Strawbridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strawbridge Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	16% 6% 2 6% 4 4 4 4 6% 8% 40%	126811
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	123230
Transparency	Day	41% 5% 12% 23% 17% 3	130339
	Night	40% 4 11% 23% 18% 4	127788
Seeing	Day	20% 14% 28% 22% 12% 3	16292
	Night	19% 11% 31% 28% 10% 1	25535

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	13% 5% 2 5% 4 3 4 3 6% 6% 49%	3597
	11th to 20th	Day	14% 9% 2 7% 5% 4 4 4 5% 5% 41%	3686
	21st to end	Day	18% 8% 2 6% 4 3 4 3 5% 6% 41%	4466
February	1st to 10th	Day	16% 7% 2 6% 5% 5% 4 3 5% 7% 39%	1800
	11th to 20th	Day	16% 9% 2 7% 3 4 4 4 6% 6% 40%	1898
	21st to end	Day	18% 9% 3 6% 4 4 3 3 5% 7% 38%	1650
March	1st to 10th	Day	20% 5% 1 5% 3 3 4 3 5% 6% 45%	4494
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 45%	4980
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 6% 43%	5781
April	1st to 10th	Day	16% 8% 2 6% 4 4 5% 4 5% 8% 39%	2400
	11th to 20th	Day	15% 6% 2 5% 5% 4 5% 4 5% 8% 41%	2374
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 6% 8% 40%	2400
May	1st to 10th	Day	26% 8% 2 5% 4 4 5% 4 5% 7% 30%	5805
	11th to 20th	Day	25% 8% 3 6% 4 4 4 3 5% 7% 32%	6318
	21st to end	Day	20% 6% 2 6% 4 4 5% 4 6% 9% 36%	6949
June	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 5% 6% 9% 37%	2800
	21st to end	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 6% 6% 5% 8% 10% 38%	6360
	11th to 20th	Day	12% 7% 3 8% 5% 6% 6% 5% 8% 10% 31%	6360
	21st to end	Day	11% 7% 2 7% 5% 4 5% 5% 6% 9% 39%	6795
August	1st to 10th	Day	19% 5% 2 7% 5% 4 5% 4 6% 8% 34%	4234
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 9% 43%	3855
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	4312
September	1st to 10th	Day	15% 6% 1 6% 4 4 4 4 5% 6% 45%	2400
	11th to 20th	Day	14% 5% 1 4 3 3 3 3 6% 8% 50%	2370
	21st to end	Day	19% 8% 3 6% 4 4 5% 3 5% 6% 37%	2197
October	1st to 10th	Day	18% 4 2 6% 3 3 3 4 5% 6% 45%	2000
	11th to 20th	Day	16% 5% 2 4 3 3 4 3 5% 7% 48%	2000
	21st to end	Day	18% 6% 3 4 3 3 3 4 5% 6% 46%	2135
November	1st to 10th	Day	13% 6% 2 4 3 3 2 3 4 7% 53%	1779
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 6% 7% 48%	1800
	21st to end	Day	16% 6% 2 5% 3 3 4 3 5% 7% 46%	1670
December	1st to 10th	Day	13% 7% 2 5% 4 3 5% 3 6% 7% 44%	2600
	11th to 20th	Day	21% 10% 2 4 2 3 3 2 4 6% 44%	2986
	21st to end	Day	16% 5% 2 5% 4 4 5% 4 6% 8% 40%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 4 5% 50%	6349
	11th to 20th	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	6195
	21st to end	Night	18% 7% 2 5% 4 3 4 4 5% 6% 42%	6490
February	1st to 10th	Night	17% 9% 3 6% 3 4 4 4 4 6% 41%	2600
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 4 6% 41%	2385
	21st to end	Night	17% 7% 3 6% 4 3 4 3 5% 5% 42%	1980
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 6% 44%	5330
	11th to 20th	Night	11% 8% 4 5% 3 3 4 3 4 5% 51%	4979
	21st to end	Night	15% 8% 3 6% 4 4 4 3 4 6% 43%	5149
April	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 6% 6% 39%	2000
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	1975
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 7% 44%	2000
May	1st to 10th	Night	23% 9% 3 7% 4 5% 4 4 5% 8% 27%	4175
	11th to 20th	Night	23% 9% 3 7% 4 4 3 3 5% 6% 32%	3575
	21st to end	Night	21% 6% 2 5% 4 3 3 3 6% 9% 37%	3926
June	1st to 10th	Night	16% 6% 3 5% 4 4 4 3 7% 8% 42%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 5% 6% 5% 6% 9% 40%	1600
	21st to end	Night	10% 4 1 5% 4 4 5% 5% 8% 11% 41%	1600
July	1st to 10th	Night	9% 7% 3 7% 5% 5% 6% 5% 7% 10% 38%	3600
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 5% 7% 9% 31%	3600
	21st to end	Night	12% 6% 2 5% 4 4 4 4 7% 8% 44%	4053
August	1st to 10th	Night	21% 6% 2 6% 4 4 5% 4 6% 8% 34%	2915
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 7% 42%	3158
	21st to end	Night	11% 4 2 5% 4 4 4 3 6% 8% 49%	3586
September	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 5% 6% 47%	2000
	11th to 20th	Night	14% 4 2 4 4 3 4 2 5% 6% 53%	2029
	21st to end	Night	19% 6% 2 6% 4 3 5% 4 5% 7% 39%	2200
October	1st to 10th	Night	18% 6% 3 6% 4 3 3 3 4 6% 44%	2400
	11th to 20th	Night	16% 5% 1 5% 3 3 3 3 4 6% 51%	2400
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 6% 49%	2705
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 4 6% 54%	2564
	11th to 20th	Night	17% 6% 2 5% 3 3 4 3 3 5% 48%	2600
	21st to end	Night	13% 6% 2 5% 4 5% 4 3 6% 7% 45%	2728
December	1st to 10th	Night	13% 8% 2 6% 3 3 4 3 4 6% 47%	4580
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 4 7% 41%	5208
	21st to end	Night	19% 7% 3 6% 4 4 3 3 5% 6% 40%	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	35% 3 10% 26% 23% 3	3598
	11th to 20th	Day	43% 4 9% 23% 17% 5%	3716
	21st to end	Day	44% 4 12% 24% 14% 1	4466
February	1st to 10th	Day	44% 5% 14% 25% 11% 2	1800
	11th to 20th	Day	44% 3 10% 24% 16% 2	1907
	21st to end	Day	45% 4 11% 24% 13% 4	1650
March	1st to 10th	Day	40% 4 9% 24% 16% 7%	4497
	11th to 20th	Day	36% 4 10% 29% 15% 5%	4980
	21st to end	Day	43% 3 9% 17% 23% 5%	5781
April	1st to 10th	Day	42% 5% 11% 22% 15% 5%	2400
	11th to 20th	Day	40% 5% 12% 22% 16% 4	2374
	21st to end	Day	40% 6% 11% 21% 18% 4	2400
May	1st to 10th	Day	52% 5% 14% 18% 10% 1	5814
	11th to 20th	Day	51% 4 10% 21% 12% 1	6322
	21st to end	Day	44% 6% 11% 24% 14% 1	6949
June	1st to 10th	Day	39% 7% 14% 23% 16% 1	2800
	11th to 20th	Day	41% 7% 13% 25% 11% 3	2800
	21st to end	Day	37% 7% 14% 25% 17% 1	2800
July	1st to 10th	Day	41% 7% 15% 19% 17% 2	6290
	11th to 20th	Day	47% 5% 16% 20% 10% 1	6285
	21st to end	Day	41% 5% 9% 21% 22% 1	6599
August	1st to 10th	Day	47% 5% 14% 22% 10% 1	4221
	11th to 20th	Day	37% 5% 12% 31% 15% 1	3891
	21st to end	Day	33% 5% 10% 28% 22% 2	4312
September	1st to 10th	Day	38% 6% 11% 21% 22% 3	2400
	11th to 20th	Day	32% 4 10% 27% 23% 4	2370
	21st to end	Day	48% 5% 8% 18% 19% 2	2197
October	1st to 10th	Day	36% 3 9% 19% 28% 5%	3260
	11th to 20th	Day	25% 3 11% 24% 29% 7%	3240
	21st to end	Day	25% 3 11% 28% 29% 5%	3425
November	1st to 10th	Day	33% 3 13% 25% 21% 4	1779
	11th to 20th	Day	36% 4 10% 26% 20% 3	1800
	21st to end	Day	37% 4 14% 31% 12% 2	1670
December	1st to 10th	Day	39% 3 16% 26% 13% 2	2600
	11th to 20th	Day	44% 3 11% 30% 11% 2	2986
	21st to end	Day	40% 6% 12% 23% 17% 2	3960
January	1st to 10th	Night	36% 3 8% 25% 21% 6%	6349
	11th to 20th	Night	38% 3 11% 24% 18% 7%	6240
	21st to end	Night	44% 3 9% 23% 19% 3	6490
February	1st to 10th	Night	44% 4 13% 25% 12% 2	2600
	11th to 20th	Night	45% 4 8% 21% 18% 5%	2387
	21st to end	Night	43% 4 9% 23% 17% 5%	1980
March	1st to 10th	Night	41% 4 9% 23% 13% 10%	5340
	11th to 20th	Night	36% 4 14% 26% 15% 5%	4980
	21st to end	Night	43% 4 7% 17% 25% 5%	5149
April	1st to 10th	Night	44% 5% 10% 20% 17% 4	2000
	11th to 20th	Night	42% 5% 10% 20% 20% 3	1975
	21st to end	Night	39% 5% 10% 21% 20% 5%	2000
May	1st to 10th	Night	54% 5% 12% 17% 11% 2	4182
	11th to 20th	Night	52% 5% 11% 18% 12% 1	3576
	21st to end	Night	44% 6% 13% 21% 14% 2	3927
June	1st to 10th	Night	39% 6% 12% 22% 20% 2	1600
	11th to 20th	Night	40% 7% 13% 25% 14% 1	1600
	21st to end	Night	34% 8% 15% 25% 15% 2	1600
July	1st to 10th	Night	42% 7% 16% 20% 13% 2	3560
	11th to 20th	Night	49% 5% 18% 19% 8% 1	3552
	21st to end	Night	37% 5% 13% 24% 20% 1	3934
August	1st to 10th	Night	48% 6% 18% 19% 9% 1	2904
	11th to 20th	Night	41% 5% 12% 26% 15% 1	3188
	21st to end	Night	33% 5% 11% 29% 21% 2	3586
September	1st to 10th	Night	38% 6% 10% 21% 22% 3	2000
	11th to 20th	Night	33% 4 9% 23% 27% 4	2029
	21st to end	Night	44% 5% 9% 20% 20% 2	2200
October	1st to 10th	Night	41% 3 10% 22% 21% 3	3920
	11th to 20th	Night	25% 3 10% 29% 29% 5%	3899
	21st to end	Night	27% 2 11% 32% 24% 4	4365
November	1st to 10th	Night	33% 4 12% 25% 21% 6%	2564
	11th to 20th	Night	39% 4 9% 22% 21% 5%	2600
	21st to end	Night	38% 4 13% 27% 16% 2	2728
December	1st to 10th	Night	38% 3 11% 31% 14% 2	4580
	11th to 20th	Night	45% 3 11% 24% 14% 4	5208
	21st to end	Night	44% 4 12% 20% 18% 2	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	26% 28% 31% 12% 2 2	200
	11th to 20th	Day	24% 36% 29% 9% 3	226
	21st to end	Day	24% 29% 28% 14% 5%	330
February	1st to 10th	Day	25% 26% 34% 11% 3	300
	11th to 20th	Day	32% 23% 32% 11% 2	446
	21st to end	Day	28% 23% 31% 14% 3	414
March	1st to 10th	Day	23% 24% 31% 18% 4	500
	11th to 20th	Day	22% 24% 34% 15% 4 1	500
	21st to end	Day	27% 21% 31% 15% 4 1	550
April	1st to 10th	Day	27% 22% 32% 12% 6% 1	500
	11th to 20th	Day	23% 21% 32% 16% 6% 2	492
	21st to end	Day	24% 19% 31% 19% 5% 2	500
May	1st to 10th	Day	30% 17% 28% 15% 9% 2	503
	11th to 20th	Day	24% 12% 30% 23% 9% 2	527
	21st to end	Day	22% 7% 24% 26% 17% 4	573
June	1st to 10th	Day	14% 14% 28% 26% 15% 3	530
	11th to 20th	Day	17% 10% 28% 27% 15% 3	530
	21st to end	Day	12% 10% 28% 30% 15% 5%	530
July	1st to 10th	Day	14% 3 20% 33% 23% 7%	525
	11th to 20th	Day	9% 4 22% 33% 27% 6%	524
	21st to end	Day	14% 4 25% 35% 18% 5%	576
August	1st to 10th	Day	12% 4 26% 32% 21% 6%	526
	11th to 20th	Day	14% 4 18% 36% 20% 8%	519
	21st to end	Day	13% 7% 25% 29% 18% 8%	583
September	1st to 10th	Day	18% 10% 22% 26% 18% 7%	530
	11th to 20th	Day	16% 5% 25% 30% 16% 9%	530
	21st to end	Day	25% 12% 21% 26% 13% 4	530
October	1st to 10th	Day	23% 7% 25% 22% 15% 8%	530
	11th to 20th	Day	18% 16% 28% 23% 12% 2	530
	21st to end	Day	22% 15% 32% 19% 10% 3	547
November	1st to 10th	Day	17% 13% 32% 24% 12% 2	401
	11th to 20th	Day	20% 19% 34% 22% 6% 1	400
	21st to end	Day	23% 19% 33% 20% 5% 1	270
December	1st to 10th	Day	25% 24% 37% 9% 6%	200
	11th to 20th	Day	32% 22% 32% 11% 5%	200
	21st to end	Day	25% 20% 33% 12% 9%	220
January	1st to 10th	Night	21% 19% 37% 20% 4	800
	11th to 20th	Night	25% 17% 31% 24% 4	800
	21st to end	Night	20% 18% 39% 22% 2	880
February	1st to 10th	Night	22% 15% 38% 24% 1	800
	11th to 20th	Night	26% 19% 38% 16% 1	783
	21st to end	Night	22% 19% 38% 18% 3	660
March	1st to 10th	Night	21% 19% 35% 23% 3	800
	11th to 20th	Night	21% 18% 37% 19% 5% 1	800
	21st to end	Night	20% 14% 40% 21% 5%	880
April	1st to 10th	Night	22% 19% 37% 18% 5%	800
	11th to 20th	Night	20% 13% 35% 26% 6%	791
	21st to end	Night	21% 12% 33% 29% 6%	800
May	1st to 10th	Night	24% 10% 32% 28% 6%	657
	11th to 20th	Night	20% 11% 33% 30% 6%	394
	21st to end	Night	19% 6% 21% 39% 13% 2	433
June	1st to 10th	Night	18% 5% 28% 37% 11% 1	400
	11th to 20th	Night	13% 6% 29% 41% 11% 1	400
	21st to end	Night	11% 4 25% 46% 14%	400
July	1st to 10th	Night	13% 4 21% 42% 19% 2	390
	11th to 20th	Night	9% 3 20% 44% 22% 2	388
	21st to end	Night	10% 4 24% 40% 20% 2	490
August	1st to 10th	Night	14% 2 17% 43% 23% 2	597
	11th to 20th	Night	15% 2 18% 38% 24% 2	763
	21st to end	Night	14% 4 19% 39% 23% 2	880
September	1st to 10th	Night	17% 5% 25% 34% 17% 3	800
	11th to 20th	Night	15% 4 20% 37% 21% 4	800
	21st to end	Night	24% 4 21% 33% 18% 1	800
October	1st to 10th	Night	22% 7% 23% 26% 19% 4	800
	11th to 20th	Night	19% 10% 27% 31% 12% 1	800
	21st to end	Night	21% 11% 33% 26% 9%	880
November	1st to 10th	Night	15% 8% 36% 28% 11% 1	789
	11th to 20th	Night	21% 14% 30% 26% 8% 1	800
	21st to end	Night	17% 17% 33% 25% 7% 1	800
December	1st to 10th	Night	25% 16% 33% 21% 6% 1	800
	11th to 20th	Night	21% 15% 36% 24% 6%	800
	21st to end	Night	22% 14% 34% 27% 3	880

Page generated at 2025-10-31 02:02:36. Version ac07

Copyright © 2025 Attilla Danko.