Lowry Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lowry Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	8% 5% 2 5% 3 3 4 3 5% 7% 55%	139872
	Night	7% 6% 2 6% 4 4 4 3 5% 7% 52%	139681
Transparency	Day	28% 5% 12% 28% 21% 5%	134749
	Night	31% 5% 12% 27% 20% 5%	135827
Seeing	Day	11% 7% 30% 32% 17% 3	17654
	Night	10% 7% 35% 38% 9%	27741

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% 10% 3 7% 3 2 3 3 5% 6% 53%	4720
	11th to 20th	Day	3 7% 3 7% 4 4 4 3 5% 8% 53%	4740
	21st to end	Day	3 6% 3 8% 6% 5% 4 3 5% 7% 49%	5188
February	1st to 10th	Day	6% 10% 3 7% 5% 4 4 3 6% 7% 44%	2057
	11th to 20th	Day	5% 8% 3 7% 5% 4 5% 3 6% 6% 48%	2049
	21st to end	Day	5% 6% 3 6% 3 4 4 3 4 6% 58%	1730
March	1st to 10th	Day	9% 7% 3 4 3 2 3 3 4 5% 56%	4600
	11th to 20th	Day	8% 5% 2 4 3 3 3 3 4 7% 60%	4858
	21st to end	Day	8% 6% 2 5% 3 3 4 3 6% 9% 51%	5907
April	1st to 10th	Day	9% 6% 2 4 4 3 3 3 5% 7% 53%	2520
	11th to 20th	Day	14% 5% 2 5% 4 3 4 4 4 8% 47%	2444
	21st to end	Day	13% 6% 2 4 4 4 4 3 5% 8% 47%	2570
May	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 8% 52%	5900
	11th to 20th	Day	10% 4 2 5% 3 3 4 3 5% 7% 55%	5862
	21st to end	Day	17% 6% 2 5% 3 3 4 3 5% 6% 47%	6882
June	1st to 10th	Day	6% 3 1 4 3 4 4 3 5% 9% 57%	2800
	11th to 20th	Day	7% 4 2 3 3 3 4 4 6% 8% 56%	2800
	21st to end	Day	4 3 1 4 3 3 4 3 6% 9% 60%	2800
July	1st to 10th	Day	6% 3 2 4 4 4 4 4 6% 7% 57%	6339
	11th to 20th	Day	6% 3 1 4 3 3 4 4 7% 10% 55%	6340
	21st to end	Day	5% 3 1 4 3 4 5% 4 7% 9% 55%	6903
August	1st to 10th	Day	11% 4 1 4 3 3 4 3 6% 8% 52%	5846
	11th to 20th	Day	7% 4 2 4 3 3 3 4 6% 7% 57%	5559
	21st to end	Day	10% 5% 2 6% 3 3 4 4 5% 7% 50%	5984
September	1st to 10th	Day	7% 4 1 3 3 3 3 3 5% 8% 61%	2400
	11th to 20th	Day	6% 3 1 4 3 3 3 4 5% 8% 60%	2223
	21st to end	Day	8% 3 1 4 2 3 3 3 4 6% 64%	2200
October	1st to 10th	Day	7% 3 1 2 2 1 2 1 2 4 75%	3590
	11th to 20th	Day	7% 3 2 4 3 3 3 3 4 5% 64%	3208
	21st to end	Day	8% 6% 1 4 3 3 4 3 4 7% 58%	3190
November	1st to 10th	Day	4 5% 2 5% 4 3 3 3 5% 6% 60%	1853
	11th to 20th	Day	5% 5% 2 4 3 3 3 3 4 5% 63%	1890
	21st to end	Day	6% 7% 2 4 3 2 3 4 5% 7% 57%	1890
December	1st to 10th	Day	3 5% 2 4 3 2 2 3 5% 7% 65%	2832
	11th to 20th	Day	3 4 1 5% 3 3 3 3 4 6% 64%	3106
	21st to end	Day	10% 9% 2 6% 3 4 4 4 5% 6% 47%	4092
January	1st to 10th	Night	5% 9% 4 9% 4 4 4 3 4 6% 47%	8201
	11th to 20th	Night	2 5% 2 7% 6% 4 5% 4 5% 7% 53%	8250
	21st to end	Night	3 7% 4 8% 4 4 4 3 5% 6% 52%	9026
February	1st to 10th	Night	7% 10% 3 7% 4 4 4 4 6% 6% 45%	2577
	11th to 20th	Night	4 10% 3 7% 5% 4 4 4 6% 7% 46%	2458
	21st to end	Night	5% 7% 2 6% 4 4 3 3 5% 6% 56%	2076
March	1st to 10th	Night	10% 6% 2 4 3 3 3 2 4 5% 58%	5494
	11th to 20th	Night	9% 7% 2 5% 3 3 3 3 4 6% 55%	5331
	21st to end	Night	8% 8% 3 7% 5% 5% 5% 5% 7% 8% 40%	5277
April	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 8% 48%	2100
	11th to 20th	Night	12% 6% 2 6% 4 4 4 5% 5% 6% 47%	2037
	21st to end	Night	16% 6% 2 5% 4 3 6% 4 5% 8% 43%	1830
May	1st to 10th	Night	13% 7% 2 6% 4 5% 4 4 5% 6% 45%	4080
	11th to 20th	Night	10% 4 1 5% 3 4 4 3 5% 8% 53%	4053
	21st to end	Night	19% 7% 2 6% 3 3 3 3 4 6% 43%	3999
June	1st to 10th	Night	7% 4 2 5% 4 3 4 4 7% 9% 51%	1600
	11th to 20th	Night	9% 4 2 5% 5% 4 4 5% 6% 8% 50%	1600
	21st to end	Night	5% 4 2 5% 5% 5% 5% 5% 7% 11% 46%	1600
July	1st to 10th	Night	8% 3 2 4 4 4 4 4 7% 8% 53%	3640
	11th to 20th	Night	8% 4 2 5% 4 4 5% 4 7% 10% 47%	3640
	21st to end	Night	6% 4 2 5% 4 5% 5% 4 7% 9% 48%	3970
August	1st to 10th	Night	10% 6% 2 7% 5% 6% 6% 4 7% 9% 37%	3963
	11th to 20th	Night	7% 4 2 6% 4 4 5% 5% 7% 10% 46%	4273
	21st to end	Night	9% 7% 2 6% 4 5% 4 3 6% 8% 45%	4993
September	1st to 10th	Night	7% 3 2 4 3 3 3 3 5% 8% 60%	2000
	11th to 20th	Night	7% 2 2 4 3 4 4 4 5% 6% 59%	2176
	21st to end	Night	8% 4 2 4 3 3 3 3 4 6% 61%	2200
October	1st to 10th	Night	7% 3 1 2 2 2 1 1 3 4 75%	3590
	11th to 20th	Night	9% 3 1 3 2 3 3 3 4 6% 63%	3890
	21st to end	Night	7% 5% 2 5% 3 3 3 3 5% 6% 58%	4610
November	1st to 10th	Night	5% 5% 2 4 3 4 2 3 5% 7% 60%	2665
	11th to 20th	Night	5% 5% 1 4 4 4 4 3 6% 6% 57%	2730
	21st to end	Night	7% 6% 2 4 4 5% 3 3 4 7% 55%	2730
December	1st to 10th	Night	3 4 2 5% 3 3 3 3 4 5% 65%	4573
	11th to 20th	Night	3 5% 2 5% 3 4 4 4 6% 8% 57%	5321
	21st to end	Night	11% 9% 2 6% 3 4 4 3 5% 7% 45%	7128

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	34% 3 12% 30% 20% 1	4723
	11th to 20th	Day	31% 4 13% 28% 20% 4	4740
	21st to end	Day	35% 4 12% 27% 19% 3	5188
February	1st to 10th	Day	39% 4 15% 23% 15% 4	2057
	11th to 20th	Day	36% 5% 11% 26% 17% 4	2059
	21st to end	Day	28% 4 10% 29% 23% 5%	1730
March	1st to 10th	Day	31% 2 7% 24% 29% 7%	4600
	11th to 20th	Day	26% 3 10% 25% 33% 3	4858
	21st to end	Day	30% 6% 19% 30% 13% 2	5907
April	1st to 10th	Day	31% 5% 12% 32% 17% 3	2520
	11th to 20th	Day	34% 5% 15% 23% 19% 4	2447
	21st to end	Day	36% 5% 12% 29% 18% 2	2570
May	1st to 10th	Day	29% 5% 12% 28% 21% 5%	5900
	11th to 20th	Day	28% 6% 12% 30% 21% 3	5866
	21st to end	Day	37% 6% 14% 27% 13% 2	6883
June	1st to 10th	Day	24% 6% 20% 32% 16% 2	2800
	11th to 20th	Day	24% 6% 14% 28% 20% 8%	2800
	21st to end	Day	20% 5% 12% 30% 27% 7%	2800
July	1st to 10th	Day	25% 5% 10% 27% 28% 6%	6271
	11th to 20th	Day	24% 5% 16% 32% 18% 5%	6265
	21st to end	Day	23% 7% 16% 22% 22% 10%	6705
August	1st to 10th	Day	31% 6% 14% 27% 18% 4	4251
	11th to 20th	Day	31% 6% 13% 23% 23% 4	4026
	21st to end	Day	23% 5% 15% 22% 22% 14%	4311
September	1st to 10th	Day	23% 3 9% 29% 24% 12%	2400
	11th to 20th	Day	22% 4 11% 28% 27% 8%	2223
	21st to end	Day	22% 4 7% 28% 31% 8%	2200
October	1st to 10th	Day	17% 2 6% 22% 34% 20%	3590
	11th to 20th	Day	23% 4 9% 36% 22% 6%	3208
	21st to end	Day	26% 3 10% 32% 22% 6%	3190
November	1st to 10th	Day	25% 4 13% 29% 22% 7%	1853
	11th to 20th	Day	24% 3 10% 32% 26% 5%	1890
	21st to end	Day	28% 3 13% 29% 23% 4	1890
December	1st to 10th	Day	20% 3 11% 35% 27% 4	2832
	11th to 20th	Day	22% 3 15% 40% 18% 3	3104
	21st to end	Day	37% 4 10% 34% 14% 2	4092
January	1st to 10th	Night	38% 4 11% 27% 16% 3	8202
	11th to 20th	Night	30% 5% 16% 27% 20% 3	8250
	21st to end	Night	33% 4 10% 27% 21% 5%	9026
February	1st to 10th	Night	37% 5% 13% 24% 18% 3	2577
	11th to 20th	Night	36% 5% 13% 27% 15% 3	2458
	21st to end	Night	30% 3 10% 26% 24% 6%	2076
March	1st to 10th	Night	29% 3 7% 23% 31% 6%	5494
	11th to 20th	Night	31% 3 8% 21% 29% 7%	5331
	21st to end	Night	38% 8% 19% 22% 12% 1	5277
April	1st to 10th	Night	35% 5% 14% 25% 16% 5%	2100
	11th to 20th	Night	37% 5% 13% 24% 18% 3	2037
	21st to end	Night	39% 6% 13% 25% 15% 2	1830
May	1st to 10th	Night	39% 6% 13% 26% 15% 2	4080
	11th to 20th	Night	29% 7% 14% 30% 18% 2	4054
	21st to end	Night	41% 7% 14% 24% 12% 1	3999
June	1st to 10th	Night	28% 8% 23% 27% 12% 3	1600
	11th to 20th	Night	30% 8% 16% 25% 15% 7%	1600
	21st to end	Night	29% 8% 14% 25% 22% 3	1600
July	1st to 10th	Night	27% 6% 14% 28% 23% 2	3600
	11th to 20th	Night	32% 6% 18% 26% 15% 3	3592
	21st to end	Night	31% 7% 17% 19% 18% 8%	3856
August	1st to 10th	Night	38% 7% 17% 23% 13% 3	2882
	11th to 20th	Night	37% 7% 13% 24% 16% 3	3095
	21st to end	Night	26% 6% 14% 25% 18% 11%	3597
September	1st to 10th	Night	24% 4 11% 25% 27% 9%	2000
	11th to 20th	Night	25% 5% 12% 26% 22% 9%	2176
	21st to end	Night	25% 4 7% 27% 29% 9%	2200
October	1st to 10th	Night	16% 3 6% 20% 41% 14%	3590
	11th to 20th	Night	23% 5% 8% 32% 27% 6%	3890
	21st to end	Night	27% 3 12% 27% 24% 6%	4610
November	1st to 10th	Night	24% 3 11% 30% 23% 8%	2665
	11th to 20th	Night	25% 4 11% 28% 25% 7%	2730
	21st to end	Night	30% 3 10% 29% 23% 5%	2730
December	1st to 10th	Night	23% 2 13% 36% 22% 4	4573
	11th to 20th	Night	26% 3 12% 38% 19% 3	5322
	21st to end	Night	39% 3 9% 32% 15% 1	7128

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	11% 7% 37% 29% 14% 2	330
	11th to 20th	Day	10% 16% 38% 31% 5%	330
	21st to end	Day	11% 8% 47% 24% 9% 2	363
February	1st to 10th	Day	10% 10% 49% 28% 3	330
	11th to 20th	Day	9% 11% 43% 28% 8% 1	330
	21st to end	Day	8% 16% 45% 23% 8%	273
March	1st to 10th	Day	12% 9% 38% 26% 14% 1	330
	11th to 20th	Day	13% 8% 33% 33% 12% 1	330
	21st to end	Day	15% 13% 39% 23% 9% 2	504
April	1st to 10th	Day	12% 16% 38% 25% 9% 1	540
	11th to 20th	Day	18% 16% 35% 24% 7% 1	517
	21st to end	Day	20% 9% 31% 26% 12% 2	670
May	1st to 10th	Day	16% 9% 26% 31% 14% 3	703
	11th to 20th	Day	17% 7% 29% 28% 16% 2	725
	21st to end	Day	16% 8% 30% 28% 15% 4	789
June	1st to 10th	Day	7% 4 26% 31% 25% 8%	730
	11th to 20th	Day	7% 8% 31% 36% 17% 2	730
	21st to end	Day	6% 4 26% 36% 25% 3	730
July	1st to 10th	Day	9% 4 19% 41% 24% 3	721
	11th to 20th	Day	5% 2 20% 39% 30% 4	718
	21st to end	Day	6% 1 18% 39% 30% 6%	794
August	1st to 10th	Day	9% 2 16% 39% 28% 5%	726
	11th to 20th	Day	7% 2 22% 40% 25% 4	609
	21st to end	Day	6% 2 21% 40% 22% 8%	583
September	1st to 10th	Day	6% 4 21% 40% 23% 6%	530
	11th to 20th	Day	3 4 19% 40% 26% 8%	353
	21st to end	Day	12% 5% 23% 26% 25% 8%	330
October	1st to 10th	Day	14% 6% 23% 27% 22% 8%	330
	11th to 20th	Day	10% 12% 33% 29% 12% 4	330
	21st to end	Day	15% 11% 31% 26% 15% 2	363
November	1st to 10th	Day	9% 8% 37% 28% 12% 5%	330
	11th to 20th	Day	7% 11% 35% 34% 12% 2	330
	21st to end	Day	13% 4 36% 32% 11% 3	330
December	1st to 10th	Day	8% 9% 45% 30% 7% 1	330
	11th to 20th	Day	12% 13% 41% 27% 6% 1	330
	21st to end	Day	14% 10% 39% 31% 5% 1	363
January	1st to 10th	Night	8% 9% 37% 37% 8% 1	1050
	11th to 20th	Night	8% 12% 40% 32% 7%	1050
	21st to end	Night	8% 8% 45% 32% 7%	1155
February	1st to 10th	Night	12% 8% 46% 29% 5%	866
	11th to 20th	Night	9% 7% 47% 32% 5%	824
	21st to end	Night	8% 8% 48% 32% 4	692
March	1st to 10th	Night	10% 10% 38% 38% 5%	840
	11th to 20th	Night	13% 7% 40% 34% 5% 1	741
	21st to end	Night	14% 8% 39% 32% 7%	693
April	1st to 10th	Night	16% 13% 37% 30% 4	630
	11th to 20th	Night	16% 10% 41% 31% 2	611
	21st to end	Night	19% 11% 29% 34% 7%	600
May	1st to 10th	Night	15% 10% 33% 34% 8%	600
	11th to 20th	Night	19% 6% 31% 35% 9% 1	591
	21st to end	Night	19% 7% 34% 31% 10%	650
June	1st to 10th	Night	9% 6% 30% 41% 14% 1	600
	11th to 20th	Night	11% 6% 37% 37% 10%	600
	21st to end	Night	7% 3 29% 48% 14%	600
July	1st to 10th	Night	10% 1 28% 45% 16%	585
	11th to 20th	Night	5% 2 26% 52% 14%	582
	21st to end	Night	6% 2 23% 51% 18% 1	657
August	1st to 10th	Night	7% 3 22% 51% 16% 1	597
	11th to 20th	Night	6% 3 27% 51% 14%	594
	21st to end	Night	5% 2 27% 49% 16%	660
September	1st to 10th	Night	8% 2 30% 45% 14%	600
	11th to 20th	Night	8% 3 30% 42% 16% 1	600
	21st to end	Night	9% 7% 29% 39% 15% 1	600
October	1st to 10th	Night	14% 6% 26% 40% 14%	600
	11th to 20th	Night	9% 9% 33% 36% 12%	793
	21st to end	Night	13% 8% 32% 37% 10% 1	1100
November	1st to 10th	Night	8% 6% 36% 41% 8% 1	1025
	11th to 20th	Night	8% 8% 38% 38% 8%	1050
	21st to end	Night	9% 7% 34% 39% 10%	1050
December	1st to 10th	Night	9% 10% 41% 33% 7%	1050
	11th to 20th	Night	9% 10% 39% 35% 7%	1050
	21st to end	Night	14% 9% 36% 36% 5%	1155

Page generated at 2026-04-14 00:44:30. Version ac07

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