Walgrove Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walgrove Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	15% 6% 2 6% 5% 4 4 4 6% 7% 41%	101490
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	98051
Transparency	Day	39% 8% 12% 23% 15% 3	105128
	Night	39% 7% 12% 23% 16% 3	102553
Seeing	Day	19% 15% 29% 22% 11% 3	15033
	Night	18% 13% 30% 27% 11% 1	24491

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	16% 6% 2 5% 3 3 4 4 5% 5% 46%	2517
	11th to 20th	Day	14% 11% 4 8% 6% 4 5% 3 5% 5% 37%	2817
	21st to end	Day	19% 9% 3 7% 5% 4 4 4 4 5% 35%	3124
February	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 6% 38%	1710
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 4 5% 6% 40%	1728
	21st to end	Day	18% 10% 3 6% 4 4 4 2 5% 5% 39%	1570
March	1st to 10th	Day	22% 7% 2 5% 4 4 4 3 5% 8% 36%	3329
	11th to 20th	Day	9% 6% 2 5% 4 4 5% 5% 5% 7% 48%	3480
	21st to end	Day	20% 8% 2 6% 4 3 4 3 4 5% 42%	3910
April	1st to 10th	Day	14% 8% 2 5% 5% 5% 4 4 7% 8% 39%	2280
	11th to 20th	Day	13% 6% 3 6% 5% 4 4 3 5% 6% 46%	2254
	21st to end	Day	18% 6% 2 5% 4 4 5% 3 6% 6% 41%	2280
May	1st to 10th	Day	27% 7% 2 5% 4 4 4 3 5% 8% 29%	4001
	11th to 20th	Day	23% 7% 3 6% 5% 5% 4 4 6% 8% 29%	4366
	21st to end	Day	11% 4 2 5% 4 4 5% 4 6% 8% 45%	4823
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 5% 7% 10% 40%	2660
	11th to 20th	Day	11% 6% 2 6% 5% 6% 5% 4 6% 9% 41%	2660
	21st to end	Day	10% 4 1 5% 4 5% 4 5% 7% 9% 45%	2660
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 10% 42%	4440
	11th to 20th	Day	9% 5% 2 5% 5% 5% 6% 5% 8% 10% 40%	4417
	21st to end	Day	11% 7% 2 7% 6% 6% 6% 5% 6% 9% 35%	4466
August	1st to 10th	Day	18% 5% 2 7% 5% 5% 5% 6% 7% 8% 33%	4104
	11th to 20th	Day	11% 5% 2 6% 6% 5% 5% 5% 7% 8% 40%	3857
	21st to end	Day	12% 5% 2 5% 4 4 4 4 5% 8% 47%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 9% 47%	2280
	21st to end	Day	21% 7% 2 6% 4 4 3 4 5% 7% 36%	2012
October	1st to 10th	Day	17% 6% 1 5% 3 3 4 4 5% 7% 45%	1900
	11th to 20th	Day	17% 4 1 4 4 3 4 4 4 5% 48%	1900
	21st to end	Day	14% 6% 2 6% 4 4 4 3 4 5% 48%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 6% 7% 51%	1810
	11th to 20th	Day	14% 6% 1 4 3 3 4 3 6% 6% 49%	1785
	21st to end	Day	15% 7% 2 5% 4 4 4 3 5% 5% 47%	1598
December	1st to 10th	Day	17% 7% 2 5% 4 4 4 4 5% 7% 38%	1520
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 5% 7% 42%	1910
	21st to end	Day	17% 7% 3 7% 4 4 4 3 6% 8% 37%	2772
January	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	4429
	11th to 20th	Night	18% 8% 2 5% 3 3 3 3 4 6% 45%	4138
	21st to end	Night	21% 9% 3 5% 4 3 3 3 5% 5% 38%	4532
February	1st to 10th	Night	15% 8% 3 6% 5% 3 4 3 5% 7% 40%	2470
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 5% 5% 42%	2337
	21st to end	Night	16% 8% 2 7% 5% 4 3 3 5% 5% 41%	1884
March	1st to 10th	Night	21% 8% 2 6% 4 4 4 3 4 6% 39%	3521
	11th to 20th	Night	11% 5% 2 5% 4 4 3 4 6% 6% 51%	3480
	21st to end	Night	15% 9% 3 6% 4 3 3 2 3 5% 48%	3722
April	1st to 10th	Night	15% 8% 3 5% 4 6% 3 3 6% 7% 40%	1900
	11th to 20th	Night	13% 5% 2 7% 4 4 3 3 6% 6% 46%	1875
	21st to end	Night	18% 6% 2 4 3 4 4 4 6% 7% 43%	1900
May	1st to 10th	Night	25% 9% 2 6% 4 3 4 3 5% 6% 32%	2921
	11th to 20th	Night	24% 9% 3 7% 5% 4 4 3 5% 6% 30%	2510
	21st to end	Night	11% 5% 1 6% 4 4 5% 4 6% 10% 43%	2732
June	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 7% 9% 45%	1520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 4 5% 7% 10% 41%	1520
	21st to end	Night	10% 5% 2 5% 3 5% 5% 4 7% 11% 43%	1520
July	1st to 10th	Night	6% 6% 3 7% 4 7% 6% 6% 6% 10% 39%	2520
	11th to 20th	Night	9% 5% 2 6% 5% 5% 6% 4 8% 10% 39%	2539
	21st to end	Night	12% 6% 3 7% 5% 4 5% 4 6% 9% 40%	3087
August	1st to 10th	Night	19% 6% 2 6% 4 4 5% 4 7% 10% 34%	2825
	11th to 20th	Night	12% 7% 3 8% 5% 6% 5% 5% 6% 7% 37%	2939
	21st to end	Night	12% 5% 2 5% 4 3 4 4 5% 8% 48%	3476
September	1st to 10th	Night	13% 6% 2 6% 3 3 3 4 5% 8% 46%	1900
	11th to 20th	Night	11% 6% 2 6% 3 3 3 3 4 8% 52%	1900
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 5% 40%	2164
October	1st to 10th	Night	21% 6% 3 5% 4 4 3 3 5% 5% 44%	2280
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 5% 6% 52%	2280
	21st to end	Night	15% 6% 1 5% 3 3 3 3 5% 6% 49%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 6% 53%	2485
	11th to 20th	Night	14% 8% 2 5% 3 3 4 3 4 6% 48%	2610
	21st to end	Night	13% 5% 2 5% 5% 3 4 4 5% 7% 47%	2798
December	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 6% 40%	2660
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 5% 6% 43%	3285
	21st to end	Night	16% 7% 2 6% 5% 5% 5% 5% 6% 9% 35%	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	37% 6% 11% 30% 13% 4	2518
	11th to 20th	Day	47% 7% 9% 21% 12% 4	2817
	21st to end	Day	48% 8% 11% 22% 9% 2	3124
February	1st to 10th	Day	43% 6% 14% 24% 11% 2	1710
	11th to 20th	Day	43% 5% 11% 25% 13% 3	1737
	21st to end	Day	47% 5% 11% 20% 13% 4	1570
March	1st to 10th	Day	43% 7% 13% 23% 12% 2	3329
	11th to 20th	Day	31% 9% 11% 27% 19% 4	3480
	21st to end	Day	45% 6% 9% 20% 17% 3	3910
April	1st to 10th	Day	40% 8% 13% 18% 16% 5%	2280
	11th to 20th	Day	38% 6% 11% 22% 17% 5%	2254
	21st to end	Day	42% 6% 11% 20% 18% 3	2280
May	1st to 10th	Day	51% 9% 10% 19% 10% 1	4038
	11th to 20th	Day	49% 11% 15% 16% 7% 1	4370
	21st to end	Day	33% 10% 14% 25% 16% 2	4837
June	1st to 10th	Day	37% 9% 14% 23% 15% 2	2660
	11th to 20th	Day	40% 8% 13% 23% 14% 2	2660
	21st to end	Day	33% 9% 15% 23% 18% 2	2660
July	1st to 10th	Day	35% 11% 16% 20% 13% 5%	4370
	11th to 20th	Day	35% 10% 16% 26% 11% 1	4342
	21st to end	Day	43% 9% 13% 17% 17% 1	4324
August	1st to 10th	Day	46% 10% 13% 23% 8%	4091
	11th to 20th	Day	37% 9% 12% 28% 14% 1	3896
	21st to end	Day	34% 8% 13% 27% 17% 2	4180
September	1st to 10th	Day	38% 7% 12% 19% 20% 4	2280
	11th to 20th	Day	34% 6% 10% 25% 21% 3	2280
	21st to end	Day	46% 6% 9% 18% 18% 2	2012
October	1st to 10th	Day	37% 5% 8% 18% 27% 5%	3160
	11th to 20th	Day	26% 5% 10% 26% 23% 10%	3140
	21st to end	Day	24% 4 10% 29% 29% 4	3428
November	1st to 10th	Day	33% 5% 13% 25% 21% 4	1810
	11th to 20th	Day	35% 5% 12% 22% 23% 3	1785
	21st to end	Day	39% 5% 11% 29% 14% 2	1598
December	1st to 10th	Day	43% 6% 15% 23% 11% 2	1520
	11th to 20th	Day	40% 6% 11% 30% 11% 2	1906
	21st to end	Day	43% 9% 16% 20% 11% 1	2772
January	1st to 10th	Night	39% 7% 12% 25% 14% 3	4429
	11th to 20th	Night	40% 5% 11% 26% 15% 2	4138
	21st to end	Night	45% 6% 8% 26% 12% 2	4532
February	1st to 10th	Night	43% 6% 14% 23% 13% 1	2470
	11th to 20th	Night	43% 4 10% 20% 18% 4	2339
	21st to end	Night	44% 6% 10% 21% 15% 5%	1884
March	1st to 10th	Night	44% 8% 9% 25% 12% 2	3521
	11th to 20th	Night	31% 8% 10% 32% 15% 3	3480
	21st to end	Night	40% 5% 8% 20% 21% 5%	3722
April	1st to 10th	Night	42% 7% 11% 19% 18% 4	1900
	11th to 20th	Night	38% 6% 11% 20% 19% 5%	1875
	21st to end	Night	39% 7% 11% 21% 17% 5%	1900
May	1st to 10th	Night	51% 7% 10% 18% 12% 2	2950
	11th to 20th	Night	51% 11% 12% 14% 10% 1	2511
	21st to end	Night	33% 11% 17% 25% 12% 3	2739
June	1st to 10th	Night	34% 8% 12% 27% 17% 2	1520
	11th to 20th	Night	36% 10% 13% 24% 14% 2	1520
	21st to end	Night	33% 9% 14% 24% 17% 3	1520
July	1st to 10th	Night	38% 11% 20% 16% 11% 4	2480
	11th to 20th	Night	36% 11% 17% 23% 11% 2	2491
	21st to end	Night	41% 8% 13% 20% 16% 1	2988
August	1st to 10th	Night	44% 9% 18% 19% 8% 1	2814
	11th to 20th	Night	42% 8% 10% 26% 13% 1	2966
	21st to end	Night	33% 7% 12% 30% 16% 2	3476
September	1st to 10th	Night	37% 6% 11% 21% 20% 5%	1900
	11th to 20th	Night	33% 5% 10% 25% 23% 3	1900
	21st to end	Night	45% 7% 9% 18% 18% 3	2164
October	1st to 10th	Night	42% 6% 10% 18% 20% 4	3800
	11th to 20th	Night	24% 4 11% 25% 29% 7%	3779
	21st to end	Night	24% 4 11% 31% 24% 5%	4120
November	1st to 10th	Night	32% 5% 9% 26% 22% 5%	2485
	11th to 20th	Night	37% 5% 9% 23% 20% 5%	2610
	21st to end	Night	36% 6% 12% 25% 19% 2	2798
December	1st to 10th	Night	44% 5% 12% 24% 14% 1	2660
	11th to 20th	Night	41% 6% 11% 22% 18% 3	3288
	21st to end	Night	41% 10% 16% 22% 10% 1	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	29% 27% 30% 10% 4	180
	11th to 20th	Day	29% 27% 32% 11% 1	265
	21st to end	Day	24% 31% 30% 12% 3	297
February	1st to 10th	Day	25% 28% 35% 9% 2	270
	11th to 20th	Day	33% 25% 36% 4 1	335
	21st to end	Day	30% 26% 29% 13% 2	382
March	1st to 10th	Day	23% 27% 31% 17% 3	460
	11th to 20th	Day	28% 24% 30% 13% 3 1	460
	21st to end	Day	25% 23% 32% 15% 4 1	506
April	1st to 10th	Day	22% 24% 35% 13% 4 2	460
	11th to 20th	Day	19% 21% 31% 17% 9% 2	452
	21st to end	Day	26% 18% 35% 14% 7% 1	460
May	1st to 10th	Day	28% 17% 30% 16% 7% 2	463
	11th to 20th	Day	23% 13% 31% 22% 9% 2	487
	21st to end	Day	17% 8% 27% 27% 19% 2	529
June	1st to 10th	Day	13% 13% 31% 22% 16% 4	490
	11th to 20th	Day	17% 8% 29% 29% 14% 3	490
	21st to end	Day	12% 9% 31% 31% 13% 4	490
July	1st to 10th	Day	12% 4 23% 33% 19% 9%	485
	11th to 20th	Day	5% 4 25% 35% 24% 7%	484
	21st to end	Day	11% 6% 28% 32% 20% 3	532
August	1st to 10th	Day	12% 7% 22% 32% 21% 7%	486
	11th to 20th	Day	13% 4 23% 33% 19% 8%	479
	21st to end	Day	10% 5% 24% 31% 20% 9%	539
September	1st to 10th	Day	17% 9% 19% 27% 19% 10%	490
	11th to 20th	Day	11% 6% 22% 32% 20% 8%	490
	21st to end	Day	26% 13% 21% 26% 11% 3	490
October	1st to 10th	Day	24% 9% 24% 22% 14% 6%	490
	11th to 20th	Day	18% 17% 27% 23% 13% 2	490
	21st to end	Day	17% 15% 37% 22% 8% 2	539
November	1st to 10th	Day	16% 15% 34% 23% 10% 2	420
	11th to 20th	Day	19% 20% 34% 17% 10%	385
	21st to end	Day	28% 19% 31% 17% 6%	200
December	1st to 10th	Day	25% 23% 36% 13% 2 1	180
	11th to 20th	Day	28% 23% 31% 14% 3	180
	21st to end	Day	26% 21% 31% 16% 6%	198
January	1st to 10th	Night	21% 22% 37% 18% 2	760
	11th to 20th	Night	23% 19% 33% 20% 4	760
	21st to end	Night	19% 21% 39% 19% 1	836
February	1st to 10th	Night	20% 20% 35% 23% 2	760
	11th to 20th	Night	25% 24% 36% 14% 1	743
	21st to end	Night	24% 21% 38% 14% 3	628
March	1st to 10th	Night	20% 19% 38% 19% 3	760
	11th to 20th	Night	22% 21% 34% 18% 5%	760
	21st to end	Night	20% 17% 41% 17% 4	836
April	1st to 10th	Night	20% 24% 35% 18% 4	760
	11th to 20th	Night	16% 17% 36% 24% 7%	751
	21st to end	Night	22% 16% 32% 23% 7%	760
May	1st to 10th	Night	23% 10% 30% 28% 9% 1	641
	11th to 20th	Night	21% 11% 29% 29% 10% 1	389
	21st to end	Night	16% 5% 24% 36% 18% 2	411
June	1st to 10th	Night	12% 4 29% 34% 20%	380
	11th to 20th	Night	12% 4 26% 39% 17% 1	380
	21st to end	Night	12% 2 26% 46% 14%	380
July	1st to 10th	Night	9% 2 20% 41% 24% 4	370
	11th to 20th	Night	8% 3 17% 42% 29% 2	374
	21st to end	Night	9% 3 20% 40% 25% 3	624
August	1st to 10th	Night	13% 2 15% 41% 26% 2	567
	11th to 20th	Night	14% 3 18% 38% 25% 3	653
	21st to end	Night	13% 4 17% 37% 25% 4	836
September	1st to 10th	Night	16% 4 21% 34% 22% 3	760
	11th to 20th	Night	11% 5% 18% 37% 25% 4	760
	21st to end	Night	24% 5% 23% 30% 17% 1	760
October	1st to 10th	Night	22% 8% 22% 27% 17% 4	760
	11th to 20th	Night	17% 12% 27% 31% 13% 1	760
	21st to end	Night	19% 14% 30% 25% 12%	836
November	1st to 10th	Night	14% 12% 33% 30% 10% 1	780
	11th to 20th	Night	18% 20% 31% 24% 7% 1	800
	21st to end	Night	17% 19% 34% 23% 7% 1	800
December	1st to 10th	Night	23% 16% 36% 18% 7%	760
	11th to 20th	Night	18% 16% 40% 21% 5%	760
	21st to end	Night	21% 17% 35% 24% 2	836

Page generated at 2024-11-27 17:30:53. Version ac07

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