A blizzard is one of winter’s most severe weather phenomena, defined by specific criteria that must be measured and sustained over time. To be officially classified as a blizzard, a storm must feature sustained winds or frequent gusts of 35 miles per hour or greater. This wind must combine with falling or blowing snow to reduce visibility to one-quarter of a mile or less, and these conditions must persist for a minimum of three hours. Accurately tracking these three metrics—wind speed, visibility, and precipitation—requires specialized instruments working across automated weather networks. Meteorologists rely on this continuous data stream to determine when a snowstorm escalates into a blizzard.
Instruments for Wind Speed and Direction
The velocity component of a blizzard is measured primarily by ruggedized anemometers designed to withstand freezing conditions and high wind loads. These instruments must accurately capture the sustained average wind speed and gusts. Cup anemometers are a traditional type, using three or four hemispherical cups that rotate around a vertical axis; the rotation rate is converted electronically into wind speed. While reliable, these mechanical devices are susceptible to icing, which can lead to inaccurate readings during a severe winter storm.
More advanced systems often employ sonic anemometers, which have no moving parts and are less vulnerable to mechanical failure or icing. These instruments measure wind speed by calculating how fast an ultrasonic sound pulse travels between pairs of transducers. The time difference indicates the wind velocity. For directional data, a wind vane is typically mounted alongside the anemometer to determine the wind’s source direction.
Quantifying Snowfall and Accumulation
Although snowfall amount is not part of the blizzard definition, precipitation measurement is crucial for hydrological forecasting and determining storm severity. Automated weighing precipitation gauges measure both liquid and solid precipitation, including snow. These devices use a load cell to continuously weigh the accumulated precipitation in a container, providing a precise measurement of the water equivalent. This method is more accurate for snow than traditional tipping-bucket gauges, which can cause evaporation.
Remote sensing technologies, such as Doppler Weather Radar, estimate the rate of snowfall across broad areas. The radar sends out microwave pulses and measures the energy reflected back by snowflakes to determine their size and density. Meteorologists use specialized algorithms to convert this reflectivity data into an estimated snowfall rate and snow water equivalent. For ground-truth verification, human observers still use rulers and specialized snow boards to manually measure the depth of new snow and determine the actual water content.
Tools for Determining Visibility
Visibility reduction to one-quarter mile or less, caused by blowing snow, is monitored by specialized optical sensors. These sensors assess the atmosphere’s ability to transmit light over a distance. Transmissometers measure the extinction coefficient of light by sending a beam from a transmitter to a receiver over a fixed baseline. The amount of light lost directly correlates to the visibility distance, providing accurate data in low visibility conditions.
Forward scatter sensors measure the amount of light scattered by particles in the air, such as snowflakes. These sensors are generally more compact and cost-effective than transmissometers, though they can be less accurate in the lowest visibility ranges. When electronic sensors are compromised by icing or malfunction, human observers at official weather stations visually confirm the distance to known reference points, providing a necessary backup.
Integrated Data Collection and Remote Sensing
The data from individual sensors are collected and processed by sophisticated integrated platforms. These include the Automated Surface Observing Systems (ASOS) and the Automated Weather Observing Systems (AWOS). These systems continuously gather measurements for wind, visibility, temperature, and precipitation type, instantly compiling them into official weather reports. ASOS stations, often found at major airports, provide minute-by-minute updates essential for tracking the rapid changes characteristic of a blizzard.
Beyond ground-based stations, remote sensing tools offer a large-scale view of the developing storm:
Weather Satellites
Geostationary satellites maintain a fixed position relative to the Earth, providing continuous, real-time imagery of cloud cover and storm system movement. This allows forecasters to track the blizzard’s path. Polar-orbiting satellites travel at a lower altitude, capturing detailed atmospheric soundings. These soundings provide vertical profiles of temperature and moisture, which helps in understanding cold air mass formation.
Radiosondes
Weather balloons, known as radiosondes, are launched twice daily worldwide. They gather upper-air data on wind speed, temperature, and pressure at various altitudes. This information is fed directly into computer models that predict the blizzard’s future intensity and duration.