Walcott Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walcott Site clear sky chart from 2005-11-06 to 2025-05-11.

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	4 4 1 4 3 3 3 3 5% 7% 63%	118148
	Night	4 4 2 4 3 3 3 3 5% 7% 62%	119617
Transparency	Day	21% 4 8% 21% 29% 18%	117906
	Night	22% 4 8% 19% 28% 19%	119449
Seeing	Day	6% 7% 27% 33% 21% 6%	15869
	Night	6% 7% 38% 37% 11% 1	25974

By Month

Percentage Cloud Cover SW (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	6% 8% 3 6% 4 3 3 2 4 6% 55%	3638
	11th to 20th	Day	2 4 1 3 2 2 3 2 4 6% 72%	3884
	21st to end	Day	5% 5% 2 5% 4 3 4 3 5% 6% 57%	4994
February	1st to 10th	Day	3 6% 2 5% 2 3 3 3 4 6% 63%	2000
	11th to 20th	Day	5% 7% 2 6% 3 3 3 2 3 6% 60%	1949
	21st to end	Day	4 3 1 4 3 3 4 3 4 5% 66%	1650
March	1st to 10th	Day	4 4 1 3 2 3 3 2 4 6% 66%	4500
	11th to 20th	Day	7% 6% 1 4 3 2 3 3 5% 7% 60%	4645
	21st to end	Day	2 5% 2 5% 4 4 4 3 5% 8% 59%	5961
April	1st to 10th	Day	5% 2 1 3 3 3 3 2 5% 6% 67%	2400
	11th to 20th	Day	6% 4 1 4 3 3 3 3 4 6% 63%	2371
	21st to end	Day	3 3 1 3 3 2 3 3 4 7% 67%	2400
May	1st to 10th	Day	5% 3 2 3 2 2 3 3 5% 6% 65%	4136
	11th to 20th	Day	10% 5% 1 3 2 3 3 3 4 5% 61%	4083
	21st to end	Day	5% 2 1 3 2 2 3 3 4 5% 70%	4489
June	1st to 10th	Day	4 4 1 4 3 3 5% 4 6% 9% 56%	2518
	11th to 20th	Day	3 2 1 3 3 3 3 4 5% 7% 67%	2660
	21st to end	Day	3 3 1 4 4 3 3 3 7% 8% 61%	2660
July	1st to 10th	Day	7% 3 1 4 4 4 4 4 6% 8% 54%	4430
	11th to 20th	Day	3 2 1 4 3 4 4 4 6% 9% 60%	4413
	21st to end	Day	4 3 1 4 3 4 4 4 7% 9% 57%	4313
August	1st to 10th	Day	2 2 1 4 4 3 4 4 7% 10% 60%	3784
	11th to 20th	Day	3 2 1 3 2 3 3 3 6% 8% 65%	3726
	21st to end	Day	5% 3 1 4 3 4 5% 4 7% 10% 54%	4180
September	1st to 10th	Day	6% 4 1 4 4 3 4 4 6% 7% 57%	2280
	11th to 20th	Day	7% 5% 1 4 2 4 4 3 5% 6% 59%	2128
	21st to end	Day	6% 3 1 3 2 2 2 2 3 5% 70%	2090
October	1st to 10th	Day	5% 2 1 2 2 1 2 2 2 4 77%	3480
	11th to 20th	Day	6% 5% 2 4 2 2 2 2 4 6% 64%	3279
	21st to end	Day	3 2 1 2 1 1 2 2 4 5% 76%	3260
November	1st to 10th	Day	4 3 1 3 2 3 2 3 4 5% 70%	1763
	11th to 20th	Day	2 3 2 4 2 2 4 3 4 5% 70%	1800
	21st to end	Day	7% 5% 2 3 3 2 2 2 4 6% 64%	1800
December	1st to 10th	Day	3 3 1 3 2 2 4 3 4 6% 68%	2940
	11th to 20th	Day	2 4 1 4 4 4 4 3 5% 6% 63%	3391
	21st to end	Day	3 4 2 4 3 3 3 3 4 5% 68%	4153
January	1st to 10th	Night	4 7% 3 6% 4 3 4 3 5% 6% 57%	6327
	11th to 20th	Night	2 3 1 4 3 3 3 3 5% 6% 68%	6121
	21st to end	Night	3 5% 3 6% 3 3 4 3 5% 7% 59%	5984
February	1st to 10th	Night	4 4 2 4 3 3 3 3 4 6% 64%	2400
	11th to 20th	Night	5% 6% 2 5% 2 3 4 3 5% 7% 59%	2338
	21st to end	Night	5% 3 1 3 2 2 3 3 5% 7% 64%	1980
March	1st to 10th	Night	3 4 1 4 2 2 3 3 5% 5% 67%	5374
	11th to 20th	Night	5% 4 2 4 3 3 3 2 4 6% 64%	5330
	21st to end	Night	3 3 1 3 2 3 3 3 5% 8% 65%	4988
April	1st to 10th	Night	5% 3 1 3 3 2 3 3 4 6% 66%	2000
	11th to 20th	Night	6% 4 2 3 3 4 4 3 5% 6% 61%	1977
	21st to end	Night	5% 3 1 3 3 2 3 3 5% 7% 65%	2000
May	1st to 10th	Night	5% 4 1 3 3 3 3 3 4 6% 66%	3053
	11th to 20th	Night	7% 5% 2 5% 4 3 3 3 5% 6% 57%	2828
	21st to end	Night	6% 4 1 4 3 2 3 3 4 7% 64%	3098
June	1st to 10th	Night	5% 3 2 4 3 4 5% 5% 9% 11% 51%	1656
	11th to 20th	Night	2 2 1 3 3 3 3 4 7% 8% 65%	1520
	21st to end	Night	3 4 1 5% 3 5% 4 4 7% 10% 54%	1520
July	1st to 10th	Night	5% 4 1 6% 4 5% 6% 4 7% 9% 50%	2540
	11th to 20th	Night	3 5% 2 6% 5% 5% 5% 4 6% 10% 49%	2540
	21st to end	Night	4 3 1 5% 4 4 4 4 8% 13% 50%	3245
August	1st to 10th	Night	2 2 1 5% 5% 6% 7% 6% 9% 12% 45%	3155
	11th to 20th	Night	3 3 1 4 3 4 5% 5% 7% 11% 55%	3113
	21st to end	Night	4 4 2 7% 5% 5% 5% 5% 7% 10% 47%	3487
September	1st to 10th	Night	5% 4 2 4 4 3 3 3 6% 9% 58%	1900
	11th to 20th	Night	6% 5% 2 4 3 3 3 4 5% 7% 57%	2052
	21st to end	Night	5% 3 1 2 2 2 2 2 5% 6% 69%	2090
October	1st to 10th	Night	4 2 1 3 1 1 2 2 3 5% 76%	3480
	11th to 20th	Night	9% 4 1 3 2 2 3 3 5% 5% 63%	3640
	21st to end	Night	5% 2 1 3 2 2 2 2 3 5% 73%	4265
November	1st to 10th	Night	5% 2 1 2 2 3 2 2 4 5% 71%	2535
	11th to 20th	Night	2 3 1 3 2 2 2 3 3 6% 73%	2600
	21st to end	Night	6% 4 1 3 3 3 3 3 4 6% 65%	2600
December	1st to 10th	Night	4 3 1 3 3 2 3 3 4 6% 67%	4250
	11th to 20th	Night	3 2 1 3 3 3 2 3 5% 8% 67%	4816
	21st to end	Night	2 4 2 4 3 3 3 3 4 6% 65%	6815

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	33% 2 5% 22% 27% 10%	3639
	11th to 20th	Day	16% 2 7% 21% 34% 20%	3884
	21st to end	Day	28% 3 5% 18% 32% 14%	4994
February	1st to 10th	Day	23% 3 8% 20% 29% 17%	2000
	11th to 20th	Day	28% 2 6% 20% 27% 17%	1959
	21st to end	Day	21% 2 3 16% 35% 23%	1650
March	1st to 10th	Day	21% 2 3 9% 29% 35%	4500
	11th to 20th	Day	24% 3 4 16% 31% 22%	4645
	21st to end	Day	24% 4 6% 21% 32% 13%	5961
April	1st to 10th	Day	19% 3 6% 17% 31% 24%	2400
	11th to 20th	Day	24% 3 6% 18% 26% 24%	2374
	21st to end	Day	18% 3 9% 23% 31% 16%	2400
May	1st to 10th	Day	20% 4 7% 20% 19% 30%	4136
	11th to 20th	Day	26% 4 7% 18% 25% 20%	4087
	21st to end	Day	17% 4 8% 17% 19% 35%	4489
June	1st to 10th	Day	22% 6% 13% 27% 18% 13%	2518
	11th to 20th	Day	16% 5% 8% 19% 25% 27%	2660
	21st to end	Day	21% 5% 11% 25% 24% 15%	2660
July	1st to 10th	Day	27% 7% 12% 22% 25% 8%	4361
	11th to 20th	Day	19% 6% 13% 28% 28% 6%	4338
	21st to end	Day	21% 7% 12% 28% 28% 5%	4176
August	1st to 10th	Day	18% 6% 15% 31% 19% 11%	3772
	11th to 20th	Day	16% 5% 10% 22% 33% 14%	3762
	21st to end	Day	24% 7% 12% 26% 26% 6%	4179
September	1st to 10th	Day	25% 6% 11% 20% 24% 13%	2280
	11th to 20th	Day	26% 5% 9% 20% 29% 12%	2128
	21st to end	Day	19% 3 5% 22% 35% 16%	2090
October	1st to 10th	Day	14% 3 5% 12% 36% 30%	3480
	11th to 20th	Day	23% 3 8% 18% 26% 23%	3279
	21st to end	Day	13% 2 3 19% 38% 26%	3260
November	1st to 10th	Day	17% 3 6% 17% 37% 21%	1763
	11th to 20th	Day	17% 3 5% 22% 34% 19%	1800
	21st to end	Day	23% 3 4 17% 30% 24%	1800
December	1st to 10th	Day	18% 3 6% 28% 31% 13%	2940
	11th to 20th	Day	22% 2 5% 18% 37% 17%	3389
	21st to end	Day	20% 2 5% 25% 32% 16%	4153
January	1st to 10th	Night	30% 2 5% 25% 30% 8%	6327
	11th to 20th	Night	18% 3 8% 19% 34% 18%	6121
	21st to end	Night	24% 3 8% 19% 30% 15%	5984
February	1st to 10th	Night	23% 3 5% 19% 29% 22%	2400
	11th to 20th	Night	26% 4 6% 19% 25% 20%	2338
	21st to end	Night	20% 2 3 17% 31% 27%	1980
March	1st to 10th	Night	19% 2 3 11% 28% 37%	5374
	11th to 20th	Night	22% 4 5% 16% 23% 31%	5330
	21st to end	Night	18% 3 8% 20% 32% 19%	4988
April	1st to 10th	Night	20% 3 6% 19% 29% 24%	2000
	11th to 20th	Night	24% 4 7% 19% 22% 23%	1977
	21st to end	Night	20% 4 10% 22% 26% 18%	2000
May	1st to 10th	Night	19% 5% 9% 15% 18% 34%	3053
	11th to 20th	Night	27% 7% 9% 19% 19% 20%	2829
	21st to end	Night	20% 5% 9% 12% 13% 40%	3099
June	1st to 10th	Night	23% 9% 14% 22% 18% 13%	1656
	11th to 20th	Night	16% 6% 12% 15% 26% 25%	1520
	21st to end	Night	23% 6% 14% 26% 18% 14%	1520
July	1st to 10th	Night	29% 7% 15% 28% 17% 5%	2500
	11th to 20th	Night	28% 6% 11% 20% 27% 7%	2492
	21st to end	Night	24% 6% 13% 29% 23% 4	3143
August	1st to 10th	Night	27% 8% 13% 22% 17% 13%	3144
	11th to 20th	Night	22% 6% 11% 26% 28% 8%	3143
	21st to end	Night	30% 7% 15% 21% 21% 6%	3487
September	1st to 10th	Night	24% 6% 13% 22% 23% 13%	1900
	11th to 20th	Night	26% 5% 11% 23% 22% 13%	2052
	21st to end	Night	17% 3 8% 22% 33% 17%	2090
October	1st to 10th	Night	13% 4 5% 10% 33% 35%	3480
	11th to 20th	Night	23% 4 8% 15% 27% 24%	3640
	21st to end	Night	16% 2 4 16% 39% 23%	4265
November	1st to 10th	Night	16% 3 6% 18% 32% 25%	2535
	11th to 20th	Night	15% 3 3 23% 36% 21%	2600
	21st to end	Night	22% 3 5% 17% 32% 22%	2600
December	1st to 10th	Night	19% 3 6% 28% 34% 11%	4250
	11th to 20th	Night	17% 2 4 16% 41% 20%	4817
	21st to end	Night	20% 3 6% 20% 36% 16%	6815

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	7% 11% 42% 28% 11% 1	300
	11th to 20th	Day	8% 16% 38% 24% 11% 2	300
	21st to end	Day	5% 10% 35% 36% 12% 2	330
February	1st to 10th	Day	5% 12% 43% 29% 10% 1	300
	11th to 20th	Day	11% 11% 44% 24% 9% 1	300
	21st to end	Day	8% 10% 49% 24% 8%	249
March	1st to 10th	Day	7% 11% 44% 29% 10%	300
	11th to 20th	Day	14% 10% 39% 25% 11% 1	300
	21st to end	Day	7% 17% 41% 22% 11% 1	542
April	1st to 10th	Day	7% 15% 42% 26% 9% 1	500
	11th to 20th	Day	8% 16% 44% 26% 5% 1	492
	21st to end	Day	5% 15% 40% 29% 10% 1	500
May	1st to 10th	Day	6% 11% 36% 33% 12% 2	628
	11th to 20th	Day	10% 8% 28% 32% 18% 3	676
	21st to end	Day	5% 8% 33% 37% 15% 1	734
June	1st to 10th	Day	5% 4 22% 40% 23% 6%	680
	11th to 20th	Day	3 2 29% 41% 20% 4	680
	21st to end	Day	4 2 19% 43% 28% 4	680
July	1st to 10th	Day	6% 1 9% 35% 36% 14%	671
	11th to 20th	Day	3 1 7% 36% 40% 14%	668
	21st to end	Day	2 1 6% 32% 43% 16%	648
August	1st to 10th	Day	2 1 8% 37% 41% 11%	487
	11th to 20th	Day	4 1 8% 33% 39% 16%	477
	21st to end	Day	6% 1 7% 36% 39% 12%	539
September	1st to 10th	Day	6% 1 10% 39% 33% 11%	490
	11th to 20th	Day	4 1 12% 38% 34% 10%	338
	21st to end	Day	10% 2 17% 37% 26% 8%	300
October	1st to 10th	Day	10% 5% 25% 31% 23% 6%	300
	11th to 20th	Day	8% 5% 30% 34% 18% 6%	300
	21st to end	Day	10% 8% 29% 26% 20% 6%	330
November	1st to 10th	Day	9% 9% 30% 31% 17% 4	300
	11th to 20th	Day	4 9% 40% 32% 10% 5%	300
	21st to end	Day	13% 11% 37% 23% 13% 3	300
December	1st to 10th	Day	6% 11% 35% 31% 14% 2	300
	11th to 20th	Day	5% 7% 43% 32% 12% 1	300
	21st to end	Day	9% 12% 45% 23% 11% 1	330
January	1st to 10th	Night	5% 11% 41% 35% 8%	1000
	11th to 20th	Night	6% 9% 42% 34% 9% 1	915
	21st to end	Night	5% 7% 45% 35% 8%	880
February	1st to 10th	Night	6% 9% 49% 30% 6%	800
	11th to 20th	Night	8% 11% 53% 23% 4 1	784
	21st to end	Night	7% 13% 51% 27% 3	660
March	1st to 10th	Night	8% 12% 48% 28% 4	800
	11th to 20th	Night	10% 13% 41% 30% 6%	755
	21st to end	Night	6% 13% 54% 25% 3	660
April	1st to 10th	Night	6% 12% 52% 28% 2	600
	11th to 20th	Night	7% 12% 57% 22% 3	593
	21st to end	Night	5% 11% 53% 25% 6%	600
May	1st to 10th	Night	6% 10% 48% 31% 5%	600
	11th to 20th	Night	6% 9% 37% 38% 9%	563
	21st to end	Night	6% 7% 47% 34% 6%	617
June	1st to 10th	Night	3 5% 38% 44% 9%	570
	11th to 20th	Night	2 4 48% 39% 6% 1	570
	21st to end	Night	3 3 31% 50% 12%	570
July	1st to 10th	Night	5% 1 17% 52% 24% 1	555
	11th to 20th	Night	3 2 12% 55% 26% 1	552
	21st to end	Night	5% 1 11% 55% 28% 1	624
August	1st to 10th	Night	2 1 12% 51% 32% 2	567
	11th to 20th	Night	4 1 14% 53% 26% 2	564
	21st to end	Night	6% 1 13% 49% 29% 2	627
September	1st to 10th	Night	5% 1 17% 53% 22% 1	570
	11th to 20th	Night	8% 1 30% 46% 15%	570
	21st to end	Night	7% 3 25% 47% 17% 1	570
October	1st to 10th	Night	7% 3 33% 44% 13%	570
	11th to 20th	Night	7% 5% 38% 39% 11% 1	674
	21st to end	Night	7% 8% 33% 35% 17% 1	919
November	1st to 10th	Night	6% 5% 36% 39% 12% 1	975
	11th to 20th	Night	3 8% 39% 37% 12%	1000
	21st to end	Night	7% 7% 38% 35% 11% 1	1000
December	1st to 10th	Night	7% 9% 38% 38% 9%	1000
	11th to 20th	Night	7% 8% 46% 33% 6% 1	1000
	21st to end	Night	7% 12% 43% 30% 8%	1100

Page generated at 2025-05-08 08:33:58. Version ac07

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