T.O.M.S. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the T.O.M.S. 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	12% 7% 2 6% 4 4 4 4 6% 7% 42%	101131
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	98467
Transparency	Day	38% 6% 11% 22% 19% 4	104879
	Night	36% 5% 10% 22% 20% 6%	102859
Seeing	Day	15% 9% 22% 25% 20% 9%	15269
	Night	16% 8% 24% 29% 19% 3	25020

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	14% 15% 3 7% 4 4 4 2 4 5% 38%	2527
	11th to 20th	Day	20% 10% 2 8% 3 3 3 3 4 4 40%	2754
	21st to end	Day	23% 10% 3 5% 3 2 4 3 4 5% 40%	3356
February	1st to 10th	Day	20% 10% 3 6% 3 3 2 2 4 6% 40%	1900
	11th to 20th	Day	20% 10% 4 6% 4 4 3 3 4 5% 38%	1848
	21st to end	Day	22% 9% 3 6% 5% 3 3 3 5% 5% 36%	1570
March	1st to 10th	Day	20% 13% 3 9% 5% 4 4 3 4 4 32%	3120
	11th to 20th	Day	12% 7% 2 7% 4 3 4 3 5% 7% 46%	3282
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 3 5% 7% 41%	4165
April	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 5% 6% 44%	2280
	11th to 20th	Day	15% 7% 3 7% 4 3 4 3 4 6% 45%	2254
	21st to end	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 44%	2280
May	1st to 10th	Day	14% 7% 2 7% 4 4 5% 4 7% 8% 39%	3911
	11th to 20th	Day	14% 7% 3 6% 5% 4 4 4 6% 8% 38%	4081
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 10% 37%	4476
June	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 5% 7% 10% 40%	2582
	11th to 20th	Day	4 4 2 6% 4 5% 5% 4 8% 9% 48%	2660
	21st to end	Day	5% 4 1 5% 5% 6% 6% 5% 8% 10% 45%	2660
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 6% 8% 11% 36%	4440
	11th to 20th	Day	9% 6% 2 6% 6% 6% 7% 6% 9% 11% 34%	4440
	21st to end	Day	15% 9% 3 9% 6% 6% 5% 4 6% 9% 27%	4302
August	1st to 10th	Day	3 2 1 4 4 5% 5% 4 7% 10% 56%	3785
	11th to 20th	Day	7% 4 2 6% 4 4 5% 5% 6% 10% 46%	3729
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 8% 54%	4180
September	1st to 10th	Day	9% 5% 2 6% 5% 5% 4 4 7% 9% 44%	2280
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 4 7% 44%	2134
	21st to end	Day	10% 3 2 5% 4 4 3 4 6% 6% 52%	2090
October	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 4 5% 58%	2090
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 4 7% 56%	1940
	21st to end	Day	16% 5% 2 5% 3 3 3 3 4 4 52%	2019
November	1st to 10th	Day	13% 5% 2 4 3 2 3 4 4 6% 54%	1762
	11th to 20th	Day	13% 8% 2 5% 3 3 3 3 4 5% 51%	1800
	21st to end	Day	15% 9% 2 5% 3 3 4 4 4 5% 45%	1797
December	1st to 10th	Day	21% 8% 3 7% 4 4 5% 3 5% 4 36%	1710
	11th to 20th	Day	16% 9% 2 6% 4 4 3 3 5% 5% 42%	2144
	21st to end	Day	14% 11% 3 8% 4 4 5% 4 5% 7% 35%	2783
January	1st to 10th	Night	19% 13% 3 5% 3 3 3 3 4 5% 40%	4428
	11th to 20th	Night	18% 10% 3 8% 3 3 3 2 3 5% 42%	4213
	21st to end	Night	23% 12% 3 4 3 2 3 2 4 4 41%	4300
February	1st to 10th	Night	20% 10% 3 6% 4 3 3 3 4 5% 38%	2280
	11th to 20th	Night	20% 10% 2 7% 3 3 4 3 3 4 41%	2218
	21st to end	Night	22% 8% 2 7% 4 3 4 3 4 5% 37%	1884
March	1st to 10th	Night	22% 10% 3 6% 4 3 3 3 4 6% 37%	3733
	11th to 20th	Night	14% 8% 2 5% 3 4 3 3 4 6% 47%	3705
	21st to end	Night	17% 7% 3 6% 5% 4 4 3 5% 5% 40%	3466
April	1st to 10th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 46%	1900
	11th to 20th	Night	14% 6% 1 5% 3 3 3 4 4 7% 49%	1875
	21st to end	Night	11% 7% 2 6% 4 4 4 3 4 7% 47%	1900
May	1st to 10th	Night	16% 7% 3 5% 4 4 4 3 5% 7% 41%	2982
	11th to 20th	Night	13% 8% 3 6% 4 4 4 3 5% 7% 42%	2812
	21st to end	Night	8% 6% 2 6% 4 5% 5% 5% 7% 10% 43%	3079
June	1st to 10th	Night	7% 6% 3 6% 4 4 5% 5% 6% 11% 44%	1591
	11th to 20th	Night	3 2 2 4 4 4 4 5% 7% 11% 53%	1520
	21st to end	Night	3 3 2 4 3 4 5% 4 6% 10% 55%	1520
July	1st to 10th	Night	6% 4 2 7% 5% 5% 6% 5% 8% 13% 40%	2530
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 7% 12% 47%	2530
	21st to end	Night	10% 8% 3 9% 6% 5% 5% 4 7% 9% 31%	3232
August	1st to 10th	Night	2 2 1 3 3 4 4 4 6% 9% 62%	3144
	11th to 20th	Night	4 3 2 4 3 3 4 4 7% 11% 54%	3100
	21st to end	Night	4 3 1 5% 4 4 4 4 6% 9% 54%	3476
September	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 7% 50%	1900
	11th to 20th	Night	11% 4 1 6% 4 4 4 4 5% 7% 48%	2025
	21st to end	Night	10% 4 2 4 4 3 3 3 5% 6% 56%	2090
October	1st to 10th	Night	9% 4 2 3 3 3 2 2 4 5% 64%	2090
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 6% 55%	2236
	21st to end	Night	16% 8% 2 4 2 3 3 2 4 5% 51%	2577
November	1st to 10th	Night	12% 5% 2 4 4 3 4 3 4 7% 52%	2536
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 5% 7% 51%	2600
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 5% 49%	2599
December	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 4 6% 39%	2470
	11th to 20th	Night	15% 8% 3 6% 3 3 2 2 4 6% 48%	3042
	21st to end	Night	16% 10% 3 7% 4 4 5% 4 6% 6% 33%	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	47% 4 6% 19% 21% 3	2528
	11th to 20th	Day	47% 4 7% 20% 20% 1	2754
	21st to end	Day	46% 3 9% 21% 20% 2	3356
February	1st to 10th	Day	47% 3 8% 19% 19% 4	1900
	11th to 20th	Day	50% 3 6% 20% 16% 5%	1858
	21st to end	Day	49% 3 8% 14% 18% 7%	1570
March	1st to 10th	Day	55% 5% 6% 12% 14% 9%	3120
	11th to 20th	Day	37% 6% 9% 24% 16% 7%	3282
	21st to end	Day	43% 6% 10% 21% 19% 2	4165
April	1st to 10th	Day	38% 6% 10% 21% 20% 5%	2280
	11th to 20th	Day	40% 4 8% 19% 22% 6%	2254
	21st to end	Day	37% 5% 10% 27% 19% 2	2280
May	1st to 10th	Day	40% 7% 11% 25% 15% 2	3946
	11th to 20th	Day	41% 8% 15% 23% 12% 1	4085
	21st to end	Day	40% 7% 12% 24% 15% 1	4489
June	1st to 10th	Day	37% 9% 12% 22% 18% 3	2582
	11th to 20th	Day	30% 7% 14% 28% 19% 3	2660
	21st to end	Day	31% 10% 15% 23% 18% 4	2660
July	1st to 10th	Day	39% 7% 21% 24% 7% 2	4371
	11th to 20th	Day	40% 6% 19% 28% 6% 1	4365
	21st to end	Day	53% 7% 12% 21% 6%	4165
August	1st to 10th	Day	22% 7% 15% 34% 21% 1	3773
	11th to 20th	Day	32% 6% 16% 27% 16% 3	3765
	21st to end	Day	30% 5% 12% 25% 24% 4	4179
September	1st to 10th	Day	35% 9% 11% 24% 16% 5%	2280
	11th to 20th	Day	39% 7% 10% 18% 21% 4	2134
	21st to end	Day	31% 6% 11% 23% 22% 7%	2090
October	1st to 10th	Day	20% 3 5% 14% 38% 19%	3480
	11th to 20th	Day	19% 3 5% 20% 37% 17%	3206
	21st to end	Day	27% 2 5% 25% 33% 9%	3309
November	1st to 10th	Day	33% 4 8% 21% 27% 7%	1762
	11th to 20th	Day	35% 3 8% 18% 28% 8%	1800
	21st to end	Day	41% 4 7% 22% 22% 4	1797
December	1st to 10th	Day	50% 5% 5% 19% 16% 5%	1710
	11th to 20th	Day	44% 4 8% 20% 20% 4	2141
	21st to end	Day	47% 5% 10% 21% 13% 3	2783
January	1st to 10th	Night	48% 3 6% 18% 22% 2	4428
	11th to 20th	Night	45% 4 6% 21% 18% 6%	4213
	21st to end	Night	48% 3 6% 22% 18% 3	4300
February	1st to 10th	Night	48% 3 8% 18% 18% 4	2280
	11th to 20th	Night	49% 3 6% 17% 19% 6%	2219
	21st to end	Night	49% 4 7% 17% 17% 7%	1884
March	1st to 10th	Night	48% 4 10% 14% 16% 8%	3733
	11th to 20th	Night	38% 5% 9% 21% 19% 9%	3705
	21st to end	Night	45% 5% 8% 18% 22% 2	3466
April	1st to 10th	Night	37% 5% 10% 23% 20% 4	1900
	11th to 20th	Night	35% 4 8% 20% 24% 9%	1875
	21st to end	Night	37% 5% 10% 25% 20% 3	1900
May	1st to 10th	Night	40% 7% 12% 22% 16% 2	3012
	11th to 20th	Night	40% 8% 15% 24% 13% 1	2813
	21st to end	Night	32% 9% 15% 27% 15% 2	3087
June	1st to 10th	Night	33% 9% 15% 23% 18% 3	1591
	11th to 20th	Night	23% 7% 16% 29% 22% 3	1520
	21st to end	Night	24% 7% 16% 30% 19% 4	1520
July	1st to 10th	Night	33% 8% 24% 26% 8% 2	2490
	11th to 20th	Night	29% 6% 23% 33% 7% 2	2482
	21st to end	Night	48% 7% 14% 21% 9% 1	3129
August	1st to 10th	Night	17% 7% 12% 38% 24% 2	3132
	11th to 20th	Night	22% 7% 24% 27% 18% 2	3130
	21st to end	Night	25% 5% 15% 23% 27% 4	3476
September	1st to 10th	Night	30% 8% 13% 23% 20% 6%	1900
	11th to 20th	Night	35% 6% 14% 16% 25% 4	2025
	21st to end	Night	28% 5% 10% 25% 24% 8%	2090
October	1st to 10th	Night	17% 4 5% 16% 36% 21%	3480
	11th to 20th	Night	20% 3 5% 15% 35% 23%	3709
	21st to end	Night	26% 2 5% 27% 25% 15%	4237
November	1st to 10th	Night	33% 4 7% 21% 25% 10%	2536
	11th to 20th	Night	33% 4 7% 19% 27% 10%	2600
	21st to end	Night	38% 3 7% 21% 25% 6%	2599
December	1st to 10th	Night	46% 5% 7% 21% 18% 4	2470
	11th to 20th	Night	39% 2 6% 22% 25% 4	3044
	21st to end	Night	47% 6% 10% 20% 12% 4	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	24% 22% 32% 16% 5%	270
	11th to 20th	Day	30% 19% 33% 15% 2 2	270
	21st to end	Day	21% 21% 41% 14% 4	297
February	1st to 10th	Day	23% 22% 32% 17% 5% 1	270
	11th to 20th	Day	37% 16% 33% 10% 3 1	270
	21st to end	Day	36% 21% 31% 9% 3	225
March	1st to 10th	Day	25% 18% 32% 19% 5%	270
	11th to 20th	Day	20% 19% 37% 21% 3	287
	21st to end	Day	24% 16% 31% 22% 8%	506
April	1st to 10th	Day	22% 17% 26% 26% 8% 1	460
	11th to 20th	Day	19% 16% 31% 22% 10% 2	452
	21st to end	Day	17% 14% 28% 24% 13% 3	460
May	1st to 10th	Day	16% 13% 25% 28% 12% 5%	463
	11th to 20th	Day	15% 4 25% 28% 18% 10%	487
	21st to end	Day	12% 4 20% 29% 22% 14%	529
June	1st to 10th	Day	11% 4 18% 28% 23% 16%	602
	11th to 20th	Day	5% 3 15% 30% 31% 16%	680
	21st to end	Day	7% 2 19% 31% 28% 13%	680
July	1st to 10th	Day	7% 3 12% 28% 32% 18%	671
	11th to 20th	Day	10% 1 11% 25% 31% 21%	668
	21st to end	Day	8% 2 11% 25% 33% 21%	582
August	1st to 10th	Day	7% 1 10% 27% 36% 18%	486
	11th to 20th	Day	12% 2 11% 29% 33% 13%	479
	21st to end	Day	12% 2 7% 21% 37% 21%	539
September	1st to 10th	Day	9% 2 12% 24% 37% 15%	490
	11th to 20th	Day	13% 2 10% 24% 32% 18%	490
	21st to end	Day	14% 4 13% 30% 28% 11%	490
October	1st to 10th	Day	11% 2 17% 33% 27% 9%	490
	11th to 20th	Day	9% 10% 32% 29% 16% 3	340
	21st to end	Day	19% 11% 28% 27% 12% 2	330
November	1st to 10th	Day	21% 8% 24% 28% 16% 3	300
	11th to 20th	Day	15% 15% 34% 29% 6% 1	300
	21st to end	Day	22% 14% 29% 20% 13% 2	299
December	1st to 10th	Day	27% 19% 29% 18% 6%	270
	11th to 20th	Day	21% 15% 33% 21% 9%	270
	21st to end	Day	26% 18% 37% 16% 4	297
January	1st to 10th	Night	23% 16% 33% 23% 4	950
	11th to 20th	Night	25% 17% 32% 22% 4	950
	21st to end	Night	22% 13% 39% 24% 2	904
February	1st to 10th	Night	24% 14% 34% 25% 4	760
	11th to 20th	Night	25% 18% 31% 24% 2	743
	21st to end	Night	27% 17% 35% 19% 2	628
March	1st to 10th	Night	21% 14% 31% 30% 4	760
	11th to 20th	Night	21% 11% 35% 28% 5% 1	722
	21st to end	Night	19% 11% 34% 26% 11%	627
April	1st to 10th	Night	15% 11% 35% 28% 11% 1	570
	11th to 20th	Night	17% 10% 31% 29% 12% 1	563
	21st to end	Night	13% 11% 32% 29% 14% 2	570
May	1st to 10th	Night	15% 5% 26% 35% 16% 2	570
	11th to 20th	Night	13% 4 23% 34% 21% 4	561
	21st to end	Night	9% 1 15% 39% 28% 7%	617
June	1st to 10th	Night	10% 2 13% 36% 32% 7%	570
	11th to 20th	Night	4 1 12% 36% 41% 7%	570
	21st to end	Night	4 1 12% 36% 38% 9%	570
July	1st to 10th	Night	8% 2 7% 26% 45% 12%	555
	11th to 20th	Night	8% 1 9% 26% 39% 16%	552
	21st to end	Night	7% 1 7% 31% 43% 12%	624
August	1st to 10th	Night	5% 1 5% 29% 49% 11%	567
	11th to 20th	Night	7% 2 8% 35% 40% 7%	564
	21st to end	Night	8% 1 8% 28% 45% 10%	627
September	1st to 10th	Night	9% 1 11% 34% 39% 6%	570
	11th to 20th	Night	13% 1 14% 28% 37% 6%	695
	21st to end	Night	11% 3 15% 32% 34% 4	760
October	1st to 10th	Night	11% 3 16% 33% 31% 5%	760
	11th to 20th	Night	15% 6% 23% 36% 19% 2	760
	21st to end	Night	20% 8% 22% 29% 19% 1	836
November	1st to 10th	Night	14% 7% 23% 36% 19% 2	780
	11th to 20th	Night	14% 10% 33% 32% 12% 1	800
	21st to end	Night	21% 10% 29% 27% 12% 1	800
December	1st to 10th	Night	25% 10% 30% 28% 7%	760
	11th to 20th	Night	21% 12% 31% 29% 7%	760
	21st to end	Night	26% 14% 31% 26% 4	1045

Page generated at 2024-11-27 16:30:49. Version ac07

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