NEO News

Analysis: Hot in the city

As the northern hemisphere approaches summer, we explore land surface temperatures that are featured in ClimateBits: Urban Heat Islands.

Note that these examples are intended for curious people looking for hands-on Earth data exploration. Primary scientific research will require additional analyses through other methods. For the basics on how to use the NEO tool, see ‘Analysis tool in 10 easy steps’.

Urban Heat Islands are places on land where buildings, roads, and other impervious surfaces trap more heat than the surrounding rural area. During summer, an urban place like New York City can be 4°C (7°F) or more warmer than surrounding rural areas. Vegetation plays a cooling role through transpiration. Cities such as Minneapolis, Chicago and St. Louis — where most trees were cleared to make way for pavement and development — are urban heat islands surrounded by cooler forests.

Demonstrate seasonal changes

Load March, June and September, 2016 for land surface temperature [day]. These are found under the ‘Land’ category. Note the difference between ‘land surface temperature’ and ‘average land surface temperature’ data sets, the latter being climatology. We use the former in this example. These are MODIS/Terra observations collected since February, 2000 at daily, 8 day and monthly temporal resolution. Here we compare [day] temperatures.

The warmest land is colored yellow; coolest land is colored light blue. Hottest places are in the tropics and during summer in areas where the land is driest. Coldest places are covered in snow and ice. Black areas are missing data — over the ocean or due to cloud cover or lack of sunlight. The values along the white transect on the large map are plotted for March (red), June (green), September (blue).

The white line drawn from south of Lake Michigan east to New York City shows that the transect was about 10°C cooler in March compared to June and September in 2016. As the month of maximum sunlight, June would be expected to be warmest, yet September temperatures were not much cooler due to the thermal inertia of the land.

Compare day/night seasonal changes

Now load March, June and September, 2016 for land surface temperature [night]. Night temperatures are also coldest for places covered in snow and ice, but have important differences from daytime temperatures for warm areas.

The same line drawn from south of Lake Michigan east to New York City corresponds to the plot of nighttime temperatures for March (red), June (green), September (blue). September temperatures were again very close to those in June, especially for the urban areas at either end of the transect (near Chicago and New York City).

Compare urban and rural day/night temperatures

Looking at a weekly map from the end of June, we can compare day and night temperatures with a focus on urban versus rural New York.

Land surface temperature [day] in red and [night] in green for the week of June 26-July 4, 2016. Histograms show temperature distributions around urban New York City (left) compared to rural upstate New York (right).

The first thing to notice is the higher daytime temperatures around New York City (maximum 37°C) compared to upstate New York (maximum 28°C). Second, are the higher nighttime temperatures around New York City (most of values are much greater than 19°C) compared to upstate New York (most of the values are less than 19°C). Notice especially that there is more overlap between daytime and nighttime temperature distributions for New York City. This is the urban heat island effect.

Related Reading

Analysis: Reflections on the Blue Planet

To better engage you on critical Earth science topics, NEO launched a new web-based analysis tool. This Analysis Blog explores NEO data sets used in ClimateBits: Albedo. Albedo is the fraction of incoming solar energy that is immediately reflected back to space.

Note that these examples are intended for curious people looking for hands-on Earth data exploration. Primary scientific research will require additional analyses through other methods. For the basics on how to use the NEO tool, see ‘Analysis tool in 10 easy steps’.

Reflected shortwave radiation

Categorized under ‘Energy’, maps of reflected shortwave radiation show the amount of solar or shortwave energy (in Watts per square meter) reflected by the Earth. These are CERES observations combined with MODIS measurements, available since July, 2006. Brighter colors indicate more reflection while dark blue indicates the least reflection. The brightest, most reflective regions are associated with clouds, snow and ice. Because clouds move quickly, they are best observed in daily maps. The 8 day and monthly composites mute transient weather patterns. More persistent features, such as polar ice caps, can be observed and compared at longer time increments. The least reflective regions are dark surfaces without cloud cover, such as forests and the ocean. The poles are dark during their winters because of the absence of sunlight then.

Reflected Shortwave Radiation (in Watts per square meter). The pale green to white regions show where more sunlight is reflected; dark blue regions are where the least sunlight is reflected.

Land albedo

Categorized under ‘Energy’ as well as ‘Land’, maps of albedo show how reflective land surfaces are from 0, meaning no reflection, to 0.9, indicating nearly all incoming solar energy is reflected. These maps are derived from MODIS measurements, available since February, 2000 at 16 day and monthly composites. Dark blue indicates the least reflection and white indicates the most. Black areas are missing data – over the ocean or due to cloud cover or lack of sunlight.

Land albedo scales from 0 (dark blue) meaning no incoming sunlight reflected to 0.9 (white) meaning almost all sunlight reflected (1 would be all). Black areas mean “no data,” either over ocean or because persistent cloudiness prevented a view of the land surface. Notice the highest albedos are due to ice caps, glaciers and snow-cover.

Comparison: different surfaces

Africa is a continent with the Sahara Desert north of savannah grasslands and then forests with thick vegetation. To see how different land cover impacts albedo and reflected radiation, we compare them during January, 2017. We limit our analysis to the area delineated by the yellow box (below, left). Use Data Probe and Plot transect to explore the whole geographic area, comparing and contrasting values of albedo and reflected radiation.

Left: Map of the region selected as the yellow box. Right: a comparison of albedo and reflected radiation from north to south along the transect (white line).

Notice that albedo and reflected radiation are highest over the Sahara Desert, except for the dark spot associated with the Tibesti mountains in northern Chad. Albedo and reflected radiation decline over the savannah grasslands, which are darker. Farther south, over the tropical rain forest, however, reflected radiation starts to rise while albedo continues to decline – likely due to evapotranspiration that promotes cloud formation.

Left: region selected (white box). Right: scatter plot of albedo versus reflected radiation within that region.

A scatter plot of the transition zone between desert and savannah demonstrates the direct relationship between albedo and reflected radiation.

 

NEO Analysis in 10 Easy Steps

Would you like to explore satellite data yourself? The new NEO analysis tool provides an easy way to compare imagery online and this new blog series highlights different Earth science concepts by pairing an introductory video with an investigation of relevant satellite imagery found here. After you’ve learned the steps, you can try these examples for yourself:

ANALYSIS: PACIFIC LIFE – HOW IS IT RELATED TO OCEAN TEMPERATURE?

ANALYSIS: HOT IN THE CITY

ANALYSIS: REFLECTIONS ON THE BLUE PLANET

Here are the basic steps to follow. Try it now!

Read more

Welcome to new NEO!

If you have been a user of NEO in the past you will, no doubt, notice many changes to the website. After many years (nearly 8!) we finally have a new look and have made several improvements to the navigation and functionality. That being said, much of the flow of the site is relatively unchanged. So let’s take a look at a couple redesigned elements of the dataset view page, which is one of the main functions of the site. (See this example for the full view.)

Starting below the main image, you will see the ability to toggle between different variations of the dataset. These variations will differ depending on which dataset you are viewing and for many the “View by satellite” toggle will not be relevant. Clicking on any of these buttons will both change the image above and the calendar view below.

View by... options

Image download options
Download options

Currently viewing: At the top of the download options block you will see the date of the image you are currently viewing. The date may be either a year, a month + year, a single day, or a date range, depending upon what dataset you are viewing.

Downloads: This is where you can choose the format, size, and color of the image you would like to download. Select the desired file type from the list (for more information about the types of files that are available, click on the “i” help button next to “Downloads”), whether you want the image in color, as you see it on the page, or in grayscale, which can be useful for applying your own color palettes. Once you have made those selections, click on the dimensions you want (e.g., 360 x 180) and you can download the image.

Download Raw Data: If the source data file for this particular dataset is available online there will be a direct link to it here. These files can be in a variety of formats and are hosted by various data providers, not by NEO. We can help point you to the data owners if you have questions but otherwise we cannot provide support for these types of data.

Calendar view
Depending upon the dataset you are viewing, you will see one of three different types of calendar views on the page (the daily view is shown below). Use the blue slider to navigate between the time periods in the given year and the “Select Year” drop-down just above the calendars to change the year. Blue links will change the image shown when clicked. The calendar menus greatly improve your ability to browse the entire collection by date. The old way was a nightmare, I know; thank you all for your patience.

Browse by day calendar

We hope that you like the new look of NEO and if you have any questions about the site please be sure to leave us a comment here or get in touch through our contact form.

Exploring the Role of Snow Cover in Shaping Climate with NEO

ICE Tool Analysis
In this chapter from the Earth Exploration Toolbook, you can investigate satellite images displaying land surface temperature, snow cover, and reflected shortwave radiation data from NEO.

Download, explore, and animate these images using ImageJ, a public domain image analysis program from the National Institutes of Health (NIH). Then use the web-based analysis tools built into NEO to observe, graph, and analyze the relationship between the three variables.

Read more on the EET website.

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