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 2026-09-14.

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%	157210
	Night	4 4 2 4 3 3 4 3 5% 8% 60%	151383
Transparency	Day	21% 4 8% 21% 28% 17%	152294
	Night	22% 4 8% 20% 28% 18%	147308
Seeing	Day	6% 7% 27% 33% 22% 6%	17802
	Night	6% 7% 38% 38% 11% 1	27682

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	5% 7% 3 6% 4 4 4 3 4 6% 55%	4720
	11th to 20th	Day	1 4 1 3 2 2 2 2 3 5% 76%	5054
	21st to end	Day	7% 5% 2 5% 4 4 4 3 5% 6% 57%	6468
February	1st to 10th	Day	3 6% 2 5% 2 3 3 3 4 6% 64%	2100
	11th to 20th	Day	5% 6% 2 6% 4 3 3 2 4 6% 60%	2049
	21st to end	Day	4 3 1 3 3 3 3 3 4 5% 67%	1730
March	1st to 10th	Day	4 4 1 3 2 3 3 2 4 6% 67%	4600
	11th to 20th	Day	7% 6% 1 4 3 2 3 2 4 7% 61%	4747
	21st to end	Day	2 5% 2 5% 3 4 4 3 5% 8% 59%	6093
April	1st to 10th	Day	4 2 1 3 3 3 3 2 5% 7% 67%	2520
	11th to 20th	Day	5% 4 1 4 3 3 3 3 5% 6% 63%	2491
	21st to end	Day	3 3 1 3 3 3 3 3 4 7% 67%	2520
May	1st to 10th	Day	8% 4 2 3 3 3 3 3 5% 7% 59%	7433
	11th to 20th	Day	7% 3 1 3 2 2 2 2 3 4 72%	7627
	21st to end	Day	8% 5% 2 5% 2 3 4 3 5% 7% 58%	8404
June	1st to 10th	Day	4 4 2 4 3 4 5% 4 6% 9% 55%	2784
	11th to 20th	Day	3 2 1 3 3 3 3 4 5% 7% 67%	2940
	21st to end	Day	3 3 1 4 4 3 4 4 6% 8% 60%	2940
July	1st to 10th	Day	5% 3 1 5% 4 4 4 4 6% 9% 54%	8249
	11th to 20th	Day	2 2 1 4 3 4 4 4 7% 10% 58%	8233
	21st to end	Day	2 2 1 3 3 3 4 4 7% 10% 60%	8046
August	1st to 10th	Day	1 2 1 3 3 3 4 4 6% 9% 65%	6988
	11th to 20th	Day	2 2 1 3 3 4 5% 4 7% 10% 58%	7003
	21st to end	Day	4 3 1 4 3 4 4 4 6% 9% 57%	7751
September	1st to 10th	Day	6% 4 1 4 4 3 4 4 6% 8% 57%	2520
	11th to 20th	Day	7% 5% 1 4 3 4 4 3 5% 6% 59%	2275
	21st to end	Day	6% 3 1 3 2 2 2 2 4 6% 69%	2200
October	1st to 10th	Day	5% 2 1 2 2 1 2 2 2 4 77%	3590
	11th to 20th	Day	6% 4 2 4 3 2 2 2 4 6% 64%	3383
	21st to end	Day	3 2 1 2 1 1 2 2 4 5% 76%	3364
November	1st to 10th	Day	4 2 1 3 2 2 2 2 4 5% 71%	1853
	11th to 20th	Day	2 3 2 3 2 2 4 3 4 5% 70%	1890
	21st to end	Day	7% 5% 2 3 3 2 2 2 4 6% 65%	1890
December	1st to 10th	Day	3 3 1 3 2 2 4 3 4 6% 69%	3030
	11th to 20th	Day	2 3 1 4 3 4 4 3 5% 6% 64%	3481
	21st to end	Day	3 4 2 4 3 3 3 3 4 5% 68%	4244
January	1st to 10th	Night	4 6% 3 6% 4 4 5% 4 5% 6% 55%	8201
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 5% 74%	7962
	21st to end	Night	4 5% 2 6% 3 3 4 3 5% 7% 57%	7746
February	1st to 10th	Night	4 4 2 4 3 3 3 3 4 6% 65%	2520
	11th to 20th	Night	4 6% 2 5% 2 3 3 3 5% 7% 59%	2458
	21st to end	Night	5% 3 1 3 2 2 3 3 5% 7% 65%	2076
March	1st to 10th	Night	3 4 1 4 2 2 3 3 5% 5% 67%	5494
	11th to 20th	Night	5% 4 1 4 3 3 3 2 4 6% 65%	5448
	21st to end	Night	3 3 1 3 2 3 3 3 5% 8% 65%	5098
April	1st to 10th	Night	4 3 1 3 3 3 3 3 4 6% 66%	2100
	11th to 20th	Night	5% 4 2 3 3 4 4 3 5% 6% 61%	2077
	21st to end	Night	5% 3 1 3 3 3 3 3 5% 7% 64%	2100
May	1st to 10th	Night	7% 4 1 4 3 3 3 3 4 6% 60%	5500
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 6% 68%	5265
	21st to end	Night	7% 6% 2 4 4 3 4 4 5% 7% 55%	5803
June	1st to 10th	Night	5% 3 1 4 3 4 5% 5% 9% 11% 51%	1830
	11th to 20th	Night	2 2 1 3 3 3 3 3 7% 8% 65%	1680
	21st to end	Night	3 4 1 5% 2 5% 4 4 7% 10% 54%	1680
July	1st to 10th	Night	4 5% 2 6% 5% 5% 6% 5% 7% 9% 46%	4740
	11th to 20th	Night	3 4 2 6% 4 5% 4 4 6% 12% 51%	4740
	21st to end	Night	3 3 1 5% 4 5% 5% 4 8% 12% 49%	6058
August	1st to 10th	Night	2 3 1 4 4 5% 6% 5% 9% 13% 49%	5834
	11th to 20th	Night	3 3 2 5% 4 5% 6% 5% 8% 11% 48%	5853
	21st to end	Night	5% 4 2 7% 5% 5% 6% 5% 8% 11% 43%	6465
September	1st to 10th	Night	5% 4 2 4 4 3 4 3 6% 8% 57%	2100
	11th to 20th	Night	6% 5% 2 4 3 3 4 4 5% 7% 57%	2191
	21st to end	Night	5% 3 1 2 2 2 3 2 5% 6% 69%	2200
October	1st to 10th	Night	4 2 1 3 1 1 2 2 3 5% 76%	3590
	11th to 20th	Night	9% 4 1 3 2 2 3 3 5% 5% 63%	3756
	21st to end	Night	4 2 1 3 2 2 2 2 3 5% 73%	4402
November	1st to 10th	Night	5% 2 1 2 2 2 2 2 4 5% 72%	2665
	11th to 20th	Night	2 3 1 3 2 2 2 3 3 7% 72%	2730
	21st to end	Night	5% 4 1 3 3 3 3 3 4 6% 65%	2730
December	1st to 10th	Night	4 3 1 3 3 2 3 3 4 6% 67%	4380
	11th to 20th	Night	3 2 1 3 3 3 2 3 5% 8% 68%	4946
	21st to end	Night	2 4 2 4 3 3 3 3 4 6% 66%	6965

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	31% 2 5% 22% 29% 10%	4723
	11th to 20th	Day	14% 2 8% 18% 37% 22%	5054
	21st to end	Day	29% 3 5% 15% 33% 15%	6468
February	1st to 10th	Day	22% 2 7% 20% 31% 17%	2100
	11th to 20th	Day	28% 2 6% 20% 28% 16%	2059
	21st to end	Day	20% 2 3 16% 35% 23%	1730
March	1st to 10th	Day	20% 2 3 9% 29% 35%	4600
	11th to 20th	Day	24% 3 4 16% 31% 22%	4747
	21st to end	Day	23% 4 6% 21% 32% 14%	6093
April	1st to 10th	Day	19% 3 6% 17% 31% 24%	2520
	11th to 20th	Day	24% 3 6% 18% 25% 24%	2494
	21st to end	Day	18% 3 9% 23% 31% 17%	2520
May	1st to 10th	Day	26% 4 9% 20% 22% 20%	7433
	11th to 20th	Day	18% 3 4 13% 27% 35%	7631
	21st to end	Day	26% 5% 10% 17% 15% 27%	8405
June	1st to 10th	Day	24% 6% 13% 27% 18% 13%	2784
	11th to 20th	Day	16% 5% 8% 20% 25% 26%	2940
	21st to end	Day	22% 5% 11% 24% 23% 14%	2940
July	1st to 10th	Day	25% 6% 11% 25% 27% 6%	8181
	11th to 20th	Day	19% 7% 16% 31% 22% 6%	8158
	21st to end	Day	18% 6% 16% 32% 26% 3	7867
August	1st to 10th	Day	15% 5% 12% 29% 30% 9%	5532
	11th to 20th	Day	17% 5% 13% 24% 31% 10%	5519
	21st to end	Day	20% 6% 13% 27% 29% 5%	6078
September	1st to 10th	Day	25% 6% 11% 22% 24% 12%	2520
	11th to 20th	Day	25% 5% 8% 21% 29% 12%	2275
	21st to end	Day	19% 3 6% 22% 35% 16%	2200
October	1st to 10th	Day	14% 3 5% 12% 36% 30%	3590
	11th to 20th	Day	23% 3 8% 18% 26% 23%	3383
	21st to end	Day	13% 2 3 19% 38% 25%	3364
November	1st to 10th	Day	17% 3 5% 17% 38% 21%	1853
	11th to 20th	Day	17% 3 5% 22% 34% 19%	1890
	21st to end	Day	23% 2 4 18% 30% 23%	1890
December	1st to 10th	Day	18% 3 7% 28% 32% 13%	3030
	11th to 20th	Day	22% 2 5% 18% 37% 17%	3479
	21st to end	Day	20% 2 5% 25% 31% 17%	4244
January	1st to 10th	Night	30% 2 4 23% 32% 8%	8202
	11th to 20th	Night	15% 2 7% 20% 37% 19%	7962
	21st to end	Night	27% 3 7% 17% 30% 16%	7746
February	1st to 10th	Night	22% 3 5% 18% 31% 21%	2520
	11th to 20th	Night	26% 3 6% 19% 26% 19%	2458
	21st to end	Night	19% 2 3 17% 33% 26%	2076
March	1st to 10th	Night	19% 2 3 11% 28% 37%	5494
	11th to 20th	Night	21% 4 5% 16% 23% 32%	5448
	21st to end	Night	18% 3 8% 20% 32% 19%	5098
April	1st to 10th	Night	20% 3 7% 19% 29% 23%	2100
	11th to 20th	Night	24% 4 7% 19% 23% 24%	2077
	21st to end	Night	20% 4 10% 22% 26% 19%	2100
May	1st to 10th	Night	25% 5% 9% 20% 21% 20%	5498
	11th to 20th	Night	18% 4 6% 15% 24% 33%	5266
	21st to end	Night	29% 6% 12% 13% 9% 31%	5805
June	1st to 10th	Night	23% 9% 15% 23% 17% 12%	1830
	11th to 20th	Night	17% 6% 12% 17% 25% 23%	1680
	21st to end	Night	23% 6% 14% 26% 18% 14%	1680
July	1st to 10th	Night	31% 7% 13% 30% 16% 3	4700
	11th to 20th	Night	25% 6% 18% 27% 19% 5%	4692
	21st to end	Night	25% 6% 13% 28% 25% 2	5924
August	1st to 10th	Night	25% 7% 11% 28% 19% 10%	4614
	11th to 20th	Night	23% 6% 11% 27% 26% 7%	4613
	21st to end	Night	30% 7% 15% 22% 21% 5%	5069
September	1st to 10th	Night	25% 6% 13% 22% 22% 12%	2100
	11th to 20th	Night	26% 6% 10% 23% 22% 13%	2191
	21st to end	Night	18% 3 8% 22% 33% 17%	2200
October	1st to 10th	Night	13% 4 6% 10% 33% 34%	3590
	11th to 20th	Night	23% 4 8% 15% 27% 23%	3756
	21st to end	Night	16% 2 4 16% 40% 23%	4402
November	1st to 10th	Night	16% 2 5% 18% 33% 26%	2665
	11th to 20th	Night	14% 3 3 23% 37% 20%	2730
	21st to end	Night	21% 3 5% 18% 32% 21%	2730
December	1st to 10th	Night	19% 3 6% 27% 34% 11%	4380
	11th to 20th	Night	16% 2 4 16% 41% 21%	4947
	21st to end	Night	20% 3 6% 20% 36% 16%	6965

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	6% 13% 42% 28% 11% 1	330
	11th to 20th	Day	7% 15% 38% 24% 13% 3	330
	21st to end	Day	6% 10% 35% 35% 12% 2	363
February	1st to 10th	Day	4 12% 43% 30% 10% 2	330
	11th to 20th	Day	12% 12% 43% 24% 9% 1	330
	21st to end	Day	8% 9% 47% 27% 8%	273
March	1st to 10th	Day	6% 11% 45% 28% 11%	330
	11th to 20th	Day	13% 9% 38% 25% 13% 1	330
	21st to end	Day	7% 16% 39% 24% 12% 2	586
April	1st to 10th	Day	7% 14% 41% 27% 10% 1	540
	11th to 20th	Day	8% 16% 45% 25% 5% 1	532
	21st to end	Day	5% 16% 40% 29% 10% 1	540
May	1st to 10th	Day	7% 11% 36% 33% 11% 1	674
	11th to 20th	Day	9% 8% 30% 32% 17% 4	775
	21st to end	Day	5% 7% 34% 37% 16% 1	844
June	1st to 10th	Day	5% 4 22% 40% 23% 5%	780
	11th to 20th	Day	4 2 28% 42% 20% 4	780
	21st to end	Day	4 3 21% 43% 26% 4	780
July	1st to 10th	Day	6% 1 10% 35% 34% 13%	771
	11th to 20th	Day	2 1 7% 35% 40% 14%	768
	21st to end	Day	2 1 5% 32% 43% 16%	748
August	1st to 10th	Day	2 1 8% 38% 40% 10%	567
	11th to 20th	Day	3 1 8% 31% 41% 15%	557
	21st to end	Day	5% 1 7% 35% 39% 13%	627
September	1st to 10th	Day	5% 1 11% 39% 33% 10%	570
	11th to 20th	Day	4 1 13% 39% 34% 9%	381
	21st to end	Day	10% 2 18% 38% 25% 8%	330
October	1st to 10th	Day	10% 5% 24% 33% 23% 5%	330
	11th to 20th	Day	8% 5% 31% 34% 18% 5%	330
	21st to end	Day	10% 8% 28% 26% 21% 6%	363
November	1st to 10th	Day	8% 8% 29% 33% 17% 4	330
	11th to 20th	Day	5% 9% 37% 31% 13% 5%	330
	21st to end	Day	12% 11% 38% 23% 13% 2	330
December	1st to 10th	Day	6% 12% 35% 32% 14% 2	330
	11th to 20th	Day	5% 6% 42% 32% 13% 1	330
	21st to end	Day	8% 12% 42% 26% 12% 1	363
January	1st to 10th	Night	5% 11% 42% 34% 8%	1050
	11th to 20th	Night	6% 9% 42% 34% 9% 1	961
	21st to end	Night	6% 7% 45% 35% 7%	924
February	1st to 10th	Night	6% 9% 49% 30% 6%	840
	11th to 20th	Night	8% 12% 52% 23% 4 1	824
	21st to end	Night	6% 13% 51% 27% 2	692
March	1st to 10th	Night	7% 12% 49% 28% 4	840
	11th to 20th	Night	9% 13% 41% 30% 6%	793
	21st to end	Night	6% 13% 54% 25% 3	693
April	1st to 10th	Night	6% 12% 51% 28% 2	630
	11th to 20th	Night	7% 12% 56% 22% 3	623
	21st to end	Night	5% 12% 53% 25% 5%	630
May	1st to 10th	Night	7% 10% 47% 32% 4	630
	11th to 20th	Night	6% 9% 40% 37% 8%	621
	21st to end	Night	7% 7% 46% 35% 6%	683
June	1st to 10th	Night	4 5% 38% 44% 8%	630
	11th to 20th	Night	2 4 47% 40% 6%	630
	21st to end	Night	3 3 33% 49% 11%	630
July	1st to 10th	Night	5% 1 19% 52% 23%	615
	11th to 20th	Night	3 2 13% 55% 27%	612
	21st to end	Night	4 1 12% 56% 27% 1	690
August	1st to 10th	Night	2 1 14% 51% 30% 2	627
	11th to 20th	Night	4 1 14% 54% 25% 2	624
	21st to end	Night	6% 1 14% 49% 29% 2	693
September	1st to 10th	Night	5% 1 17% 54% 21% 1	630
	11th to 20th	Night	7% 1 29% 47% 15%	609
	21st to end	Night	7% 3 25% 48% 16% 1	600
October	1st to 10th	Night	7% 3 33% 45% 13%	600
	11th to 20th	Night	7% 5% 39% 38% 11% 1	710
	21st to end	Night	7% 8% 33% 36% 16% 1	968
November	1st to 10th	Night	6% 5% 36% 39% 12% 1	1025
	11th to 20th	Night	3 8% 39% 37% 13%	1050
	21st to end	Night	7% 7% 38% 36% 11% 1	1050
December	1st to 10th	Night	7% 8% 39% 37% 8%	1050
	11th to 20th	Night	6% 8% 45% 34% 6%	1050
	21st to end	Night	6% 12% 43% 31% 7%	1155

Page generated at 2026-09-11 03:11:54. Version ac07

Copyright © 2026 Attilla Danko.