Urban Heat and Drought Watch
How much hotter is your city than its countryside, day and night? Is this year’s spring drier than every spring since 2001? MODIS answers for any place.
Two studies from the MODIS sensor on Terra. Urban heat island: the land-surface temperature of a chosen month at 1 km, day and night, averaged over its 8-day composites and compared between the built-up area and the countryside around it, which are taken from the annual 10 m land-cover map; with the heat-island intensity, the hottest built-up places and temperature against the share of buildings. Drought: the 250 m greenness (NDVI) of a chosen month in every year since 2001, turned into the vegetation condition index, with drought classes and their areas, the anomaly from the usual greenness, and the greenness of that month year by year.
Overview
Iraqi summers are among the hottest on Earth, and cities make them worse at night, when roofs and roads give back the heat of the day. Knowing where the hottest districts are helps to plan shade, trees, water and power.
Drought in Iraq shows first in the rain-fed wheat of Nineveh and Kirkuk and in the orchards of Diyala and the Euphrates. Comparing a spring with every spring since 2001 shows at once whether a year is dry, and where.
What you enter · What you get
What you enter
- Study: urban heat or drought
- A city or farmland from the list, found by name or drawn (5 to 150 km)
- Year and month
What you get
- Day and night surface temperature maps
- Heat-island intensity, day and night
- Hottest built-up places and their area
- Vegetation condition index, NDVI anomaly and NDVI maps
- Drought classes in km²
- Greenness of the month since 2001
How it works in the field
Method and equations
Surface temperature
MOD11A2 version 6.1: 8-day land-surface temperature at 1 km from the split-window algorithm, day (about 10:30) and night (about 22:30), scale 0.02 K. The cloud-free composites of the month are averaged pixel by pixel.
Heat island
The share of buildings within about a kilometre is computed from the annual 10 m land-cover map. City pixels are built-up with more than half of their surroundings built over; countryside pixels have less than 5 % and are not water. The surface urban heat island intensity is the difference of their means.
Vegetation condition
MOD13Q1 version 6.1: 16-day NDVI at 250 m. For the composite that starts in the chosen month of each year since 2001, the minimum and maximum of every pixel define the vegetation condition index; land that never exceeds NDVI 0.2 is left out. Classes follow Kogan: below 10 extreme, 10–20 severe, 20–30 moderate, 30–40 mild drought.
Accuracy and error limits
- MODIS surface temperature agrees with ground measurements within about 1–2 K over most land; at 1 km a pixel mixes roofs, streets, gardens and water, so single buildings and small parks are not resolved. The heat-island figure depends on how the countryside is defined: in desert cities bare land can be hotter than the city by day. NDVI at 250 m mixes fields, so small farms are averaged; the VCI reflects the record since 2001 only.
- What is not covered: air temperature and heat felt by people, cloud-covered days (left out of the composites), crop type, irrigation and the cause of a low NDVI (drought, fallow, harvest, salinity or building). Land cover before 2017 is replaced by that of 2017.
- To reduce the error: compare the same month across years, use July or August for the heat island and the month of peak greenness for drought, read night temperatures for health impacts, and check drought areas against rainfall records and agricultural plans.
Who uses it
Standards and references
- Wan, Z. (2014). New refinements and validation of the collection-6 MODIS land-surface temperature/emissivity product. Remote Sensing of Environment, 140, 36–45.
- Didan, K. (2021). MODIS/Terra Vegetation Indices 16-Day L3 Global 250m SIN Grid V061. NASA EOSDIS Land Processes DAAC.
- Kogan, F. N. (1995). Application of vegetation index and brightness temperature for drought detection. Advances in Space Research, 15(11), 91–100.
- Peng, S. et al. (2012). Surface urban heat island across 419 global big cities. Environmental Science & Technology, 46, 696–703.
- Voogt, J. A. & Oke, T. R. (2003). Thermal remote sensing of urban climates. Remote Sensing of Environment, 86, 370–384.
- Karra, K. et al. (2021). Global land use/land cover with Sentinel 2 and deep learning. IGARSS 2021, 4704–4707.
At a glance
- Category
- Climate & emissions
- Runs on
- Phone, tablet and desktop browser
- Export
- Maps as PNG, JPG and PDF; chart as PNG, JPG and PDF; results as CSV; full PDF report
- Access
- Free
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