Archive for March, 2018

Climate Monday: Weather Forecast Maps

March 26th, 2018

The College of Wooster is now back in session for six more weeks, which means we have six more climate visualizations to share this semester. Today is bright, sunny, and quickly approaching 50°F in Indiana, Ohio, and Pennsylvania, but we’re due for a rainy week, so it seemed like a good time to highlight weather forecasts.  In the USA, we see forecasts for our localities frequently.  Maps or descriptions of current and future weather conditions are pervasive throughout the various forms of media, from phone apps to newspapers to radio broadcasts.

Below is the current weather surface according to The Weather Channel.  There’s high pressure currently centered over the northeast — hence the clear skies, but a storm is brewing over the Oklahoma panhandle.  Over the next few days, that low pressure center is going to move eastward and northward, shoving that precipitation you currently see over Missouri along with it and generating more along those fronts as it strengthens.  I like The Weather Channel current surface maps.  They’re neat, colorful, easy to read, and only show the essentials needed to understand the current weather state.  However, they don’t have a similar map for forecasts.

Current US Surface Weather Map

For example, if you go to the “Classic Weather Maps” section of their website, and then scroll down to learn what’s in store for Wednesday, I you see this:

Day 3 Forecast

 

That not too bad for many people.  Find a city near you, look at the symbol and the high temperature, and you get a good sense of how to dress. However, if you’re halfway between cities shown, like Wooster or the middle of Iowa or Oregon, this map is less helpful. Is is going to rain in Wooster like Cincinnati or just be cloudy like Detroit? Also, there’s a lack of context — there’s no marking of high and low pressure to help give a sense of the atmospheric circulation behind these weather forecasts.  If you’re interested in your locality, you probably want a map that’s zoomed in further, so this national map isn’t helpful. And if you’re a weather geek, you probably want more detail.  So this forecast map may not be what you’re looking for.

Another option is Weather Underground. This website is a bit geekier than The Weather Channel, and it’s especially cool that you can link up your own personal weather station data to their server for free and share it with the world.  Their main forecast map for Wednesday morning (shown below) is dominated by the precipitation.  No cities or cloud/sun symbols are shown, but you probably know where you live and can surmise that a place receiving that light green shade will be cloudy with some showers, whereas the dark green is a sure thing for steady rain at least part of the day.  It’s a little easier to gauge the broader context here, too.  That big band of rain from Texas to upstate New York is a classic signature of a winter storm (yes, “winter”… “extratropical cyclone” is more accurate, but also less commonly said) that’s moved across the Heartland and currently sits in the northeast, a big cold front extending down to the southwest.  But the front isn’t drawn; neither is the low pressure symbol.  There could be more.

Both The Weather Channel and Weather Underground are primarily weather communicators and collectors. They do not actually make the forecasts; rather, they receive forecasts from the National Weather Service (a branch of the National Oceanic and Atmospheric Administration, or NOAA). If you aim for Wednesday from NOAA, you get this map:

For the weather geeks, this is the best map.  It has the precipitation forecast, but it also shows the fronts and the pressure.  The context that can be inferred from other maps is plain and explicit here.  This is definitely not the prettiest map, and if you’re not a weather fan, it might seem pretty cluttered.  However, if you like clutter, check out this forecast map from Unisys:

NAM - US - SL Pres/Prec - 48hr

Personally, I think that is not a nice color scheme, and the number of “Highs” and “Lows” indicated is a tad excessive.  But there’s a lot of data on this map, and that can be useful for analysis even if it fails at communication. In the end, the map you choose likely depends on your personal preference!

A geological and archaeological hike in northeastern Ohio on the last day of winter

March 19th, 2018

It was a beautiful latest-winter day in Wooster. Nick Wiesenberg had the great idea of taking an afternoon to hike through Pee Wee Hollow, a wooded area of ravines, streams and rocky exposures a few miles northwest of Wooster near the village of Congress. Greg Wiles, his faithful dog Arrow, and I went along. We had an excellent time with no agenda but to explore. Above is Dr. Wiles standing at an outcrop of Lower Carboniferous sandstones, shales and conglomerates making up the Logan Formation. The rocks are similar to those exposed in Spangler Park.

Pee Wee Hollow has three small Native American mounds on an upper plateau. Nick and Arrow are standing on one above. They were excavated in the 1950s, and possibly pillaged long before that. Dr. Nick Kardulias, Dr. Wiles and several others wrote a paper on these mounds. I can quote the abstract entirely: “While a great deal is known about the many earthworks of central and southern Ohio, there is a gap in our data about such features in the northern part of the state. The present report is an effort to bring work on one such site in Wayne County into the literature. The Pee Wee Hollow Mound group consists of three small circular earthen structures and a possible fortification trench on a high bluff overlooking the main stream that drains the county. Systematic excavation by avocational archaeologists in the 1950s revealed the structure of the mounds and retrieved a small assemblage of artifacts, some charcoal, and pockets of red ochre. Recent analysis of the artifacts, coupled with radiocarbon dating, indicates that the site was a location of some local importance from the Late Archaic through the Middle to Late Woodland periods.” (Pennsylvania Archaeologist 84(1):62-75; 2014)

Another of the mounds with Greg and Arrow for scale.
The very fine sandstones of the Logan Formation are especially well exposed in the creek beds. Here are a set of joints our structural geologist Dr. Shelley Judge would appreciate.

There are even some nice Bigfoot field structures. Who knew?We spent most of our time walking up Shade Creek. The creek bed is mostly Logan Formation sandstones.

Greg is standing here on a bedding planes of sandstone with nice ancient ripple marks. Note, by the way, the chunk of ice above his head. Still winter, but not for long.

Here’s a closer view of those ripples.Arrow here contemplates a thick exposure of dark gray shale. Greg found some nice crinoid columns in it, and I found several molds of bivalves.

The more resistant units in the Logan have the best fossils. This slab of very fine sandstone cemented with iron carbonates (a type of siderite concretion) has several internal molds of brachiopods and white calcitic crinoid columns. I described the remarkable preservation of similar crinoids in an earlier series of blog posts.

A nice, uncomplicated walk in a beautiful bit of nature.

Climate Monday: Visualizing the South Asian Monsoon

March 5th, 2018

Last Monday I posted some diagrams, animations, and predictions for El Niño and La Niña. So this week we’ll shift from the Pacific Ocean to the Indian Ocean and check in on the South Asian monsoon.  “Monsoon” is really just another word (of Arabic origin) for “season”, but it’s typically used to describe places with distinct wet and dry seasons caused by a reversal in the dominant regional winds.  There are several factors that impact any monsoon, and in India three important ones are:

  1. The position of the “Intertropical Convergence Zone” (ITCZ)
  2. Land heats up and cools down much more easily than water.
  3. The Himalaya

Although the relative importance of #1 & #2 for South Asia is still debatable, most traditional explanations focus on #2, possibly because it is easier to explain…

Figure 1 is a diagram from Thomas Reuters that depicts the traditional explanation for why monsoons in South Asia (and elsewhere) occur.  The theory goes that:

  1. Land heats up rapidly during summer, while the ocean heats up slowly, so the land surface ends up hotter than the ocean surface.
  2. Hot air is less dense, making it buoyant and likely to rise.
  3. Rising air over land is replaced by cooler ocean air from the southwest, which brings ample moisture with it.
  4. This moisture-bearing air then rises over the Indian sub-continent, cooling down, which causes condensation (cloud formation) and rain, rain, rain.

In winter, this all works in the opposite direction:

  1. Land cools down more quickly than the ocean, so by mid-winter the air over the ocean is warmer.
  2. Rising air is limited to the ocean, and India experiences sinking air instead.
  3. On top of that, winds blow from the northeast over India to replace the air that’s rising to the south, and those northeasterly winds are dry because they come from interior Asia.

In this way, land-sea contrasts help form the monsoon — a seasonal oscillation of southwest to northeast winds and wet to dry seasons.  You’ll see this same description in many animations of the monsoon, too, like this one from NASA:

However, these explanations are incomplete.  Land-sea contrasts are just one factor impacting monsoons.  If they were the only factor, we’d expect monsoons to exist everywhere with a strong warm/cold season and a land/sea boundary. We’d also expect monsoons to be absent anywhere without a strong land/sea contrast or warm/cold season.  Neither of these is true.  The Sahel in Chad is far from any ocean but has a monsoon climate, and islands like the Galápagos and New Caledonia have a monsoon despite being surrounded by the Pacific Ocean.  Meanwhile, places like North Carolina and France have strong winter/summer contrasts in temperature but no clear wet/dry season, and even coastal places like San Francisco, USA or Luanda, Angola, which have distinct wet/dry seasons, lack the wind reversal characteristic of a monsoon.

Figure 2: Seasonal shifts in the Intertropical Convergence Zone (ITCZ) — the main tropical rain belt. (Image Credit: Mats Halldin)

The South Asian monsoon cannot be understood without another aspect: the Intertropical Convergence Zone (ITCZ). This is a zone of hot, rising air throughout the tropics.  This air cools at it rises, causing condensation and rainfall.  It occurs primarily because the tropics receive more direct sunlight than anywhere else in the world, and because of that solar control, the ITCZ drifts northward in May through July and southward in November through January, following the Sun.  It happens over land and water alike, but the shifting tends to be more prominent over land areas, which can heat up and cool down more quickly. In other words, when you combine the concept of land-sea contrast with the concept of the ITCZ, its understandable that the monsoon in South Asian is particularly strong. Both are working in concert.

You can see the progression of the monsoon northward across India throughout June and July (Figure 3).  It’s mostly a south-to-north progression, but also largely east to west.  Again, this is due to a convergence of factors, not just land/ocean heating contrasts.

Figure 3: Progress of the 2016 summer monsoon in India compared to normal. (It was a late monsoon year.) Source: India Meteorological Department.

However, the South Asian monsoon also would not be nearly so strong without the Himalaya — the highest mountains in the world.  These mountains are so imposing that they effectively block advancement of winds blowing from the southwest.  Warm, moist air from the Indian Ocean stalls out in the Himalayan foothills, making Bangladesh the wettest place on Earth.

This video and animation from JeetoBharat, an Indian mentoring and test-prep organization, does a better job incorporating the multiple facets of the South Asian monsoon: