Thomas Conway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas Conway Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	13% 9% 3 8% 5% 4 5% 4 6% 7% 37%	101416
	Night	14% 10% 4 7% 5% 4 4 3 5% 6% 37%	98190
Transparency	Day	44% 6% 12% 22% 14% 2	104999
	Night	45% 5% 11% 22% 15% 2	102735
Seeing	Day	16% 10% 29% 25% 15% 5%	15823
	Night	18% 10% 30% 30% 12% 1	25672

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	15% 14% 9% 12% 6% 5% 6% 4 5% 5% 20%	2527
	11th to 20th	Day	14% 15% 7% 15% 6% 5% 5% 7% 7% 4 16%	2530
	21st to end	Day	21% 25% 6% 10% 4 3 3 2 3 5% 17%	2820
February	1st to 10th	Day	17% 14% 4 9% 7% 5% 4 3 5% 6% 26%	1816
	11th to 20th	Day	14% 12% 4 8% 6% 5% 4 3 5% 6% 33%	1848
	21st to end	Day	17% 13% 5% 10% 5% 3 4 3 5% 6% 28%	1570
March	1st to 10th	Day	18% 15% 4 8% 5% 4 3 3 4 5% 31%	3120
	11th to 20th	Day	10% 8% 4 7% 5% 5% 5% 4 7% 7% 38%	3256
	21st to end	Day	18% 11% 6% 12% 6% 5% 5% 3 5% 6% 23%	4148
April	1st to 10th	Day	17% 12% 3 8% 4 4 4 4 5% 6% 33%	2280
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 5% 7% 37%	2254
	21st to end	Day	20% 8% 3 7% 4 4 5% 4 5% 7% 33%	2408
May	1st to 10th	Day	18% 8% 3 6% 5% 5% 5% 4 6% 8% 32%	4082
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 3 6% 8% 40%	4162
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 5% 9% 46%	4824
June	1st to 10th	Day	10% 6% 2 5% 4 4 5% 5% 6% 10% 43%	2660
	11th to 20th	Day	8% 5% 1 6% 5% 5% 5% 4 7% 8% 46%	2660
	21st to end	Day	10% 5% 3 7% 5% 6% 5% 5% 7% 9% 38%	2660
July	1st to 10th	Day	12% 6% 3 8% 5% 5% 6% 5% 8% 10% 34%	4440
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 7% 11% 46%	4440
	21st to end	Day	9% 6% 3 7% 4 5% 5% 5% 7% 8% 42%	4813
August	1st to 10th	Day	7% 5% 2 7% 4 4 5% 4 6% 8% 48%	4213
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 4 6% 8% 46%	3796
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	4180
September	1st to 10th	Day	10% 5% 2 5% 4 5% 4 4 6% 8% 48%	2280
	11th to 20th	Day	11% 5% 2 5% 3 4 5% 3 6% 7% 50%	2123
	21st to end	Day	11% 6% 3 6% 4 4 4 4 7% 9% 43%	2090
October	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 6% 7% 46%	2090
	11th to 20th	Day	19% 7% 2 6% 4 4 5% 3 5% 6% 38%	1870
	21st to end	Day	20% 9% 3 7% 5% 5% 4 4 6% 7% 31%	1881
November	1st to 10th	Day	13% 7% 2 6% 3 4 4 3 6% 8% 45%	1762
	11th to 20th	Day	18% 10% 3 8% 5% 4 3 3 4 7% 35%	1800
	21st to end	Day	17% 12% 4 8% 4 4 3 3 4 4 35%	1773
December	1st to 10th	Day	16% 15% 2 10% 6% 5% 5% 3 5% 5% 28%	1539
	11th to 20th	Day	14% 13% 5% 12% 5% 4 4 4 5% 5% 28%	1918
	21st to end	Day	22% 22% 8% 11% 5% 3 4 2 4 4 13%	2783
January	1st to 10th	Night	16% 16% 8% 10% 5% 4 4 3 3 4 25%	4428
	11th to 20th	Night	15% 19% 8% 13% 5% 4 5% 4 6% 4 17%	4440
	21st to end	Night	25% 17% 7% 10% 4 3 3 2 4 4 21%	4843
February	1st to 10th	Night	17% 14% 4 8% 6% 5% 4 3 4 6% 30%	2362
	11th to 20th	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 35%	2218
	21st to end	Night	20% 13% 3 8% 5% 3 3 3 5% 5% 32%	1884
March	1st to 10th	Night	17% 11% 4 7% 4 4 3 3 4 6% 37%	3733
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 3 5% 7% 47%	3704
	21st to end	Night	19% 9% 6% 8% 5% 5% 5% 4 5% 6% 28%	3512
April	1st to 10th	Night	14% 8% 4 7% 5% 4 4 4 5% 7% 38%	1900
	11th to 20th	Night	18% 9% 3 7% 4 3 4 3 4 6% 39%	1875
	21st to end	Night	17% 8% 2 8% 5% 5% 4 4 6% 5% 36%	1772
May	1st to 10th	Night	17% 8% 3 7% 5% 4 4 3 6% 9% 34%	2813
	11th to 20th	Night	16% 7% 2 5% 4 4 4 4 5% 8% 40%	2732
	21st to end	Night	10% 7% 2 7% 5% 5% 5% 3 6% 7% 44%	2743
June	1st to 10th	Night	9% 5% 1 6% 4 4 5% 4 5% 8% 49%	1520
	11th to 20th	Night	8% 6% 3 5% 4 4 4 4 7% 8% 47%	1520
	21st to end	Night	10% 6% 3 7% 6% 4 6% 4 6% 6% 43%	1520
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 3 7% 10% 42%	2530
	11th to 20th	Night	7% 4 2 5% 4 4 3 3 5% 8% 55%	2530
	21st to end	Night	5% 6% 3 6% 5% 4 5% 4 7% 9% 46%	2750
August	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 4 7% 10% 47%	2716
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 6% 8% 52%	3030
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 8% 52%	3476
September	1st to 10th	Night	10% 4 2 5% 4 3 4 3 5% 6% 53%	1900
	11th to 20th	Night	8% 3 1 4 3 3 4 4 6% 8% 54%	2038
	21st to end	Night	10% 6% 2 6% 4 5% 4 4 5% 8% 46%	2090
October	1st to 10th	Night	11% 5% 2 6% 4 4 3 3 5% 6% 52%	2090
	11th to 20th	Night	16% 8% 2 5% 3 3 2 3 4 6% 46%	2307
	21st to end	Night	20% 9% 2 7% 4 4 3 3 4 6% 38%	2717
November	1st to 10th	Night	13% 8% 3 6% 4 4 3 3 6% 7% 45%	2536
	11th to 20th	Night	14% 10% 3 8% 4 4 5% 5% 5% 6% 37%	2600
	21st to end	Night	18% 11% 4 7% 5% 4 4 3 5% 5% 35%	2562
December	1st to 10th	Night	16% 13% 3 10% 4 4 4 3 5% 6% 32%	2636
	11th to 20th	Night	16% 14% 4 8% 6% 5% 4 3 4 5% 32%	3279
	21st to end	Night	24% 19% 6% 11% 6% 4 4 3 4 4 16%	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	63% 7% 12% 12% 6%	2528
	11th to 20th	Day	68% 5% 8% 11% 8%	2530
	21st to end	Day	72% 3 8% 13% 4	2820
February	1st to 10th	Day	59% 5% 11% 16% 8% 1	1816
	11th to 20th	Day	51% 5% 10% 22% 10% 1	1858
	21st to end	Day	56% 5% 12% 20% 6% 2	1570
March	1st to 10th	Day	55% 7% 9% 19% 10% 1	3120
	11th to 20th	Day	43% 6% 12% 27% 12% 1	3256
	21st to end	Day	61% 6% 16% 14% 3	4148
April	1st to 10th	Day	51% 5% 10% 21% 13%	2280
	11th to 20th	Day	47% 5% 11% 23% 12% 3	2254
	21st to end	Day	50% 6% 12% 18% 12% 2	2408
May	1st to 10th	Day	47% 8% 12% 24% 9%	4120
	11th to 20th	Day	41% 6% 14% 28% 10% 1	4166
	21st to end	Day	33% 7% 13% 22% 22% 3	4837
June	1st to 10th	Day	36% 7% 11% 25% 17% 3	2660
	11th to 20th	Day	34% 7% 11% 23% 21% 3	2660
	21st to end	Day	39% 7% 16% 19% 15% 3	2660
July	1st to 10th	Day	44% 6% 13% 20% 14% 2	4371
	11th to 20th	Day	30% 7% 17% 28% 17% 1	4365
	21st to end	Day	38% 7% 16% 17% 19% 3	4662
August	1st to 10th	Day	33% 7% 15% 28% 16% 1	4198
	11th to 20th	Day	33% 7% 12% 31% 16% 1	3834
	21st to end	Day	30% 6% 14% 26% 19% 4	4179
September	1st to 10th	Day	34% 7% 12% 26% 17% 4	2280
	11th to 20th	Day	31% 6% 12% 25% 22% 4	2123
	21st to end	Day	37% 6% 10% 26% 20% 2	2090
October	1st to 10th	Day	26% 4 7% 32% 29% 2	3480
	11th to 20th	Day	35% 5% 11% 23% 24% 1	3073
	21st to end	Day	44% 4 10% 26% 14% 2	3081
November	1st to 10th	Day	38% 4 15% 24% 16% 2	1762
	11th to 20th	Day	50% 4 13% 21% 12% 1	1800
	21st to end	Day	52% 5% 11% 21% 10%	1773
December	1st to 10th	Day	58% 5% 13% 16% 8%	1539
	11th to 20th	Day	57% 6% 10% 21% 5% 1	1915
	21st to end	Day	76% 4 9% 9% 3	2783
January	1st to 10th	Night	62% 6% 11% 14% 6% 1	4428
	11th to 20th	Night	70% 4 11% 10% 5%	4440
	21st to end	Night	68% 3 9% 14% 5%	4843
February	1st to 10th	Night	57% 4 11% 17% 10% 2	2362
	11th to 20th	Night	48% 5% 11% 21% 13% 2	2219
	21st to end	Night	54% 5% 10% 18% 10% 3	1884
March	1st to 10th	Night	49% 5% 11% 23% 9% 3	3733
	11th to 20th	Night	37% 5% 11% 27% 18% 2	3704
	21st to end	Night	56% 6% 12% 20% 5%	3512
April	1st to 10th	Night	44% 5% 12% 23% 13% 2	1900
	11th to 20th	Night	47% 4 8% 21% 16% 4	1875
	21st to end	Night	47% 6% 10% 20% 14% 3	1772
May	1st to 10th	Night	47% 6% 12% 22% 12% 1	2840
	11th to 20th	Night	42% 7% 15% 25% 11%	2733
	21st to end	Night	38% 7% 11% 17% 21% 5%	2751
June	1st to 10th	Night	32% 6% 12% 26% 21% 3	1520
	11th to 20th	Night	33% 6% 12% 23% 24% 3	1520
	21st to end	Night	39% 8% 14% 22% 14% 4	1520
July	1st to 10th	Night	38% 6% 14% 24% 12% 5%	2490
	11th to 20th	Night	28% 5% 12% 27% 26% 2	2482
	21st to end	Night	33% 6% 17% 19% 19% 5%	2662
August	1st to 10th	Night	32% 7% 17% 27% 16% 1	2707
	11th to 20th	Night	29% 6% 14% 29% 20% 1	3058
	21st to end	Night	31% 5% 11% 31% 19% 3	3476
September	1st to 10th	Night	31% 6% 12% 25% 21% 6%	1900
	11th to 20th	Night	27% 6% 10% 27% 26% 5%	2038
	21st to end	Night	37% 5% 9% 27% 19% 3	2090
October	1st to 10th	Night	22% 3 8% 33% 31% 2	3480
	11th to 20th	Night	34% 4 9% 23% 28% 2	3830
	21st to end	Night	39% 3 9% 24% 21% 4	4467
November	1st to 10th	Night	39% 5% 10% 26% 18% 3	2536
	11th to 20th	Night	46% 5% 12% 21% 13% 2	2600
	21st to end	Night	51% 4 11% 23% 11%	2562
December	1st to 10th	Night	54% 4 12% 20% 9% 1	2636
	11th to 20th	Night	55% 4 11% 18% 10% 1	3281
	21st to end	Night	72% 4 9% 10% 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	23% 14% 34% 21% 7% 1	270
	11th to 20th	Day	24% 14% 41% 16% 4 1	270
	21st to end	Day	23% 19% 32% 20% 6%	297
February	1st to 10th	Day	24% 18% 37% 16% 4	270
	11th to 20th	Day	26% 14% 38% 20% 3 1	270
	21st to end	Day	26% 16% 41% 13% 4	225
March	1st to 10th	Day	22% 22% 34% 19% 3	270
	11th to 20th	Day	21% 15% 38% 19% 7%	327
	21st to end	Day	27% 16% 30% 20% 6% 1	506
April	1st to 10th	Day	23% 13% 34% 22% 7% 1	460
	11th to 20th	Day	21% 17% 31% 17% 11% 3	452
	21st to end	Day	23% 15% 33% 19% 7% 2	460
May	1st to 10th	Day	23% 12% 31% 21% 11% 2	463
	11th to 20th	Day	20% 7% 33% 22% 14% 4	529
	21st to end	Day	12% 10% 30% 24% 17% 8%	734
June	1st to 10th	Day	12% 8% 29% 29% 17% 6%	680
	11th to 20th	Day	11% 5% 31% 31% 19% 4	680
	21st to end	Day	14% 8% 32% 29% 13% 6%	680
July	1st to 10th	Day	10% 4 21% 29% 21% 15%	671
	11th to 20th	Day	10% 3 22% 33% 21% 11%	668
	21st to end	Day	8% 3 24% 33% 24% 7%	739
August	1st to 10th	Day	8% 3 18% 30% 28% 13%	557
	11th to 20th	Day	10% 4 20% 29% 25% 12%	479
	21st to end	Day	9% 3 16% 35% 26% 12%	539
September	1st to 10th	Day	11% 5% 19% 31% 25% 8%	490
	11th to 20th	Day	10% 4 16% 28% 29% 14%	490
	21st to end	Day	12% 9% 26% 24% 20% 9%	490
October	1st to 10th	Day	14% 5% 31% 28% 18% 5%	490
	11th to 20th	Day	19% 21% 27% 22% 9% 1	309
	21st to end	Day	30% 14% 34% 18% 4 1	330
November	1st to 10th	Day	16% 12% 37% 23% 9% 3	300
	11th to 20th	Day	22% 14% 39% 15% 9% 1	300
	21st to end	Day	25% 12% 40% 19% 4	291
December	1st to 10th	Day	20% 19% 37% 18% 7%	270
	11th to 20th	Day	20% 17% 37% 20% 6%	270
	21st to end	Day	28% 19% 36% 14% 3	297
January	1st to 10th	Night	21% 15% 37% 22% 4	950
	11th to 20th	Night	23% 16% 36% 21% 5%	950
	21st to end	Night	23% 13% 35% 25% 4	1045
February	1st to 10th	Night	24% 12% 38% 22% 3	842
	11th to 20th	Night	20% 14% 36% 26% 4	743
	21st to end	Night	25% 14% 38% 21% 3	628
March	1st to 10th	Night	21% 13% 38% 24% 5%	760
	11th to 20th	Night	17% 11% 41% 24% 6% 1	760
	21st to end	Night	23% 12% 35% 22% 7%	660
April	1st to 10th	Night	17% 14% 38% 23% 6% 1	570
	11th to 20th	Night	20% 9% 37% 25% 7% 1	563
	21st to end	Night	21% 9% 33% 28% 8% 1	570
May	1st to 10th	Night	20% 7% 34% 30% 9% 1	570
	11th to 20th	Night	19% 4 28% 32% 16% 1	561
	21st to end	Night	11% 4 26% 41% 17% 1	617
June	1st to 10th	Night	10% 3 25% 47% 16% 1	570
	11th to 20th	Night	10% 4 30% 38% 16% 1	570
	21st to end	Night	11% 6% 25% 37% 21% 1	570
July	1st to 10th	Night	10% 2 16% 41% 28% 4	555
	11th to 20th	Night	8% 2 19% 38% 29% 4	552
	21st to end	Night	5% 2 14% 47% 31% 1	624
August	1st to 10th	Night	7% 1 11% 46% 30% 5%	567
	11th to 20th	Night	7% 2 14% 42% 30% 5%	564
	21st to end	Night	8% 2 15% 40% 32% 3	627
September	1st to 10th	Night	11% 3 15% 39% 27% 4	570
	11th to 20th	Night	8% 4 17% 39% 28% 4	708
	21st to end	Night	12% 6% 24% 32% 23% 3	760
October	1st to 10th	Night	13% 5% 25% 38% 18% 1	760
	11th to 20th	Night	19% 13% 29% 28% 10%	760
	21st to end	Night	25% 13% 31% 25% 7%	836
November	1st to 10th	Night	17% 12% 35% 26% 10% 1	780
	11th to 20th	Night	19% 16% 34% 25% 6%	800
	21st to end	Night	22% 12% 36% 24% 6%	789
December	1st to 10th	Night	23% 13% 35% 24% 4	926
	11th to 20th	Night	25% 12% 32% 26% 5%	950
	21st to end	Night	29% 14% 32% 22% 2	1045

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