Geography h.w report

Lab 9: Lightning and Arizona Climate/Weather Introduction

Since the early stages of Arizona statehood, the climate of Arizona has been considered one of the primary means of economic boon to the state. The climate encourages the growth of cotton, citrus, cattle, and tourism. For Phoenix, where many visitors and temporary residents are located, the nuances of the Arizona weather phenomena missed or unknown. This lab will explore the unique climate and weather features that contribute to Arizona’s variable and unique environment.

Key Terms Monsoon Haboob Thunderstorm Stages Climate Zones Charge Separation Urban Heat Island Stepped Leader Return Stroke Thunderstorm Formation

Fundamentally, thunderstorms are created by the conflation of two things: rising air and moisture. Both of these are necessary conditions to get a storm, and “uplift mechanisms” help to get the air rising. These include convectional uplift (rising warm air - think of a hot air balloon), frontal or boundaries (air boundaries overtaking each other or colliding - think a cold front), and orographic (mountain uplift - air being pushed up the mountain).

As this lifted air rises, it cools. If the air cools enough, it will reach dewpoint and begin to form clouds. This is when the first stage of a thunderstorm begins. The rising air pushes upward, creating billowing, puffy clouds. This is stage 1 of the thunderstorm’s life cycle: the towering cumulus stage. This stage is defined primarily by updrafts. Once the air begins to cool at the top of the atmosphere, it descends, creating a downdraft in which water and ice begin to fall back down to earth. This is called the mature stage, defined by both updrafts and downdrafts being essentially equal. In this stage is when the thunderstorm is capable of producing tornadoes, hail, winds, and flooding. The final stage of a thunderstorm is the dissipating stage, where the downdraft becomes strong enough to cut off the updraft, leaving behind a cloud top in the upper atmosphere.

Figure 1 Thunderstorm Stages

Charge Separation & Static Discharge Thunderstorms have very turbulent environments. Strong updrafts and downdrafts occur with regularity and within close proximity to each other. The updrafts transport small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet, miles above the freezing level. The water freezes and then falls back in the thunderstorm downdraft as ice and hail. The particles that ascend have their electrons sheared off by those falling back down. Because electrons carry a negative charge, the result is a storm cloud with a negatively charged based and a positively charged top. This negative base also has the impact of pushing electrons in the normally neutral ground away enough to create a positively charged ground underneath the thunderstorm. This charge separation in the thunderstorm and the ground creates an electric field.

You can see this similar action occurring at a much smaller scale if you shuffle your feet across carpet while wearing socks. As you move across the ground, you shear off more and more electrons, causing you to be negatively charged, while the ground or your friend has a relatively more positive charge. When there is enough of an imbalance in charge and you stick your finger out to an object and get close enough, these charges attempt to equalize, creating a static shock which discharges the negative charge from you into the highly conductive metal doorknob. This process is also known as a static discharge, and on a much larger scale, this is seen as lightning in thunderstorms. However, because the atmosphere is a very good insulator, it takes a massive amount of this charge imbalance to create lightning. The lightning stabilizes these massive imbalances between charges within the cloud itself and between the cloud and ground.

Typically, lightning that strikes the ground has a negative charge. It comes from the negatively charged particles inside the lower central part of the cloud and strikes the positively charged ground below. However, not all lightning originate from this central region. Some lightning can strike outside of the area which the thunderstorm is moving across, where the ground is negatively charged compared the strong upper positive charge in the top of the thunderstorm, and is called positive lightning. This happens around 5% of the time, but positive lightning is particularly dangerous due to the much stronger electric field needed to pass through the greater distance of air, sometimes many miles from the thunderstorm.

Lightning Components In negative cloud-to-ground lightning, like the one in the video, the mechanism for moving the negative charge to towards the ground is called a stepped leader. This moves in incredibly fast segments, drawing the negative charge towards the ground by ionizing segmented regions of more positively charged molecules in the air. Once close enough to the ground, a positive charged leader moves from the ground, typically from a structure or feature taller than the surrounding environment, like a tall building or tree (although it’s more complicated than that, and there can be many positive charged leaders). One of the leaders from the ground will then attach to the negative leader and form a channel that the current will flow through. The current, in what’s called a “return stroke” moves up the channel, while the electrons constantly flow down towards the ground. This return stroke is what our eyes perceive as the lightning flash, and is the brightest component. The process is so fast and the return stroke so bright that we can’t perceive the preceding components with our own eyes. The return stroke can also happen multiple times. A feature called a dart leader will act as a secondary stepped leader. A dart leader will travel back down

Figure 2 Thunderstorm Charge Separation

the channel made by the original stepped leader, drawing down negative charge from the cloud and then initiating another return stroke. This is why lightning sometimes appears to flicker.

Figure 3 Lightning Strike Components

Monsoon A monsoon is a pronounced seasonal reversal in wind direction (N to S, E to W). The seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea; in summer, it blows from sea to land. Monsoons are commonly mistaken as a rainy season due to the Indian Sub-Continental monsoon and the heavy seasonal precipitation there. The North American monsoon is a wind pattern that produces a dry spring with a relatively wet summer across the southwestern US and northwestern Mexico. In June, a high pressure ridge in the upper atmosphere blocks moisture from moving into the Southwest. As the summer progresses, the high pressure progresses north. This causes a change in winds over the Southwest. The dry conditions, combined with high solar angles produce extremely hot conditions over Arizona. This hot air lowers the pressure over the southwestern US creating a thermal low at the surface. This thermal low, along with the high pressure aloft then begins to draw air from the south, bringing warm, moist air from the Gulf of

June July

Figure 4 Monsoon Winds and Pressure

California and even the Gulf of Mexico toward Arizona. The increase in moisture, combined with the hot conditions leads to an increase in precipitation (usually thunderstorms and haboobs) over the southwestern US and northwestern Mexico around July and August. The thunderstorms which develop during the monsoon season are known for spectacular light shows. Compared to storms further east, the amount of moisture is still relatively low (despite the added moisture from Mexico). This causes storm clouds to have their bases higher in the atmosphere and also results in less precipitation. Because of this, the lightning produced by these storms is more prodigious and photogenic. In one year (2015), over a million cloud to ground lightning strikes were reported in Arizona.

Haboobs

Haboobs are dust storms created by downdrafts and outflows from thunderstorms that quickly move across the desert. They come from the Arabic word habb, meaning “wind”. Typically, “Haboob” is used to describe the strong dust storms experienced in the Saharan Desert and on the Arabian Peninsula. However, Haboobs can occur in the southwestern US and other parts of the Middle East and Africa Haboob Safety: Arizona Department of Transportation and the National Weather Service have begun using infographics to warn motorists of the dangers of dust storms. Important instructions given include:

1. Pulling over off the road 2. Turning off the vehicle 3. Rolling up the windows, close vents 4. TURN OFF LIGHTS (including hazard lights).

This last piece of advice is given so that in the event another motorist is NOT heeding these warnings, they will not attempt to follow your lights, rear-ending your vehicle.

Figure 5 Phoenix Haboob July 5 2011

Climate Zones Sonoran Desert

Central, southern, and southwestern Arizona are iconically Arizona. The Valley of the Sun stretches from the Alpine Rim and Southeast Mountains all the way to the Colorado River Valley. Along the way, small individual peaks and ranges pop up briefly, though none are very tall. The Sonoran Plateau to the south in Mexico removes moisture from the air, leaving dry, hot air as it descends in the valley. The lack of moisture and flat surface of an ancient interior sea bed allow wind to drag small sand and dust particles through the atmosphere, causing the threat for dust storms in the valley. The added sand and dust in the air also help produce the distinct red sunsets in Arizona. The added particles to the atmosphere refract the light differently than clear air or water vapor. The red hue of an Arizona sunset lends itself the Sonoran Desert. Both Tucson and Phoenix lie on the eastern edges of the desert.

Alpine Rim

The land rises quickly between the Sonoran Desert and the High Plateau country, the rapid ascent cools the atmosphere, condensing any remaining moisture out of the air, dumping rain and snow along the Rim. This thin band of rugged climbs receives both strong winds and high yearly precipitation. Alpine vegetation thrives here under the windy, chilly, and relatively wetter climate. Pine trees mix with the hardy desert cacti species like the Prickly Pear Cactus in this transitional climate zone. Flagstaff, Payson, and Show Low are strung out along the rim. Flagstaff ranks between Buffalo, N.Y. and Erie, PA in terms of annual snowfall with 100.3 inches per year

High Plateau

Northeastern Arizona, mainly Navajo Nation and Hopi Reservation land is a high plateau. To the east, rise the Rocky Mountains, to the west, the landscape slowly descends into the Great Basin to the northwest and the Sonoran Desert to the southeast. Wind flowing from the southwest is dry after being orographically lifted up to the plateau, leaving its moisture behind along the Rim, before racing northwest toward the Rockies. A northwesterly wind howls through the landscape with little resistance for miles to the northwest. There is only small scrub vegetation in the landscape. The major topographical feature to impede the wind are the Sierra Nevada mountains, in western Nevada and eastern California. The Colorado river drain Rocky Mountain snowmelt through the northern reaches of the High Plateau in Arizona, through the slot canyons and into the Grand Canyon before descending further into the Colorado River Valley.

Southeast mountains

The rugged, hot, and dry topography in the southeastern reaches of Arizona provide a foreboding place with incredible lithospheric landscapes. This area is marked by boulder canyons, rugged peaks, and sharp valley descents. Once the monsoon season happens, this part of Arizona is one of the first to begin experiencing the rainy effects of the wind reversal. Gulf moisture moves across the Sinaloa, Sonora, and Chihuahua states of Mexico into southeast Arizona. The topography encourages rapid orographic lift for precipitation to occur, and the dry, warm air quickly descends into the valleys on the other side. The small clusters of mountains throughout the area are testaments to this: with general more vegetation on the southern and eastern sides of the mountains.

Figure 6 Precipitation Map of Arizona

Colorado River Valley

In western Arizona, the Colorado River defines the border between Arizona and California. This boundary is also a key means of water resources for the state and for irrigating agricultural fields. Along the Colorado River, the added moisture from the flowing water slightly decreases the atmosphere’s ability to rapidly heat and cool. This restriction can often lead to warmer nights and higher heat stress values for areas along the river compared to locations just east and a little higher in elevation (Lake Havasu City and Bullhead City compared to Kingman). Due to the slightly higher moisture content, the Colorado River Valley has been a prime location for growing cotton here in Arizona (one of the original “5 C’s” of Arizona’s economy).

Phoenix and Tucson UHI/built environment

While elevation, precipitation, and general topography certainly play a role in defining Arizona’s weather and climate, the anthropogenic influences of roadways, buildings, canals, cars, and even our metabolic habits play a role in affecting the urban weather and climate. Many physical characteristics of buildings and roads define how heat is stored, transmitted, reflected, re-emitted by the surface. One of the most important characteristics is albedo. This plays a role in cities too. Asphalt has a very low albedo (absorbing most of the solar energy it receives), and consequently the pavement heats up dramatically. On a warm day (90-95° F), an asphalt surface can be as warm as 140-150° F. This heat storage by materials commonly found across cities is a contributing factor in the Urban Heat Island. Urban Heat Island (UHI) is a phenomenon where city centers are distinctly warmer than the surrounding rural areas. This is due to a concentration of surfaces that efficiently retain heat (like asphalt), buildings and structures that reduce wind flow to remove heat from the city center,

waste heat emissions from energy usage in buildings, and a larger concentration of human metabolism producing heat. This heat island is most pronounced at night when the buildings and energy consumption in cities impede the atmosphere from quickly taking heat from the surface of the Earth and moving it upward, away from the surface. In Phoenix, the temperature difference between the city center and outlying communities such as Buckeye, New River, Queen Creek, etc. can be as high as 15- 20° F.

Figure 7 Urban Heat Island Temperature

Monsoon Season San Francisco Peaks Exercise

The San Francisco Peaks are a set of peaks in northern Arizona, with the tallest on the northern portion of the mountain, Mt Humphreys, standing at over 12,500 feet. On the western portion the mountain is Snowbowl, a ski resort. The town of Flagstaff sits to the south at around 6,000 feet. The ground is covered in pine trees, turning into scrub vegetation and then into alpine on the San Francisco Peaks as you ascend over the tree line. To start, we will take some data of the region north of Flagstaff in August 2014 and do a quick analysis of it in Google Fusion Tables; please follow the steps below:

A. Download the FlagstaffLightningJuly.csv file. This has a table of the date, time, latitude, and longitude of

over 1,000 cloud-to-ground lightning strikes that took place in August 2014.

B. Then, go to this link: https://fusiontables.google.com/DataSource?dsrcid=implicit . Click on “Choose File” and choose the FlagstaffLightningAugust.csv file.

C. Keep the separated character as “comma” and then click “Next”

D. When the data table appears, click “Next”

E. Click “Finish”

F. Click on the tab “Map of Latitude” near the top of the page.

G. In the upper left hand corner of the map, drag your mouse on top of “Map” and click the box for “Terrain”

H. You should now see a map with points displayed and elevation contours. You may have to zoom out in order to see all the strikes. These are the cloud-to-ground lightning strikes. Do you see anything interesting here? Is there any clustering in certain areas? No clustering in others?

I. For better visualization of this clustering, click “Heatmap” on the left side of the page and set the radius to 20, and make sure you zoom in enough that the map takes up most of the page, giving more detail.

J. What do you see now? This heatmap displays clustering of data points. The more points there are in an

area, the redder the map appears. Feel free to explore this area and toggle between the heatmap and the lightning point map.