Colorado Climate Center
Colorado State University

Colorado Drought Status Briefing

·Current Briefing


·Attend a future webinar



For Fun: Questions & Answers

For Fun: Click Here for Trivia

Question: Why is the sun becoming so much "brighter" than it was in the past?

First of all, there are many measurements taken of the solar intensity. The earth's orbit is not precisely circular so there is a time of year when we are closest to the sun and a time when we are farther away. My recollection is we are closest during northern hemisphere's winter season. We are closest to the sun in early January. Our period between Equinoxes is one week shorter (Sept-March) than during the other part of the year as a result of our orbital location.

Also, the output of the sun does vary, but imperceptably as far as you or I being able to feel a difference. Only the precisest instrumentation can detect tiny changes.

There are 3 factors that definitely do apply. It is true that brightness and glare become a much great problem for us as we get older. I'm really noticing that reality right now as I never have worn sun glasses in the past (although I probably should have).

Secondly, fall is an excellent time to notice the sun. Our many east-west streets have "sun in the eye" problems at this time of year (early Sept into mid Oct or so) because the sun is often right in line with the streets during morning and evening rush hour. We routinely see an increase in traffic accidents at this time of year on clear days. This is also a time of year, climatologically, when there is a greater likelihood for perfectly clear skies both morning and evening. So sunshine is particularly persistent. Also, the atmosphere is much drier now than it was a few weeks ago which means there is greater penetration of certain energy wavelenghts.

Thirdly, the angle of the sun is a big factor now. It is relatively low in the sky even at noon which means more sunlight reaches our faces and bodies instead of just the top of our head and shoulders. I was just outside a few minutes ago and was amazed at how "intense" the sunlight seemed. We will reach a comparable sun angle again at the end of February, and while the sun's energy will be the same it will not feel as intense because air temperatures at that time will be quite a bit cooler.

So while I agree with your observation, the scientific data we have point to the fact that 1) we are getting older and noticing it more and 2) the fall -- particularly mid Sept to Mid Oct. is when it is most noticeable, but that in reality the sun's intensity has not changed. If, however, you now live in a different area than you did when you were younger, then you might be perfectly correctly. I grew up in Illinois, much closer to sea level than my current home in Colorado. Much more of the sunlight is "attenuated" at low sun angles when you are closer to sea level than when you are here a mile or more above sea level. That does make a measurable difference. --Answered by Nolan Doesken (10/03).

Question: How much sunshine do we get in Colorado? On average for a year, how many days does the sun shine in Colorado?
This is a question that comes up several times per year. You will find in many Chamber of Commerce publications from all areas of Colorado that we get at least 300 days of sunshine each year. The only problem is, there is no official definition of "days of sunshine" so there is no data set that you can easily turn to.

Have you ever wondered if anyone actually keeps track of stuff like this? It turns out that for many years, three locations in Colorado have operated an instrument called a "sunshine switch" -- Pueblo, Denver and Colorado Springs. If this instrument is cleaned and perfectly calibrated (which it rarely is), it can tell you minute by minute each day when the sun was shining. We did a study over 10 years ago based on these three stations and found that for Denver if you count every day when the sun came out for at least one hour, that then you could come up with an average of around 300 "days of sunshine" each year.

But my assumption is that most people, if they heard "day of sunshine" would assume that meant it was a sunny day. The National Weather Service did establish a criterion for determining clear, cloudy and partly cloudy days based on sky cover. Any day, with an average skycover of 30% or less was considered a clear day, while if the sky cover was 80% or more, (averaged from hourly sky condition reports between sunrise and sunset), it was considered a cloudy day. Anything in between counts as "partly cloudy". Based on this definition, there are 115 clear days, 130 partly cloudy ones and 120 cloudy days, on average, each year. Over in Grand Junction the number of clear days is great (137) but the number of cloudy days is almost the same (121).

But the fact is, here in Colorado and much of the Rocky Mountain region, there are relatively few totally clear days but a whole lot of days when the sun peeks out at least a little. Therefore, we tend to brag about our sunshine -- but mislead folks along the way.

I am circling around your questions. Of course the answer will differ from one location to another in Colorado with the most sunshine occurring down around Alamosa with the least around Boulder and in the northern mountains of the state. -- In the Denver area there are probably only 30-40 totally overcast days per year, and some of them are even fairly bright -- about 300 days would have at least one hour of sunshine sometime during the day, but only about 115 days per year fit the classic definition of "clear". -- Answered by Nolan Doesken (9/03).


What country gets the most snow and why? We've searched and come up with no answers.

This is a harder question than it seems. The problem is that there are snow places in many countries, but the whole country may not be very snowy. The US. for example, is less snowy than Canada, on average -- by a lot. But there are places in the U.S. that receive more snow than most places in Canada. So the question is, are you looking for snowy countries, or just snowy places in countries. The U.S. has a long history of measuring "inches of snowfall" while that has not been a standard meteorological measurement in many other countries.

That being said, where are the snowy places. Basically, the best combination is the northern mid latitudes to southern high latitudes (north of perhaps 40 deg N latitude but not all the way up to the Arctic and close to a rich source of moisture (like an ocean). Throw in some mountains to help squeeze out the moisture, and then you're in business. There are snowy places in high mountains in other parts of the world (Asia, for example) but snowfall generally falls short of those areas that are closer to relatively warm oceans. So some of the possibilities become the U.S. Pacific NW, British Columbia in Canada. The areas of SE Alaska where mountain ranges are close to the Pacific. Then head over to Europe -- the Alps harvest a lot of Atlantic and Mediterranean Sea moisture. The mountains of Norway get tons of snow. And so on and so on. In the southern Hemisphere there are fewer candidates, but the best is the southern Andes mountains, and the mountain ranges of New Zealand.

You can pretty much narrow down the possibilities like this. Then, perhaps, you may be able to find some specific data from those locations. -- Answered by Nolan Doesken


Why is it so windy in Huerfano County?
Episodic strong winds are a part of life for all areas in the immediate lee (just east of) the high Rocky Mountain chain. Most of these strong winds are relatively brief but severe associated with rapidly descending air cascading over the crest of the Rockies and racing out to the plains. These "Down Slope Wind storms" are most common from late autumn into spring and accompany upper level disturbances in the strong winter-teim jet stream.

Fort Collins, Boulder, Denver, Colorado Springs and Pueblo are all prone to these windstorm events.

There are a few "preferred" areas that see strong winds much more commonly. Particular topographic features along the Front Range of the Rockies make certain areas more prone to strong winds than others. You are in one of those wind zones. There are three topographic features of your area that work together to produce a "wind tunnel". A clue to this tunnel is the location of the Great Sand Dunes. The long, relatively straight and broad valley of the Huerfano River happens to run parallel to the strongest upper level winds that blow over the Rockies in the winter. The bend in the Sangre De Cristo Mountains (and the protruding Blanca Peak massive) channels the winds toward Mosca Pass. Mosca Pass provides a low pass for the concentrated winds to blow through. Then, on the eastern side is a long, broad and straight valley headed straight for the open plains. And thar' she blows!

This is not a year-round wind tunnel. From late spring through mid autumn when upper level winds are light, the Huerfano County winds are not strong. But as long as the upper-level "Westerlies" are blowing, your wind tunnel will often be working. One of the benefits of these winds are markedly warmer winter temperatures. Compare temperatures at Westcliffe to those of Gardner on a breezy winter morning. The difference can be huge -- as long as you don't consider the wind chill effect.

Under certain circumstances winds can reverse and blow up the valley. When easterly "upslope" winds blow, the Huerfano valley becomes a preferred lcoation for heavy snows. In the summer, upvalley winds create preferred locations for thunderstorm development. The wettest areas in Colorado in July and August are often found in the Wet and Sangre de Cristo Mountains where these upslope easterly daytime winds converge with monsoonal winds blowing up from the south or southwest at mountain top level. -- Answered by Nolan Doesken

What exactly is a 100-year drought?
A 100-year "***" refers to any event that has a one-percent chance of occurrence in any year at a given point. Probabilities like this are normally determined based on past historic data. 100-year events have usually been reserved for heavy rain amounts for a specified duration (at a specified point or small local area) or peak or annual stream flow volumes at a particular point along a river or stream. Flood plains are often designated based on estimated 100-year flood flow volumes. The values of a 100-year event are area and duration dependent and cannot easily be generalized.

Based on statistical studies, you would like to have 200-400 years of data in order to make a reasonable stab at estimating the magnitude of a 100-year "***" In otherwords, there is quite a bit of uncertainty in many estimates of 100-year events. We only have about 100 years of historic observations of precipitation and streamflow, so we can estimate 25-year events reasonably well, but our estimates of 100-year events are much less certain. For example, for Loveland the 100-year rain storm for a duration of 24 hours is currently estimated to be about 5.0 inches and for a 6-hour duration about 3.6 inches.

One could do the same sort of analysis for drought. It is fairly easy to do for annual streamflow volumes, assuming the flow on the stream has been undisturbed for the past 100+ years (not the case on the Big THompson). For precipitation it's a bit more difficult as there are infinite possibilities on how to define drought based on quantities, durations and aras. But once you arrive at an acceptable definition, you go through the historic data and determine what the probablity of occurrence of any precipitation shortage over a prescribed period of time and a defined area might be. There would be a whole myriad of "100-year drought" numbers you could come up with based on your assumptions.

The Colorado Water Conservation Board has examined Colorado streamflow records and can assign probabilities to annual and peak streamflow volumes. From this, you could estimate a 100-year drought based on single-year streamflow statistics. Likewise, the paleoclimatologists studying tree rings could do the same. We have not tried to do the same with precipitation at this time simply because of the large number of potential combinations and the fact that no definition satisfies all users. -- Answered by Nolan Doesken

What is a river basin?
This is simply the land area that could contribute water to a river simply by natural gravity. There must be a clearer and simpler definition than that, but I can't think of it. The South Platte River Basin consists of all the areas from the crest of the mountains (continental divide) where local water if allowed to flow naturally by gravity without evaporating would end up in the South Platte River. Thus Castle Rock is in the South Platte River Basin as is Cheyenne, Wyoming and Estes Park -- as well as the cities of Denver, Greeley, Fort Morgan, etc. that are immediately along the river. -- Answered by Nolan Doesken
What is snow pack? What is an average snow pack for this time of year?
Snowpack here in Colorado refers to the accumulation of snow that graduallly adds up over the course of the winter, mostly in our mountains. The water content of that snowpack is what water officials monitor so closely since that relates closely to the amount of water that will end up in our rivers and stream, reservoirs and irrigation canals during the late spring and summer. The water content in our mountain snowpack normally reaches its greatest value in mid April and sometimes doesn't peak until late April or even early May. Based on an average of over 70 monitoring locations in high and moderately high snow accumulation areas of our mountains, the average snowpack water content is about 18 inches at its peak. This year the snow pack water content reached its maximum in late March at about 10 inches (again, remember this is just an average of about 70 or more individual monitoring points in Colorado -- depending on where you measure, you get a different answer -- but as long as you measure at the same places every year then you can compare one year to another). Snow has been melting steadily since then and as of today the statewide average snow water equivalent remaining in the mountain snowpack was 5 inches compared to an average on this date of about 17 inches. This infomration is provided by the USDA Natural Resouces Conservation Service who administers this snowpack monitoring program. -- Answered by Nolan Doesken
What is stream flow and what does it measure?
Stream flow is the volume of water (typically measured in units of cubic feet per second to define instantaneous flow rates or in acre-feet to define the total volume of water over longer periods of time).passing a specified point along a river or stream. In our state's system of water law, accurate measurements of streamflow are critical for determining how much water is available to each water owner. Hydrologists and engineers also track streamflows carefully in order to know how large to build structures (bridges, culverts, canals, dams, spillways, etc.) to safely convey water from snowmelt and from heavy rains. Scientists have long established useful relationships between measured snowpack levels and ensuing streamflow volumes so that winter and spring snowpack measurements result in reasonably accurate predictions of spring and summer streamflow from our mountain rivers and streams. -- Answered by Nolan Doesken
What is the annual average temp and annual average precipitation for Colorado?
This is actually a pair of numbers that we don't pay much attention to -- since there is no place in Colorado that receives the average temperature and average precipitation. That being said, 17" of precipitation per year (rain and melted snow) is a reasonable estimate. If you include the mountains (which in Colorado, you must), the statewide average annual temperature is somewhere between 45 and 49 degrees F -- you could use 47. But, again, there is no place in Colorado that I know of where the average precip. is 17" and the average temperature is 47 F, so this is just an exercise in research. -- Answered by Nolan Doesken
How deep is the frostline in the winter in various parts of Colorado?
A good question for which little quantitative data are available. It is a function both of temperature (related in a general sense to elevation but not strictly) and snow cover. Where snow accumulates deeply, little frost penetration occurs. Here in Fort Collins where we measure soil temperatures, there was one winter back around 1930 with extreme persisting cold and very little snow when frost penetrated to more than 30 inches. In most years, the max frost penetration on level, non-shaded ground is only 10-14" (much more in shaded). The greatest penetration in the past 20 years has been 18" at our site. We have had many mild winters recently, and we've had some with only a few inches (3-8") penetration. The ground is most likely frozen from just after Thanksgiving until sometime in early March at our station. This year, the ground remained unfrozen all January except in the top 2 inches at night and in the morning. This was very unusual.

A location known for deep frost penetration is the San Luis Valley since they have little winter snow but prolonged cold temperatures. Penetrations as great as 5-6 feet have been noted, although 3 feet is more common.

In snowier locations, frost penetration is not a big deal -- only a foot or two -- except for where roads, driveways, sidewalks, etc are kept clear of snow. There the frost penetration is much greater. We always recommend contacting county building officials in each individual county, since they are much more familiar based on regular contacts with experienced excavators -- they don't have data, but they do have experience, and that has usually been place into county building codes. -- Answered by Nolan Doesken


Please check back often as we add more questions & answers to this section.


Mailing Address:
Colorado Climate Center
Dept. of Atmospheric Science
1371 Campus Delivery
Colorado State University
Fort Collins, CO 80523-1371

Department of Atmospheric ScienceColorado State University
Equal Opportunity StatementDisclaimer Statement
© Colorado Climate Center 2010. All rights reserved.
No portion of this site may be copied or reproduced without express written permission.
Contact: Colorado Climate Center.