 4 5% 26%	109833
Transparency	Day	57% 6% 11% 18% 7% 1	111941
	Night	61% 5% 10% 16% 8% 1	114481
Seeing	Day	27% 17% 29% 18% 7% 2	14876
	Night	26% 16% 27% 23% 8% 1	23932

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% 24% 9% 19% 7% 6% 3 2 2 3 12%	3597
	11th to 20th	Day	8% 17% 9% 24% 12% 6% 4 3 4 4 9%	3570
	21st to end	Day	11% 19% 8% 14% 6% 10% 6% 3 4 5% 15%	3960
February	1st to 10th	Day	15% 19% 6% 14% 9% 6% 5% 3 3 4 17%	1660
	11th to 20th	Day	13% 21% 7% 14% 8% 4 5% 3 4 4 17%	1824
	21st to end	Day	14% 18% 6% 12% 7% 7% 4 3 5% 5% 20%	1570
March	1st to 10th	Day	22% 15% 5% 10% 5% 5% 3 3 4 5% 22%	3116
	11th to 20th	Day	14% 24% 6% 12% 5% 4 3 2 3 4 22%	3256
	21st to end	Day	17% 14% 6% 11% 4 3 3 2 3 5% 32%	4166
April	1st to 10th	Day	23% 16% 4 10% 4 4 4 2 4 4 25%	2280
	11th to 20th	Day	20% 14% 3 7% 4 4 4 3 4 5% 32%	2262
	21st to end	Day	25% 12% 3 7% 5% 4 4 3 4 5% 27%	2470
May	1st to 10th	Day	24% 12% 3 7% 4 4 4 4 5% 6% 26%	4224
	11th to 20th	Day	20% 8% 3 9% 5% 5% 6% 4 6% 6% 27%	4398
	21st to end	Day	15% 8% 3 7% 5% 4 5% 5% 7% 8% 33%	4837
June	1st to 10th	Day	27% 9% 2 5% 4 4 4 5% 5% 7% 28%	2660
	11th to 20th	Day	19% 7% 2 6% 4 4 4 4 7% 8% 34%	2660
	21st to end	Day	25% 8% 2 7% 4 5% 5% 4 5% 6% 28%	2660
July	1st to 10th	Day	15% 6% 3 7% 5% 4 6% 5% 7% 9% 33%	4440
	11th to 20th	Day	16% 7% 2 7% 5% 5% 6% 5% 6% 9% 32%	4440
	21st to end	Day	16% 7% 2 6% 4 5% 5% 4 6% 7% 39%	4814
August	1st to 10th	Day	20% 8% 3 8% 4 5% 5% 4 7% 8% 27%	4423
	11th to 20th	Day	19% 6% 3 7% 5% 5% 5% 4 7% 8% 31%	3996
	21st to end	Day	20% 6% 2 5% 3 4 4 4 6% 7% 38%	4180
September	1st to 10th	Day	17% 6% 3 6% 3 4 4 4 5% 7% 40%	2280
	11th to 20th	Day	18% 6% 2 4 4 3 4 3 5% 7% 44%	2153
	21st to end	Day	20% 6% 3 5% 4 4 4 5% 6% 8% 36%	2090
October	1st to 10th	Day	24% 10% 3 7% 4 4 5% 4 5% 6% 29%	2033
	11th to 20th	Day	23% 12% 3 7% 4 4 5% 3 5% 5% 28%	1815
	21st to end	Day	27% 18% 4 8% 3 3 4 2 3 4 24%	1881
November	1st to 10th	Day	20% 16% 4 7% 4 4 3 4 5% 5% 28%	1761
	11th to 20th	Day	17% 20% 6% 11% 4 4 4 3 4 4 24%	1800
	21st to end	Day	18% 21% 6% 12% 5% 4 4 3 5% 5% 17%	1603
December	1st to 10th	Day	16% 23% 8% 19% 7% 4 3 2 4 3 10%	2600
	11th to 20th	Day	18% 20% 7% 15% 4 4 3 2 4 4 19%	2986
	21st to end	Day	18% 19% 5% 14% 6% 5% 4 3 4 4 19%	3960
January	1st to 10th	Night	12% 23% 9% 20% 8% 5% 4 2 3 3 12%	6349
	11th to 20th	Night	9% 21% 10% 21% 9% 6% 4 3 3 3 11%	6315
	21st to end	Night	11% 18% 7% 14% 9% 7% 4 3 4 3 19%	6996
February	1st to 10th	Night	15% 19% 6% 15% 7% 5% 4 3 4 4 18%	2628
	11th to 20th	Night	14% 17% 7% 14% 8% 6% 4 3 4 4 20%	2242
	21st to end	Night	13% 18% 6% 12% 6% 5% 4 3 4 4 25%	1884
March	1st to 10th	Night	22% 14% 4 8% 5% 4 3 3 4 5% 29%	3724
	11th to 20th	Night	16% 22% 6% 10% 6% 4 3 2 3 4 24%	3699
	21st to end	Night	14% 20% 6% 11% 4 4 4 3 3 3 29%	3466
April	1st to 10th	Night	23% 14% 5% 9% 4 4 3 3 5% 4 27%	1900
	11th to 20th	Night	23% 12% 3 7% 5% 4 4 2 3 5% 32%	1866
	21st to end	Night	22% 12% 3 8% 5% 3 3 3 5% 5% 32%	1710
May	1st to 10th	Night	24% 11% 3 9% 4 4 4 4 4 6% 26%	2743
	11th to 20th	Night	22% 9% 3 7% 4 5% 4 3 5% 6% 33%	2495
	21st to end	Night	18% 7% 3 5% 5% 4 5% 4 5% 8% 37%	2738
June	1st to 10th	Night	27% 8% 3 5% 4 3 4 4 5% 8% 29%	1520
	11th to 20th	Night	21% 6% 2 6% 3 4 5% 3 6% 8% 36%	1520
	21st to end	Night	21% 9% 2 7% 4 4 4 5% 7% 6% 30%	1520
July	1st to 10th	Night	16% 8% 3 8% 5% 5% 6% 4 7% 7% 32%	2520
	11th to 20th	Night	18% 8% 2 6% 5% 4 4 4 6% 7% 36%	2520
	21st to end	Night	17% 7% 2 7% 5% 5% 4 4 5% 6% 38%	2739
August	1st to 10th	Night	24% 8% 3 7% 4 3 4 3 6% 6% 32%	2506
	11th to 20th	Night	17% 8% 3 8% 5% 5% 6% 4 6% 7% 32%	2799
	21st to end	Night	18% 6% 2 5% 4 4 4 3 5% 6% 42%	3476
September	1st to 10th	Night	17% 7% 2 7% 4 4 4 3 4 6% 42%	1900
	11th to 20th	Night	16% 5% 2 5% 3 3 4 4 5% 6% 46%	2026
	21st to end	Night	21% 6% 2 5% 4 3 4 3 4 6% 41%	2090
October	1st to 10th	Night	22% 9% 3 7% 5% 4 4 3 5% 5% 34%	2144
	11th to 20th	Night	23% 12% 4 8% 5% 4 4 3 4 5% 29%	2365
	21st to end	Night	26% 14% 3 7% 4 4 4 3 4 5% 25%	2717
November	1st to 10th	Night	21% 15% 4 7% 5% 4 5% 3 5% 6% 26%	2537
	11th to 20th	Night	19% 16% 5% 9% 5% 4 4 3 4 5% 27%	2600
	21st to end	Night	20% 20% 7% 11% 5% 3 3 2 4 4 21%	2795
December	1st to 10th	Night	15% 26% 11% 17% 6% 3 3 2 3 3 10%	4580
	11th to 20th	Night	19% 22% 7% 15% 5% 4 3 2 3 3 16%	5208
	21st to end	Night	19% 19% 5% 12% 6% 4 4 2 3 4 21%	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	80% 4 7% 8% 1	3598
	11th to 20th	Day	79% 5% 7% 6% 3	3600
	21st to end	Day	73% 4 7% 10% 6% 1	3960
February	1st to 10th	Day	72% 5% 6% 12% 4 1	1660
	11th to 20th	Day	72% 5% 8% 10% 4 1	1833
	21st to end	Day	65% 6% 9% 12% 6% 1	1570
March	1st to 10th	Day	64% 6% 9% 12% 7% 1	3119
	11th to 20th	Day	65% 6% 9% 9% 9% 1	3256
	21st to end	Day	57% 4 7% 16% 15% 1	4166
April	1st to 10th	Day	63% 5% 8% 16% 8% 1	2280
	11th to 20th	Day	56% 5% 8% 19% 12% 1	2262
	21st to end	Day	59% 5% 9% 16% 9% 2	2470
May	1st to 10th	Day	56% 7% 11% 18% 7% 1	4238
	11th to 20th	Day	52% 11% 15% 14% 7% 1	4402
	21st to end	Day	44% 10% 13% 22% 10% 1	4837
June	1st to 10th	Day	54% 7% 12% 18% 8% 1	2660
	11th to 20th	Day	46% 7% 12% 22% 11% 2	2660
	21st to end	Day	56% 8% 11% 18% 8%	2660
July	1st to 10th	Day	46% 5% 15% 26% 8%	4370
	11th to 20th	Day	48% 5% 15% 28% 4	4365
	21st to end	Day	45% 6% 13% 30% 6%	4661
August	1st to 10th	Day	54% 5% 16% 19% 6%	4409
	11th to 20th	Day	52% 5% 18% 20% 5%	4035
	21st to end	Day	45% 5% 17% 19% 13% 1	4180
September	1st to 10th	Day	42% 7% 14% 26% 10% 1	2280
	11th to 20th	Day	41% 4 12% 31% 12%	2153
	21st to end	Day	45% 7% 14% 24% 9% 1	2090
October	1st to 10th	Day	50% 4 13% 26% 7%	3384
	11th to 20th	Day	54% 4 14% 21% 7%	2992
	21st to end	Day	55% 3 15% 20% 7% 1	3081
November	1st to 10th	Day	58% 7% 11% 17% 7%	1761
	11th to 20th	Day	65% 6% 8% 13% 7% 1	1800
	21st to end	Day	69% 6% 12% 10% 3	1603
December	1st to 10th	Day	82% 3 7% 6% 1	2600
	11th to 20th	Day	70% 3 7% 14% 6%	2986
	21st to end	Day	71% 4 6% 10% 8% 1	3960
January	1st to 10th	Night	78% 6% 8% 7% 1	6349
	11th to 20th	Night	78% 5% 7% 7% 2	6360
	21st to end	Night	71% 4 8% 8% 9% 1	6996
February	1st to 10th	Night	71% 5% 7% 12% 5%	2628
	11th to 20th	Night	68% 5% 8% 13% 5%	2244
	21st to end	Night	63% 5% 8% 14% 9% 1	1884
March	1st to 10th	Night	58% 5% 11% 16% 7% 2	3734
	11th to 20th	Night	64% 6% 7% 13% 9% 1	3700
	21st to end	Night	60% 5% 6% 18% 10% 2	3466
April	1st to 10th	Night	60% 4 9% 15% 10% 1	1900
	11th to 20th	Night	55% 5% 9% 18% 12%	1866
	21st to end	Night	54% 6% 9% 17% 12% 2	1710
May	1st to 10th	Night	56% 9% 10% 16% 8% 1	2749
	11th to 20th	Night	49% 9% 14% 17% 9% 1	2496
	21st to end	Night	44% 9% 12% 20% 15% 1	2739
June	1st to 10th	Night	53% 7% 10% 21% 8% 1	1520
	11th to 20th	Night	44% 7% 13% 21% 11% 3	1520
	21st to end	Night	50% 9% 13% 18% 8% 1	1520
July	1st to 10th	Night	50% 5% 13% 21% 11%	2480
	11th to 20th	Night	47% 6% 14% 27% 7%	2472
	21st to end	Night	46% 6% 14% 24% 10%	2651
August	1st to 10th	Night	52% 6% 14% 22% 6%	2496
	11th to 20th	Night	51% 7% 17% 18% 6%	2826
	21st to end	Night	43% 5% 14% 24% 13% 1	3476
September	1st to 10th	Night	42% 7% 13% 26% 11% 1	1900
	11th to 20th	Night	38% 6% 16% 29% 11%	2026
	21st to end	Night	43% 7% 12% 25% 13% 1	2090
October	1st to 10th	Night	54% 4 10% 24% 9%	3573
	11th to 20th	Night	53% 4 10% 23% 9%	3927
	21st to end	Night	55% 4 13% 17% 10% 1	4467
November	1st to 10th	Night	59% 6% 11% 17% 6% 1	2537
	11th to 20th	Night	60% 6% 9% 14% 9% 2	2600
	21st to end	Night	68% 5% 11% 13% 3	2795
December	1st to 10th	Night	82% 4 7% 6% 2	4580
	11th to 20th	Night	75% 3 6% 10% 6%	5208
	21st to end	Night	69% 5% 6% 12% 8%	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	31% 20% 34% 12% 5%	200
	11th to 20th	Day	30% 27% 30% 12% 2 1	200
	21st to end	Day	28% 23% 32% 12% 5% 1	220
February	1st to 10th	Day	37% 30% 24% 8% 1	238
	11th to 20th	Day	32% 26% 32% 8% 2	432
	21st to end	Day	32% 20% 31% 15% 1	382
March	1st to 10th	Day	33% 18% 31% 15% 3	460
	11th to 20th	Day	31% 22% 31% 12% 4 1	460
	21st to end	Day	33% 22% 24% 14% 6% 1	506
April	1st to 10th	Day	36% 18% 26% 15% 5%	460
	11th to 20th	Day	35% 20% 25% 13% 5% 2	452
	21st to end	Day	34% 18% 29% 12% 6% 2	460
May	1st to 10th	Day	33% 18% 26% 15% 6% 2	463
	11th to 20th	Day	22% 18% 31% 20% 7% 2	487
	21st to end	Day	21% 19% 31% 22% 6% 1	529
June	1st to 10th	Day	31% 14% 31% 17% 7% 1	490
	11th to 20th	Day	20% 16% 31% 22% 9% 1	490
	21st to end	Day	29% 12% 34% 16% 7% 2	490
July	1st to 10th	Day	19% 10% 33% 26% 10% 1	485
	11th to 20th	Day	18% 10% 36% 26% 8% 2	484
	21st to end	Day	19% 11% 34% 27% 8% 1	532
August	1st to 10th	Day	20% 10% 28% 27% 13% 1	486
	11th to 20th	Day	21% 9% 27% 24% 15% 3	479
	21st to end	Day	22% 13% 24% 24% 12% 5%	539
September	1st to 10th	Day	19% 13% 29% 21% 13% 4	490
	11th to 20th	Day	19% 11% 23% 22% 19% 7%	490
	21st to end	Day	19% 14% 26% 25% 12% 4	490
October	1st to 10th	Day	27% 16% 25% 18% 12% 2	490
	11th to 20th	Day	31% 19% 22% 19% 8% 1	445
	21st to end	Day	39% 17% 24% 15% 4	429
November	1st to 10th	Day	36% 16% 28% 15% 4 1	395
	11th to 20th	Day	32% 23% 25% 14% 5% 2	400
	21st to end	Day	34% 21% 33% 11% 1	203
December	1st to 10th	Day	33% 21% 29% 13% 4 1	200
	11th to 20th	Day	32% 28% 30% 9% 1 1	200
	21st to end	Day	36% 20% 30% 10% 3	220
January	1st to 10th	Night	28% 22% 32% 16% 3	800
	11th to 20th	Night	28% 23% 29% 16% 4	800
	21st to end	Night	20% 20% 35% 20% 5% 1	880
February	1st to 10th	Night	26% 22% 31% 18% 3	781
	11th to 20th	Night	22% 24% 33% 17% 3	743
	21st to end	Night	27% 19% 32% 21% 2	628
March	1st to 10th	Night	29% 17% 30% 20% 3 1	760
	11th to 20th	Night	29% 19% 26% 20% 6%	760
	21st to end	Night	27% 19% 27% 19% 8% 1	836
April	1st to 10th	Night	30% 17% 28% 21% 4	760
	11th to 20th	Night	31% 17% 26% 19% 8%	742
	21st to end	Night	29% 16% 29% 19% 6%	570
May	1st to 10th	Night	27% 11% 31% 23% 7%	518
	11th to 20th	Night	23% 11% 30% 27% 7% 1	374
	21st to end	Night	20% 12% 29% 31% 8% 1	411
June	1st to 10th	Night	30% 8% 26% 27% 8% 1	380
	11th to 20th	Night	22% 7% 26% 33% 13%	380
	21st to end	Night	20% 9% 25% 36% 10% 1	380
July	1st to 10th	Night	16% 5% 30% 36% 12% 1	370
	11th to 20th	Night	13% 4 26% 38% 18% 1	368
	21st to end	Night	16% 6% 24% 41% 13%	416
August	1st to 10th	Night	19% 7% 17% 37% 20% 1	378
	11th to 20th	Night	16% 7% 22% 35% 18% 2	565
	21st to end	Night	17% 8% 25% 31% 19% 2	836
September	1st to 10th	Night	20% 7% 26% 33% 13% 2	760
	11th to 20th	Night	19% 7% 19% 30% 23% 2	760
	21st to end	Night	23% 8% 27% 24% 17% 1	760
October	1st to 10th	Night	27% 12% 23% 25% 11% 1	760
	11th to 20th	Night	29% 14% 25% 21% 10% 1	760
	21st to end	Night	34% 15% 26% 20% 3	836
November	1st to 10th	Night	31% 17% 28% 19% 5% 1	780
	11th to 20th	Night	27% 20% 27% 16% 8% 1	800
	21st to end	Night	31% 23% 24% 17% 5% 1	800
December	1st to 10th	Night	34% 22% 25% 15% 4	800
	11th to 20th	Night	31% 22% 27% 16% 4 1	800
	21st to end	Night	32% 21% 27% 17% 3	880

Page generated at 2025-02-03 03:05:56. Version ac07

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