Introduction

This analysis tool allows one to track through time the height of the freezing level (0 C or 32 F) above sea level. Freezing level has important effects on hydrology in mountain environments. This level affects 1) the elevation of the rain/snow line, 2) whether precipitation at a specified level falls as rain or as snow, 3) whether the ground is frozen or thawed when the first autumn snows fall, 4) the efficiency of snowpack accumulation through the winter months, 4) the internal temperature and rate of “ripening” and melting of the snowpack in the spring, and 6) the length of the snow free season at different elevations. Freezing level also affects ecological function through biological growth rates (both plants and animals) at different elevations. Other temperature thresholds of interest are available (10 C, 20 C, 30 C / 50 F, 68 F, 86 F) as well. These temperatures can be substituted for the term “freezing level” below.

Definition

In the upper atmosphere temperatures are always below freezing everywhere on earth. Starting from the upper atmosphere and working down, the freezing level is taken to be the elevation above sea level in the free atmosphere at which a temperature of 0 C (or 32 F) is first encountered. The mean daily temperature profile used for this process is formed from the four six-hour averages available from Global Reanalysis (see below).

Monthly and seasonal means

Freezing level heights are found for each day. When the entire atmosphere is cooler than freezing, values are set to zero meters above sea level for that day. Monthly and seasonal values are the mean of all the daily values during that interval. Topographic information is not included in the determination of freezing level. This allows freezing level in mountainous or elevated regions to often be found below the topography. Below the topography, the vertical rate of change of temperature ("lapse rate") is calculated by the Reanalysis, and is generally between 4.5 and 6.5 degrees C per kilometer.

Monthly Precip as Snow

The purpose of this product is an estimate of the percent of precipitation at a given elevation that is falling as snow rather than rain during a certain portion of the year. A user selects location, number of months, end month, running mean smoother, and an elevation. Precipitation is modeled amount from Global Reanalysis interpolated to point in question. If temperature at the elevation and time is above 0 C, precipitation is considered to be rain, otherwise is snow. For the time interval in question, the amount of precip falling as rain is divided by the total amount of precipitation. This ratio is expressed as a fraction from 0 to 100 percent snow. Lower numbers indicate more rain, less snow.

Data source

Values are based on the NCEP/NCAR Global Reanalysis determined every six hours from January 1948 until the last few days. The grid spacing is approximately 2.5 x 2.5 degrees of latitude/longitude. To see the grid, click the box under “settings” in the left hand frame. Values are interpolated to the latitude/longitude selected.

Means are from 1970-2000.

Acknowledgement

Development of the North American Freezing Level Tracker was funded by the National Oceanic and Atmospheric Administration (NOAA) through the Regional Integrated Sciences and Assessments (RISA) Program under the Coping With Drought element.

For questions, questions, or web problems, contact wrcc@dri.edu, 775-674-7010, or kelly.redmond@dri.edu. John Abatzoglou at the University of Idaho (jabatzoglou@uidaho.edu) programmed most of the code.

Click anywhere on the map to place a marker. This marker can subsequently be dragged around the map. Use the Products panel on the right to generate products for the marker's location.

To see available locations, select the Display Options tab, and check Show Grid Points.

Select products below. Products appear in middle panel. Previous products can be retrieved from middle of top line, or deleted by clicking “X” in upper right of each graphic. Use “save as” to retain graphic. To plot another product after completion of a previous product, click “Map” in middle tab and then click another desired location on the map. Or, drag first marker to desired location.

North American Freezing Level Tracker

Monthly/seasonal Time Series

This product shows the mean height of the selected temperature value for the same portion of each year through the latest available year. Use the pulldowns to select a month or season. A “season” is a period of 2 or more months extending through the end of the selected month. To select December through February, for example, select the three months (“span”) ending in February (“End Month”). Four temperature values are available: 0 C (the “freezing level”), +10 C, +20 C, and +30 C. These other values are of interest to ecologists and biologists. Latitude and longitude can be selected by clicking on the map, or by direct entry in the form. The running average is computed as the mean over the indicated number of years (“Running Average”), and is plotted at the midpoint of each such period, and shown as a black line. The long-term mean value for the overall period is plotted as a horizontal line. Years with values reflecting warmer than usual conditions are shown in red, and values reflecting cooler than usual conditions are shown in blue.

Last 12 Months

Shows the recent history of the height of the selected temperature value for each of the last twelve full months. Latitude and longitude can be selected by clicking on the map, or by direct entry in the form. Red: Values for the most recent 12 months. Black: Long-term median for each of the most recent 12 months. Blue shading: from bottom, 5th, 10th, 25th, 50th, 75th, 90th, 95th percentile for each month, for period of record (1948 through latest month).