Colorado’s climate is governed by five major factors which combine to produce generally cool temperatures, bountiful sunshine and low precipitation, but with sharp local gradients, large seasonal cycles and dramatic day to day changes (Topper et al., 2003).
Mid-latitude location halfway between the equator and the North Pole
Colorado’s southern border is located at 37 degrees and the northern border at 41 degrees north latitude, positioning the state roughly halfway between the equator and North Pole (Doesken, Pielke & Bliss, 2003). The distance from the equator coupled with the Earth’s tilt of its axis of rotation dictate the angle the sun’s rays are received, the amount of the sun’s radiative energy received and the length of the day for a particular location and time of year. Regions at higher latitudes than Colorado receive less direct sunlight, because the sun’s rays hit those locations at more of an angle, which causes them to become more dispersed and creates a cooler climate. Colorado’s middle latitude location provides anywhere from nine to 15 hours of daylight (University of Nebraska-Lincoln Astronomy Education, 2010), depending on the season, and large daily and seasonal temperature swings (Doesken, Pielke & Bliss, 2003).
Significant distances from large bodies of water and encased by large, varied land masses
Colorado’s position far inland and great distance from any large bodies of water, results in a usually dry climate. Also, because there are no oceans to moderate daily and seasonal temperatures, Colorado typically has hot summers and cold winters with relatively large temperature differences throughout each day. Moisture from the Pacific Ocean and Gulf of Mexico must travel long distances before arriving in Colorado, with much being intercepted by mountain ranges before reaching the state (Doesken, 2011; Doesken, 2013; Lukas, et al., 2014).
Complex and varied features ranging from vast plains to the highest mountains in the Continental Divide
Colorado’s topography can be divided into three major regions: the plains in the east, the mountains through the center and the rugged plateaus and mesas in the western portion of the state. Each region has its unique patterns of air movement, precipitation and temperature (Jones & Cech, 2009). While air moves freely over the eastern plains, the high mountains act as barriers, often creating orographic (terrain-driven) wind patterns that affect both temperature and precipitation (Lukas et al., 2014). For almost any weather pattern, one part of the state will be on the upwind side of the mountains while other areas are downwind. Thus, there are almost always drier and wetter regions, and hotter and colder areas in very close proximity. The state’s river valleys also add to the high climactic variability throughout the three regions. For example, the Arkansas (southeastern) and South Platte (northeastern) River valleys dissect the eastern plains creating higher ground between the valleys, extending eastward along the New Mexico border to the south and along the Wyoming and Nebraska borders to the north. These features impact temperatures, wind patterns and storm tracks throughout the area (Doesken, Pielke & Bliss, 2003).
Substantial variation ranging from less than 4,000 feet to more than 14,000 feet above sea
Located at the heart of the Rocky Mountains, Colorado has an average elevation of 6,800 feet. Starting at 3,350 feet in the lowest valleys along the eastern border with Kansas, the elevation of Colorado increases towards the west climbing to 5,000 to 6,500 feet at the eastern base of the Rocky Mountains. Continuing westward, the elevations rise dramatically from 7,000 to 9,000 feet in the foothills, to 9,000 to over 14,000 feet above sea level in the vast mountain ranges of the Rockies (Doesken, Pielke & Bliss, 2003). Farther west, the elevation begins to decrease into rugged canyons and plateaus extending into the surrounding states of Utah, New Mexico and Arizona; although still residing at elevations higher than the eastern plains, at around 5,000 to 7,000 feet (Scott, Collins & Diggs, 2003). Air masses must rise over the Rocky Mountains and higher elevations, and as they rise the pressure and temperature both decrease. Since cooler air can hold less water than warmer air, rain or snow occurs whenever moist air masses moves over a range of mountains. As a result, the windward sides of mountains receive more precipitation than the leeward sides of mountains (Doesken, Pielke & Bliss, 2003).
Near a strong mid-latitude jet stream produced by large temperature changes converging 14,000 feet above sea level
Colorado climate is generally influenced by prevailing westerly winds, and during the winter months, they create a strong mid-latitude jet stream as the result of large temperature differences meeting between the tropics and North Pole (Doesken, 2013). This mid-latitude jet stream is also known as the Polar Jet Stream, which becomes stronger and moves farther south from Canada into the U.S. as the North Pole becomes colder in the winter months, creating a larger temperature gradient. Moving at high altitudes and high speeds eastward, the jet stream carries air masses from the Pacific in a wave-like pattern as it follows the troughs of low pressure and ridges of high pressure across the U.S. (National Weather Service, 2011). Winter storms typically follow the jet stream’s path, and although the usual path of the jet stream tends to be slightly north of Colorado, it occasionally deeps southward moving winter storms across and sometimes even south of Colorado.
Precipitation in Colorado
In Colorado, precipitation occurs in various forms (rain, snow, hail, etc.) and the amount varies seasonally, annually and by location. Statewide the average yearly precipitation is approximately 17 inches; however, there is a large range in the average annual precipitation across the state due various topographical features. For example, the San Luis Valley and parts of south central Colorado receive an average of less than 7 inches of precipitation each year, while many mountainous regions receive 25-40” climbing to a maximum of near 60 inches per year just east of Steamboat Springs in north central Colorado (Collins, Doesken & Stanton, 1991; Doesken, Pielke & Bliss, 2003). Average precipitation slightly decreases moving from the eastern border with Kansas into the Colorado plains, but then tends to significantly increase moving further west into the higher elevations of the foothills and mountain ranges. Three major precipitation patterns occur in Colorado as a result of seasonal, large-scale atmospheric circulation interacting with the state’s topography (Collins, Doesken & Stanton, 1991):
- Pacific Moisture: The winter pattern delivers moisture from the prevailing westerlies flowing in from the Pacific Ocean. As the mountains block the westerlies’ path and harvest most of the available moisture as snow, the eastern portion of the state is left with very little precipitation. In general, precipitation increases with altitude west of the Continental Divide, but significantly decreases east of the Divide in the winter precipitation pattern (Collins, Doesken & Stanton, 1991; Lukas, et al. 2014).
- Gulf and Subtropical Atlantic and Land-Recycled Moisture: While the mountains and areas west of the Divide receive most of their annual precipitation as snow during the winter months, most lower elevation precipitation falls as rain during the late spring and summer months (Doesken, 2011). The second precipitation pattern accompanies the transition into spring as warmer temperatures arrive, as well as slow-moving storms carrying moisture from the Gulf of Mexico into the eastern portion of the state from the south and southeast (Collins, Doesken & Stanton, 1991). As the temperatures continue to increase into summer, convective processes significantly contribute to daily cloud formation and precipitation statewide, but most notably east of the Rockies, whereby rapid and high surface heating forces warm air to rise, expand and cool (Lukas, et al. 2014). So while the mountains act more as barriers in the winter as they squeeze out available moisture from the westerlies, they begin to act more like thunderstorm generators moving into the spring and summer months (Doesken, 2011, 2013).
- Subtropical Pacific Moisture: The third major precipitation pattern most greatly impacts the southern half of the state in July through late summer, although its effects can frequently be felt throughout the entire state. A weak monsoon-like circulation drives subtropical moisture from the Pacific Ocean up into Colorado, resulting in frequent summer thunderstorms in the southern portion. This moisture pattern typically weakens and retreats south in late August through October; however, sometimes significant moisture from dissipating tropical cyclones which jet up into the southern part of the state. During all of these precipitation patters, some moisture can also be attributed to land-recycled moisture from evaporation carried on incoming winds (Collins, Doesken & Stanton, 1991).
The state’s complex topography also influences precipitation patterns at adjacent lower areas depending upon their proximity. If the lower elevation areas are close to the mountains, precipitation that did not fall in the mountainous regions will fall in these adjacent lower areas. If the lower areas are farther from the mountains, less precipitation will fall, as is the case in North Park, South Park and the San Luis Valley. The following seasonal precipitation variations are generally true for Colorado, although events outside of these patterns can and do happen frequently due to the state’s complex topography, middle latitude and interior continental location (Doesken, 2011, 2013; Doesken, Pielke & Bliss, 2003):
- Mid-winter is the wettest time of year for the high mountains and most of western Colorado
- Eastern Colorado is driest during this season
- Early summer can be wet in eastern Colorado
- June tends to be the driest month in the central mountains
- July is typically the wettest month in the southeast and southern mountain valleys
- August tends to be the wettest month for southwestern Colorado
- Wettest season for the eastern foothills
- May and early June can be the wettest months for parts of the Front Range and northeastern Colorado
- Late summer and early fall can be the wettest time of year for areas far west near the UT border
- Snow typically begins to build in the mountains in October
Although these precipitation patterns typically make an appearance annually throughout the state, year-to year variation in precipitation is the rule for Colorado, as minor fluctuations in the dominant patterns can and have caused flooding, droughts and/or more localized, extreme storms (Collins, Doesken & Stanton, 1991). This natural variation in precipitation creates serious concern over water management for municipalities, agriculture and industry. Because Colorado is a headwaters state, it greatly relies on winter snowmelt to provide adequate flows for its rivers, with melting snow providing as much as 80 percent of surface water supplies. Historically, most rivers experience peak streamflows in May and June due to the retreating snow from the mountains (Doesken, 2013).
The timeline below briefly outlines significant historical flood and drought events throughout Colorado.
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Temperatures in Colorado
Colorado’s mid latitude and mid continental position, coupled with its rugged topography and high elevations create highly variable seasonal and daily temperatures throughout the state. East of the Rockies annual temperature variations are generally controlled by the exchange among Pacific, subtropical and Polar air masses, while in the mountains, annual temperature variations are dictated more by atmospheric ridges and troughs moving through the state (Doesken, 2013). Temperature variations are less significant west of the mountains, with snow presence or absence being the main influencing factor on annual variations (Doesken, Pielke & Bliss, 2003; Doesken, 2013; Lukas et al., 2014). Statewide, daytime temperatures are more variable than nighttime, and winter temperatures are more variable than those during the summer; therefore, the least variation in temperatures usually occurs during summer nights (Doesken, 2013).
January tends to be the coldest month across the state while July, or sometimes August, is usually the warmest, with a 40 – 55 degree Fahrenheit swing in average temperature between the warmest and coldest month two months (Doesken, 2013; Lukas et al., 2014). Daily temperature changes of 30 degrees Fahrenheit or more frequently occur throughout much of the state, with daily maximums being reached by the afternoon as the air is warmed by surface heat building from the sun’s radiation. Due to low humidity, little solar energy is absorbed by the air throughout the day in Colorado, allowing for large temperature fluctuations.
In general, air temperature decreases approximately 3.5 degrees Fahrenheit (2 degrees Celsius) per 1,000 feet of elevation increase (Doesken, Pielke & Bliss, 2003; Lukas et al., 2014). Despite this trend, the largest daily temperature changes typically occur in mountain valleys. This is due to mountain-valley wind patterns and temperature inversions created, whereby cold, heavy air pools in still mountain valleys resulting in very cold temperatures. Inversions occur most frequently during winter nights in the absence of wind. Along the Front Range, an inversion may result during days with high air pollution, because the pollutants become trapped in the layer of cold air.
Additionally, differences in elevation, slope and aspect (the direction that a slope faces – north, south, east or west) result in varying amounts of solar heating in the mountains and surrounding terrain. South facing slopes receive more sunlight than north facing slopes, and as a result, are usually warmer and drier. Steep slopes may cast shadows over regions below, resulting in cooler temperatures (Doesken, 2103). In contrast, chinook winds are responsible for rapid increases in temperature along the Front Range. Air pressure is less at higher elevations and greater at lower elevations, so when winds move down from the mountains to the Front Range, they are subjected to increasing pressure. This increased pressure compresses and warms the winds as they move eastward. These warmer chinook winds have been known to raise temperatures by 50 degrees Fahrenheit in only a few hours (Doesken, 2013). While these winds bring warmer temperatures along the Front Range during the winter months, they can often be devastating with sustained, severely turbulent winds and gusts sometimes reaching 70 miles an hour or more.
Temperatures of streams and lakes fluctuate less from day to day than air or soil temperatures. In early summer, water is on average colder than the air or soil, but by late summer it is warmer. The seasonal rise in daily temperatures creates snowmelt and runoff, beginning first with the lower elevations and south facing slopes (Doesken, 2013; Lukas et al., 2014). Temperature has significant effects on the following:
- Precipitation, causing moisture to fall as rain or snow
- Supply of water in its relation to the snowmelt process
- The demand for irrigation and domestic water
Climate Change in Colorado
Amid climate models, data and theories, scientists are aware of two things: the climate is changing and the effects may dramatically change Colorado’s water outlook (Ozzello, 2008).
Scientists have proven conclusively that very large emissions of greenhouse gases caused primarily by the combustion of fossil fuels are changing the Earth’s climate (IPCC, 1990, 1995, 2001, 2007, 2013). Within the past century, the average temperatures of the earth’s surface have increased by 1.6°F since 1900 and .8°F since 1980 due to increasing atmospheric concentrations of greenhouse gases mainly from anthropogenic influences (Intergovernmental Panel on Climate Change [IPCC], 2013). This warming trend is also apparent in Colorado where the statewide annual average temperatures over the past 30 years have increased by 2°F, with increases across all seasons: summer temperatures by 2.5°F, fall by 2.5°F, spring by 2.2°F and winter by 1.6°F (Lukas, et al., 2014). The warming spring temperatures, coupled with lower snow-water equivalent (SWE) since 2000 and enhanced solar absorption from dust-on-snow, have been identified by researchers as the driving forces behind a shift in the timing of snowmelt and peak runoff of one to four weeks earlier in the spring within the past 30 years. Additionally, researchers have identified a trend toward more frequent soil moisture drought conditions in the state over the past 30 years (Lukas, et al., 2014).
As long as humans continue to emit greenhouse gases, this warming trend will continue. And once humans stop emitting greenhouse gases, the warming reached at that time will be locked in for approximately 10,000 years (Clark et al, 2016). According to climate models, under a medium-low emissions scenario the statewide average annual temperature is expected to increase by 2.5°F to 5°F (relative to a 1971–2000 baseline) by 2050, and under a high emissions scenario, the statewide average annual temperature is projected to increase by 3.5°F to 6.5°F by 2050 (Lukas, et al., 2014; State of Colorado, 2015). These increasing temperatures pose significant challenges to managing Colorado’s water resources. The following table summarizes the projected changes and potential impacts to Colorado’s water resources:
Table 2. Projected water-related impacts from climate change in different areas and sectors of Colorado. Adapted from Lukas et al., 2014.
|Element||Projected changes & potential impacts to Colorado’s water resources|
|Overall surface water supply||Most projections of future hydrology for Colorado’s river basins show decreasing annual runoff and less overall water supply, but some projections show increasing runoff. Warming temperatures could continue the recent trend towards earlier peak runoff and lower late summer flows.|
|Water infrastructure operations||Changes in the snowpack and in streamflow timing could affect reservoir operations, including flood control and storage. Changes in the timing and magnitude of runoff could affect the functioning of diversion, storage, and conveyance structures.|
|Crop water demand, outdoor urban watering||Warming temperatures could increase the loss of water from plants and soil, lengthen growing seasons, and increase overall water demand.|
|Legal water systems||Earlier and/or lower runoff could complicate the administration of water rights and interstate water compacts, and could affect which rights holders receive water.|
|Water quality||Warmer water temperatures could cause many indicators of water quality to decline. Lower streamflows could lead to increasing concentrations of pollutants.|
|Groundwater resources||Groundwater usage for agriculture could increase with warmer temperatures. Changes in precipitation could affect groundwater recharge rates.|
|Energy demand and operating costs||Warmer temperatures could place higher demands on hydropower facilities for peaking power in summer. Warmer lake and stream temperatures, and earlier runoff, could affect water use for cooling power plants and in other industries.|
|Forest disturbances in headwaters regions||Warmer temperatures could increase the frequency and severity of wildfire, and make trees more vulnerable to insect infestation. Both have implications for water quality and watershed health.|
|Riparian habitats and fisheries||Warmer stream temperatures could have direct and indirect effects on aquatic ecosystems, including the spread of non-native species and diseases to higher elevations. Changes in streamflow timing could also affect riparian ecosystems.|
|Water- and snow-based recreation||Earlier streamflow timing could affect rafting and fishing. Changes in reservoir storage could affect recreation on-site and downstream. Declining snowpacks could impact winter mountain recreation and tourism.|
Climate change is affecting and will continue to affect the demand for Colorado water and the ways in which it is used within and across basins and economic sectors. Addressing climate change will require a concerted effort by Coloradoans and involve a two-pronged approach: greenhouse gas reductions (mitigation) and adaptation (State of Colorado, 2015). While it is of necessity that Colorado reduce its total greenhouse gas emissions, a warming climate cannot be entirely avoided, and therefore, the state must adapt.
Climate Change 2014: Synthesis Report by the Intergovernmental Panel on Climate Change (2014)
The Synthesis Report (SYR), constituting the final product of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), distils, synthesizes and integrates key findings of the contributions to the AR5 in a concise document for the benefit of decision makers in the government, the private sector as well as the public at large. The SYR, therefore, is a comprehensive up-to-date compilation of assessments dealing with climate change, based on the most recent scientific, technical and socio-economic literature in the field.
Climate Change in Colorado: A Synthesis to Support Water Resources Management and Adaptation by Lukas, et al. (2014)
This report is a synthesis of climate science relevant for management and planning for Colorado’s water resources. It focuses on observed climate trends, climate modeling, and projections of temperature, precipitation, snowpack, and streamflow. Climate projections are reported for the mid-21st century because this time frame is the focus of adaptation strategies being developed by the State of Colorado and other water entities.
Colorado Climate Change Vulnerability Study by University of Colorado Boulder and Colorado State University for the Colorado Energy Office (2015)
The Colorado Climate Change Vulnerability Study provides an overview of key vulnerabilities that climate variability and change will pose for Colorado’s economy and resources. The purpose of the study is to provide state agencies, local governments, and others with background for preparedness planning.
Colorado climate plan: state level policies and strategies to mitigate and adapt by the State of Colorado (2015)
The goal of this document is to promote state policy recommendations and actions that help to improve Colorado’s ability to adapt to future climate change impacts and increase Colorado’s state agencies level of preparedness, while simultaneously identifying opportunities to mitigate greenhouse gas emissions (GHG) at the agency level. In this plan, the major sectors of the state government are addressed, specific actions are called for, and policy recommendations are made.
Colorado Climate Preparedness Project (Final Report) by the Western Water Assessment for the State of Colorado (2011)
Using documents and information obtained through a series of 22 structured interviews, the Colorado Climate Preparedness Project provides a catalog of climate impacts and adaptation activities and options in five climate-sensitive sectors in the state of Colorado: water; wildlife, ecosystems and forests; electricity; agriculture; and outdoor recreation.
Climate Literacy: The Essential Principles of Climate Science by the U.S. Global Change Research Program (2009)
An interagency guide that provides a framework and essential principles for formal and informal education about climate change. It presents important information for individuals and communities to understand Earth’s climate, impacts of climate change and approaches for adapting and mitigating change.
Colorado Climate Center
This website maintains recent climate data, publications and other Colorado climate resources.
Colorado Agricultural Meteorological Network (CoAgMet)
With a growing number of data collecting stations, CoAgMet provides localized weather data in irrigated agricultural areas. In a similar vein is the Community Collaborative Rain, Hail, and Snow Network, a volunteer network of citizens gathering climate data.
Colorado State University Department of Atmospheric Sciences
Contains information about CSU’s Department of Atmospheric Sciences, the Colorado Climate Center, and links to other weather related websites.
Colorado Water Conservation Board Climate Change website
The Colorado Water Conservation Board (CWCB) purpose is to provide policy direction on water issues and to stand as Colorado’s most comprehensive water information resource. This webpage provides the latest information related to climate change and Colorado’s water resources.
Extreme Weather Map by Environment Colorado
Environment Colorado’s interactive extreme weather map shows weather-related disasters in the United States over the last five years and tells the stories of the people and communities who have endured some of those disasters and other extreme weather events.
High Plains Regional Climate Center
The High Plains Regional Climate Center is one of six NOAA Regional Climate Centers in the U.S. The Center seeks to increase use and availability of climate data in the High Plains regions by providing climate services, developing climate data and information products, and engaging stakeholders.
National Climate Assessment
The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future. A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.
National Drought Mitigation Center
The National Drought Mitigation Center, based in the School of Natural Resources at the University of Nebraska– Lincoln, was established in 1995 to help reduce vulnerability to drought. The Center works with states and tribal governments across the U.S. and with national governments around the world to develop better drought risk management strategies related to monitoring, early warning and planning. The NDMC also maintains an extensive website with resources for monitoring, assessing impacts, planning and K-12 education.
NOAA Climate.gov is a source of timely and authoritative scientific data and information about climate. They strive to promote public understanding of climate science and climate-related events, to make data products and services easy to access and use, to provide climate-related support to the private sector and the Nation’s economy, and to serve people making climate-related decisions with tools and resources that help them answer specific questions.
United States Climate Page
Find plots of mean daily maximum and minimum temperatures, precipitation, and snowfall for selected cities and towns in the U.S. In Colorado, plots for Akron, Denver, Eagle, Grand Junction, Colorado Springs, Pueblo, La Junta, Alamosa, and Trinidad exist. (Hint: Try the fast lane–the state maps are great!) The site provides current forecasts and weather conditions for Denver, Colorado, and the United States. It also links to other weather related information.
U.S. Climate Resilience Toolkit
The U.S. Climate Resilience Toolkit provides scientific tools, information, and expertise to help people manage their climate-related risks and opportunities, and improve their resilience to extreme events. The site is designed to serve interested citizens, communities, businesses, resource managers, planners and policy leaders at all levels of government.
Western Regional Climate Center
The Western Regional Climate Center is one of six NOAA Regional Climate Centers in the U.S. The Center serves as a focal point for coordination of applied climate activities in the West and conducts applied research on climate issues affecting the West.
Western water and climate change by Dettinger, Udall & Georgakakos (2015)
This paper is a distillation of findings regarding western water and climate change, coupled with several vignettes of issues developing in iconic western rives to add specificity to those finds and to illustrate the diversity of conditions facing the region.
Western Water Assessment Dashboard
The Western Water Assessment is a university-based applied research program that addresses societal vulnerabilities to climate variability and climate change, particularly those related to water resources. While based in Boulder, Colorado, they work across the Intermountain West—Colorado, Utah, and Wyoming.
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