Red snow algal blooms reduce albedo and increase snowmelt, but little is known of their extent, duration, and radiative forcing. We calibrated an established index by comparing snow algal field spectroradiometer measurements with direct counts of algal cell abundance in British Columbia, Canada. We applied the field calibrated index to Sentinel-2, Landsat-8, and MODIS/Terra images to monitor snow algae on the Vowell and Catamount Glaciers (Purcells, British Columbia) in summer 2020. The maximum extent of snow algal bloom cover was 1.4 and 2.0 km² respectively, about one third of the total surface area of the two glaciers, making these among the largest contiguous bloom areas yet reported. Blooms were first detected following the onset of above-freezing temperatures in early July and persisted for about two months. Algal abundance increased through July, after which the red snow algal bloom area decreased due to snow cover loss. At their peak in late July the blooms reduced albedo by 0.04 ± 0.01 on average. Snow algae caused an additional 5.25 ± 1.0 × 10⁷ J/m² of solar energy to be absorbed by the snowpack in July–August, which is enough energy to melt 31.5 cm of snow. This is equivalent to an average snow algal radiative forcing of 8.25 ± 1.6 W/m² through July and August. Our results suggest that the extent, duration, and radiative forcing of snow algal blooms are sufficient to enhance glacial melt rates.

红雪藻大量繁殖会降低反照率并增加融雪量，但对其范围、持续时间和辐射强迫知之甚少。我们通过将雪藻场光谱辐射计测量值与加拿大不列颠哥伦比亚省藻类细胞丰度的直接计数进行比较来校准已建立的指数。我们将现场校准指数应用于 Sentinel-2、Landsat-8 和 MODIS/Terra 图像，以监测 2020 年夏季 Vowell 和 Catamount 冰川（Purcells，不列颠哥伦比亚省）上的雪藻。雪藻水华覆盖的最大范围为 1.4和 2.0 公里²分别占两个冰川总表面积的三分之一左右，使它们成为迄今为止报道的最大的连续水华区域。在 7 月初出现高于冰点的温度后首次发现花朵并持续了大约两个月。到 7 月，藻类丰度增加，之后由于积雪减少，红雪藻华面积减少。在 7 月下旬的高峰期，花朵的反照率平均降低了 0.04 ± 0.01。雪藻在 7 月至 8 月之间额外吸收了 5.25 ± 1.0 × 10 ⁷  J/m ²的太阳能，足以融化 31.5 厘米的雪。这相当于 8.25 ± 1.6 W/m ^{2的平均雪藻辐射强迫}通过七月和八月。我们的研究结果表明，雪藻大量繁殖的程度、持续时间和辐射强迫足以提高冰川融化速率。